Re: parallel packs?

["David Roden" <roden ald net>]

> No, battery packs which are more fully charged will have a higher
> voltage under load, so the power will come from the charged battery
> pack rather than the uncharged one.

This is OK with new, matched packs.  If the packs don't match in 
capacity, the higher capacity pack will try to keep the lower capacity 
pack at about the same state of charge.  It's like connecting two tires 
together with an air hose -- the higher pressure tire pushes air into the 
lower pressure tire until they're equal.

Now suppose one tire leaks.  The good tire will deflate itself trying to 
keep the leaky one filled.

This is what can happen if you parallel packs, and your older pack 
develops a shorted cell.  Now the voltage is 2 volts lower -- 6% 
difference.  The newer pack tries to charge the older one.  Now instead 
of the energy going to do useful work, it's going into gassing and 
overcharging the remaining good cells in the old pack.

There's also the fact that an older, sulfated battery will have a lower 
voltage at full charge (at least as full as it can get) than a new 
battery.  This is true even if it has no really bad cells.  So, if you 
leave the packs in parallel during and after charging, the new one will 
try to charge the old one and will be pulled down below full charge.

I still believe that it's best not to parallel packs of different ages, 
and I would also hesitate with packs of similar ages but different 
capacities (GE's booster pack notwithstanding).  If you do so, be sure to 
check the batteries' SG regularly (and load test periodically too), 
replacing any that show signs of a weak cell.

Of course I'm not an electrical engineer, so listen carefully to the 
folks with EE degrees (and remember, your mileage may vary).

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