Battery status: Voltage vs. Amperage used

[Ostinato]

What’s more important in measuring the status of batteries when they are under load – voltage or amperage used?

I know what 12 volts batteries should measure when fully charged or at other points when they are not under load. I also know how many amp hours I have in my house bank and can measure how many I have used. However, I am not sure which I should pay most attention to when cruising and the house bank is under load.

Anyone have any thoughts?

[accomplice]

To a first approximation, the voltage will tell you the current charge state of the batteries. Open circuit voltage, however, will tell you nothing of the capacity of the batteries. There are multiple methods for estimating capacity. Those methods which use a matrix of impedance tests tend to perform better.

[a64pilot]

Originally Posted by Ostinato

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Anyone have any thoughts?

Hello, I'm primitive Pete, I don't have any way of knowing how many amps I've used etc., but I won't pull my batteries below 12V, with a small load, that should approximate a SOC of about 50%, I'm using approximate and about as there are way to many variable for a primitive like me

[skipmac]

Momentary voltage of batteries under load tells you very little. There are too many variables and the voltage difference between good and bad, fully or partially charged, etc is very small.

The same for momentary amp draw. Only tells you what's going in or out at that exact time.

A battery monitor that tracks amps in and out will give you much more information although that also requires proper calibration to the size and condition of your battery bank and some intelligent analysis of the information to give really meaningful information about the exact state of charge of your batteries.

I have read that the Balmar Smartgauge is the best option to get the most accurate information on your batteries.

[NewMoon]

Originally Posted by Ostinato

What’s more important in measuring the status of batteries when they are under load – voltage or amperage used?

I know what 12 volts batteries should measure when fully charged or at other points when they are not under load. I also know how many amp hours I have in my house bank and can measure how many I have used. However, I am not sure which I should pay most attention to when cruising and the house bank is under load.

Anyone have any thoughts?

As others have said, voltage is often misleading, especially under load. If you have a battery monitor which accurately tracks amp-hours used, that's the way to look at remaining battery capacity. For example, if your house bank capacity is 200 AH, and you've used 100AH, you're down to 50%, the point at which you need to start recharging.

[transmitterdan]

Check out SmartGauge by Balmar. It only uses voltage and gives accurate state of charge. Does not require shunt nor repeated calibration. It also monitors a second starter battery.

[Stu Jackson]

To the OP:

Both.

The more information you can get about your batteries the better.

For example, lets say you have bad batteries that are dying but you don't know it. The amp hour counter will show you're fine. 11.2V will show you something drastically different.

Use all the tools ya got.

[ontherocks83]

Originally Posted by Stu Jackson

To the OP:

Both.

The more information you can get about your batteries the better.

For example, lets say you have bad batteries that are dying but you don't know it. The amp hour counter will show you're fine. 11.2V will show you something drastically different.

Use all the tools ya got.

Seconded. Having both and comparing them is the best. If you have both I would say leave it on Amps used and just reference the voltage every now and then to make sure the "math" adds up. Like stuff said if at half your bank amps used your voltage is much below 12 to 12.2 then you got problems. Likewise if youve only used 1/4 of your total amps and your already at 12.2 you have problems.

I only have a volt meter so I only can guesstimate how many amps I have left. Wish I had both but being on a lake I can't justify the cost so I'm fine with estimates. On the ocean I would want both.

[hellosailor]

"Use all the tools ya got."
I like that, Stu.
We keep the batteries on a scale that reads out in micrograms, and just keep track of how many micrograms of electrons have been removed or lost or restored. (WEG)

[Stu Jackson]

Originally Posted by hellosailor

"Use all the tools ya got."
I like that, Stu.
We keep the batteries on a scale that reads out in micrograms, and just keep track of how many micrograms of electrons have been removed or lost or restored. (WEG)

Ah, good one, now I know where that phrase "pesky electrons" came from.

[Ostinato]

Ok - but you can really draw down voltages pretty easily on a momentary or longer basis. Pumps and inverters can very demanding. So I'm not sure that voltage under those conditions is a good indicator under most circumstances.

Does anyone have any suggestions about how long to wait before voltage recovers?

[hellosailor]

"Does anyone have any suggestions about how long to wait before voltage recovers"
The folks who make batteries often say a day, or overnight, or 24 hours. Part of that depends on the battery type and how what kind of electrolyte has to even itself out.

You can do a little testing on your own by just measuring the voltage with everything off, after use. Then measure again at 2-4-8-16 hours and see if it has been recovering, or peaks and stops at a certain time.