charging 9 volt batteries

The important thing is the current, not the voltage. Nicad and NiMH cells are usually charged at constant current.

Nicad and NiMH cells are usually charged at 0.1C (20mA in this case), though NiMH can be charged at 0.3C to 0.5C, sometimes faster. I'd use the second charger.

If the battery is way down, it will take close to 20 hours for a full recharge. Considering that the rate is quite low, you don't have to worry much about overcharging or damaging the battery.

Is that really 7.2 or 8.2v? Many rechargeables are 8.2v nominal rather than 9 volts. You can't actually do an exact 9 volt nominal as the cell voltage is 1.2 - as opposed to 1.5 for dry cells.

You need a custom NiMH charger for best results - one designed for Ni-Cad won't work properly. Many newer ones do both.

FWIW the important thing is not to wreck the battery with a poor charger - far more important than fully charging one if you require a decent life from it.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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I have an Eveready battery charger and some Eveready rechargable batteries, including one Everedy 9 volt rechargable battery.The battery charger has a place to recharge the 9 volt battery. cuhulin

And how does this assist the OP in deciding how to go about charging his 9V NiMH batteries?

Well, obviously, it doesn't. That's why the lamer didn't quote any previous material.

The one that is designed to recharged NiMH batteries. The charger needs to cease charging NiMH batteries when they reach full charge and then not charge thereafter (i.e., no trickle charge to top-off the battery). If a thermistor is not used to detect a full charge then a timer should be built into the recharger to ensure the charger turns off (to no longer continually try charging the battery).

NiMH should be charged at a very low rate. It is unclear if 15mA x2 means 15mA is used for each of 2 battery slots or if it gets split and

7.5mA is used for each battery. You didn't indicate if the both chargers were expressly designed to recharge NiMH batteries or if they might be for rechaging NiCad batteries.

old

be

Not so.

A thermistor cannot detect full charge. It can only detect the temperature rise near the full-charge point. This is not quite the same thing.

This is simply untrue. I work at Microsoft Hardware, and have been learning about the differences between NiMH and nicad charging.

NiMH cells will tolerate very high charge rates. Indeed, the high rate is desirable not only for convenience, but because it produces are large voltage _drop_ at the end of charge. The better chargers can detect this, and shut off.

In general, any charger that will handle nicads will also charge NiMH cells safely.

Sure, if you don't mind severely shortening the lifespan of a NiMH battery.

Yeah, sure you are. You don't even understand that a thermistor is NOT just a temperature sensing device but can also be used for a self-resetting overcurrent protector within a circuit and never senses the temperature of the recharged battery. RTDs are made of metal while thermistors are made of ceramic or polymer. RTDs are used over a larger temperature range while thermistors are accurate only over a small range.

Tell me, in just what recharger have you ever seen a thermistor contact that rests against the side of the battery to monitor its temperature? If the thermistor is inside the case, just how is it going to measure the temperature of the battery, a battery that is outside and obvious affected by air blowing over it versus when not blowing over it?

I'd call that rather a sweeping statement. ;-)

Plenty of NiCad so called chargers can't handle NiCads properly either. Some are just an unregulated DC supply with a simple series resistor. Which may work after a fashion at lower charge rates but is risky at higher ones.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

It is, but it's broadly true. Most nicad chargers charge at 0.1C, which is perfectly safe for NiMH cells.

A rapid charger that pumps a lot of current is a different matter. Nicad and NiMH cells have different end-of-charge criteria, and a nicad charger might not shut off at the right time -- though, as I understand it, it's more likely to be premature than late.

MAHA (Powerex) specifically states that low charge rates might not produce full charge. I'm assuming this is chemistry/thermodynamics, because a low charge rate will not trigger as large a delta-V at the end of the charge, and the charger will keep charging (rather than undercharge).

I wouldn't disagree.

I hate to spoil your day, but I work for a company that provides engineering services to Microsoft. I talk regularly with the people who design charging systems for NiMH cells, and edited the document describing the charging system for a new product.

The companies manufacturing these cells recommend high charge rates, and Microsoft has done extensive research that indicates the cells work properly and have an acceptable service life.

Of course they do. The most common heavy duty use of any rechargeable is in power tools - and nimh replaced nicad fairly seamlessly. Although lion types are now appearing at the top end.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

You are mostly correct on NiMH charge rates, but using Microsoft as a keyword lessens your impact.

I have never tried to recharge a NiMh battery with my Everyready recharger before.It might work ok, it might not.What I would do is go to a store and buy a proper recharger for such like batteries. Li-On batteries? UNT UHHHHH!!! Not in my house! cuhulin

Yeah, Will changes from working at Microsoft Hardware (inferring he works at Microsoft) to then working for someone contracted by Microsoft.

This discussion has changed from 9V "radio" or other 1-cell hand-sized NiMH batteries that charge at a rate to prevent venting versus high-rate compositions for the electrolyte that include a catalyst to handle the oxygen outgassing. Suddenly we went from the small hand-sized batteries that must charge at slow rates to the high-rate constructions. Different animals, folks. Saying "NiMH" doesn't equate to all batteries that use a metal-hydride negative electrode, a nickel compound for the positive electrode, and some varying composition of alkaline electrolyte between with varying physical construction and perhaps includes a catalyst to prevent venting. That's like saying all gunpowder is the same simply because you used the term "gunpowder" to cover all possible compositions.

The delta-V method of detecting overcharge (when the cell begins to reverse polarity) shouldn't be used on high charge rate NiMH batteries produced since bumps in the charging cycle can cause premature shutoff; see

The short: use the charger that was designed for the particular NiMH batteries that it charges. It might not be the ideal charger for those batteries types within the NiMH family of batteries but it'll probably work better than you emulating the Red Green show in adopting something else.

and

lion

Do you understand the difference between at and for? No? I do, indeed, work at Microsoft.

Apparently not. They're don't seem to be "regular" and "high-rate" NiMH cells.

Tell that to MAHA, et al.

d

eries) and whose output says it is 15ma x2.

be

There are two types of rechargeables. 7.2 volts and 8.4 volts. You obviously can't charge a 8.4 with a 7.2 volt charger. Know your battery type. it should say on the battery what the actual voltage is. A 230 mahr battery would need 23 hours at 10 ma.

Really? You don't know much about charging and chargerrs.