Breaking into APC’s BR24BP battery pack

by on Oct.27, 2009, under Hardware, How-To's, Investigative Dissassembly

apcLogo_141x68.jpgIn this post, we will show what was necessary on how to get access into the BR24BP battery pack for APC’s BackUPS RS/XS series of battery backups.  Read more for additional details, caveats and some good to know general informaton on how you can replace the stock batteries in the battery pack with your own batteries to save money and time.

BIG MONSTEROUS LEGAL DISCLAIMER: This information is provided as anecdotal as-is information. It is recommended when working with electronic components to replace the defective component with a component of the same type, model and rating.  Battery backups are no exception to this rule and as such we recommend that you follow the exact ratings as specified on your UPS, even if they differ from the information on this site.  When in doubt, go with a 1-to-1 replacement, or better yet, purchase a replacement battery pack from APC directly.  By following the steps in this guide, you indicate that you can not sue firestorm_v1 or those of us at YourWarrantyIsVoid.Com if you burn your eyebrows off or cause damage to loved ones or property, etc..

Now that the legalese is done with, let’s talk about this a bit.


Ever since the dawn of time, or at least the dawn of the computer age, mankind has been faced with one monsterous problem:  How to keep the computer running when the power goes out.  Even in the 21st century, we are still not immune to the power failures, surges and brownout/blackouts that plague our planet’s power grid.  The solution was to use uninterruptable power supplies (UPSes) also commonly called “battery backups” to keep the juice flowing, even though the power from the electric company had ceased.  The idea was that power would be stored in batteries and would be used through specialized circuitry to recreate the line voltage that our beloved machines needed to operate.  The idea was a grand saviour to the information age  and since then have saved countless months of uptime loss, unavailability and other such lack of availability.

However, with every great solution is a thorn in its side.  In our case with the batttery backups, the thorn is the batteries.  Every now and then, through regular use and standby charging, it becomes necessary to change them out.  Usually this cost is a lot less than just buying another battery backup and is a preferred method to keeping old but still usable UPSes out of the trash can. In my specific case, I have an APC Battery Back-UPS XS 1500 and it has served me extremely well through the years.  Unfortunately an extended power failure had knocked it and the addon battery pack, BR24BP out of comission and had rendered the UPS useless.

Initial examination:

Pior to just ripping the face place off of anything, even if it pisses me off, I perform a good bit of research using Google and Yahoo to attempt to find disassembly instructions for something.  In the case of the battery backup’s battery pack, I had nothing but a bunch of forum posts with people looking for the same details.  Unfortunately no solution was to be had so I started investigating on my own and was ultimately successful.  This writeup is a testament to those findings and a howto for anyone that was as lost as I was with trying to find a way in.

BR24BP battery pack side view

BR24BP side vew

This is a shot of the BR24BP in all it’s glory.  Despite it’s innocent looking exterior, it’s a mofo to get into.  With no visible way of getting in, I set out with my metal screwdrivers and started prying like a madman.  Eventually, I was able to get the white front cover off and the secret to this beast was unlocked.

Front panel finally off

Front cover finally off

(The grey piece on the back was held together much in the same fashion, pardon editor’s fault for the back showing ajar. 😛 )  In this shot, we learn something important.  The front is not held on by any fancy method, locking mechanism or other trickery.  It is held to the front of the battery backup by means of a pair of snaps.  One at the top of the cover pointing down, and one at the bottom cover pointing up.  It would be almost trivial to modify the case so that you could get into the battery box at some point again to do a second swap out fo the batteries.

Back of front cover. The back is identical except it's grey and has a hole fro the power cord.

Back of front cover

Here is a shot of the front cover.  The back cover is identical to this except it is grey and has a hole for the power cable to go through.  It could be theorized that the ends of the snaps are sliced to prevent from someone gaining access to the innards of the battery box.  It could also be theorized that a quick session with a Dremel could prodice a hole with which to pry up on the snaps to gain access in the future without having to pry the case open like your life depended on it.

