On 4/7/25 11:55, jordi.palet--- via NANOG wrote:
I didn’t meant for a UPS replacement for lead-acid you should add a BMS,
because they already have an small internal BMS, as they have inside multiple
LFP cells, and yes, they also have choices with bluetooth. In fact, this is the
way I replaced my car lead-acid battery, which previously was dying every 2-3
years.
While you can buy UPS's with Li-Ion cells, most UPS's are sold with LA
batteries.
Of course, a Li-Ion-based UPS will come with a BMS and balancer included.
Not such a big issue for a car or any other application that requires
only one 12V battery (like a gate or garage motor), since no balancing
is required. You just need to buy the right 12V battery that has
sufficient capacity to handle a motor engine crank.
Personally, even though LFP is safe with a relatively high thermal
runaway temperature threshold, I continue to use LA for my cars, even in
cars where the battery is not located in the engine bay.
It is true that if your UPS has multiple lead-acid cells, because they most of
the time are in series, they will not be automatically balanced. What I do in
those cases is fully charge each cell before using them. If they are new and
grade “A”, normally, unless there is faulty cell, they will remain balanced
across multiple years. In fact, I tested that with an APC UPS with hast 4 20Ah
lead-acid cells, and 4 external ones. Replaced with LFP ones, and after a
couple of years, they were still balanced. Checked again after 2 more years,
and still are balanced. Of course, if they have also bluetooth, you will be
able to check that without removing them, as it allows to see the SoC of each
individual cell (voltage is not good enough with LFP).
Voltage drift is much less of an issue with LA than with Li-Ion. True,
including a balancer for your LA batteries is nice, but not terribly
critical as compared to Li-Ion.
Li-Ion voltage drift can be a real problem if you have a large pack
built in parallel to maximize capacity. If one of the batteries is
running at a lower voltage, it will shut your entire system off even if
the rest of the batteries still have enough charge in them.
For large systems, you'd be running some kind of CAN bus between each of
the batteries in the pack to communicate voltage and SoC info, so that
the entire pack can balance itself.
Another issue that can develop with poor balancing is chemical and
digital drift, where the calculated SoC is different from what the
actual chemical voltage is. The impact here is a potentially incorrect
SoH (State of Health), which may lead you to thinking your battery is
aging faster than it really is.
Mark.
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