trying to make a box, in which i wil have battereis, and i want to be able to seal it off... so i need to be able to charge the battereis...im working with 3X AAA batteries..how do i charge them, and what kinda battery will i need? (Ni-mh/NI-cd?)...on radioshack most of them look to output 1.2V, normal is 1.5....will this present a problem? PS i will be working with a 12V dc power source...from my car cig lighter...
with Ni-Mh and Ni-Cd you can trickle charge them both safely and easily. It just takes 8-12 hours to charge. You simply provide them with a C/10 charge. Where C is the capacity in mAh. So for 3, 750 mAh batteries in series, you would provide a constant 75mA of current through them to charge them up. The capacity stays the same in series, and adds up in parallel. Though you should never charge cells in parallel. That would be the easiest thing to implement. And for the 1.2v/1.5v thing, 1.2v is normal for rechargable cells, and 1.5 is normal for non-rechargable cells. However the 1.5v quickly drops to around 1.2 and sits there for a while, then drops off again as the battery discharges. So its pretty much the same.
so voltage doesnt matter? or does it have tobe the same as the voltage that the batteries are outputing?
so does this have to be an exact 75 Ma? and will it die from being plugged in to much or will it be OK being plugged into a charging source constantly?(will be unplugged, but probably will rarely use the battery features so they will stay quite charged, and get charged for a while)
The 75 mA was an example for if you are using 750 mAh batteries, and no the voltage does matter but, the constant current circuit will adjust the output voltage so that the current is always the same. With this trickle charge, you can leave it hooked up for an indefinite amount of time, because once it hits full, the current is then dissipated as heat, but it is so little current that it doesnt overheat the battery. Though it would probably be better if it wasn't on charge 24/7, but plugging it in an leaving it for a while wouldnt hurt. Maybe you could use something called a PTC to aide in that. As the PTC warms up, its resistance increases, so that if the thing it is on gets warm, or too much current gets pulled through it, it increases resistance and and limits the current, or even cuts it to a very small amount. That could be setup to lower the current to the battery pack when it is getting warm. Or you could possibly use a regular NTC thermsistor. Its all in the charging circuit. Provided the case doesn't make heat build up too much, a constant trickle charge probably wont hurt, though limiting it to less than C/10 would slow charge times, but it would probably prolong the battery life. Or you could use a temp sensor to keep the batteries from sitting and being too hot for too long. What exactly would be the best? I dont know, thats getting past my knowledge, but I'm sure someone else here knows a lot more.
The simplest constant-current charger is a 317T and one resistor Code: 12V---In 317 Out---Res--+------+ Adj | | | | | +-------------+ Batt | 0V-----------------------------+ Supply voltage needs to be at least 3V more than the battery total, current = 1.25/Resistor value, so 18R gives you 70mA.
