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Replacing the APC BP24BPG external battery with 870-2183 (RBC-33) harness


OLD CS1

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SPD_MMAE-83NRKD_B_V_520x520.JPG.bbfaf660933cd4b340d759668da44727.JPG APC BR-1500G UPS frontThe APC BackUPS Pro 1500VA UPS, model BR1500G, has an external battery port on its back.  This port is interfaces with the BP24BPG, which is a 24V external battery pack holding four 9Ah or 7.5Ah 12V sealed lead-acid (SLA) batteries.  These batteries are identical to the two which are in the UPS, itself, but in 2x2 series-parallel configuration, giving 24V.

 

The external battery pack looks like, is the same size, and costs just as much as the UPS: $180 new from APC and most retailers.

 

 

 

For anyone who works with the APC SC-series UPSes and has replaced those batteries, this port will look familiar.  It is physically compatible with the plug on the RBC-33, which is two batteries strapped together in series using the 870-2183(a) harness.  In fact, it is the identical plug used to connect the BP24BPG external battery pack to the BR1500G.  However, plugging the RBC-33 into the external battery port does not give any results as the UPS simply ignores the battery pack.

 

Back of UPS showing external battery portPlug end of RBC-33Examination of both the port and the plug reveals three connectors.  Obviously two are for the positive and negative terminals of the batteries.  As both the BP24BPG and RBC-33 are 24V packs, it was fair to consider the third connection could be a "sense" line with some value of resistance to indicate the unit's purpose.

 

 

 

 

APC 870-2183 harness without batteriesRBC-33 wiring showing positive, negative, and senseTaking a look at the 870-2183 harness proves this idea out, showing the heavy gauge black and red wires, and smaller gauge yellow wire coming out of the cable into the plastic bracket which holds the two batteries together.  There is also a short heavy gauge yellow wire which jumpers the negative and positive terminals of the batteries.  This configuration puts the batteries in series for 24V output.

 

 

 

870-2183 harness positive battery terminal with yellow sense wire attachedYellow sense wire is crimped into the positive connectorFortunately, I did not just jump on the idea of the sense line being some level of resistance.  It terminates directly on the positive connector, putting a full 24V down that line, which could have blown my ohm meter right out.  Here you can see the yellow wire routed on one side of the plastic bezel of the RBC-33, then on the other side where it is crimped right into the positive connector.

 

 

 

 

Polarity of the lines at the plug-end of the RBC-33's 870-2183 harness cableAt the plug, the wires in the cable match up to the three "prongs" exactly as they run in the cable.  This plug is marked with each connector's polarity.  The multi-meter confirms there is no resistance between the plug end of the yellow wire and where it terminates at the positive battery terminal.  But this still does not answer the question of, why does the UPS not recognize this as an external battery pack?  What does the BP24BPG do differently?  It cost me $160 plus shipping to find out.  Following the same reverse-engineering procedures on this pack as on the RBC-33, I found the connector uses the same polarity and the sense line is grounded instead of connected to the 24V positive.

 

This is an easy enough modification to make: simply move the sense wire to the negative terminal on the RBC-33 battery pack's 870-2183 battery harness.  Once connected to UPS, the UPS makes a "beep" to indicate it recognizes the external battery.

 

APC advertises the battery pack as tripling the run-time of the UPS.  This makes sense as it does triple the number of batteries connected to the unit: four external and two internal.  Using the modified RBC-33 only adds two external batteries, which should give double the run time.  This is bore out in testing by changing the external pack in use on a UPS under load.  While running my chest freezer, the UPS shows 114 minutes of run-time available.  With the BR24BPG connected that run time increases to 338 minutes, and with the modified RBC-33 connected the UPS reports 232 minutes.

 

I had concerns about how the UPS would behave charging just two external batteries rather than the proper four, so I ran the unit supervised for three days in this new configuration.  I found it does properly manage the two external batteries.

 

Though the connecting cable is shorter, this modification is a viable alternative to much more bulky full battery pack.  The harness can be found inexpensively on its own from various sources, including some local battery stores, making it economical and perfectly usable.  Even buying a new RBC-33 is half the price of the BP24BPG, commensurate with the extra run time provided versus the full pack.  I have several of the harnesses around, and this low-cost alternative will be very handy during the next extended power outage from a storm or hurricane.

