On Fri, 15 Oct 2010 12:21:15 -0400, Wayne.B
wrote:

I'm sure that it's possible to get away with an 80% discharge for some
period of time. I've seen some fairly convincing life cycle charts
however that seem to indicate greatly increased battery life at the
50% level as opposed to 80%. I like to baby our house batts as much
as possible given the expense and difficulty of replacing them.

See "Cycles vs Lifespan" at:

http://www.windsun.com/Batteries/Battery_FAQ.htm

http://img32.imageshack.us/img32/9595/batterydod.gif

(from site: http://www.mpoweruk.com/life.htm )


I agree about babying one's house bank, as long as it doesn't limit
the use of them. Like everything else, it's a trade off.

Here's data from real world experience. It's simply what was, with no
claims:

Back in my EV days, we had a group of about 30 that had built our own
cars or pickups. Each one of them used 20 golf cart batteries in
series.

Note that we all recharged at the C/10 rate ( Amp Hour Capacity
divided by 10 ) and recharged daily. Next to overcharging, the next
best way to destroy a lead / acid battery is to delay charging,
allowing sulfation. We made very sure to keep the electrolyte above
the plates and we did an equalizing charge monthly.

We got an average of 3,600 charges, with a few getting about 7,000.

To explain, our batteries lasted 8 to 12 years. These were our regular
cars. Some of us commuted to work daily. Some barely took the surface
charge off the batts most days, some slmost killed them dead each
workday. That doubled figure were for the few that had really long
commutes and/or their workplace supplied sockets to recharge.

Two of the group rigged up a generator on a trailer to extend their
range. Note they couldn't pull a large enough generator to go
indefinitely, just enough to double or triple the range. The motors
drew about 140 Amps or so at highway speeds.

This seems to say that lead / acid batteries are somewhat like a clock
- wind it up once and it's going to work for X length of time. The
fact that regular car batteries seem to last about 2 years says the
same thing.

Note that we all had several batteries fail at one time or another
during that period. I had one go after a couple months that was
replaced under warranty. I don't remember now just when, but I had two
others go out over the years.

To repeat, we all got 8 to 12 years from our batteries, whether we
discharged them to 10 or 20 or 50 or 70 or 90 per cent daily. We got
many more cycles than the graphs say - most of us should have gotten a
year or two or three. Maybe because we did otherwise baby them, or
maybe because the published figures are worse case, or both?


In any event, if I were to be a weekend sailor and planned on
discharging to 20% or so, I don't think it would be worth it to buy an
extra pair of batteries and have the extra weight and space taken up.
I'd still be getting a pretty long life out of them. Long term
cruising is a different story - I packed 'um in, as many as I could!

Rick