Mark,

Good info. I have been looking for something like this for some time.
Today I took my digital multimeter down and checked the voltage. The
boat has been sat for 3 weeks with no power drain or charging. They will
have been left fully charged when I last used the boat. The voltage on
the house bank was 12.8V (still isolated and not touched for 3 weeks).
Cabin temperature was around 12'C.

This seems odd to me after 3 weeks of coldish weather (3-10'C). I would
have expected some self discharge. The house bank is two AC-Delco
Voyager M30HMF batteries that came with the boat. I don't think that
they are anything fancy like gel type. They wouldn't have been my choice
and the bank is half the size I would ideally want. They are a two or
three years old and have probably never been discharged at all since the
last owner lived aboard, the boat never left dock and he constantly ran
a charger.

How much self discharge should I expect?

The following could be explanations that I can think of...
1) the charge lost to self discharge is a lot lower than I thought it
would be. I would have imagined at least 10% over that period.
2) my multimeter is not accurate although I believe that it is good to
at least 0.1 volts from other stuff I have used it for.
3) the batteries are not just standard wet cell lead acid and hence the
table is not valid for them.
4) someone has broken into my boat and recharged the batteries for me.

I have tried to get info from ACDelco (who are general motors) about
these batteries for charging voltages for various charge stages etc but
I was not convinced they knew what I was talking about. Initially they
said the information was confidential and couldn't be released then
after a lengthy explanation from me that that didn't make sense and more
detail on what I was asking for the answer I got back was... "13.8 volts
for charging and float". This is less than most alternators kick out.

Anyone have any ideas or comments?

Thanks,

Steve


Mark wrote:
Resting state voltage readings can provide a rough guide of battery
charge state. For accurate voltage readings, batteries must remain
idle (no charging, no discharging) for at least 6 hrs, preferably 24
hrs. If loads have been light for a few hours, you can shorten these
times; for example, after running a 1 amp anchor light only, overnight,
using your 300 amp bank (0.3% of C discharge rate), turn off all loads,
wait an hour or so, then test.

Assuming generic wet cell lead acid batteries a room temperature
environment, Trojan Battery company publishes this table:

% Charge V
100 12.73
90 12.62
80 12.50
70 12.37
60 12.24
50 12.10
40 11.96
30 11.81
20 11.66
10 11.51


Decide how long you want your battery bank to last, considering the
following, per Trojan:

1. Shallow discharges will result in a longer battery life.
2. 50% (or less) discharges are recommended.
3. 80% discharge is the maximum safe discharge.
4. Do not fully discharge flooded batteries (80% or more). This will
damage (or kill) the battery.
5. Many experts recommend operating batteries only between the 50% to
85% of full charge range. A periodic equalization charge is a must when
using this practice.
6. Do not leave batteries deeply discharged for any length of time.
7. lead acid batteries do not develop a memory and need not be fully
discharged before recharging.
8. Batteries should be charged after each period of use.

So a reasonable approach from a dollars per amp-hour standpoint would
be to shoot for a recharge cycle at the 50% discharge state, don't get
worked up about the occasional discharge to 80%, recharge anyway even
if you've not gotten to 50% discharge if a few days have passed, and
leave your batteries fully charged if you'll be gone for more than a
few days. Such an approach means you would fire up the engine when you
see the resting state voltage drops to about 12.1 volts.


varnish wrote:

I've got a 300AH domestic battery bank and monitor the voltage across it
while cruising. It's usually around 12.8 when charged. What does it
have to drop to before I run the engine to recharge it?