Remaining Battery Capacity

[mattheww1950]

@Barkuti

Little question for HKJ and other technically :) inclined fellows. As far as I understand, a typical consumer UPS (an old Riello iDialog Plus 60 whose Pb battery died) could be effectively overhauled with standard li-ion cells, 4S BMS and 4S LiFePO₄ balance boards. Since it comes with a 6S Pb battery, the UPS should charge its battery to 14.4 14.6V, this means the 4S li-ion setup would get at least around 3.6 to 3.65V per cell. From what I can infer from HKJ’s graphs, even with a 30A load a cell like the PLB 5000mAh would still deliver 1.5+Ah down to 2.5V, typical li-ion BMS cut-off voltage. Since we limit ourselves to around ⅓ of peak cell capacity, even for a continuous discharge cell temperature would never exceed any limits. Thus, a 4S2P INR26650-50A battery pack would for sure deliver an average of ≈2.8V at 30A, for 84W/cell and 3+ minutes of battery life at peak load. Even at cut-off, 10V × 60A = 600VA at the input. Target SAI load is a network connected terminal computer for ≈300VA at most, with no possibility of SAI communication and always on.

Makes sense? To me it does, and 8x/12x cells plus 2x 40A BMS and balance boards would cost little money.

Cheers

There are in fact two issues, and you need to be aware of both of them. Lead Acid batteries have stable voltage under load. You cannot tell the charge state of a Lead Acid battery from its voltage. You actually have to measure the specific gravity of the acid in the cell to make that determination. That also means a UPS expects that if it has a 12 volt battery, it expects to see about 13.2 volts from startup until the battery is dead, by contrast Li-Ion cell charge state is determined by battery voltage, so as it discharges, the voltage will fall quite markedly from the original 4.2 or 4.35 volts per cell, and that is likely to be a problem for a device that expects to see constant voltage throughout the discharge cycle. In addition with lead acid cells, you can build them two ways. They can optimized for deep discharge cycling (this is how batteries to operate wheel chairs and electric trolling motors for boars are built), or optimized for overcharge tolerance (car batteries are designed this way). The life expectancy of any Li-Ion cell is close tied to how often and how deeply it is discharged. In addition the charging circuits are very different. Realistically because of the Li-Ion discharge properties, you need a UPS that is designed to deal with the changing input voltage as the cells discharge.