Thx Joe - the first paragraph of tha paper summarizes my dilimma exactly.
I had a chance to do a fairly loose test today, using the paralleled bank where bad and a good were matched up. The voltage decline under a 30-40amp load seemed much more gradual than before and I got about 20AH more than previously before my cut-off voltage. Still well under the pack capacity, but this was "round 1" of trying to balance them.
I'm also not sure if there may be an advantage to paralleling 2, 48V strings vs creating battery pairs (12V) connected in series to 48V. There are fewer cables involved in the former, so I would think it would be more efficient. It also avoids a rather complex mix of parallel and serial connections (if you consider the cells connected in series inside the battery as part of the system). So that may be my next try.
re: equalization - reading up there are chargers that do an automatic equalization at prescribed intervals - that seems best, but as a fallback I'm going to try an adjustable DC power supply rated at 20-30 amps.
Thx
-Keith
--- In electricboats@
>
> Uh, what about this:
>
> http://www.battcon.
>
> Monitoring the current of each of the two paralleled batteries will tell you
> what's happening to each of them. Inasmuch as you're probably sitting on a
> float charger, self-discharge may not be an issue at all. I would be
> inclined to say the amp-hour capacity of the the paralleled pair is the sum
> of the amp-hour capacities of each of the two, despite their being quite
> different.
>
> JoeS.
>
>
> > From: Tad G <tad@...>
> > Reply-To: <electricboats@
> > Date: Thu, 16 Jul 2009 18:00:07 -0400 (EDT)
> > To: <electricboats@
> > Subject: Re: [Electric Boats] When good batteries go bad
> >
> > One of the reasons to not do that is that you end up with the good battery
> > charging the bad one, the worse case scenario in terms of losses. As soon as
> > the charger isn't keeping the bad one topped off the good one will start do to
> > so, but obviously it won't be able to and they will both loose charge until
> > they are depleted. That's one of the reasons for battery isolators. Two
> > batteries that aren't exactly the same will self discharge faster when
> > connected together than when disconnected.
> >
> > -Tad
> >
> >> ----- Original Message -----
> >> From: "aweekdaysailor" <aweekdaysailor@
> >> To: electricboats@
> >> Sent: Thursday, July 16, 2009 12:47:52 PM GMT -05:00 US/Canada Eastern
> >> Subject: [Electric Boats] When good batteries go bad
> >>
> > (snip)
> >>
> >> My theory is...if I pair a new battery with the worst of the pack (in
> >> parallel) then that unit now acts like a medium-good battery in relation to
> >> the others (the good battery will be trying to continually charge the bad one
> >> during discharge - conversely during charge the good battery will be helping
> >> the charger see it's "full" and go into float). I think I still won't get the
> >> full pack amperage, but some "average" value determined by the weakest pair.
> >>
> >> There are lots of warnings about not doing this - but no clear explanations.
> >>
> >> Any battery gurus care to comment on the strategy?
>
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