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I've got a UPS that charges its batteries with 84 V 1.5 A (6S1P SLAs). I want to replace the batteries with li-ion 20S2P. I'll use an external charger to charge them in cases of prolonged power-outages, but mainly they would be charged by the UPS.

Just to by safe instead of a simple BMS, I can install a smart BMS (Ant-BMS or Daly) and set it up to function between 3.4-4.1 V in cells. Which still would give me ~90% of capacity.

The question is - is it safe? The UPS was made for lead-acid, so it definitely has float voltage. Plus it may have a equalization mode(unlikely). But the BMS should be able to cut off both float and equalization, right? I can reduce the BMS's upper voltage for cells even more (4.05 instead of 4.1 V), which would definitely be lower than (13.5-13.8 V)*6 float voltage of the UPS's original batteries.

I understand that it would be better to get a li-ion UPS. But money is tight now.

If my setup would reduce life of the batteries, even by half - I don't mind. What I need is - for it to work reliably and safely for the next 2-3 years with semi-regular power outages that sometimes exceed 48h.

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    \$\begingroup\$ No, it's not safe. Float charging Li-ion cells will damage them and possibly cause fire or explosion. A BMS does not generally control charging, just protection against extremes. \$\endgroup\$
    – Finbarr
    Jan 11, 2023 at 10:11
  • \$\begingroup\$ Perhaps ask over on diysolar or secondlifestorage forums where people do this regularly? \$\endgroup\$
    – winny
    Jan 11, 2023 at 13:53
  • \$\begingroup\$ @Finbarr That sounds like an answer to me! \$\endgroup\$
    – marcelm
    Jan 11, 2023 at 15:44

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The use case of a UPS seems better-suited to lead cells than to LiIon: keeping a lead cell fully charged tends to be best for longevity: that makes it well-suited for starter batteries (charge topped up soon by alternator) and UPS. In contrast, maintaining full charge is detrimental for LiIon chemistries.

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Please, just don't. Like others have said, a BMS is meant to protect from extremes (over discharge, short circuit), not for charging. I also don't understand why you would want to replace the lead acids with lithium-ion. The lead acids almost always have much more current and capacity. They are also so much easier to charge. You don't have to make some fancy, dedicated charger. You can literally just throw 13.8V at it.

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That may not work well, and this is why:

With a balanced string:

You say that the UPS expects six lead acid batteries. That's a Constant Voltage of 82.8 V.

You say that you will use 20 cells in series. If the string is balanced, 82.8 V will divide into the cells as 4.14 V / cell.

The problem is that you plan to set the BMS for a lower cut off voltage, so the BMS will shut off the battery before the constant voltage stage is reached. Solution: set the BMS cut-off voltage to 4.2 V / cell

Perfect.

The second problem is if the string is unbalanced. A cell will reach 4.2 V first, the BMS will shut off the battery, balance the cells, turn back on, repeat a few times, until the string is balanced.

It is possible that the UPS with shut down and not restart once it sees that there is no battery (because the BMS shut off the battery while balancing). In that case, balance the string by charging each cell individually to 4.1 V. Then the BMS should not shut off and the UPS would stay on.

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