 

UPDATE: At the time I posted this, the UPS in question had been running for two weeks with the modified RBC-33 attached.  Today it started running its fan constantly.  The only relief is to remove the battery pack.  Not certain what is causing that, or if the behavior is isolated to this UPS.  It could be the self-test is detecting the unauthorized pack.  I will have to test out another UPS and see if it duplicates this behavior.  If so, I might be able to get it to "tell" me what is wrong.

 

The fan did eventually stop.  Again, not sure why it started up, but it seems perhaps it was giving a charge to the battery.  I noticed the UPS with the BP24 acted similarly after having run on battery for a minute.  The modified RBC-33 still works out to be a good configuration.

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Did you check for any resistors embedded in the "sense" wire that might be covered by heat shrink in the harness of the BP24BPG?

from viewing other UPS mods on the web, this would be something to look for.

I am trying to do a similar mod, but instead i want to connect 2 large AGM deep cycle batteries to the BR1500G, while getting the unit to display the correct runtime with the added batteries...

do you have any photos of the BP24BPG harness in detail?

Edited by Peter W
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On 1/22/2021 at 10:41 AM, Peter W said:

Did you check for any resistors embedded in the "sense" wire that might be covered by heat shrink in the harness of the BP24BPG?

from viewing other UPS mods on the web, this would be something to look for.

I am trying to do a similar mod, but instead i want to connect 2 large AGM deep cycle batteries to the BR1500G, while getting the unit to display the correct runtime with the added batteries...

do you have any photos of the BP24BPG harness in detail?

Yes.  I detected no resistor in the sense wire of the BP24BPG.  I do not have any additional photos of the external battery pack harness, but I can do some if needed.

 

I expect that the unit will display the correct run-time when using the deep-cycle batteries.  With no special configuration, the UPS is able to calculate the difference between the two-battery and four-battery packs.  The former it is not designed to work with.

 

Sorry for such a late reply -- I do not recall getting a notification of your comment.  Hope all has gone well with your project.

 

Oh, BTW, it is not a shrink-wrapped wiring harness but rather it is molded.

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I just stumbled across this blog post; it was a super interesting read!

 

Our org almost exclusively uses the rack-mount flavor of the APC UPS chassis, most of them are SMT1500RM2U (or similar).  Prior to me transitioning to a different group internally, we had started the push to the li/on models versus the lead-acid ones.

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@digdugnate I use the SM1500RM2U at the data center for my servers, and two of the SMT1500RM2U like yours on my home office rack.  The newer models with the LCD panel use a different power scheme internally which does not keep the main transformer energized all of the time.  This cuts heat output enough to change the ambient temperature in my office from around 82°F to 78°F -- the office used to stay around 6-8° hotter than the house, but now only about 2-3°.

 

I looked at the LiIon stuff.  They seem kind-of pricey for the same power output as the SLAs.  How have they been for you guys?

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42 minutes ago, OLD CS1 said:

@digdugnate I use the SM1500RM2U at the data center for my servers, and two of the SMT1500RM2U like yours on my home office rack.  The newer models with the LCD panel use a different power scheme internally which does not keep the main transformer energized all of the time.  This cuts heat output enough to change the ambient temperature in my office from around 82°F to 78°F -- the office used to stay around 6-8° hotter than the house, but now only about 2-3°.

 

I looked at the LiIon stuff.  They seem kind-of pricey for the same power output as the SLAs.  How have they been for you guys?

The cost is a fair bit higher than the LAs right now but doing the analysis it looks like we'd come out ahead over five to seven years of the lifecycle of the UPS. 

 

I LOVE the li/ons from an installation standpoint because they're shorter in depth (which is great for us because not all our IDF cabinets are standardized sizes) and weigh significantly less.  We've had them in production only about six to eight months but so far they seem to hold up pretty well to temperature extremes.

 

The batteries didn't used to be user-replaceable but now they're considered CRUs.  Even though APC will try to steer you towards the AP9640 network management cards, you can still use the old AP9630/31 if you were so inclined.  (we manage all our UPSes inhouse instead of using APC's software).

