Connect cordless drill batteries in parallel

Im wondering what it takes to connect two batteries (or battery packs rather) for a cordless drill in parallel and make them to share the load?
It seems there is a risk that 'stronger' battery will supply more current..

Yes that is the risk, but the bigger risk is that the stronger battery starts charging the weaker one. You can couple the batteries through resistors or diodes to prevent help this.

Aha, there must be a diode in series with each battery.
Thank you Mike!

That doesn't do anything to even out the load sharing but it does at least prevent cross charging.


Load shairing to a certain extent will be done with a seriese resistor. The battery supplying more current will have a bigger volts drop across the resistor and so the other one will supply more of the load. You have to balance load shairing with the power losses in the resistors. Keep the diodes.

Generally you only parallel identical cells/batteries bought together, and leave them
paralleled for life - the first one to fail means you should replace both. Identical cells with
identical charge histories tend to play well together.

Parallel cells with one failed might not be noticed other than the capacity drops a lot.
The good cell helps out the bad one somewhat (the opposite situation for cells in series,
where a bad cell rapidly gets completely trashed by the others back-driving it)