new hybrid batteries and overcharging

According to the manufacturer web pages I've read, overcharging *any* NiMH cell significantly can shorten its life... they're somewhat less tolerant to this than nickel-cadmium cells. Also, it's somewhat more difficult to detect the "full charge" state in a NiMH than it is in a NiCd, especially at low charge rates.

Based on what I've read, there seem to be two charging schemes for NiMH which the cells will tolerate fairly well:

- Slow charge (0.1 C or so) with a timed cutoff after 12 to 16 hours. If you touch the cells during charging and find that they are significantly warm, then they're probably "full". [They do warm up somewhat during the normal charging process, so judging whether they're warm enough to indicate full-charge is not always easy.]

- Fast charge (0.5C to 1C, or in some cases even higher) with primary cutoff based on temperature rise, secondary cutoff based on zero delta-V (i.e. the cell voltage stops rising when full-charge is reached) and a timed cutoff as a failsafe. Intermediate rates (above .1C and below .5C) have some disadvantages... this amount of current may not result in a rapid temperature rise at full-charge (thus making full-charge harder to detect reliably) but is high enough to affect the cell's lifetime if you do end up overcharging the cell.

It sounds to me as if your Energizer charger has its full-charge detection circuit tuned properly for this newer type of NiMH cell. If it's a slow (overnight) charger, you probably won't hurt the cells significantly using it as long as you shut it down manually at the proper time. If it's a "quick" or "fast" charger, it may very well be overheating the cells enough to reduce their lifetime.

If you plan to use a lot of NiMH cells, or to recharge them frequently, it might very well be a good investment to buy a high-quality charger specifically designed for reliable fast-charging of such cells. I like the Powerex MH-C9000 myself, as it's fast and reliable and has a lot of useful features.

How do you define "overcharging"? Switching to a trickle charge at the end of the charge cycle is, technically, overcharging, but no one considers it abuse.

NiMH cells can tolerate huge charging currents. MAHA specifically states that do not recommend charging at _less_ than 1/3 C, and permit charge rates as high as 1.0 C!

Whether this applies only to their cells, or pretty much everyones, I don't know. But NiMH cells don't appear to be particularly "delicate".

"Overcharging" probably means, as others have suggested, continuing the charge past "negative delta V" and continuing to charge at a high rate to the point where the cell badly overheats.

But, as I said in my "moody" missive, this is something you should ask the manufacturer, as only it knows how its only cells respond to various charging protocols.

The people in this group do not.

Yes and no. NiMH chargers can use either a rise in temperature (which might be hard to judge when the sensor is not part of the battery pack) or a drop in voltage to signal "full charge". The latter is supposedly larger and more distinct at higher charge rates.

I don't believe either of these apply to nicad charging.

Pretty much the way you do later, and the way that the manufacturers seem to. "Overcharging" is when one continues to force charging current into the cell, once the cell's electrochemistry has reached the point of saturation and no further useful electrochemical conversion can be performed.

Actually, some of the manufacturer data sheets I've read seem to recommend against it.

True. That's one (not the only) definite advantage of NiMH cells - they can be recharged very quickly.

They're not particularly delicate in terms of their rate of charge absorbtion _during_ proper charging. As you say, they can eat a lot of current.

They are, however, more easily damaged than NiCd cells by the overheating which occurs if you continue to pump energy into them after their electrochemistry has saturated.

I read "overcharging" as any continued charging past the point of "full". High-rate and low-rate overcharging does affect NiMH cells differently, as the latter doesn't heat up the cells very much.

True. Unfortunately, without further information about how the specific charger operates and behaves, even the cell's manufacturer probably won't be able to give a useful answer.

Actually, both of them do, although NiMH and NiCd cells differ somewhat in both of these respects.

During the normal charging cycle (when they're still accepting charge), NiCd batteries do not heat up very much at all... the electrochemical process in these batteries is said to be endothermic during charge acceptance. The cell's terminal voltage rises slowly during this phase of charging. Once the plates are fully charged up, the electrochemical reaction changes, and a secondary reaction develops which releases the energy as heat... and so the NiCd cell heats up significantly. As a result of the change and the heating, the cell's terminal voltage stops rising, and actually drops significantly. This reversal of the voltage curve with time isn't hard to detect, and most NiCd fast-chargers seem to use a "negative delta-V" detection circuit to determine that the cell has reached full charge and to shut off the current (or drop it to a trickle).

NiMH cells behave a bit differently. They do warm up somewhat during the main phase of charging - the electrochemical reaction is exothermic. Like a NiCd, their terminal voltage rises slowly during the charge cycle. Also like a NiCd, when they reach full charge they start dissipating most of the incoming charge energy as heat, and (in a fast-charge scenario) they can get quite warm quite quickly. However, the effect of this on their terminal voltage is a bit different... it stops rising, but it doesn't begin to fall significantly until you've gone pretty far past the full-charge point and gotten them pretty hot... and the manufacturer data sheets I've read say that this degree of overcharging will shorten their life appreciably.

So, the manufacturer data sheets I've read recommend using the temperature rise (absolute and/or delta-temperature-over-time) directly, using a thermistor, as the primary means of detecting full charge in a NiMH. Zero-delta-V-over-time makes a good secondary shutoff mechanism, and a timed shutoff for safety is also recommended.

I have two chargers designed to charge AA NiMH in three hours or less. I wish they sensed temperature, but it seems they work strictly by voltage changes.

The first one was designed for NiCds as well. I don't recall any trouble with NiCds, and what you've written may explain it. With NiMH, each charger has occasionally stayed on longer than expected, and I removed the cells because they felt hot. I haven't seen any signs of damage from these incidents.

Before I bought my first NiMH cells, I looked at data published by an amateur photographer using several brands of cells and more than one charger. Sometimes when he took pictures he would find that a set of cells hadn't taken a normal charge. I think that's a drawback in charging more than one cell in series in a circuit designed to shut off when a cell is charged. Even when each cell is charged in its own circuit, I think gas bubbles formed in a cell during charging may cause a voltage fluctuation that may shut off a charger. I wonder if that happens more often with new cells.

I only use Ray O Vac alkaline batteries. cuhulin

Some of us are more financially conservative and environmentally savvy. How do you dispose of your alkaline batteries? It's against the law to just throw them into the trash here in California.

I have a lot of Eveready rechargable batteries and the proper Eveready battery gizmo recharger here.I am too lazy to fool around with that stuff all the time, unlless a long power outage occurs.I can use one of my inverters and charge those batteries up from one of my van batteries if I need to. cuhulin

Thank gawd I don't live on the left coast or new england either! Those people are Crazy! cuhulin

Which goes counter to what the trash is for, too...

Then again, it's California, where *everything* has been shown to cause cancer and must have a warning.

Nah, we just don't like to pollute our ground water.