Back cover pried off

Back cover pried off

This is a shot of the rear cover after being pried off.  The thin holes at the top and the bottom are the holes for the snaps.  Once finally freed of both the front and the rear faceplates, we are left with the battery box and six phillips screws from victory.

Battery pack sans front/back covers

battery pack sans front/back covers

In this shot, we can finally remove the only screws in this battery pack’s setup and pull off the cover to reveal the batteries inside.  One special note about the covers.  They are omni-directional (Editor’s note: omni-sidal seemed to not be a word. 😛 ) meaning that the “left” side could easily be the “right side”.  The only thing that determined direction was that the indentation for the pedestal foot was at the bottom (pointed towards you if on a table) and that the cord came out of the “back”.  Aside from that, it was anyone’s game.

Finally unboxed

Finally unboxed

(I apologize for the blurriness of the full-size picture, just use the thumbnail for general positioning data.)  Behold, here is the batteries in all its glory.  Keep in mind that my top two batteries are “poofy” and need to be replaced.  The bottom ones, are not poofy so will get taken to a battery place to get charged and tested.

Battery type and model #

Battery type and model #

Before we get into wiring diagrams and all that nonsense, please make sure you use the right battery.  These are 12 volt,  3.4A batteries.  CSB# HR-1234W-F2 and are the Sealed Lead Acid (SLA) type.  IT IS CRITICALLY IMPORTANT THAT YOU USE THE SAME TYPE AND RATINGS OF BATTERIES IN YOUR UPS.  EXTENDING YOUR UPS MAY GIVE YOU ADDITIONAL UPTIME BUT WILL CAUSE YOUR ADDITIONAL UPTIME TO FAIL AS THE CHARGER CIRCUIT CAN NOT HANDLE THE EXTENDED CAPACITY! (Can I say this enough?) If you are in doubt about whether or not you have the right battery, take one from the pack to an Interstate Batteries or a Batteries Plus and get four just like it.  Bring them home and wire up as covered below.

Wiring it all up:

The interesting thing about this pack is that despite the link that I found for APC, this is actually a 24VDC system, not a 12VDC system as advertised. Granted, the batteries themselves are 12VDC, but they are hooked into a 12x2x2 array meaning that the batteries are connected in series (to make 24VDC) and then are connected in parallel to another pair connected in series.  The entire thing ends up being a 2x 24v array as shown in our next picture:

battery hookup

Battery Hookup

Ok, so let’s review this hookup.  The umbilical cord that goes to the main BackUPS is in my hand.  For now, ignore the little yellow wire.  It has nothing to do with our hookup at the moment.  Clockwise from top left, we have the UL (upper left) battery, UR (Upper Right) battery, LR (Lower Right) and LL (Lower Left) batteries.  The umbilical has two positives and two negative leads on it. WHEN HOOKING UP BATTERIES, YOU MUST OBEY THIS IMPORTANT RULE: AT NO TIME WILL THE POSITIVE AND NEGATIVE OF THE UMBILICAL BE ON THE SAME BATTERY!!! THIS WILL CAUSE AN EXPLOSION AS THE 24V BATTERIES TRY TO (AND SUCCEED) OVER CHARGE THE SINGLE 12V BATTERY! That being said, connect one of the RED wires to the positive terminal on two batteries and connect the BLACK wires to the other two batteries’ negative terminals.  At this point, all four batteries should be connected with one connection from the umbilical.  Connect the BLUE wires from the positive battery’s NEGATIVE terminal to the negative battery’s POSITIVE terminal.   In essence, you have created two 24 volt battery packs made of UL and UR, and LL and LR respectively.

Little Yellow Wire

Little Yellow Wire

Now, back to this little yellow wire (and the 80A fuze)  This wire acts as a “Sense” for the battery backup to determine the battery pack’s overall health. The wire has a resistor shrinkwrapped inline with it and should not be tampered with.  Doing so may adversely affect the UPS’s operation.


