Connecting batteries in series

Question

I have a 90 V motor I'm testing on a bike. I ran it with two Bosch corldess 18 V batteries in series. With 18 volts it ran about 10 miles per hour. With 36 volts (both batteries together) about 20 miles per hour. I have two 14 volt batteries from another power tool set and I'd like to connect them together in series to bring it up to 64 volts. Can I do that? I want to test it first to see if it's about what I want for the speed before I buy my permanent battery.

Can I connect 18 volt and 14.4 volt batteries together in series?

Answer 1

It is never a good idea to mix batteries. You don't even want to mix old and new batteries of the same make/model. The reason for this is bad things can happen as the batteries run down-- even causing some batteries to be "charged" with a reverse polarity (this is very bad).

As you get more and more batteries in series you also start wanting some sort of active monitoring system to prevent these issues (even with the all same type of batteries). Sometimes this monitoring system can even do some active cell balancing for you.

You might be able to skip this and just "go for it", but you run the risk of reduced battery life or possibly fire/sparks/smoke. Maybe that's acceptable, maybe not. That's up to you to decide.

Answer 2

• Quick test: Probably OK with sensible use

• Permanent use - careful design needed.
  Usually not a good idea.

What David says is good advice for permanent us.

For testing purposes as you suggest it is probably OK for short durations.
Be SURE all battery packs are fully charged.
As Dave says, if you run any cell in the whole "string" too low it can be reverse biased by the current flow from the other batteries and can be damaged permanently..