# BMS for two parallel protected 14500 Li-ion batteries

I have two li-ion 14500 850mAh protected 14500 Li-ion batteries and a BMS 1S (for example)

I would like to connect the protected 14500 Li-ion batteries in parallel in order to get more current (for twice longer use)

Thought of connecting BMS (mostly for the balancing feature, since they already have protection) so the protected 14500 Li-ion batteries will be balanced.

And this is the point that makes me bit confused- how does the BMS knows the voltage of each protected 14500 Li-ion battery if they are connected in parallel? the BMS 'sees' only one positive and one negative.

If one cell has 3.5 V and the other has 2.8 V, what voltage the BMS will see?

I saw another option for connecting a BMS for each protected 14500 Li-ion battery, but again, if the batteries are connected in parallel what voltage each BMS will see?

In case of series connection it strait-forward because the BMS can compare all the voltages of all the series cell, and than decide to reduce the higher voltage (passive balancer for example)

I'll be happy for explanation of how to connect a BMS for two parallel protected 14500 Li-ion batteries.

• BMS is for balancing series batteries. Parallel batteries self 'balance' by having equal voltage. If they are unequal voltage before you put them in parallel, you must equalise their voltages, perhaps by connecting a small resistor between them for a while, as simply connecting unequal voltage batteries directly in parallel could cause damaging currents to flow. Once the voltages are equal, connect them in parallel, and you're done. Treat them as a single, higher capacity, cell, for both charge and discharge. Commented May 14, 2023 at 12:57

EDIT: OP just made me aware of the fact that these are protected batteries, not plain cells. That changes the answer completely.

DO NOT connect protected cells directly in parallel. When either cell turns back on, an inrush in current will flow from the most charged cell to the least charged one, at best degrading them, at worst damaging them.

Please ignore the rest of this answer: it only applies to regular cells.

how does the BMS knows the voltage of each *cell if they are connected in parallel?

The voltage of two cells connected directly in parallel is the same. That's physics. That is true of any two components directly in parallel, nit just cells.

the BMS 'sees' only one positive and one negative.

No, the BMS sees a voltage. That voltage is the voltage of both cells because they both have the same voltage because they are connected directly in parallel. That's physics.

if one *cell has 3.5v and the other has 2.8v, ..

.., and you connect them directly in parallel you may start a fire. DON'T DO THAT! Only connect cells in parallel if they have the same voltage.

what voltage the BMS will see?

Fire issues aside, the moment you connect two cells directly in parallel, their voltages will suddenly become the same. That's physics. That voltage will be somewhere between 2.8 and 3.5 V. That's the one and only one voltage that the BMS will see.

I'll be happy for explanation of how to connect a BMS for two parallel cells.

1. Discharge the highest-voltage cell to the voltage of the lowest-voltage cell. Or, equivalently, charge the lowest-voltage cell to the voltage of the highest-voltage cell.
2. Once the cells have the same voltage, connect them in parallel
3. Connect the B+ wire from the BMS to the common of the two positive terminals of the two cells.
4. Connect the B- wire from the BMS to the common of the two negative terminals of the two cells.
5. Connect the charger and the load to the P+/- terminals of the BMS.
• Thank you for the detailed answer :) what if one cell have some issue so its protector will cut it off, meanwhile the second battery is on and it's voltage decreased in 0.5v for example, suddenly the first battery is on again..? this scenario is possible? can they have voltage differences while in use (for example the natural balancing is slower than the voltage difference being created)? Commented May 15, 2023 at 11:36
• so given the fact that each cell has its own protection, and both fully charged, there is no need in BMS..? Commented May 15, 2023 at 11:37
• "each cell has its own protection". Oh! I didn't realize that. Thank you for making me aware of it. DO NOT connect protected cells directly in parallel. When either cell turns back on, an inrush in current will flow from the most charged cell to the least charged one, at best degrading them, at worst damaging them. Commented May 15, 2023 at 13:29
• so the only scenario of connecting cells in parallel is to connect them (unprotected) with one common protection in parallel. BMS has no use. or, just using one cell and replace it with the second one, when the cell is over. Commented May 15, 2023 at 18:09
• No. You have two options: 1) Use 2 unprotected cells plus a BMS: 2) Use a single, larger protected cell, no BMS. Commented May 15, 2023 at 21:59

Two cells in parallel is just one cell with double the capacity. The two cells can't have a different voltage as they are in parallel.

And you cannot connect the cells in parallel if they have different voltage before you connect them.

The voltages must be equalized so that at the moment you short the batteries together only a very small current flows from one battery to the other, instead of high current enough to damage the wires or the cells.

So after paralleling any amount of cells it still is a 1S cell and connects to 1S BMS like a single cell.

• Thanks you for your answer :) so the purpose of the BMS in that case is for protection only and not for balancing. what if one cell suddenly fails and its voltage reduced by 0.5v? I should not worry about that? is there more cases that voltage differences can occur in the middle of use? is it ok to connect one protection in parallel to two cells? (sorry for the multiple questions, im still confused) Commented May 15, 2023 at 18:11