It’s been fun dissecting this battery pack however it was hard as hell initially.  APC does not make it easy to pry open the front/back of their UPS battery packs.  I’m hoping that someone aside from myself finds this information useful as it was not easy obtaining it.  If you have any information on where to get good replacement batteries, or you wish to share your experience, feel free to fire back at me in the comments section.

Until the next post, happy hacking!


UPDATE!!! 11/10/09

Readers Derek and Shaaz commented concerning the resistor that was inline on the yellow lead of the power cable.  After performing some investigative research (edit: I tore off the shrinkwrap), I have found that it is a 1% tolerance metal oxide film resistor.  The resistor color code is Red, Black, Black, Red, Brown which indicates a value of 20 ohms with a tolerance of 1%.  The tolerance is key as I am still of the opinion that this is how the APC backup monitors the battery level and is able to determine the level of power left. I would not suggest using a standard resistor instead of this 1% tolerance resistor as this will affect how long the UPS thinks it has on charge remaining.

Reader “Steevo” wrote in to catch me on a critical error in regards to this resistor.  M initial calculations were incorrect, the value of this resistor is not 20K ohm, but rather 20 ohm, Please make sure you use the correct resistor if you ever need to replace it.

Finally the mystery is unveiled

The secret is revealed!

UPDATE: More details about the power connector from the BBU power pack to the UPS

After requests for information in regards to the power pack’s connector, I realized that I had made a serious oversight.  I had not documented the power connector that connects the battery pack to the UPS body.  With this in mind, I took some more pictures of the connector:

Power connector

Power connector

The power connector has four pins holding the strain relief on.  After getting the pieces pried apart, the end connector slides off and you can then see the wiring on the inside.  Barring that, I went ahead and was able to extract the outside case off of the three spade connectors as shown here. In order to extract the three spade connectors, you will need to take a long sharp screwdriver and pry the plastic clips holding the spade connectors down.

connector endcap

connector endcap

Here’s the connector with the spades having been successfully extracted:

successful extraction

successful extraction

Keep in mind that the plug orientation is not certain as I’ve reinstalled the UPS and it’s buried under the desk.  The red mark on the connector jacket is there for orientation purposes, but interfacing with the UPS may require that the black wire is on the left, and the yellow wire is on the right.

:, , , ,

49 Comments for this entry

  • J Craig

    Excellent article–saved me replacing the wrong batteries–the ones in the main UPS were okay–it was the external pack that was bad. I didn’t figure that out until I’d already bought batteries (not a problem–same type).

    RE getting off the front and back covers: I found it easiest to pry from the bottom (there’s a notch that I got a flat screwdriver into with a little forcing and then just pried outward on the cover). With the bottom end off, you can slide the cover up toward the top and, if you’re lucky, just unhook the teeth of the top tab–or pry a little bit while pushing up (so the teeth on the tab mostly disengage.

    Before I put it back together, I sanded off the back of the teeth on the snap-together-tabs at the top and bottom of the front and back covers. The effect of that was to make the flat back of the teeth effectively shorter: the teeth angle toward the point on their backs now too (it’s not flat for the whole height of the teeth now). Which minor adjustment, I hope, will make it easier to pry off another time.

    I also noted that there are molded-in screw holes in the inside case (at least on my model): top and bottom; front and back–if you measured carefully, you could drill holes in the outside front and back cover and use a narrow self-tapping screw to hold the front and back on–and just cut off the frustrating locking tabs–or really grind down the back of the teeth so they’d just hold it in place so it’s easier to get the screws in. It wouldn’t be quite as smooth on the outside, with the screws there, but it looks like a moderately straightforward option to improve access to the guts. If I were going to do that, I’d want to use some washers under the screw heads because the cover does not have a molded-in hole to accept the screw and the face of the cover isn’t very thick.

