Guido explain this to me like I'm a first grader

Guido, explain this to me like I'm a first grader. No secret I've been a fan of 18v dewalt tools for past 20 years. I like they way the tools hold up for my purposes, but hate Dewalts Ni-cd and Li-ion. So I buy Ni-Mh batteries off ebay. Last order I spent a little more for a square deal 3 year warranty and cheaper than battery plus rebuilding them with Ni-cd cells.

Some of my original Ni-Mh batteries won't take a charge using the special dewalt charger designed for all 3 battery types. So I use an old .25 amp black and decker charger to keep those batteries charged and use them in the pole barn. BTW my oldest battery is my best one. I've been thinking there is something wrong with the temperature sensor (thermistor)inside the battery. Well I decided to use my shop radio to slowly discharge the Ni-Mh batteries in an attempt to erase their memory. When voltage gets to a low the radio shuts off.

To my surprise, when I plugged the radio back into the 110v the batteries now will take a charge from the radio. My question is why? In the summer thermistor's resistance goes down at temp goes up. In the cold pole barn, just the opposite, temp goes down resistance goes up. I decided to measure the resistance of the thermistors in all my batteries, both good and bad batteries. Most range from 29K to 100K. One of my bad batteries will now take a charge and the thermistor's resistance is infinity. So what's the need for the thermistor, except for having it there to tell charger battery is too hot to charge? What shuts the dewalt charger off? Obviously not the thermistor. Does it measure the current and the battery voltage to determine when it time to shut off?
geo

Hello Geo-Th,In,

I just read your post, lots of questions! Anyhow I am having trouble seeing right now. I call it quadruple double vision. I will attempt to explain later,

Guido.

Hello Geo-Th,In

The "smart" charger needs to see voltage before it charges. Your radio is probably just a power out, That output will start charging period! It May possibly be power tapering as well. The thermistor is there ONLY to stop the charging process when the pack gets hot to a predetermined parameter. The value of the thermistor is 10K Ohms @ 68 degrees F*. Temperature then determines the value. The charger is mated to that value. The charger will charge old packs with out the third thermistor terminal, that is why it will charge a pack with an open thermistor. Take a pack that you know it won't take a charge and while it is plugged in the charger-radio, unplug and plug the A/C line a few times. Let me know what happens, I know. There is lot I can had, trying to keep it elementary as you said. I told you before HiMh are the best for powering cordless tools.

Guido.

Guido,
For years 6 MiNh batteries wouldn't take a charge with dewalt charger or shop radio which has a built-in charger. I could only charge them with old B&D 0.25 amp manual charger.

The thermoster also tells charger if the battery is hot due to using battery.

Now it's cold and theromisters are measuring 28k to 100k they charge just fine. One battery a thermoster measures infinitely, open, and all is well. Why? CHARGER obviously doesn't rely on theromisters to shut off. That sounds crazy.
George

Hello Geo-TH, In,

You are sayin what I said. Open thermistor no problem. Thermistor with more resistance working. Manual charger has no problem charging. Thermistor tells charger to stop charging

Guido.

Glad you guys brought this up. I can't get 4 screwdrivers to take any kind of a charge. Can you help me out? They look like this-

Hello Bret4207,

You need cosmic rays to charge them. I guess you forgot? You probably need a solar powered flash ligh to
look for the cosmic rays collector!😵

Guido.

Thanks Guido! I was worried the flux capacitors were shot...

The thermistors don't have anything to do with charging.

NiCd and NiMH chargers are looking for a "peak" in the pack voltage during the charge process where the voltage drops slightly. This indicates that the pack has taken all the charge that it is going to take.

The main difference is the peak in a NiMH is much more subtle than NiCd. A NiCd charger probably won't sense the peak and will overcharge NiMH batteries.

When packs get old, and develop a memory, you get what are called "false peaks." It usually happens immediately after you plug the pack in. Depending on the charger it will register as an error or as a fully charged pack depending on the design.

By running the battery right down you got it to where the battery would not immediately false peak.

Hello barnyardengineering,

Good explanation! When peak voltage is achived voltage then starts to drop, and heat starts to build up.
Some charger have a default of 5 milli volts, then it stops charging. My charger has an adjustable delta
peak. Even that sensitivity needs monitoring. Some pack get really hot even with such a small voltage drop.
Each pack should really have its own charging parameters, it would them be a better charged pack without
overheating,

Guido.