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Old 29-05-2023, 07:24   #1
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Preventing LFP bank from being fully charged

Hi everyone,

I'm a full time live aboard and planning to leave the boat in a marina for a couple weeks. I'm going to leave the DC fridge and a few other things running while I'm away.

The issue is I have a LFP house bank and I'd like to prevent it from getting fully charged. I can't simply disconnect part of my solar panels because if there are several cloudy days in a row (normal as we're in the rainy season) I could end up depleting the house bank. I have victron mppts and a BMV-712 monitor. Is there any way I can use them to achieve that?

Thanks in advance!
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Old 29-05-2023, 10:10   #2
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Re: Preventing LFP bank from being fully charged

Are you using a GX device (such as a Cerbo GX) and DVCC? If so, it’s easy. Just go onto your Cerbo and set an override on the DVCC voltage. When my boat is alongside for king periods of time, I usually set this to 13.2v or 13.3v which translates to an SoC of around 60%. I colloquially refer to this as “Storage Mode” and will then remote in the day before I intend to use the boat, remove the override, and let it come back up to 100% charge.

If you don’t have this kind of a setup, just adjust the configuration on your charging sources directly, and limit the voltage that way.
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Old 29-05-2023, 10:16   #3
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Re: Preventing LFP bank from being fully charged

Yeah I do something similar with my cerbo and DVCC, except my rec BMS controls it so I can just adjust one value on the BMS and it'll sit at 65% indefinitely at the dock.

I don't think you can use BMV to control DVCC so you'll have to go into each MPPT and change them all individually
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Old 29-05-2023, 10:45   #4
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Re: Preventing LFP bank from being fully charged

As others have said voltage corresponds to SoC% and a battery can't be charged to a higher voltage than the charger puts out.

Note while most LFP cells can be charged to 3.65V per cell (14.6V for "12V" battery) it doesn't have to. In fact in most cases one can get away with a lot less and that improves battery longevity.

You may also want to check the absorption time. Most chargers will charge LFP to an excessive long time in absorption which while not terrible is going to result in SoC% rising.

Depending on your setup you may be able to set everything in a central controller like CerboGX or will need to do it seperately in each charging source.
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Old 29-05-2023, 11:51   #5
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Re: Preventing LFP bank from being fully charged

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Originally Posted by Statistical View Post
As others have said voltage corresponds to SoC% and a battery can't be charged to a higher voltage than the charger puts out.

Note while most LFP cells can be charged to 3.65V per cell (14.6V for "12V" battery) it doesn't have to. In fact in most cases one can get away with a lot less and that improves battery longevity.

You may also want to check the absorption time. Most chargers will charge LFP to an excessive long time in absorption which while not terrible is going to result in SoC% rising.

Depending on your setup you may be able to set everything in a central controller like CerboGX or will need to do it seperately in each charging source.
I'll have to change it on each charging source.

So I'd just set the absorption voltage to something like 13.4? Do I need to change the float voltage as well?
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Old 29-05-2023, 11:56   #6
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Re: Preventing LFP bank from being fully charged

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I'll have to change it on each charging source.

So I'd just set the absorption voltage to something like 13.4? Do I need to change the float voltage as well?
Yes. Sometimes chargers have separate bulk and absorption numbers if so change both to the same lower value. Sometimes they just have a single absorption value which controls both bulk and absorption. If there is an absorption time it may be very high (like 4+ hours) which will cause the Soc to rise significantly so you may want to reduce that as well down to 15 minute to an hour depending on the charger output compared to battery capacity.

Ideally float voltage is already down around 13.4V or lower. LFP don't really like to float charge. So "float" isn't really being used to charge the battery but rather power low always on loads so the battery doesn't discharge.

Resting voltage of LFP is around 13.4V. Even if you charge it to beyond that and the battery voltage rises higher (13.5V to 14.6V measured when resting = no charge or loads) that ends up being a surface charge which quickly bleeds away and the voltage falls back to around 13.4V once charging source and loads are removed usually within a few hours but at most 24 hours.

So 13.4V or even 13.2V "float" is a good value for LFP. It is right at or just below the resting voltage of the battery and very likely will never charge the battery just ensure it doesn't discharge. If heavy loads come on the float output won't be enough to cover them and then energy will flow out of the battery so voltage will drop causing the charger to go back to bulk/asborb to refill the used capacity.
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Old 29-05-2023, 16:27   #7
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Re: Preventing LFP bank from being fully charged

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Originally Posted by alctel View Post
Yeah I do something similar with my cerbo and DVCC, except my rec BMS controls it so I can just adjust one value on the BMS and it'll sit at 65% indefinitely at the dock.

I don't think you can use BMV to control DVCC so you'll have to go into each MPPT and change them all individually
Iím using a REC ABMS myself, but thought you could use a BMV or Smartshunt as your system controller as well. I could easily be wrong on that.
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Old 29-05-2023, 16:46   #8
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Re: Preventing LFP bank from being fully charged

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Iím using a REC ABMS myself, but thought you could use a BMV or Smartshunt as your system controller as well. I could easily be wrong on that.
BMV can provide data but it can't control anything. For DVCC you need a Cerbo GX. Granted in any simple system with 1 inverter/charger and 1 or 2 solar charge controller it isn't that hard to just set them all.
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Old 30-05-2023, 22:49   #9
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Re: Preventing LFP bank from being fully charged

you can lower the bulk / absorb voltage in the mppt's 13.5v or so while away
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Old 31-05-2023, 04:45   #10
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Re: Preventing LFP bank from being fully charged

Quote:
Originally Posted by LifesBetterWhenYoureBeating View Post
Hi everyone,

I'm a full time live aboard and planning to leave the boat in a marina for a couple weeks. I'm going to leave the DC fridge and a few other things running while I'm away.

The issue is I have a LFP house bank and I'd like to prevent it from getting fully charged. I can't simply disconnect part of my solar panels because if there are several cloudy days in a row (normal as we're in the rainy season) I could end up depleting the house bank. I have victron mppts and a BMV-712 monitor. Is there any way I can use them to achieve that?

Thanks in advance!
I just lowered my solar setpoints
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