I don’t have a lot of electronics knowledge, but I can follow directions, so I was looking for a circuit online to charge a couple of NiMH AA batteries (1.2V 2450mAh) with the things I have onhand - a lot of resistors of assorted values, mostly. I do have a couple zener diodes and a bunch of IN914s if they would help and some miscellaneous junk.
Anyway, most of the circuits I’ve found require types of diodes or ICs that I don’t have, but I understand that a very simple charger that you have to babysit to keep it from overcharging the batteries can be made with basically just a power source and some resistance.
I have an old 12V wall wart, and I’ve calculated that to give the batteries the 240mA they need, I’ll need resistance of somewhere in the neighborhood of 37 ohms, 2 watts. So, to be safe, 4 watts. Now most of my resistors are 1/4W and 1/2W, but I do have a ceramic 10W 1 ohm resistor. My question is, can I put this in parallel with a 1/2W resistor to get what I need, or…? How does that work?
You can build simple chargers for lead acid batteries. They look a lot like power supplies. But I wouldn’t do that for NiMH or NiCad’s.
If you want to build a charger for NiMH or NiCad batteries, you had better build it according to a design that someone has already verified. The design will probably employ a type of feedback circuit, and may even include one or more application-specific IC’s. There are lots of schematics on the web.
Personally, if I wanted to build a charger for NiMH or NiCad batteries, I would use a “battery charger IC”. Quite a few manufactures sell these (e.g. Linear, Maxim, etc.). Their application notes will provide complete schematics.
Particularly NiMH. NiCd is somewhat forgiving of “close enough” charging conditions, but NiMH needs to be charged under rather strict conditions of voltage and current or you’ll suffer a loss of capacity and/or irreversible cell damage.
When you put resistors in series, the same current goes through all of them and the resistances add together. The voltage is going to get divided proportionally over the resistors based on their resistance.
When you put resistors in parallel, the voltage across the resistors will all be the same, but the current is going to get divided proportionally based on the resistance.