Marty,
>Anyway, if I connect these batteries in parallel, I'll get 12V, and
>*should* get close to an 8AH rating for the pair -- correct?
True, but operating batteries in parallel is risky, unless you place an
isolation diode in series with each battery upstream of the "paralleling"
connection. Batteries are never identical, so without the isolation, the
battery with the higher terminal voltage will try to recharge the one with
lower voltage. Ordinarily, this isn't much of a problem, because they will
tend to equalize as they discharge. The real hazard comes if a cell in
one battery shorts - the resulting mismatch can cause excessive current
flow and result in fire or explosion.
>Question is: Would the charge rate for these batteries need to be
>adjusted if they were left in parallel? Each battery is charged
Never charge batteries in parallel (for the same reasons stated above).
The battery with the lowest internal resistance will hog the charging
current with potential overheating. At trickle charge rates (C/10, or
lower), you can get by with it, but at anything higher, it hazardous.
We once had 2 12v automotive batteries connected in parallel for backup
power for a 2M repeater. They were floated across the 12V power power
bus. For some reason, they became unbalanced, and the electrolyte in one
completely boiled away. Fortunately, the repeater was in a concrete block
building, so nothing caught fire.
>The plan would be to leave them charged, and take them on weekend
>trips (I think 8AH would get me through a weekend of QRP operating).
This is a good plan, but instead of leaving them on continuous trickle
charge, recharge them individually, and disconnect them. Then top them
off on Friday before you leave for the weekend. Then use them
individually. This way, if one fails, you still have the other, and the
weekend won't be a total loss.
As an aside, we do use paralleled Ni-Cads in some spacecraft power
systems. The difference is that the batteries are made up of matched
cells, then in use the individual battery currents are continuously
monitored, and a battery is taken off line if a problem occurs.
The cells in off-the-shelf Lead Acid batteries aren't selectively
matched - if they were, we couldn't afford them!
72/73 Bill wb0cld
Bill Launer
St. Charles, MO
launerb@crl.com
wb0cld@wb0cld.ampr.org [44.46.66.25]
qrp-l #279 qrp arci #3551
Grid Square EM48RT
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