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'[EE:] NiMH batteries - charging?'
2004\03\22@083401 by

In my tried and tested behaviour I am commandeering this thread... :)

What is the minimum circuitry to charge a NiMH battery? I have seen them
charged by DC (other batteries) with no intervening circuit, but this
was in a throwaway type situation.

I thinking of using a NiMH 9v battery for my paintball trigger, I have
visions of killing an alkaline in a single game and the current draw
seems to fit the NiMH better (bursts of high current, fairly low
otherwise)

If I go this route, I'd like to go as simple as plug the gun into the
wall between games and keep the battery topped off. I can't really
--
Anthony Toft <toftatcowshed.8m.com>

--

On Monday, Mar 22, 2004, at 05:26 US/Pacific, Anthony Toft wrote:

> What is the minimum circuitry to charge a NiMH battery?

Constant current at 0.1C for 12-14 hours.  Ie a reasonably stable
voltage supply and a resistor...

BillW

--

At 08:33 AM 3/22/2004 -0800, William Chops Westfield wrote:

>On Monday, Mar 22, 2004, at 05:26 US/Pacific, Anthony Toft wrote:
>
>>What is the minimum circuitry to charge a NiMH battery?
>
>Constant current at 0.1C for 12-14 hours.  Ie a reasonably stable
>voltage supply and a resistor...

Check the cell spec.  0.1C constant charge takes a special NIMH cell.
Sanyo's C series are rated for this.
Some NIMH cells are rated for ZERO constant (unterminated) charge current, and they aren't kidding.

--

On Monday, Mar 22, 2004, at 08:51 US/Pacific, David VanHorn wrote:

>> Constant current at 0.1C for 12-14 hours.  Ie a reasonably stable
>> voltage supply and a resistor...
>
> Check the cell spec.  0.1C constant charge takes a special NIMH cell.
> Sanyo's C series are rated for this.
> Some NIMH cells are rated for ZERO constant (unterminated) charge
> current, and they aren't kidding.
>
Thus the 12-14 hour comment.  Which assumes dead or nearly dead cells
what you'll find in an awful lot of the consumer chargers on the market.
A lot of the remaining ("Fancy") chargers do something special for a
fast charge, and then "drop down" to .1c or .05c for "float", whether
it's a good idea or not.  Note that battery manufacturers aren't really
motivated to provide algorithms that maximize cell lifetime :-(

Given their annoyingly fast self-discharge, it's very frustrating to
not be able to float-charge NiMH without adverse effects...

BillW

--

Linear Technology has an interesting charger for Lithium Ions
(actually, they have several interesting chagers - I've now designed
two of them into products). One I haven't used yet, but looks
interesting, is the LTC4052-4.2 . This is a "pulse charger" that uses
a current limited "wall wart" power supply. The charger just
switches, connecting the wall wart to the battery and disconnecting
it. The wall wart becomes a "constant current" charger. As the
battery approaches 4.2V, the duty cycle of the switching is
decreased, decreasing the average current going into the battery. The
chip has an output indicating when the average charge current has
fallen to 10% of maximum (I'm guessing they're detecting 10% duty
cycle) for a "near end of charge" indication. Charging is terminated
by a timer.
So, looking at NiMH, I'm thinking of doing something similar under
PIC control. I'd use a current limited wall wart and switch it on and
off the battery, watching the battery voltage (turning the charger on
at some voltage, then off when the battery reached some higher
voltage). Depending on the hysteresis, it seems that this could
handle the entire charging process. As the battery self-discharges,
the charger would pulse the battery again for a short period of time.
We could also do a charger terminate based on the 10% duty cycle.
Thoughts?

Harold

--
FCC Rules Online at http://www.hallikainen.com

--

At 09:05 AM 3/22/2004 -0800, William Chops Westfield wrote:

>On Monday, Mar 22, 2004, at 08:51 US/Pacific, David VanHorn wrote:
>
>>>Constant current at 0.1C for 12-14 hours.  Ie a reasonably stable
>>>voltage supply and a resistor...
>>
>>Check the cell spec.  0.1C constant charge takes a special NIMH cell.
>>Sanyo's C series are rated for this.
>>Some NIMH cells are rated for ZERO constant (unterminated) charge
>>current, and they aren't kidding.
>Thus the 12-14 hour comment.  Which assumes dead or nearly dead cells
>what you'll find in an awful lot of the consumer chargers on the market.

Yes, My phone is like this, resulting in batteries that signal "almost dead" a few minutes after being taken off charge.  Two 1C cycles of discharge and charge restore them to normal function. (Nicad)

>A lot of the remaining ("Fancy") chargers do something special for a
>fast charge, and then "drop down" to .1c or .05c for "float", whether
>it's a good idea or not.  Note that battery manufacturers aren't really
>motivated to provide algorithms that maximize cell lifetime :-(

Hmm.. I've done this in my Nomad printer.

Sanity checks are done on input voltage, battery voltage, and battery temperature. Battery condition is checked, and we launch into either conditioning charge (c/3) or high rate (c) on that result.

High rate checks the battery under load once a second, and monitors absolute and delta on voltage and temperature, as well as an absolute limit on time in high rate.

A really flat battery will go conditioning, high rate, topping, and low rate, with pauses between states to assure that the battery temperature is both below 40C, and descending, before continuing to the next state.

Abnormal conditions are handled as they occur, and the specific error in a given state vectors to the appropriate handler.

>Given their annoyingly fast self-discharge, it's very frustrating to
>not be able to float-charge NiMH without adverse effects...

If you use the Sanyo C series cells, then you can give them as much as C/10 constant without large problems.

If you can accurately measure the current that low, you can set them to zero or slightly positive current budget.  I use a pulsed trickle, letting them discharge a bit, then providing a short 1C charge pulse. This is adaptive, seeking a zero energy budget.

AVR Tiny-26 doing 500kHz PWM controlling the current every 10mS, and managing everything else, acting as a buck constant current source.

--

> > What is the minimum circuitry to charge a NiMH battery?
>
> Constant current at 0.1C for 12-14 hours.  Ie a reasonably stable
> voltage supply and a resistor...

Thanks for the replies folks, it would appear that my "minimum
circuitry" solution won't do what I think I will need, that is a quick
'top off' between games (15-30 minutes or so) from an unknown charge
state. As you can imagine, the battery going dead mid game is an
absolute no-no.

With that in mind I am going to build a charger based on the max712.
There seems to be something I can make work in the datasheet, if I can
just understand it enough to select all the discreet stuff required to
make sure it doesn't cook off the battery.
--
Anthony Toft <toftatcowshed.8m.com>

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestmitvma.mit.edu

> Thanks for the replies folks, it would appear that my "minimum
> circuitry" solution won't do what I think I will need, that is a quick
> 'top off' between games (15-30 minutes or so) from an unknown charge
> state. As you can imagine, the battery going dead mid game is an
> absolute no-no.

Cell temperature rise is a legitimate end point method. In your case it
would be easy to implement and you may not mind trading off some cell

I have a commercial charger which I use for my AA camera batteries. I think
it uses 3 separate end point methods BUT when the charge light goes out the
cells have always become very warm - probably about 50C.

Russell McMahon

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