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How to get Li ion batteries to work in (SLA) UPS


100 W
Apr 14, 2013
Santa Monica
Repurposing APC SmartUPS (SUA1000, SMC1000 etc) for home appliance backups (fridge, mom's oxygen machine etc).

Hi all,
My apology, I know this is not directly related to endless, but I couldn't find another place with as much educated and helpful and kind members.

I (re-)searched this subject and found no reliable and actionable information.
Many who ask, some replying the obvious (throwing the question back to the inquirer).
Some claim to have been successful without any demonstration or actual info on how they got there.
UPS's in businesses are normally discarded after a few years (write off after 3 years) and just dumped (including their lead batteries).
Given the availability of Li batteries I truly think it would be great to give those UPS units a second chance of useful service.

So here it is
The actual UPS voltage of my own fully charged and floating SLA batteries is between 27.1 ... 27.5
That results in 3.87 ... 3.93 Volt per Li-ion cell in a 7s configuration.
(I chose li-ion since they are less expensive and meanwhile quite reliable if treated correctly, please.)

If that's not enough, these APC UPS can be programmed to higher charging/floating voltages.

I've read about CV/CC charge profile for Li-ion batteries:
Would the UPS SLA charge output work for Li-ion batteries?
i.e. will the BMS keep matters in check?

Thank you!!
If the batteries were designed for the same voltage range as the SLA and have a BMS designed to cut off charging so the float voltage the SLA charger will leave on there all the time won't continue trickling current into them, and the batteries can handle the current load for the entire time you need the UPS to run, then they'll work.

I'd recommend picking the BMS for reliability rather than cost. Cheaper ones are usually also lower-cost designs, with less failsafes, etc.

If you can use a contactor-based BMS, rather than a FET based one, that should be more reliable because FETs often fail stuck-on and the failure is "silent", meaning you don't usually have any way to know they've done this without specific testing procedures, until you find the damaged cells from overcharge or overdischarge.

IIRC JBD makes contactor-based units, mostly intended for high-current applications. But you can use the lowest-current-rating device, and not actually get the contactor (if they offfer that option to save $$), and instead find or use your own relay or contactor if you don't need anywhere near as much current as the contactors they usually sell with them. It's unlikley you will ever have an overcurrent situation if you specify your parts correctly based on the max load the UPS will see and draw from them; it's not something that should change (unlike some motor-cotnroller applications), so the relay just has to be able to handle the voltage and current expected, and have a coil rating the same as the BMS is designed to drive.

Be sure to pick cells (or pack size) that can support more than that worst-case current load needed, so that the cells are not heavily loaded (so they don't heat up, since the UPS can already get pretty toasty inside), and so that they will still handle the load you need even as they age over time. (same for capacity...get more than you need).

Some BMS also have Bluetooth connectivity and an app you can run on an android/etc device, so you can monitor the cells' states without opening hte UPS up, whenever you like. Some have serial cable connectivity for the same purpose, via various protocols, and so could be connected to a backed-up device that monitors the pack and shuts itself down to unload it if a problem is detected, before the BMS would just shut off the output and cut power to the backed-up device that's still running (similar to the APS programs you can install that talk to the UPS itself, or the UPS services built into some OS's, except that you'd have to write this program yourself).

As you note the voltage range of the SLA would result in less than full charge of the LI, and as long as you can just add more cells in parallel (or use bigger ones) to get the capacity you need, it's actually better to do this (longer lifespan for hte cells, since almost all the time they will sit fully charged, which is higher degradation than sitting at say, half charge, unlike SLA which has to stay full to minimize degradation).

What cutoff voltage does the UPS use to shutodwn for low battery? It's probably between 20 and 24v, but you should chekc. If the UPS goes farther down than the safe cell LVC, the BMS will have to protect them every tme the UPS is run to empty. If it doesn't go anywhere near the LVC, it will not be able to use the capacity between those points, and may require more parallel cells (or bigger ones) to deliver t he runtime needed.

Some small "brick" UPS's runtime is not only dictated by the capacity of the battery inside, but also by the thermal capacity of the stuff in the unventilated casing. It's often designed to run out of places to stuff heat not long after the battery would run out of power. Rackmount versions are often fan-cooled and don't usually have this problem.

FWIW, there have been some other topics about lithiumizing UPSs, but i don't recall the results.
The new forum version search isn't working correclty ATM, so here's a search of the archived version
with some relevant threads (not all of them are, and not all posts about it show up in this search either).

