charging 9 volt batteries

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I have a few 9v  Ni-MH batteries 200mAh.

I have two chargers one with output at 9v of 10ma and the other a very old
one that says output is 7.2v (even though its designed to take two 9v
batteries) and whose output says it is 15ma x2.

Which would be the best to use on the 9v Ni-MH and what would be roughly be
the minimum charge time to ensure batteries are fully charged please?
thanks for advice.



Re: 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.



Re: charging 9 volt batteries
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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.  

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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.

--
*Two wrongs are only the beginning *

    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
We've slightly trimmed the long signature. Click to see the full one.
Re: charging 9 volt batteries
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


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



Re: charging 9 volt batteries
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Well, obviously, it doesn't.
That's why the lamer didn't quote any previous material.

Re: charging 9 volt batteries

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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.

http://www.powerstream.com/NiMH.htm
http://www.greenbatteries.com/bachfa.html
http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery

Re: charging 9 volt batteries
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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.


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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.



Re: charging 9 volt batteries

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

Re: charging 9 volt batteries

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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?  

Re: charging 9 volt batteries
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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.



Re: charging 9 volt batteries
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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.

--
*Red meat is not bad for you. Fuzzy green meat is bad for you.  

    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
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Re: charging 9 volt batteries

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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 http://www.powerstream.com/NiMH.htm

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.

Re: charging 9 volt batteries
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and
lion

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


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


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Tell that to MAHA, et al.



Re: charging 9 volt batteries
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You are mostly correct on NiMH charge rates, but using Microsoft as a
keyword lessens your impact.

Re: charging 9 volt batteries
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


Re: charging 9 volt batteries
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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.

--
*If you don't pay your exorcist you get repossessed.*

    Dave Plowman         snipped-for-privacy@davenoise.co.uk           London SW
We've slightly trimmed the long signature. Click to see the full one.
Re: charging 9 volt batteries

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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).


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I wouldn't disagree.



Re: charging 9 volt batteries
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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.

Re: charging 9 volt batteries
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Really? You don't know much about charging and chargerrs.



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