Just recently I put together a 12.5 Volt battery pack using ten AA NiMH 1.25 Volt batteries in series. It has a current capacity of 2300mAh. After creating the first pack, I then created an identical second, and connected the output of that (in parallel) with the first. Then simply used epoxy to put the whole unit together as one 12.5 Volt, 4600mAh battery pack. (Total of 20 AA NiMH batteries.)
My question is, what should now be the optimum charging current for this newly created battery pack?
I know, at least with typical NiCAD's, that the charging current is supposed to be approximately 10% of the battery's current capacity (in this case, 460mA) for about
14 hours. Would this be correct for this situation as well? What charging current would you use? Any advice and comments much appreciated.
So I assume I can just charge it at 1C (when possible) for approximately 1 hour, or anything (lower current) below for a greater amount of time. When you say long life, are you referring more to the overall longevity and life-span of the battery, or to the length of time the battery will last for a particular charge?
Since my smaller DC supply only puts out 1000mA, I'd probably have to charge the pack at around 1/4C or less anyway. Perhaps a good balance between long life and fast charge?
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. 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'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.
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.
DO NOT attempt to fast charge based on time alone. You can never be sure of the initial state of charge. If you expect to time it manually, you'll make a mess the first time you get a phone call that diverts you.
You didn't disclose the configuration of the pack. 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.
There's a bunch of information on the cadex site. mike
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I'm referring to how many charge/discharge cycles the battery will withstand.
charge the pack
life and fast
For various reasons, the datasheet recommends 1C, C/2, C/3 or C/10 but does not recommend C/4 thru C/9.
ammeter, the
minutes time, the
started to make
that I
Possibly. NiMH batteries have a slight voltage dip as they reach full charge. This could account for the slow increase in current drawn. However, this voltage dip is so subtle on NiMH's that they recommend using a backup procedure (such as cell temp or timer) to prevent over charging. Read the datasheet.
I think mike is giving you good advice here, so I'm only clarifying what went before and answering your new questions.
ammeter, the
time, the
to make
connection
ammeters
start
the
It's luck that you cannot and should not count on. That is part of the reason for mike's advice.
very
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.
battery pack
points
The cells are in series, so controlling current in the string controls it in each cell, automagically.
voltage of
will reach
the
that
Delta
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.]
(in
are then
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)
possible
contact
temperature
throughout
hot
still
You may want to provide a little extra "insulation" for the corner cells. They are more exposed to ambient than the middle cells.
it
pack, I
to
With that interest, you might want temperature sensors on the battery as well.
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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 could be that I got lucky and got some very closely matched batteries with very similar internal resistances.
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?
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.
Never.
Agreed.
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.
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.
Recently I purchased the MAHA MH-777P-II Universal charger...
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... 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.
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