    One item that may be of interest to others, when I went to Batteries Plus, the guy basically said, if the battery is the same physical size, it’s almost certainly the right one; they had them in stock. The replacement batteries are widely available on line also (a lot less if ordered although I did not calculate shipping). If you search for BX1500 replacements, you’ll find the right size (generally sold on-line in pairs–with the intention of reusing the harness that is holds the main unit’s batteries together). The same batteries apparently work in either the main unit or the external pack.

    The Batteries Plus fellow also told me that a good, fully charged, battery will show a voltage of at least 10 volts (they had a tester that would check the capacity, but the ones I took in were so dead they didn’t even register). So, if you take a DC volt-meter and put the positive end into the middle prong on the big plug (whose guts are illustrated in the later pictures), you can put the negative side in the prong on the left (or right–at least one side works), and check either the battery pair in the main UPS or the 4 in the external pack–you should get a reading of somewhat more than 20 volts (my best ones–which are still some years old, mind you) show around 25 volts. (I’ve got 4 of these units with matching external packs.) The ones I’ll work on next show around 20 to 21 volts–probably at least one of the batteries in those packs is no good or perhaps, as in the one I operated on today, a pair is no good.

    With some extension wires/test leads, a place like Batteries Plus would be able to test the capacity of the dual battery pair in the main unit or the external pack itself to give you even more info–without actually taking anything apart (other than pulling the battery pack from the main unit–a simple no-tools job). That would at least let know what kind of life the batteries have (if the straight voltage test isn’t a it’s-definitely-dead reading). Based on what I know from testing batteries of other technologies (such a ni-cd or li-ion), a battery can still show a reasonable voltage, but not hold much charge. However, if it doesn’t give you the voltage at all, you’ve got a dead one.

    (Thanks again for the pictures and carefully created text! So far as I was able to find, this is a unique set of directions.)

  • Richard Benjamin

    12V 7 Ah batteries are available at HobbyTown USA stores for 19.99. They are used to start model airplane engines.

  • firestorm_v1

    Hello J:

    Thank you for your comments. I really appreciate the additional info and I’m sure that this will help others in the same situation. One of the reasons I even wrote this article was because of the limited information on the backup expansion pack. There was almost no documentation to start with and all I could find were sites that wanted to sell you a new battery pack at almost the same price as what I paid for the UPS/Battery Pack combo. When I went to the Batteries Plus store, the guy there was really helpful with identifying similar and compatible products and many people I know recommend them for all their specialty battery needs. I’ll keep that in mind about the online ordering however shipping could be a killer.


  • John Peoples

    Thanks to all for this very valuable info. I’ll be making use of it in the very near future. Tried to pry off those covers but didn’t want to put any real heavy torx on them until checking the web for info like this!

    What did we do before the web?

  • Scott

    Since I’m looking at the inside of the master unit right now I can definitively say that the wires go yellow/red/black starting from the USB connector (or left to right looking at the back of the unit). Thanks for the info.

  • firestorm_v1

    Hello Scott:

    Thank you for the clarification. I’m sure this will help a lot of people.


  • firestorm_v1

    Hello John:

    You are very welcome for the information! Honestly, I’m not sure what we did before the web aside from the usual “Well if it doesn’t come off, pry harder! If it broke, well it needed replacing anyways” approaches.


  • David Megnin

    I just bought a Back-UPS Pro 1500G and an extra RBC124 battery pack thinking I would be able to plug that in to the external battery connector where the BR24BPG goes. The RBC124 did not come with a connector, but I have one from a RBC32 pack. It includes the yellow wire.

    Can I plug the RBC124 into the back of the Back-UPS Pro 1500G to get a little extra run time?

  • firestorm_v1

    Hello David:

    The RBC124 may contain the right batteries to repopulate a BR24BPG or at least reload the ones that came with your UPS however I don’t think it would work in its existing configuration without further investigation. It really depends on how the battery pack is wired and brief googling didn’t turn up much in the way of wiring diagrams. Be safe and be careful. If something bad happens, it could catastrophically go bad.

