Assemble one battery from two batteries

I have two 18V 1.5Ah batteries for power tool, which consists of five 18650 elements in serial (5s1p configuration). 18650 elements are of the same model, age and condition. I want to make one battery from these two (so it become 5s2p), but my concerns are about charging the new battery. Can I use the same charging controller? Or I need to buy a new one? I want to use the same charger for the battery.

As long as you connect them correctly, this should be no problem.

You need to connect the point between cell 1 and cell 2 of battery pack A to the point between cell 1 and 2 of battery pack B. Connect the point between cell 2 and 3 of A to the point between cell 2 and 3 of B.

Pack A:
[cell1] | [cell2] | [cell3] | [cell4] | [cell5]
Pack B:
[cell1] | [cell2] | [cell3] | [cell4] | [cell5]

^         ^         ^         ^
this       this      this      this

And so on.

What you'll end up with is a 5s2p battery that can be balance charged in the same way as a 5s1p battery.

Pack AB:
[cell1] | [cell2] | [cell3] | [cell4] | [cell5]
[cell1] | [cell2] | [cell3] | [cell4] | [cell5]

And of course, connect the very ends of pack A to the corresponding ends of pack B as well.

Pack AB:
| [cell1] | [cell2] | [cell3] | [cell4] | [cell5] |
| [cell1] | [cell2] | [cell3] | [cell4] | [cell5] |

Before you make a connection, you should ensure that both connection points is at the same voltage potential (in other words, before you hook up the point between cell 1 and cell 2 to each other, check that cell 1 in battery pack A is at the same voltage as cell 1 in battery pack B).

You don't want huge amounts of current to flow to even out the charges as soon as you hook the cells up to each other, because that can become hot and/or damage the cells.

At this point, the charger should not know or care whether it's a 5s1p battery or a 5s2p battery, other than that the capacity is roughly doubled.

• The charger could possibly care, e.g. it could hit a safety timeout on a larger capacity pack, and any fuel gauge will probably be confused, and balancing may suffer, etc (depending on which parts of the BMS are on the pack vs. in the charger) – Bill Dubuque Jun 11 '16 at 14:31