 

I managed our UPSes for our org for several years to standardize installs and configs but was more than happy to pass the baton to someone else.  I still do some small work with ours.

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@digdugnate Yeah, I still use 9630s with SNMP and other monitoring utils (apcd and NUT.)  I am not a big fan of the outside system monitoring, especially after I spent so much time developing my own in-house resources.  They are not perfect, but they work well enough for me.

 

I can see the shallow chassis being a benefit in my environments, as well as the decreased weight.  The SLA packs are good for about three to five years, usually on the high end for me as the UPSes are only there to keep the rack going until the building generator spins up.  I would like to see how long, in practice, the LiIon packs last.  It costs me about $140 to replace the batteries in one unit, so every three years or so that is not terrible, but if I can get longer service from the lithium batteries for a comparable price, it might be worth it.  Possibly at home, as well, to help keep the heat down.

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expected battery life for li/ons is five to ten years, according to their data sheets, compared to four to six years for lead/acid. 

 

from my notes:

 

Difference in battery life from lead/acid to li/on - 4 years greater for li/on

Difference in chassis weight from lead/acid to li/on - 20-30 lbs lighter for li/on

Difference in maximum depth from lead/acid to li/on - 5-6 inches shorter for li/on

Total cost for lead/acid battery replacement - $542.14

Total cost for lead/acid chassis replacement - $1637.15

Difference in cost for chassis lead/acid to li/on - $559.99

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7 minutes ago, digdugnate said:

Total cost for lead/acid chassis replacement - $1637.15

Oh, they are having a giggle with this.  I am running the same chassis I bought 10 to 15 years ago :)  I guess that means replacing the SLA unit with an LiIon?

 

Probably worth having another look.  I imagine the heat output is even less, I might be able to bring my office in-line with the rest of the house.  Two degrees hotter in practice, one degree on average (night temp at 70°F kind-of skews the average,) is worth the electrical/energy costs alone.

 

env-temp-day.thumb.png.1ea996a6009c512dc0400af5ec37e95f.png

(Blue = house, green = office)

 

I appreciate the info.  I will spend some time with my vendor in the coming weeks.

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it does. :)  

 

The li/ons don't put out diddly for heat, near as I can tell, either.  Happy to help!  Funny that we got together on AA of all places to talk about UPSes, lol.

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One issue I've seen with SLA-form-factor 12V LiFEP04 packs purchased as a replacement for original lead-acid batteries is that the old SLAs (typically rated at 12V @ 9AH) can provide a lot more current than 9A.  However the LiFeP04 batteries have a battery management system (BMS) to protect the cells from over/under charge and over current.  They may only provid 11-12A of maxiumum current (compared to *WAY MORE* with a old-fashioned lead-acid battery.  The observed behavior is that a UPS with a BMS-based Lithium pack may fall quite a bit short of being able to deliver the full load that the UPS was able to with the SLAs.  A big UPS might offer 600W continuous load capability.  At the internal 24V battery terminals thats >25A  (let's ignore conversion losses, it will be even higher with them).  You aren't getting 25A out of a LiFeP04 SLA-form-factor replacement battery from Amazon, the UPS will just shut off without warning.

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They must have changed something since yours and mine. Just tried to rig up 24 VDC worth of AGM gel cells to the external jack using spade lugs, I connected to the bottom terminal, negative, when I went to attach the sense line (jumpered to the negative lead), sparks! Luckily it was quick enough it didnt seem to have done any damage to the UPS.

Any ideas?

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On 6/9/2024 at 6:57 PM, K5KTF said:

They must have changed something since yours and mine. Just tried to rig up 24 VDC worth of AGM gel cells to the external jack using spade lugs, I connected to the bottom terminal, negative, when I went to attach the sense line (jumpered to the negative lead), sparks! Luckily it was quick enough it didnt seem to have done any damage to the UPS.

Any ideas?

No idea.  Same model UPS as mine?  I suppose it is possible Schneider made some changes, but that would make new UPSes incompatible with the old battery packs.  Is there any voltage on the sense connection in the UPS port?

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