I started to do this some years back with a rackmount APC and some Thundersag...er, Thundersky LiFePO4 cells, which approximated the voltage range needed to emulate the SLA...but had problems with the cells testing them in another application, so didn't proceed with it. I still have the UPS in the shed, and the cells...someday should continue it. But most of the devices I use nowadays that could suffer dataloss are battery powered, so don't need one, and our power is so reliable that there has only been one outage in years that I can remember (during a storm). (internet, now that goes down every few weeks, sometimes for days, but not power). So hasn't been much motivation to do it. :)
Dear Amberwolf,
I love you! Really hoped you reply. Thank you!

Since power outages here in SoCal can take half a day, I was planing on 7S30P Samsung Li-ion 2500mah 18650 cells (27.3V * 75Ah). These 210 cells fit perfectly in the pictured space.
Don't know how to arrange/connect them (either left or right in the red blue picture) but that's for later.
Heat (if any since low draw) is also unclar to me, but again, later hopefully.
Actual usage is ca 300W draw (fridge 200W when active, oxygen machine 100W) and plenty of time for charging.

I can adjust float and cutoff in the mentioned APC UPS but as you mentioned rely on the BMS as a backup, just to make sure.

Was planning to use a Daly 50A BMS, will dig into the ones (is it jiabaidabms?) you kindly recommended

Thank you REALLY much also for the "Graveyard - Search", was losing my mind, I thought I had read some of those here, believed I have Covid or ageing memory loss or so.

Yes, as you mentioned nearly all UPS related never give actual reports on outcome, I plan to change that with your and other's kind help.

Hope the 30x parallel arangement doesn't introduce new problems?
There is plenty time for them to be recharged.

Thanks again for your invaluable insights and kind sharing of your knowledge.


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The current draw is only around 300W i.e. 12A @ 25V.
Would a Smart BMS with 30A be enough, considering the actual max output of SMC1000-2UC is only 600W (24A@25V)?

"IIRC JBD makes contactor-based units..."
I tried to dig into JBD's Smart BMS but got lost.
jbd-which one.jpg
So many products but little info on what the actual differences are?
Could you please point out which series/unit you recommend?

Also, is the 30p (30 * 2.5Ah =75Ah) a factor to consider?
There will be 210 Samsung Li-ion 2500mah 18650 cells arranged as 7s 30p (27.3V * 75Ah).
These batteries don't need to be fast charged, actually prefer a slow charge to reduce heat and stresses.

Depending on how hot this all gets, more fans can be added to the existing temperature controlled fan, it's driving stage is strong enough to power 8 more similar fans I have at full speed.

Thank you
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at 12A battery shouldn't have any heating problems, especially with such big number of cells.
And i think 7S li-ion is the right voltage (typical for 24V acid battery replacement). Shouldn't be a problem to find a bms for that.
Balancing such large number of cells in parallel would be a challenge because typical resistive balancers offer very small balancing current around 20-50mA, would take forever to balance such big battery. So it might make sense to use an additional/external balancer.
Charging - you don't have to use CC/CV charger if the internal power supply of the UPS has voltage lower than 29V. In such case the battery wont be charged to its max voltage and current reduction will happen just by itself when battery voltage equalizes with power supply.
I have good experience with cheap Chinese '7S BMS' modules, they aren't smart but work just fine and cost $10-$20. And balancing is done by separate balancer boards (also some cheap Chinese stuff) that i have modified by adding extra resistors to allow balancing current around 200mA. For 7S8P battery works OK.
Thank you for your reply.
Thanks for worry free charging via SLA UPS. I can adjust the "float" voltage quite accurately, planing for around 3.9V per cell.
Makes sense regarding balancing. I now understand why on JBD one of their BMS was featured with 150mA balancing current.

Which are the BMS and separate balancer boards you recommend?
i bought cheaply a big quantity of no-name 4S balancer boards from local electronics seller, they have nice wiring and space for soldering additional resistors for increasing current. So im using these for all my batteries. Unfortunately i cant find these boards anywhere for sale.

There are also some basic balancer boards from Aliexpress, can be bought cheaply but it's not so easy to modify the resistance on them and with 62ohm resistors they have quite small discharge current (around 60mA). But can be connected in parallel so then you double the current. However, i don't like to use these because of soldering (no connector).

For the first battery build id recommend you get a BMS with balancer (even a small one) because the wiring is simpler and you don't need more opportunities for making mistakes. And maybe build a smaller battery for tests? The one you're thinking about is huge, if you're going to make it yourself then better to build something smaller just to learn how everything works.


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I had built battery packs in the past (soldering the NiMh taps), now borrowed a sukhi spot welder.
Used Daly 30A BMS for a different project, software is weird, but great to have insights.

Is this what Amberwolf and you think of?
150mA passive balance
Is that enough for slow charging/balancing?

Also checked active (inductive) balancers
FWIW, if the pack is made of matched cells (or groups of cells that as groups are equal to each other in characteristics), you won't need to balance it.

