Servo - September 2010

Versapak battery packs are rated at a C of 1.2 amp hours. Therefore, a charge rate of C/10 would be a charge current of 120 mA. Some people claim that continuously charging NiCd batteries at C/10 (which is what the stock chargers do) won’t hurt them; others claim that it will shorten their lives, but does not cause other problems.

I would say from both personal experience and talking to friends, that it will DRAMATICALLY reduce the life of the battery packs. So much so that you should never ever leave these batteries on the chargers after they are fully charged (which is probably in the instructions I didn’t read ...).

Of course, NiCd batteries also “self discharge” by up to 10% per month which means when you go to use the batteries that you have taken off the charger to keep them from being killed, there is a very good chance they will need charging. You can’t win! Or can you?

I decided that I was going to build a smart charger for the Versapak batteries. These are chargers that detect when NiCD or NiMH batteries are fully charged, and shut down the charging current. While some so-called smart chargers charge two cells in parallel, I didn’t think this was such a good idea. So, I decided to build a smart charger for each individual Versapak battery pack, and do it all inside the standard holder/charger. Yes, I would be voiding any warranty on the charger and probably the batteries, but it would be fun (and useful) to build, so I didn’t care. BTW, these battery packs are really just three sub-C 4/5 size batteries, but that could be a whole other article!

I wanted to be able to use the existing holders for the Versapaks, and after taking one apart I discovered that there is plenty of room inside for additional circuit boards (or even expanding the component count and size of the current board). The battery contacts need a board to hold them in place, so I would need to have one under the batteries where the original board was anyway. I decided to