# Can a 1S BMS be used for a parallel arrangement of 4 Li-ion batteries?

Currently I am on a Mini UPS for a modem router project. So my router needs a 12V and 0.6A current. I have four 3.7V, 2800mAh Li-ion batteries.

So I need to keep the voltage at 3.7V but I need to increase the total battery pack capacity for long periods of use.

So I connected all the batteries in parallel to achieve this.

2800 × 4 = 11200mAh and 3.7V

Finally, I used a DC step-up converter to transform 3.7V to 12V.

My question is: Can I use a 1S 3.7V charging BMS to charge all batteries that are connected in parallel? Could it cause any damage to the batteries?

Or is a 1S BMS only for a single cell?

• Are you describing a schematic in the hope that you can transmit its image (and the device data sheets) into people's brains remotely? Commented Jan 8, 2021 at 16:38
• Welcome to EE.SE. There's a schematic button on the editor toolbar with battery symbols, etc. Double-click the component to edit its properties. Use the Custom Component for your BMS. Commented Jan 8, 2021 at 17:31

As far as LV cutoff is concerned. OK But for CV cutoff to say 5% of CC with 4 cells sharing current, each cell must go to 1/4 of 5% CC to trip the CV cutoff. This extends the aging time during CV.

So if you can reduce CV from 4.2 to 4.0 that might be better as it will spend closer to the same time charging the higher ESR double layer charge as 1 cell.

Otherwise the CC rate will be shared and thus take 4 times longer from 3.8 to CV levels.

Consider the simple equivalent cct as C*ESR // C/x * ESR * y so the secondary charge time constant ought to be the same as for 1 cell to give an equal compromise to longevity vs storage capacity. Where x,y are variables of the double layer charge effect that causes the effective memory when you change the CV instantly then shortly after remove all charger or load and it restores back to nearly the same previous voltage.

In Short (no pun) it will work but I recommend you tweak the charging profile.. Max CC and reduce CV

The only other factor is does the BMS actually measure battery current or just charger current? If the Latter, beware of overextended CV times during load > I cutoff.

• Your answer seems to be electronic theories. I'm not an expert in this field so I don't understand what is CV, LV, and formulas that you referred. Could you explain it to me in a simple way? Commented Jan 9, 2021 at 5:22
• All battery chargers start with constant current CC, until max voltage then constant voltage , CV until current reaches low a low threshold of current . Load is cutoff at Low voltage threshold LV Commented Jan 9, 2021 at 6:47
• So what is the answer @TonyStewartEE75 needs 1 BMS ? needs 4 of them or better none? Commented Feb 19 at 19:48