Flemming Torp wrote:

"Larry" skrev i en meddelelse
...

The discharge-charge cycle is a ratio of 1:5. It takes 5 times as long
to CHARGE a lead acid battery as it does to DISCHARGE it. That ratio
hasn't changed in my lifetime. It's chemistry.....a slow chemical
reaction.

SNIP ...

Hi Larry, I've seen this rule of thumb before. But it cannot - in my
experience - be a valid general statement.

The 1:5 ratio has absolutely no meaning for cruisers. Its true that
the last 15% takes a long time to to charge, but this need not be done
on a daily basis. Over the range that most cruisers discharge and
charge, the charge rate, assuming a large source and quality
regulator, will at the 25% of capacity rate, tapering down to about
half that.

It is true that a 400 Ah bank that can accept 100 Amps in the "sweet
spot" can probably put out 500 Amps just before it melts something,
but this is of little concern.


Today, my battery bank consists of two 70Ah batteries + a start battery
of 60Ah. If I spend - let's say - eight hours at sea using my
instruments, chartploter and autopilot, and the cooling compressor
running for 24 hours, I will have used something like 70Ah i.e. roughly
50% af the capacity of the bank ...

I'd say you need more capacity if you want to stay out for extended
periods. I've found that 3 times the daily use is better, because
that allows you to cycle between 50% and 85% daily, with a run time of
about 90 minutes. More, of course, is better, ignoring cost, weight,
and space issues.

There is no easy way to do better than this, not counting wind and
solar. Each cruiser is in a different situation. I like to find a
nice harbor, and "squat" for a week or two, perhaps traveling a few
miles to the next island, so I'm faced with daily charging. More
solar panels (I have 150 Watts) would ease the situation - if I was
out full time I'd double or triple the solar, and add wind generation.

For some cruisers, running a genset for extended periods is an option,
but in many areas that I cruise that would be anti-social. I do,
however, carry a 2K Honda as backup so that a vacation isn't ruined by
mechanical problems. Also, boats that spend more time sailing then
hanging out can use a shaft driven alternator.



When I connect the land charger (in my case 220V) charging the 12V
batteries with 35A it does not take five times as long to "fill" the
battery bank back to close to 100% ...

Well, its a little hard to measure the last few percent. At what
point do you declare that it is now "fully charged" and you're now
just trickle charging to make up for self discharge?

It is true that chronic undercharging can be a problem, but that is
minimized if you fully charge the bank periodically, either on shore
power or with an extended motor sail.


What is the problem with the rule og thumb? Is there some kind of
"hidden assumption"? ... Or under what circumstances is the ratio 1:5
valid for charging the battery bank?

It might work well for submarines.


If I decide to increase the capacity of the bank to - say - 600Ah - will
I have to install a larger charger - i.e. giving more than 35A - in
order to give the batteries a full recharge - or is it just a matter of
time until the bank is "full" again?

It depends on your usage. I tend to go out for 1-3 weeks and never
plug in to shore power. Then the boat stays at the slip for a week,
so the charge rate is not a big issue. A 35 Amp charger would put
out the full 35 Amps for about 6 hours if I came in 50% charged, and
then spend several hours tapering off, so this would probably be big
enough even you wanted to travel the next day with a full battery.


I have decided to add an alternator
to my Volvo Penta 2002. It will give 60A. This one - in combination with
the existing alternator - the total charge current will be around 100+A
... with a bank of 600Ah, that should be no problem - right? I mean: No
harm to the batteries?

If you have a large bank and a large alternator, the quality of the
regulator, monitor, combiner (or equivalent), fuses and general wiring
become important. You'll spend as much, or more for these components
as for the alternator and batteries. With proper regulation, a large
alternator should effectively shut down as the bank gets charged.

A particular problem that you will have with this setup is that if you
use two regulators, one on each alternator, they will confuse each
other and there's a fair chance that only one will turn on. It is
possible to control them with one regulator, and there are special
devices to balance two engines, but you should consider this issue
carefully.

BTW, many small "stock alternators" (such as the Hitachi's on a
Yanmar) have an internal regulator and are "N-type," and those
designed for external regulators are usually P-type. While they can
be used together, you can't have a single regulator controlling both
types.