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?

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?

I guess around 11.5 would be the critical voltage below which some devices
would cut out, such as inverters, instruments etc.


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(varnish) wrote in
. uk:

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?


Voltage doesn't really tell us what the charge left is on a battery. If a
battery is lightly loaded, for instance, it will hold quite high voltage
WAY past when it should have been charged. If it's heavily loaded, it will
show low voltage, even though it's still at 80% capacity. So, measuring
voltage is a poor place to see how much is left on the battery....damned
near useless.

Here's an excellent webpage for battery maintenance:
http://www.independent-power.com/bat...aintenance.htm

Here's another:
http://www.nwes.com/using_batteries.htm

From this last webpage, you can see the most important reading of charge,
specific gravity of the electrolyte:

"The term specific gravity describes the ratio of the density of
electrolyte to the density of water. Electrolyte weighing 1.2 times as much
as the same volume of water has a specific gravity of 1.200. The full
charge gravity of a cell is a matter of design and depends on several
factors. The specific gravity must be high enough to contain the amount of
sulfuric acid necessary to meet the chemical needs of a cell. If the
sulfuric acid content is too high, damage may result to the cell. The
standard full charge gravity for lead acid batteries used in an R-E system
is 1.250 to 1.285 depending on which type of battery you are using. Since
the acid content of the electrolyte decreases linearly as the cell is
discharged, the decrease in gravity is directly proportionate to the amount
in ampere-hours taken out. The specific gravity at any point in the
discharge indicates the depth of discharge, and can be translated into amp
hours taken out. A cell having a full charge specific gravity of 1.280 and
a final specific gravity of 1.130 has a gravity drop of 150 points.

Example: assume the specific gravity is 1.180 at 77°F at the end of a
discharge. That is 100 points specific gravity below the full charge
gravity, therefore, 100 ÷ 150=67% discharged of rated capacity. So if your
battery were rated at 1000 amp hours you would have taken 670 amp hrs out
of the battery. Use this formula and the readings from your amp/hr meter to
get a good idea as to the battery’s State Of Charge (SOC)."

So, we'll need a good, TEMPERATURE-COMPENSATED, hydrometer. I like to keep
gravity above 1.190, around 50% discharge, as the lower limit of how far
I'll discharge the cells....ESPECIALLY if I'm going to have to leave them
discharged for longer than overnight. Battery companies say you can
discharge safely to 80% of rated capacity, but I wouldn't do that unless
you're standing at the switch of the recharging equipment. Don't leave
them less than 50% discharged for any period of time and they'll last a
LONG time.

On Lionheart, we use the Xantrex Link 10 ampere-hour meter to monitor
volts/load current at the battery and ampere-hours used. It works OK, my
captain takes it as gospel, but I don't. But, you can't measure the
gravity with my hydrometer in 8' seas, so I compromise...(c;

http://www.wholesalesolar.com/products.folder/meter-
folder/Xantrex_link10.html

12.2V is a good point. It represents about 50% discharge. Discharging below
50% starts to shorten the life of the battery.

Doug

"varnish" wrote in message
. uk...
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?

Hi,

you shoud not discharge more than 50% of nominal capacity. Discharging more
than 50 % will reduce the number of possible cycles for
charging/discharging. Gel-batteries can be discharged more than 50 %, up to
80 % without reducing cycles.

50 % of discharge are reached at 11,8 volts at normal batteries and 12,2
volts at gel batteries.

If You want to start your engine for charging, be sure, that you don`t
discharge too much, because an empty battery won`t start the engine.
Starting the engine will discharge battery additionally and you reach depp
discharge area, wich damages the batterie. So the voltages befor are
absolutely minimal values.

Best will be to have 2 batteries, one for the engine, one for your power
consumption on bord.

regards

juergen

"varnish" schrieb im Newsbeitrag
. uk...
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?

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?

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?

Steve wrote in :

Good info.

Measuring open circuit, unloaded voltage of any lead acid battery is no
indication of state-of-charge. If there's any acid left in it, it'll
measure a little over 2V per cell on all cells with some acid left.

Now, put a 20A load on a charged battery and a discharged battery. THEN,
you can see the charged battery hardly varies at all in voltage as the
discharged battery's INTERNAL RESISTANCE causes a substantial voltage drop
due to the lack of electrolyte to react with lead plates. When you remove
the load from the discharged battery, the voltage goes right back up,
unless you've used all the available electrolyte during the discharge.

The ONLY true way to measure the charge on a lead-acid battery is specific
gravity, compensated for temperature properly. Of course, in the sealed-up
sooper-dooper $500 "maintenance-free", this is moot point.

