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Old 05-01-2013, 08:03   #1
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Loaded battery voltage. What's correct?

Hi Folks,

This has probably been discussed a zillion times but I can't find it in the searches.

I never like to run my house bank below 50% (Aprox 12.24V) when the battery is at rest.

Given: no solar charging etc.

What voltage can I run the battery down to whilst under load (say the refrigerator is going) where, when it stops it will go back above 12.24 v within a minute or two?

Or should I only run it to the loaded Voltage of 12.24?

Mark
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Old 05-01-2013, 08:32   #2
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When your battery is under load it Will show a lower reading than the actual battery state. If wIth your refrIgerator runnIng It reads 12.0 volt but then rebounds back to 12.3 volts that Is fIne. The rebound voltage is the actual State of your battery. The amounT of rebound should be consisTenT if The draw on The baTTery is consisTent. Deep cycle batteries or more able to hanDle deep draw downs than regular batteries. I use the high quality lifeline batteries on my own personal boat and have gotten about 9 years of hard use out of them
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Old 05-01-2013, 08:41   #3
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Re: Loaded battery voltage. What's correct?

On my boat I find the voltage with loads on (daytime underway loads) and at about 50% SOC, that the voltage reading is about 0.15V lower than if I turn off the loads and let it rest a few minutes.

I don't feel the need to push it if I'm going to have to run the engine to recharge regardless, so for me if the loaded load drops below 12V I start the engine.

I also don't worry too much about whether the batteries are at 55% or 45% for all those that are going to do the math.
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Old 05-01-2013, 09:13   #4
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Re: Loaded battery voltage. What's correct?

Mark,

the voltage drop under load is the result of internal resistance in the battery, and so will be proportional to the current. If you are actually measuring voltage at the battery terminals then you should be able to measure the internal resistance of your batteries. You might also get it from the manufacturer, but that depends.

With the batteries rested, put a load with known current on the system for a short period (~30 seconds or less?). Check the voltage under load.

E=IR > R= E/I, so:

Internal Resistance = (Resting Voltage - Working Voltage)/Load Current

In other words, divide the measured voltage drop by the load current you applied and you will get the internal resistance.

For lead-acid I would expect something under 0.02 Ohm. With that you can build a little table for voltage drop vs. current. If I use 0.01 Ohm as an example then:
  1. 10A = 0.1V
  2. 20A = 0.2V
  3. 30A = 0.3V

Internal resistance in your battery will change with age and with state of charge, so do this little test when you think you are near 50% DoD and it should give you a general idea of the voltage drop to expect. IEEE 450 actually requires (slightly more sophisticated) testing of IR on a regular basis, and uses the changes in measured IR to determine when batteries should be replaced.

Most of this goes out the window if your volt meter is not connected pretty much directly at the battery posts, as you will then have to account for additional drop factors in the wiring, etc. so will not be measuring the IR of the battery, but rather that of the system.
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Old 05-01-2013, 10:04   #5
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Re: Loaded battery voltage. What's correct?

Quote:
Originally Posted by MarkJ View Post
Hi Folks,

This has probably been discussed a zillion times but I can't find it in the searches.

I never like to run my house bank below 50% (Aprox 12.24V) when the battery is at rest.

Given: no solar charging etc.

What voltage can I run the battery down to whilst under load (say the refrigerator is going) where, when it stops it will go back above 12.24 v within a minute or two?

Or should I only run it to the loaded Voltage of 12.24?

Mark
Mark,

No easy one size fits all answer. You will have to determine this on your own for your own bank. Problem is that it's an ever moving target as the batteries age and lose capacity.. I would just apply differing known loads at 50% SOC and build yourself a cheat sheet..

To do this right you need to do the experimenting or just get a battery monitor and be sure to program it correctly. The Victron BMV-600 is below $180.00 these days and if properly programmed will account for Peukert in the % charged screen..
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Old 05-01-2013, 10:15   #6
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Re: Loaded battery voltage. What's correct?

Thanks guys

I'm glad it wasnt an idiot question.


Mark
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