[mxmtech]

Nicad batteries have a temperature sensor that stops the current, right? So does it make a critical difference where you do your charging? If I charge my batteries in the garage where the temperature is near freezing I'll get a different result than if I bring them in and charge in the furnace room where it is uncomfortably hot?
What if I charge my batteries up, then wait awhile and plug the charger in again when the batteries have cooled down from the previous charging cycle? Will I damage the batteries from overcharging by doing this?

[MN Driver]

I don't know of any nicad's that have temperature sensors, at least the most common cylindrical ones don't. I didn't think the flooded prizmatic NiCd's did either, but the two require a completely different charge algorithm. The consumer grade cells usually don't gather much heat when charging below 1C. I know that at 10 amps for the 6.5Ah NiMh cells used in the Honda Insight I've heard they get a bit warm sitting on the bench but under fan cooling they get 50amps but those batteries are designed for very high charge/discharge levels.

If they are cylindrical I'd avoid charging them at rates that get near the range of being hotter than you would want to touch as that is way too hot for them. NiCds usually keep pretty cool in my experience though and then at the end of charge they start getting warm pretty quickly but it all depends on the charge rates. ...again I have no experience with NiCd prizmatics so if that is what you are talking about I probably haven't helped you much but I do know those need topping of water every time as they are supposed to reach gassing voltage and gas just a little as often as possible, every time is best according to the Saft documentation I read once.

Edit: What are the specs on your batteries, the documentation that came with them, if it had any, should help with charge current and charging methods. I've always thought the Nickel chemistries preferred a one-shot full charge and NiCd's don't take kindly to constant partial charges.

[Sunking]

Originally Posted by mxmtech

Nicad batteries have a temperature sensor that stops the current, right?

Wrong! NiCads require special charging techniques. Depending on the manufacture, but generally they all require a constant current method. The two best ways to determine when the battery is fully charged is a delta voltage rise, or temperature rise. Tempurture is hard to do. Voltage is easy, it just takes some logic or A/D comparators.

Here is a fantastic circuit using a PIC. It can be modified to operate at any voltage or current (well up to 42 volts, higher and you would need to use a different comparator or modify to limit voltage) . I have built a few of these myself. It does not matter what the DOD of the battery is to start with and will never over charge a battery, and when finnished will deliver a trickle charge to keep it 100%. If your battery can handle a 1C charge rate, maximum time is 1 hour and 15 minutes.

[neanderthal]

Sweet! Where did you find a bunch of affordable nicads?

But yeah I agree, it would be helpful to know if they are sealed cylindrical cells, or flooded, prismatic cells.

[mxmtech]

These batteries are a pack of F cells. I took a pack of C cells apart and they had what I assumed was a temperature actuated switch wired in sequence. I assume that these are the same.
The battery pack and the charger came from the same dealer so I assume that they are compatible.
The battery pack is 24 volt 8 AH and the charger is 1.8 A @ 24 V. The batteries get quite warm at the end of a charge.
They came with zero documentation and the girl at the store sounds like a newbee. Without a doubt someone in the store is an expert but I've been unable to get past the clerk.
I guess the term "affordable" is relative. I paid a little less than $300 for the package including shipping and handling from the makers of the Cycle Analyst. They don't advertize their nicad packs on their website but they do sell them if you ask.
I'll run a few charge cycles using a room temperature thermostat, and a current usage meter, and see what I can learn from it.

[mxmtech]

Charged my pack 3 times last night. Room temperature is 70 degrees F or 22 degrees C. First charge took .09 kwh, second .03 kwh, and third .04 kwh.
There is definitely something wrong with either the charger or the sensor on the battery pack.
I don't know what I lose to heat but .16 kwh would equal 6.66 AH on my 8 AH pack which sounds like a full charge to me.