For the boater who has to have it all, there's:
http://www.cellcare.com/products_ser...e200/index.asp
even has a data logger to track cell state.
Comes with tracking software for your PC....(c;

A more practical means of measuring "How much is left?" is a Link 10 or 20
from Xantrex, available from Waste Marine or on the net.
http://www.xantrex.com/web/id/97/p/1/pt/5/product.asp
I've just installed another Link 10 in another sailboat after replacing 2-
year-old house batteries he deep cycled, regularly, into oblivion.
Installation is simple. Disconnect the negative terminal from the battery
bank, this means ALL the connections to the negative terminal, not just the
big cable. Put ALL the connections on the load end of the included 500 Amp
shunt. Connect a #2 battery jumper from the battery end of the shunt (not
included) to the negative end of the battery bank. This makes ALL the
current to and from the battery go through this current measurement,
including charging, please! There is a negative power connection to power
the Link 10/20 and two wires, one power and one voltage measurement that go
to the positive terminal (NOT THE BREAKER PANEL OR SOME OTHER PLACE JUST
FOR YOUR CONVENIENCE) of the battery bank. Now that we can measure battery
voltage and all the current in or out, the Link 10's little computer can
keep up with your amp-hour drain and charge, which is a fair way to measure
state-of-charge. But, it is NOT a power company. Battery amp-hour
capacity wanes with time, changes with temperature (which an accessory
temperature probe can make the Link compensate for, but isn't really
necessary.) So, fairly close, you can now discharge it to 50% of capacity,
even set a accessory alarm (not included) to warn you of impending doom to
get the engine charging cranked up. It's close enough for boats and not
thousands of dollars for a commercial gravity meter.

Link or any amp-hour meter runs all the time, so connect its power leads
directly to the battery with a multiconductor cable through two fuses...one
for volts sensing and one for power (2A in the manual). The thing comes
set to turn off the display in factory default to make you feel better, but
all it does is turn off the LED display. It uses no appreciable power and
would take a hundred years to discharge a set of L-16 monsters, probably
using less power than the battery's natural leakage. I always want the
display LEDs on so owners can glance over to see how many AMPS we're
drawing NOW all the time. Battery power lasts longer when they see the
amps screw up from turning all the lights on like at home. It makes them
more self-concious of power usage...There's a 4-bar "meter" the computer
provides that goes from green to yellow to red as the bars drop to make him
feel even more guilty...(c;

Steve wrote:
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).

Self-discharge rate is a function of temperature; the colder the
battery, the more slowly it self-discharges. At room temperature,
typical wet-cell lead acid batteries self discharge at a rate of about
10% per month. The self-discharge rate at 40 degrees Farenheit will be
more than halved because of the lower temperature. So after 3 weeks at
40 degrees F. your batteries probably lost about 4% of their capacity,
not detectable by your voltmeter measurement.

Also, the resting state voltage rises as the temperature drops; at -40
degrees centigrade (brr!) the resting state voltage of a fully charged
battery approaches 15 volts. Your 12.8 volts reading is a bit higher
than what you'd read at room temperature because of this.

Concerning long term storage, at room temperature a fully charged
battery would approach 50% discharged after about three months and
should be recharged, drop the temperature 15 degrees (F.) and it would
take 6 months for the battery to discharge to 50%. So, depending on
temperature, it would be wise to recharge resting batteries every
couple of months or so, if long life is your goal.

Ahhhh, that could explain it.

The temperature on the day I took the reading was just under 10'C (50'F)
so could have accounted for that higher measurement especially since it
has been very cold (for Wellington NZ) which would have reduced the self
discharge rate. I don't think that the temperature got down to 40'F for
very long but it didn't get much above 50'F at all.

So does anyone have a temperature compensated version of that table?

Thanks for the advise about the recharging. Luckily in the summer the
batteries are recharged at least weekly but the winter has longer
periods of not being used. I have been concerned about the possibility
of self discharge causing damage over these long storage periods but
what you say about self discharge at low temperatures clears up that worry.

I have a question about what the best option would be to increase the
size of the house bank but I will start a new thread for that.

Thanks

Steve


Mark wrote:
Steve wrote:

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).


Self-discharge rate is a function of temperature; the colder the
battery, the more slowly it self-discharges. At room temperature,
typical wet-cell lead acid batteries self discharge at a rate of about
10% per month. The self-discharge rate at 40 degrees Farenheit will be
more than halved because of the lower temperature. So after 3 weeks at
40 degrees F. your batteries probably lost about 4% of their capacity,
not detectable by your voltmeter measurement.

Also, the resting state voltage rises as the temperature drops; at -40
degrees centigrade (brr!) the resting state voltage of a fully charged
battery approaches 15 volts. Your 12.8 volts reading is a bit higher
than what you'd read at room temperature because of this.

Concerning long term storage, at room temperature a fully charged
battery would approach 50% discharged after about three months and
should be recharged, drop the temperature 15 degrees (F.) and it would
take 6 months for the battery to discharge to 50%. So, depending on
temperature, it would be wise to recharge resting batteries every
couple of months or so, if long life is your goal.