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?
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.
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?
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.
DO NOT attempt to fast charge based on time alone.
Never.
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.
Agreed.
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.
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.
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.
There's a bunch of information on the cadex site.
mike
Thank you for the info.