If it's kept in a narrow range of voltage, not brought near the full or empty regions, it's also much less likely to need balancing even if it is not perfeclty matched, but still pretty close.

If it doesn't see frequent cycling it will also have plenty of time to rebalance itself while sitting there waiting for another outage, so it probably won't matter how low the balance current is.

If you think you'll need high current balancers, you can save yourself getting them / etc by doing as much cell matching as you can stand, to get the groups all as similar as possible.
The batteries will definetly not brought near the full or empty regions, I will make sure of that with the UPS low and high triggers.
The batteries will have a long time (days) to recharge/balance, never experienced more power outages shortly after a long one (+5h).
So what you are mentioning is great, I'd rather get a smart BMS with slow balancing than a dumb one with stronger balancing currents.
Don't know if it's ok to post sources here, but I'm hooked up for pictured (never cycled, but don't know how old)
View attachment batteries-hooked.webp

I will try to put them together in strings of 10 or maybe 3x10, plan to let them sit so they "self" balance.
Should I still get a JBD BMS or is Daly ok for this slow discharge/charge application?
I don't recall seeing problem posts for JBD BMS, but there have been multiple reports of problems wtih Daly...and Daly may also be being counterfeited, which means you have no idea what quality the cf is (hard to tell if you have one, too). Methods has some threads about testing one version of a Daly with interesting info.

None of my ebike BMS experiences have been positive so far; at present I'm just running my SB Cruiser packs "nekkid", since they are well-matched ex-EV cells (though ancient) not being used near their limitations.
Gotcha, looking at JBD BMS offerings, all 7s units are sold out or unavailable, for over a week now.
There are ebay and alibaba offerings but considering the counterfits, i'd rather get them directly.

Question, does JBD have a secondary official outlet, a reseller in the US or Europe?

Also, how reliable are their "automatically identify 5-16S", "Automized identity"?


And can I use other units with higher cell channels and just short the lower (or higher?) channels to get to 7 channels?

Thanks !!
Unfortunately I don't know the details of their BMS/etc; just passing on info I've gleaned from other posters here and elsewhere that have discussed their systems. :oops:

Best bet to find official unit sales sites and details of operation is to ask them directly:
Like most companies it'll probably take some time and back-and-forth to get across what you want and make sure the answers you get are applicable. :(
diving into BMS, reading their specs (if any), seems most start balancing only when over 4 Volts!
Wanted to avoid going that high.
Do I need an external balancer (in hope they balance all time-ish)?
Cool, didn't know that, so smart BMS it is.
Many here don't recommend ali-xyz, but to get it from OEM only.
I'll check the products from recommended JBD again, hope they have them in stock meanwhile.
Many here don't recommend ali-xyz, but to get it from OEM only.
Yeah, you might get what was advertised, and you might not. If you get it from the OEM via their actual official Ali* site, it'll be real, but resellers are not always resellers, they are sometimes cloners / counterfeiters, or they are selling defective stuff they got somewhere, etc., and they dont' really have any reason to help you out after you paid for it.

I've bought a fair bit of really cheap stuff (dollar or two a board) from Aliexpress over the last three years or so (and a bit before that), and some of it from some sellers was great, but the "identical" item from a different seller can be garbage, or not the same version (but advertised as the same one). Some of the sellers respond to messages about a problem, and some don't--usually the sellers with good stuff that just happens to have a problem do, and the others don't, or they just respond with delaying tactics.

If the seller doesn't have a real shop name, just a number, don't even bother. Some of the sellers use the same shop name on ebay and even amazon, and they are probably trustworthy, of the ones I've gotten anything from results have been good. If they haven't been around a while, and gotten great feedback and a lot of followers, it's a greater risk. The actual reviews you can't trust on any of them, because almost always they review receiving it or not, and not how it actually functions over time. I'm sure plenty of people are like me, too, where when a seller's stuff turns out to be garbage they don't even bother rating it.
JBD has their own website selling their products but they also sell on AliB!
Many items are sold out on their own website but available on AliB, so strange.

Don't want to start another thread, looking for battery cable wire to beef up my friend's spot welder transformer and add handheld welding probes.
Was searching, no results: anybody used WindyNation?
Or do you have a source for gauge 4 (ca 6mm diameter) fine stranded wire for Transformer and then going directly to the welding probes? Roughly 5 foots long.
Also looking for thin (clear?) shrink tube for it.
Thanks and have a nice long weekend!
Was searching, no results: anybody used WindyNation?
The present Xenforo search isn't *quite* useless...but there is an archive of the old PHPBB forum with a slightly more useful search (it just can't find antyhing posted since the conversion earlier this year). So if you can't find something in XF, try this page

Here's some results in the archive, for example: