Re: ReCharging NiMH Batteries
From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/29/05
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Date: Tue, 29 Mar 2005 09:55:29 -0800
"QDR" <QDR@nospam.com> wrote in message
news:bbvi41db53mnjsc0cmhtr9mqc1tnp1sm2j@4ax.com...
> On Thu, 24 Mar 2005 21:54:05 -0800, mike <spamme0@netscape.net> wrote:
I think mike is giving you good advice here, so I'm
only clarifying what went before and answering
your new questions.
>>> When I connected my DC supply (18VDC) to the battery pack through an ammeter, the
>>> charging current started out slightly above 1000mA. In about 35-40 minutes time, the
>>> current had dropped to around 805mA, if I'm not mistaken, and then started to make
>>> its way back up to 813, 814, 815mA and so on. It was at this point that I
>>> discontinued the charging. Is this normal for a NiMH battery?
>
>>YOU DO NOT WANT TO DO THIS!!!
>>It's not a good idea to FAST charge batteries in parallel. Small
>>mismatches can cause almost ALL the current to one side. You don't have
>>any way to measure the current in each string separately.
>
> Very good advice, and it should be a concern. By severing the battery pack connection
> (no longer connected in parallel) and connecting them both to two separate ammeters
> while they are being charged by the same charger, I find the current difference
> between the battery packs to be generally less than 5-7%. The difference can start
> out larger as the batteries first begin to charge, and then becomes negligible the
> closer they get to being fully charged. What's your take on this?
It's luck that you cannot and should not count on.
That is part of the reason for mike's advice.
>>You'll probably get away with it at low charge rates.
>>Check the vendor specs on long-term slow charging. Early NiMH didn't
>>tolerate this well. Maybe better by now.
>
> It could be that I got lucky and got some very closely matched batteries with very
> similar internal resistances.
To continue the luck would require similar histories
for the cells. Varying discharge depths and rates,
and varying recharge methods, will make the cells
even less matched than when they were when new.
>>It's not a good idea to EVER connect a constant voltage source to charge
>>a NiMH pack. You need CONTROL of the current in each cell.
>
> I'm not sure I understand this fully. May be a dumb question, but if a battery pack
> is comprised of, say, 10 cells, and the battery pack has only two connection points
> (one negative, one positive), then how would you control the current to each
> individual cell in the pack?
The cells are in series, so controlling current in the
string controls it in each cell, automagically.
>>You need to monitor something to terminate fast charging.
>>Temperature works if you measure ALL the cells and don't mind cooking
>>them every time they charge. I prefer voltage slope sensing with
>>temperature backup in case something fails.
>
> What I've done thus far is this: I use a volt meter to closely monitor the voltage of
> the battery pack as it charges. In this particular case, the battery pack will reach
> a point where the voltage is at around 14.47 volts. It is at this point that the
> voltage begins to drop back down several one-hundreths of a volt. And also at that
> point that I disconnect the charger; Sort of a human intervention Negative Delta
> Voltage charging method, if you will.
You may be seeing a heating effect as the cells reach
full charge. When that happens, more applied charge
is converted to heat rather than chemical energy.
[Fast charge cautions agreed upon.]
>>You didn't disclose the configuration of the pack.
>
> The configuration of the two packs is that they are comprised of 10 identical (in
> make and model) Duracell AA batteries to create 12.5 volts. These two packs are then
> connected in parallel to give a capacity of 4600mAh rather than only 2300mAh.
That connection is problematic because it makes the
problem of unequal current sharing discussed above
all the harder. Can you split the parallel strings for
charging purposes? Or can you afford equalizing
resistors among the strings? (slow discharge)
>>For fast charging,
>>you need equal temperatures, meaning at least equal exposure to ambient.
>>Depending on how you epoxied them together, you may have created a problem.
>
> The pack is put together in such a way as to create as equal temperature as possible
> between all 20 batteries. In fact, all batteries are all within physical contact
> within the pack. I have been charging it at approximately C/5. As far as temperature
> goes, it seems to be both equal and constant on both sides of the pack throughout
> charging, even though it never reaches anything near 140 degrees. There are no hot
> spots or cold spots. Very even temperature. A rate of C/5 I think might be still
> considered relatively slow, even for a technically termed fast charge.
You may want to provide a little extra "insulation"
for the corner cells. They are more exposed to
ambient than the middle cells.
> Recently I purchased the MAHA MH-777P-II Universal charger...
>
> http://www.modelairplanenews.com/click_trips/mar03/maha_charger.asp
> http://www.cetsolar.com/c777plus.htm
> http://www.bills2way.com/equip/mhc777plusII.html
>
>
> ... to deal with packs (and others such as this), but as of yet have not used it
> with this pack. Generally upon buying (or creating) a rechargeable battery pack, I
> like to charge it manually first, with ammeters and voltmeters attached so as to
> physically see the real-world charging characteristics of the pack.
With that interest, you might want temperature sensors
on the battery as well.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
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