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Old 03-06-2023, 18:40   #1
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How low can you get?

Hi everyone,
I have two quite new AGM batteries(4 months) which I want to treat well so they last. I also have a Victron battery monitor that is supposed to help me with that.

I read that to keep the batteries well I should not use more than 25% of their capacity, which means I should never go under a 75% reading on the monitor. I also read that 75% of the capacity is around 12.4V. So, apparently, I should never go under 75% or 12.4 V.
BUT- my problem is that at around 85-87% I already get a reading of around 12.1V, much lower than I intended to keep.
Is something wrong? If not, can I keep using the batteries until a 75% reading, no matter what the voltage shows? Or, on the contrary, should I only use the batteries until I get a 12.4V reading even if thatís, say, 92%?
Now, I think I know part of the answer, and that is that the voltage value of 12.4V for 75% is in a resting state of the battery, meaning that it has no load for a few hours. But my problem is that on a daily basis I donít know and cannot check the resting state of the battery.
So, back to my question: can I ignore completely the voltage value and just go by the percentage, or should I also be concerned with the voltage? And if I should be concerned about it, how low can I get?
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Old 03-06-2023, 18:56   #2
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Re: How low can you get?

Typically yes, you can ignore the voltage. Especially if you have some fairly high loads on the battery.

Just go with the percent discharge figure. It’s more accurate.

And 50% depth of discharge is a little bit more normal. You can use twice as much capacity as you are using right now safely and the batteries will be just fine.

Just make sure that all loads and all charging are going through the shunt for your battery capacity monitor. If you have any loads that are sneaking around the shunt, that will cause a lot of problems. You will be further down than the monitor says.
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Old 03-06-2023, 20:03   #3
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Re: How low can you get?

Quote:
Originally Posted by Udira View Post
I have two quite new AGM batteries(4 months) which I want to treat well so they last.
Just as important is regular charging to 100%.
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Old 05-06-2023, 09:29   #4
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Re: How low can you get?

Thanks for your replies!
Itís reassuring to know that I can use the batteries more liberally..
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Old 05-06-2023, 09:39   #5
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Re: How low can you get?

Quote:
Originally Posted by Udira View Post
Thanks for your replies!
It’s reassuring to know that I can use the batteries more liberally..
AGM also likes to be charged hard at least 0.2C (0.4C would be even better although for smaller boats that is very challenging). C is simply a measure of amps vs amp-hours. So if you have a 300Ah battery then 0.2C would be charging at 300 * 0.2 = 60A.

Avoid slow bulk charging and ensure you don't short charge (that is not getting to 100% SoC). The occassional slow or short charge is fine but periodically the batteries should be charge hard and get up to 100% SoC.

Note this is specific to AGM chemistry other chemistries to include LFP are different.

https://marinehowto.com/how-fast-can...ry-be-charged/
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