Recharging NiMH Batteries

[lk1932]

My daughter borrowed my camcorder and one of the original batteries died. She took the old one in and wanted to buy a new one. She got a new 2 hour battery and has charged it up a few times on my camcorder battery charger. My question is this: The battery that was sold to her is a NiMH battery and the old one was a Nicad battery. What kind of a problem, if any, will she encounter by charging the battery on a nicad charger?

[Q.E.D.]

None, really. It may take a little longer to charge the new NiMH battery, since they generally have higher capacity, but they're also more forgiving of overcharging.

[lk1932]

Thanks for the info, Q.E.D. I was a little worried about ruining the Nicad charger. They are a might expensive!

[Crafter_Man]

We recently built some wireless data acquisition modules using NiMH batteries. We had to build our own chargers. We learned that there are differences between NiCAD and NiMH when it comes to charging, but (as Q.E.D pointed out) they’re subtle.

First of all, most chargers are modeled as constant current sources. While pumping current into the battery the controller continuously monitors the voltage, and stops charging (or trickle charges) when the voltage reaches a certain value. (I’m leaving out some minor details here. I’m just trying to explain the concept.) When you charge NiCADs the voltage will reach a maximum and then it will decrease a little. This is due to self-heating of the battery. With NiCAD you want to stop charging sometime after this happens. With NiMH, you want to (ideally) stop charging at the maximum voltage.

Secondly, you will often find NiMH battery packs have a built in temperature sensor which aids in charging. The controller looks at two things: 1) absolute temperature, 2) slope of temperature/time. You will sometimes find this same technique used with NiCAD chargers, but it’s more common in NiMH battery packs.

Does the NiMH have a built in temperature sensor? Does the NiCAD have a built in temperature sensor? If neither do, then you’re probably O.K. If they both have temperature sensors, then it will only work if both sensors provide the same type of output for a given temperature. If one has a temperature sensor and the other doesn’t, then I would be wary.

Another thing to keep in mind: It is true (as Q.E.D. mentioned) that NiMHs have more energy density than NiCADs. But NiCADs also have lower source impedance, which means they can produce more current for a given load. If the load requires a fairly high continuous current then the NiMH might not work. If it requires high current only in short bursts (pulse profile) then I would add a big frick’n capacitor.

[lk1932]

I noticed that when charging either the nicad or NiMH batteries the green "End Charge" light on the charger comes on indicating that the charge is complete. Don't know if that means anything or not.

[Q.E.D.]

Basically it means the battery has told the charger that it's full (really the charger is measuring the battery voltage, and turns on the green light when it reaches the necessary level). One caveat I forgot to mention, though: a NiCd cell tops out at 1.2 volts, while a NiMH cell tops out at 1.25. This means that the charger might not be giving the NiMH cell an absolutely full charge, but the difference is small enough not to have any noticeable impact, in most cases. In any case, damage to the charger is highly unlikely.

[lk1932]

Thanks, guys, for setting my mind at ease. We will now recharge both batteries without worrying about damageing something. Thanks again.

[BurnMeUp]

i guess this could have been the Secret of NiMH