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Adjustable balancer for repurposed 18650 cells
Cheers to the forum.

I am trying to replace the stand-by battery that is being used in my home building elevator system for emergency lighting and lift release functions. The original battery is a sealed lead-acid 1Ah battery. The system charges the battery under 100mA and it keeps the battery charged at 13.8V. The battery is rarely used, since there are almost never power cuts, but it is being used once a month from the elevator janitor. The lead battery dies every 3 years for some reason.

I was thinking of putting together a 4S battery using 18650 cells. However, under 13.8V they would be charged only up to 3.45V per cell and they will almost definitely drift apart, since there is no balancing function at that voltage on any BMS readily available. So I was thinking that we are in need of a way to balance a battery at custom voltages. For example, it would be extra useful in this stand-by application if a balancer could bleed the cells at 3.6V, so when the weakest cell reaches that threshold (or any other adjustable is some way threshold), it would stay there and wait for the rest of them.

So, if I get one of these "balancer" boards, is there any way I could solder onto them some component that would change the bleed voltage? Change some resistors that set the balancing voltage maybe?
Having the cells at 3.45V would result in no capacity left so its useless and you need a BMS.
No those have set values of balancing. Also you need protection

With the cost of a 1Ah LA batteri (ONLY 1AH??) i would say its easier to just replace that every 3 years. ALOT easier. You can even buy a tad better LA battery that will last 5 or 8 years as long as the charger doesnt overcharge them.

Im sorry to say that I dont think going to Lithium is a way to get less headache. Do it if you want to learn and twinkle but not for any thing else.
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Maybe use LiFePo4 not Li-ion? = better fit for 4s = 12V system
AliExpress search with "lifepo4+balancer+4s" gave good results.
You'd need to check the charger - it's probably a dump simple one & over-charging the original LA battery so it might do the same to a Lithium one.

What's the impact if the battery dies?
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When in need of a stand-by battery, it should be constantly charging. If however you keep it at full voltage, that’s gonna damage the cells. That’s why I believe an adjustable balancer would come handy in application where the cells would never reach their maximum. We could for example replace acids with li-ion in emergency lights and alarm systems, but it wouldn’t be wise to keep the cells all the way up. And if they are not fully charged, they will never balance.
The bluetooth BMS discussed on this board is programmable to your needs, but i doubt they do for 4s.

But the doubts said above are valid.
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Electronics ? No clue. Am machinery engineer.
Redpacket is right, go LiFePo4.
LI-ion & LiFePo4 don't have to be held at manufacturers spec "max" charge voltage - that voltage is usually bad for the cell anyway & holding it there = bad for sure.
If you hold a LiFePo4 cell at about 3.40V (not 3.65V) it will be very close 100% charged capacity wise but the cell will be happier longer term.

You could use a programmable LA charger to hold float at 13.6V (3.4V/cell), absorb briefly at say 14V (3.5V/cell) = should work nicely.
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Finally someone who understands.

It would be excellent if we could hold a "stand-by" Li-Ion battery's cells at 3.9V or 4.0V, it would possibly last for ever. You can easily do this by using an adjustable CC/CV converter. However, the cells would most probably drift, since there would be no balancing taking place.
That's why I believe there is a market for a small and cheap 1S cell balancer: You could stick 3 of these on a 3sXp pack and they would keep all cell groups at -say- 3.9V. Something like SONY's self-balancing LiMn cells, but for any voltage. Of course, you would be dissipating heat continuously once the battery reaches its "full" voltage, that's what the PSU should be selected accordingly with a very low current and appropriate maximum voltage. I was thinking of a small board that would have a bunch of dip switches or solder joints to temporarily or permanently set the balancing voltage to 3.8V, 3.9V, 4.0V, 4.1V or 4.2V.
A reasonable quality 12v 1.2Ah SLA battery will cost you about £5/ year on the basis you are changing it every 3 years.

A good quality battery will last longer, but the /year cost will likely remain very similar - plus, however much you value 10 minutes of your time to change it every few years.

Other than, because they can, I can't see any compelling reasons why changing the battery chemistry would be beneficial.
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