    Good luck!


  • D Scott

    correction, according to Mrg. these batteries are 9.0Ah not “3.4A” as described above.

  • L T

    Nice writeup! I came across your article while searching how to open up the BR24BP, since it eluded me a bit as well seeing no visible entry.

    The lab I work with was scrapping 3 Back UPS XS 1500 VA, and one of these battery packs. Some of the batteries were pretty bad… the plastic had completely cracked exposing some of the insides! I ended up finding 2 good sets (4 batteries) in between all the 10. Two are questionable, as the UPS says bad battery, but they are not ‘poofy’ nor cracked. I’ll see what a voltmeter tells me with these… they might just need to be recharged, perhaps.

    I pondered in keeping the battery enclosure, but I needed something to store the bad batteries in so I don’t get stuck with them and my lab can recycle them as asset dispositions, ha.

  • L T

    Oh, for anyone looking for 8 AH, batteries (1yr warranty), I found Apex Battery carries a relatively inexpensive set of 2:

    (No cables, but if you have the UPS already, you can reuse them)

    10% off code: cod-10offapex
    Free shipping, and 4% cash back from Mr. Rebates.

  • AJ

    Just a note to thank you for putting this resource out here. I have 3 of these and 2 were complaining about the batteries. One thing I found… some of the battery packs were deformed and bulging. Not good. I will replace with the same. Hopefully no problems for a while after that. Thanks again for your blog on this.

  • slgwv

    Thanks so much for figuring this out and writing it up! I have a couple of old BR24BP units that I was just about to give up on and recycle, and now it looks as though I can fix them. One of the units contains 4 Vision CP 1290 batteries instead of the HR 1234W units, the difference seeming to be that the CP 1290’s are 9 A hr rather than 7.5. The physical dimensions are essentially the same. Also, the main unit that the BR24BP plugs into, the Back-UPS XS 1500, uses a “RBC 33” battery cartridge. Turns out the RBC33 is just two of the CP 1290s with a plastic spacer containing some cables, so you don’t need to buy a whole new RBC 33, either. Btw, the CP 1290s in the RBC 33 are also hooked in series.

  • Kilow

    Big Mahalo! You da big Kahuna! Help me a lot!

  • firestorm_v1

    Hello slgwv:

    I’m glad that you were able to find this information useful. Thank you for the information regarding the RBC33 packs. Mine appeared to be wired up in series which baffles me even further as to the wiring schema of the BR24BP. It’s like a parallel of two RBC33’s for a 24 volt pack. As several other users have pointed out, there are many places to get alternatives to the batteries that APCC sells, and oftentimes you can get a cheaper rate or discount on multiple purchases.

    Thank you for the information!


  • firestorm_v1

    Hello AJ:

    That seems to be a common sight among battery failures in UPS devices. It would appear that as the cells age, they have a tendency to build up gases during charging (or in overcharging in a lot of cases) that cause the unnatural bulging. While they are “sealed” batteries, they have emergency vents that will pop to allow for gases to escape. Much better than an exploding battery. It’s recommended to replace any battery that’s showing a bulge as this is usually a sign that the battery has gone bad and will actually shorten your battery backup time.

    Thank you.


  • firestorm_v1

    Hello LT:

    Thanks for the heads up. This is a bit cheaper than what the Batteries Plus was offering them for.


  • firestorm_v1

    Hello LT:

    You might want to get an external battery charger for those and try to charge them. At the very least, they won’t accept a charge but you may be able to get two more good batteries out of them. SLA batteries are thankfully a bit more forgiving of abuse than other battery types.

    Good Luck!


  • zorggy

    it’s a long time i’m not come here. (and bad english, i’m french sorry lol)
    after maybe One year, my custom battery pack run always good. (connector external on BR1500 APC with only 2 “car battery” 12v in serial (and in a made box).
    i’m always happy from theses quality/cost (very good time on car batt, and cheap). thanks for picture from this BR24BP for example 🙂

  • firestorm_v1

    Hello Zorggy:

    Thank you for your response! It’s always nice to hear this working for people and being able to avoid tossing away a perfectly good battery backup.

    Thank you.


  • ML

    Simple answer to complicated question: you really want to use only the CSB HRL1234W battery in UPS applications for the 8-9AH class batteries. It is the almost the only battery on the market designed and specified for high rate UPS service with long float life. Otherwise performance and float life will suffer markedly. If you are interested, a tome follows.

    Lead acid batteries have a particular characteristic due to their construction and chemistry which is that the faster they are discharged, the less total energy they can deliver. This is true of most types of batteries but more so for lead acid than for Ni-cd, Ni-mH, and most lithium chemistries. This presents itself as a substantial loss of capacity (amo-hours or AH) as you increase the rate of discharge.

    Lead acid battery construction varies based on the kind of service for which they are designed, and between manufacturers. Starter batteries need to provide huge currents but are rarely discharged more than 20%, so they have lots of thin plates which present a lot of surface area to the electrolyte but which are fragile and don’t last long under deep discharge or float conditions. That’s why your car battery lasts maybe 3 years. Deep discharge batteries have robust plates but less surface area, so their internal resistance is higher and so they can’t provide as much peak current. But they can stand hundreds of cycles of 100% discharge. Some, like the ones designed for solar power storage or telephone system backup, can last for 20 years or more. So the mfg. has to get the secret sauce just right for the specific application.

    Most lead acid batteries (other than starter batteries) are specified at and designed for a 20 hour discharge rate. That is, the capacity a 9AH battery is specified at a discharge rate of .45 amp for 20 hours. Discharge it at a faster rate and you get less capacity; discharge it in a few minutes like UPS does and you get a LOT less capacity, maybe a third or less.

    Problem is, a UPS really wants both capabilities: a battery that can be discharged in minutes with minimum loss of capacity, but which can handle hundreds of 100% depth of discharge cycles and also be left floating for years. This is where the CSB HRL series comes in.

    The HRL 1234W design is optimized and rated specifically for UPS-type service. The float life is almost double of the typical 1290 SLA battery – for instance, a PowerSonic P1290 is rated 4-5 years of float life while the CSB HRL1234W is rated 8 years. In both cases this is at 20 deg. C and 2.275V/cell, both of which are critical to float life. In the real world, at 30-35C inside your UPS, its more like 2 years vs. 4 years. But double is still double.

    The 1234Ws are not rated at 9AH as D Scott mentioned earlier. Rather, the “1234W” refers to battery capacity in UPS-type applications, specified as 34 watts per cell for 15 minutes at constant power discharge, or about 350 watts of output for 15 minutes in a 2 battery inverter once you consider inverter efficiency. This does equate out to something like a 9AH at the 20 Hour rating that is used for most other batteries in the 1290 class. But moat of those batteries will not meet the 15 minute rating as they are not optimized for this service and lose more capacity under high load. And they will only last half as long.

    The HRLs are premium batteries and they come at a premium price; about 40% more than typical 1290 batteries. But, as with everything else, you get what you pay for. You can buy them in assembled packs from APC (yes, APC uses only CSB HRLs) or on the open market as individual batteries at a much lower cost. Either way, these are really the batteries you should use for max hold up time and battery life.

    Disclaimer: I have been involved with electronic power design for 20+ years but I have no financial or other relationship with any of the companies mentioned above.

  • Fatboyjones

    Thank you for this write-up as I just acquired an APC 1500-RS that has some issue and want to put more batteries in to revive and test. And if I was successful I wanted to add additional batteries which I can now see how it is done electrically.

    Thanks 🙂

  • Deepak Insight

    I came across your article while searching how to open up the BR24BP. Thanks to all for this very valuable info.

  • Bill Wetzel

    Better check your resistor value again. It sure looks like a 20 K-ohm resistor, not 20 ohm. I checked several resistor-code web sites to be sure.


  • Julio

    Those CSB HR 1234W batteries do not have 3.4 Ah capacity, they have 7.2 Ah and they are the most popular UPS batteries in the world, any battery vendor can tell you this.

  • Julio

    The batteries in the APC pack pictures are HR 1234W, not HRL 1234W so it seems that APC has lowered its standards.

  • Michael

    The resistor code shows a 20 k ohm resistor, not a 20 ohm resistor. The confusion is because 1% resistors have one extra digit to indicate the more-exact 1% value.

    In this case:
    Red, Black, Black, Red, Brown
    2 0 0 10^2 1% = 20.000 ohm = 20k ohm

    Just to clarify the confusion, a 20 ohm 5% resistor would have the code:
    Red, Black, Black, Gold
    2 0 10^0 5%

    Please, PLEASE, correct this mistake.

  • Wolfgang Rupprecht

    Thanks for the info on cracking open the case. I had 6 new CSB HR1234W’s in hand and was scratching my head as to how they wanted this silly side-car opened. The force required was well past what I thought would break things. I guess I’m not used to 1″ wide snaps made out of thick plastic like this.

  • Michael

    Does somebody have any part number for the battery power connector so I can make my own extended battery pack without having to break into the BR1500?
    And if somebody can also tell me an online site where to buy the connectro, that would be just great.

    By the way, many thanks for your post!!

  • Sumeet

    I too checked and it seems the resistor is 20K Ohms not 20 Ohms. I am trying to build this battery pack using this information on this page for my UPS as the BR24 is no longer available being obsolete.
    I am curious to know the role of this resistor. I searched all over but could not get a clue. Could somebody with a good understanding of electronics give a clue?

  • Sumeet

    There is one more explanation for the resistor: It could be a Negative temperature coefficient (NTC) thermistor. The resistance will decrease with increase in temperature. Thus it may serve as a sensor for high temperature. Only the placement and appearance of the resistor does not suggest so!
    I got this idea from where such a resistor is infact being used with an identical circuit diagram.
    Any comments?

  • Sumeet

    Yet one more explanation is that the resistor is Metal oxide varistor which can drop to a very low resistance when a high voltage is applied, making it useful for protecting the equipment by absorbing dangerous voltage surges. Ref:

  • Sumeet Patney

    Comments are invted on my earlier posts.

  • Bryan C

    First thing and most importantly, thank you for your post. After a bit of hesitant prying on my plastic case I turned to the greatest invention ever, Google. Well at least a close second to sliced bread. 🙂

    One bit of information I was looking for after reading your post was where to put the hole that would allow easy removal of the covers. After extrapolating dada from your photos I came up with a ¼” (6.35mm) hole with a center point 7/8” (22.23mm) from the ends (top/bottom) of the case. It worked well. You can buy white nylon plugs if you wish to plug the holes for aesthetics, not mechanical reasons to plug them.
    For your interested readers a quick lesson on resistor color code. Preface: this was how I was taught so no haters please. Bad Boys Raped Our Young Girls But Violet Gives Willing.

    Black = 0
    Brown = 1
    Red = 2
    Orange = 3
    Yellow = 4
    Green = 5
    Blue = 6
    Violet = 7
    Gray = 8
    White = 9

    Resistors have 3, 4, 5 or 6 bands. For the purpose of this discussion we will only deal with 4 and 5 bands.
    3 bands are 20% resistors and 6 bands are for temperature coefficient, nether of which most of us would be dealing with.

    The last band is the tolerance.
    Next to the last band is the multiplier.
    The rest of the bands are numbers.

    Take this resistor. Red (2), Black (0), Black (0), Red (2), Brown (1%)

    First, the best way to remember the multiplier is just add the number of (0) as represented by the color.

    That will give us 2, 0, 0, + 00 (or x100) ohms at 1% or 20,000 represented as 20KΩ (ohms).

    Other tolerances: Silver = 10%, Gold = 5%, Red = 2%.
    Again, your post was a tremendous help. Thanks

  • Dan Oetting

    That little yellow wire…

    It’s there to sense something but what could it sense? There are obvious answers like battery voltage or battery current. A voltage sense that eliminates the voltage drop across connectors and long cables would require two wires. A current sense as someone else pointed out cannot cross connectors that would add too much variability. A very usefull sense for an external battery back would be the pack temperature. But that isn’t a thermistor under the shrink wrap.

    What’s left to be sensed? The resistor value, the connection to the red or black wire and possibly the state of the fuse. What this tells the UPS is which battery pack has been plugged into the connector. The UPS may use this information to adjust the charging current. Or, it may use the information to panic and void the warrantee if the user plugged in the wrong battery pack trying to extend the runtime beyond the designed specification.

  • Smokin69

    As Bill Wetzer(couple replies above) said, better recheck that resistor value, I get the same as him. it is a 20 K ohm not a 20 ohm

  • Smokin69

    Can you check the resistor value again?? It looks to be a 20 k OHM not a 20 OHM. I am going to build one of these for my BR1500g and want to make sure I use the right resistor. red black black red brown is a 20 K OHM 1% not a 20 OHM

  • Norman Walker

    Thanks a bunch for this information. After finally prying the front (and then the back) off, I realized that the catches are backwards from what I expected. Had they been reversed, one could simply stick in a small screwdriver in the grove and press the “hook” away from the catch. As it is, that only makes it TIGHTER! Thanks again…

  • Emanuel

    You affirm that the resistor is 20 ohm

    In photo is clearly seen Colour code resistor RED-BLACK-BLACK-RED-BROWN = 2-0-0-x100-1% => 200×100=20.000. =20Kohmi 1%

    what is the truth???

  • firestorm_v1

    Hello Wolfgang:

    I was pretty surprised with how much force I had to put into the pack to get it open as well and in the process of gaining access, I ended up breaking a screwdriver. The upside is that at least it’s not the brittle mess that people usually make for enclosures so it’ll take some abuse. Glad to know you were able to get in yours.


  • Rich...

    Doesn’t seem to matter too much what size resistor you use. I tried 20, 22k, and 44k and got the same results from all of them. Seems to just to tell the unit that there is an external battery box. It does however turn on the fan. I guess the fan needs to run whenever there is an external battery. Tied 2 9ah batteries, 2 18ah batteries and, 4 18ah batteries and got the same estimated run time no matter what combination of batteries or resistors I used.

  • Matthias

    I was able to measure the resistor. It has factful 20 kOhm! So, please correct this in you description!

  • Mark

    Your comment “It would be almost trivial to modify the case” was exactly how I started out. I drilled 2 1/4″ holes (side-by-side) for where I perceived the snaps to be and then used a flattip screwdriver to pry the faceplates off. The holes are about 3/4 of an inch fron the edge. This made taking the covers off simple and like you stated available if we ever need to do this again. Considering I have 6 of these to work on this is a big time saver. Thanks for the other information as well.

  • Erich

    Thanks for the write up!! Almost four years ago and still a valuable tool!

  • firestorm_v1

    You are very welcome. This article remains one of the most popular articles and it’s always nice to hear that people are still finding it useful.

    Thank you.


  • Theron

    Easy way to get the front and back covers off:
    You will need an average flat-blade screwdriver, small will not work.
    Lay the unit on your lap, front facing away from you. Push on the front cover, near one end, just enough so that you can get the blade of the screwdriver against the inner edge of the cover. Then just hit the screwdriver on the back of the handle with your palm. It pops the tab out easily on that end of the cover, and popping out the tab for the other end is easy. For the back cover, repeat the process.

  • Joe

    I replaced the batteries in the APC RS1500 and the “replace battery” indicator light turned off. How does one know when to replace the batteries in the battery pack BR24BP?

  • ShadowClown

    Thanks! Old one but great information!

1 Trackback or Pingback for this entry