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Old 14-07-2015, 19:45   #1
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when to charge battery

Hi, I've read that I shouldnt let battery fall below 50% charge, but how do I know when it's at 50 %? Thanks
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Old 14-07-2015, 20:05   #2
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Re: when to charge battery

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Originally Posted by amoret View Post
Hi, I've read that I shouldnt let battery fall below 50% charge, but how do I know when it's at 50 %? Thanks
Assuming it's a 12-volt flooded lead-acid battery, 12.2VDC at the battery terminals without load is about 50% charged. Your measurement needs to be accurate....a digital voltmeter or multimeter of known accuracy.

AGMs and gelled batteries will measure a slightly higher voltage....12.3VDC is about 50% charged.

This isn't a hard-and-fast rule, just a good one to follow as best you can.

Also, don't let your batteries sit with less than a full charge; they'll sulfate and lose capacity (shortening their lifetime).

Bill
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Old 14-07-2015, 20:12   #3
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Re: when to charge battery

Yep, measuring voltage as above is the only simple way. The only easy thing that can be done more accurately is a profiling charge controller that will measure voltage as the battery charges and is dissipated to create a profile of exactly how that specific battery reports voltage throughout it's charge/drain cycle. It's also technically possible to measure the specific gravity of the sulfide in a lead acid battery, but that's not trivial.

Here's a table for flooded lead acid batteries (Gel cells will be higher):

100% -- 12.65
75% --- 12.45
50% --- 12.24
25% --- 12.06
0% ---- 11.89 or lower

Keep in mind that this is > open circuit < voltage, i.e., disconnected from any loads and other batteries. Also, while charging, voltage will read as high as 14.3 but that's coming from the alternator, not the battery.
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Old 14-07-2015, 20:18   #4
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Re: when to charge battery

It is even better to stay less than 50% discharge, and to hold it at full charge when not using the boat. Lead acid batteries will last a very long time with a less than 50% discharge, bulk charging at above 14.4 volts, and keeping them at float charge voltages of 13.2 volts during long periods of off season disuse,
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Old 14-07-2015, 20:24   #5
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Re: when to charge battery

Depends what type of battery it is. The best way to know id to use a battery meter. Alternatively voltage can be used, but before measurement, you have to leave the battery with everything switched off, for at least 1/2 an hour, so you can measure the real voltage. The voltage required depends on battery type.
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Old 14-07-2015, 20:28   #6
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Re: when to charge battery

you use a battery monitor like a victron 700


the voltage doesn't really work as you need to shut off the battery for 24 hours then measure... can't do it on a boat in use.
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Old 14-07-2015, 20:32   #7
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Re: when to charge battery

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Originally Posted by mstrebe View Post
Yep, measuring voltage as above is the only simple way. The only easy thing that can be done more accurately is a profiling charge controller that will measure voltage as the battery charges and is dissipated to create a profile of exactly how that specific battery reports voltage throughout it's charge/drain cycle. It's also technically possible to measure the specific gravity of the sulfide in a lead acid battery, but that's not trivial.

Here's a table for flooded lead acid batteries (Gel cells will be higher):

100% -- 12.65
75% --- 12.45
50% --- 12.24
25% --- 12.06
0% ---- 11.89 or lower

Keep in mind that this is > open circuit < voltage, i.e., disconnected from any loads and other batteries. Also, while charging, voltage will read as high as 14.3 but that's coming from the alternator, not the battery.
As above. Use voltage for a quick check. No load of course.

Otherwise use a hydrometer. Specific gravity is the most accurate means of determining battery state of charge. Flooded lead acid only.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app
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Old 14-07-2015, 21:28   #8
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Re: when to charge battery

Thanks to everyone for the very helpful replies. And yep, it's a deep cycle lead battery. Cheers
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Old 14-07-2015, 22:50   #9
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Re: when to charge battery

Been using a digital voltmeter for years and have had long battery life. For ordinary flooded lead acid batteries (the best IMHO) the chart above is a good guide. You should not be charging or discharging the battery for maybe half an hour before taking the reading. I rarely let my battery bank go below 70%.

There are more expensive and complicated ways of doing this, but I believe in keeping things simple.
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Old 15-07-2015, 00:36   #10
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Re: when to charge battery

Some handy graphs in here, bearing in mind temperature will change things.
https://pssurvival.com/PS/Batteries/...ltage_1993.pdf
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Old 15-07-2015, 08:41   #11
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Re: when to charge battery

I have been stumped by my battery not holding a charge. I have one house bank 12V 8D battery. It is too large to remove in the winter during layup, so I charge it with shore power for a week before putting her to bed. A few times during the winter I run a cord to the A/C and charge for a weekend. I have never, even when new, seen the battery at 13.6V. It is now 3 years old. I just went to Bermuda and back with it, but I was really nursing it along by not using much power. The charging voltage is at 14.4V. Is there a way to bring this back? Simple charging doesn't seem to do it.
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Old 15-07-2015, 09:00   #12
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Re: when to charge battery

The Ample Power Primer http://www.amplepower.com/primer/primer.pdf
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Old 15-07-2015, 09:01   #13
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Re: when to charge battery

I agree that your batteries will serve you well if always kept above 50% discharge. Also, if you have ever let them go to 10.5 volts they are dead and cannot be revived. Go to the battery store.
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Old 15-07-2015, 09:11   #14
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Re: when to charge battery

I have had luck bringing back old motorcycle batteries by using chargers with a pulse cycle. I guess it shakes the sulfation loose.
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Old 15-07-2015, 09:48   #15
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Re: when to charge battery

I live full time aboard and charge through solar and wind. some times I have to shut the wind off. I've never let it get below 70 percent. If I have to leave for an extended period I attach a trickle charger at 2 amps to keep it charged. If you let a battery set for 24 hours or more at less than 90 percent sulfating will start on the plates. causing that part of the pate to never make electricity again. The reason a starting battery last so long, is it is recharged right away. one of my 4d banks is 8 years old now and still has about 75 % of its rated amp hours. I'm not frivolous with my electrical usage, but I don't skimp either....fans, lighting,refrigeration,electronics, entertainment.....

Maintain the wiring, maintain the charging and they will last.
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