NiCad charging. 8.4v

Factual Questions
1

I’m building an 8.4v power supply for a movie camera. The construction itself is very easy. All I have left to do is replace some temporary wires with soldered ones.

But how do I charge it? The chargers I see at Radio Shack can charge 7.2v and 9.6v, but they don’t have a setting for 8.4 (seven 1.2v NiCad C-cells). I could charge the cells in a charger, but the charger only holds four cells and takes 14 hours to charge them. That’s two days of charging!

What I’d like to do is keep the cells in the battery pack, and just plug a charger into the battery pack. (This is how it works with the 12v battery my other camera uses.) What would happen if I tried to charge 8.4v worth of cells with a 9.6v charger? Alternatively, is there a way an amateur can modify a 9.6v charger to charge 8.4v?

2

and pick up a battery charger for remote controlled radio controlled vehicles.
They have specific settings for 7.2 and 8.4 volt battery packs, and you can get a good one fairly cheaply.

3

I thought the hobby shop chargers were only for 7.2v and 9.6v. Thanks, I’ll take a look.

Oh, you wouldn’t have a brand name for an 8.4 volt charger, would you?

4

Just found one: Tower Hobbies model 420 charger. (I guess I should have done a search before I posted. Bloody caffeine deficit, anyway!) Thanks for the reply.

5

Now that I have the charger ordered, I’ll need to make a cord that will charge the pack. The typical hobby-type connector is a little plastic block with a red (+) and a black (-) wire. When I plug it into the pack, do I put the positive (red) to the positive terminal on the battery pack, and the negative to the negative?

6

I’m not sure I understand your question. If the connector on the charger has an obvious red and black, then you do keep positive and positive aligned. If the charger has one of those funky barrel connectors, you need to figure out which contact is positive from the charger spec, and wire the batteries appropriately so that positive on the charger goes to the positive end of your series battery stack.

7

OK, that wasn’t clear. Always have positive on the charger go to positive on the batteries. Red should be positive.

8

Thanks, douglips. That’s what I thought it was, but I needed an expert opinion. Just like jumping a car, eh?

9

A battery charger is basically a constant current supply. The charge current depends on battery capacity as well as battery voltage, so you should check what capacity the charger is designed for. Too large a current can shorten battery life. Too small a acurrent shouldn’t do any damage, but it may not fully charge the battery, especially if the charger’s cutoff circuit is a simple timer.

10

here’s a link which might interest you:
http://www.geocities.com/SiliconValley/3569/begin/batt-00.htm#charg

11

Johnny, what size cells are you using? I’ve got a Ray-O-Vac renewall charger that will do eight AA NiCd or Nickel Metal Hydride cells at a time.

12

Thanks, Padeye. But I’m using C-cells. Gotta turn the motor, and still have enough power left over for transpoting the film in 400’ mags.

BTW: I did get all of the batteries charged on the 4-cell slow charger. It took about 21 hours per four cells. I made a dummy battery from a piece of dowel, a couple of those circular connectors soldered to a wire, and a couple of screws. My home-made battery pack is working fine with the camera!