Connecting Lithium Ion Batteries in Parallel

[KaptianWiz]

So in have 3 laptop batteries that I don't need. Each battery pack has 6 Lithium Ion batteries inside it. Here are the specs
1st pack: 10.8V (Series), 4V (Parallel), 4400mAh/48Wh, 6 cells
2nd pack: 10.8V (Series), 4V (Parallel), 5200mAh, 6 cells
3rd pack is the same as 2nd pack
So I want to combine all the cells (18 cells) in all 3 packs to make a power bank.
Is combining them in parallel a bad thing?
I still want 4 volts output, only an increase in capacity with all them connected.
Does combining the batteries with different capacities do anything bad? Should I not put in the 4400mAh with the other 5200mAh batteries?

 

[(*steve*)]

There is nothing intrinsically bad about connecting batteries in parallel. However if the state of charge is unequal, the one with the highest voltage will try to charge the one with the lowest voltage. In the best case a small current will for for a short time until the voltages equalise. In the worst case you will end up with both batteries catching fire.

Before connecting them, ensure that their voltages are within maybe 0.25V of each other. If the voltages differ by more than that, try connecting them via a 1 ohm resistor. Assuming a difference of a volt out less, this will allow them to safely equalize.

In theory there is no problem putting batteries of the same voltage but different capacity in parallel. In practice of prefer both batteries to be as similar as possible but the ones you mention should not pose a problem.

 

[KaptianWiz]

There is nothing intrinsically bad about connecting batteries in parallel. However if the state of charge is unequal, the one with the highest voltage will try to charge the one with the lowest voltage. In the best case a small current will for for a short time until the voltages equalise. In the worst case you will end up with both batteries catching fire.

Before connecting them, ensure that their voltages are within maybe 0.25V of each other. If the voltages differ by more than that, try connecting them via a 1 ohm resistor. Assuming a difference of a volt out less, this will allow them to safely equalize.

In theory there is no problem putting batteries of the same voltage but different capacity in parallel. In practice of prefer both batteries to be as similar as possible but the ones you mention should not pose a problem.

Ok, thanks for the response. I think it is safe enough to connect them. Thanks again!