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PeterFromSwe avatar image

Solar charge lithium to 80% SOC. MPPT 75/15 BMV-712.

When parking boat in a no load scenario, it would be good to solar charge LiFePo4 only up to only 80% SOC. This is good for battery life and when starting engine and leaving dock 100% is reached pretty soon anyway. Can this be accomplished with mentioned products?

MPPT - Solar Charge ControllerBMV Battery MonitorLithium Batterysolar
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adev avatar image

yes you can just set the absorbtion voltage lower on the MPPT charge controller - exact voltage would depend on battery voltage and chemistry...

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If you're not too fussed about how exact the 80% is, +1 to adev's suggestion. Getting the (and trusting) the 80% value on a BMV will require effort in setting up and possibly ongoing effort as well.

LFP Charge Voltage

As far as I understand it, the voltage must be very low to stop charging completely, otherwise 100% SOC will be reached eventually. My idea was to use SOC instead of voltage. (well synchronized periodically by using shore power).

I've read the attached link, and think that its consistent with what we're saying; the first answer in the link I think could also have been worded better.

Some background - for any given battery (and this is different between batteries, even in the same manufacturing batch), there is a voltage level that would equate to 100% SOC. When the battery is charged to this point, you would get the nameplate capacity (xxx amp-hours) out of it if you were to completely flatten the battery, in the conditions given by the manufacturer, and everything else in the environment being identical.

For all practical purposes, you would not be able to (or even want to) precisely measure this voltage. You rely on the typical charging voltage given to you by the manufacturer.

The point of the above background is that if you were to charge to a voltage level below the typical charging voltage, you would reach XX% SOC, where XX < 100. This is essentially what the 2nd answer is saying in the link.

In the 1st answer, when they say "the charging time takes longer", it means that it takes longer to charge to YY% SOC, where YY < 100. Not that the charging time to 100% SOC takes longer. When you charge at a lower voltage, you will never reach 100% SOC.

FWIW, I believe the "word" is that 50% SOC is better for longevity. But obviously there's a tradeoff between longevity and practical use of the battery.

As you rightly say, "you would not be able to (or even want to) precisely measure this voltage". Thats the key driver to rely on BMV SOC estimation instead.

With two different voltages 100% SOC is reached but it takes longer time for lower voltage. As long as the voltage is above minumum required (as link says 3.3 V per cell).

Ok, let me try on more time :)

I'm just going to use some numbers pulled out of the air; its the relative differences that matter and I didn't look at the SOC vs V curve. Lets say you're charging a 100AH battery.

3.65V charge => 100% SOC = 100 amp-hour till flat (dead)

3.60V charge => 95% SOC = 95 amp-hour till flat (dead)

In both cases, they're charged to 100% from the perspective of available amp-hours. If you charge at 3.65v, your 100% means 100 amp-hours. If you charge at 3.60v, your 100% means 95 amp-hours. i.e. you're at 100% of the 95% SOC.


If you do want to go down the SOC path, sure, grab a BMV, wire the shunt to the return path and fire it up. Configure the tail current parameters so it syncs reliably. Monitor it initially to make sure that you can trust the SOC readings, then off you go. Cross-check the system manually from time to time to make sure nothing's gone awry.

I think you are wrong, claiming that 3.60 V charge will not charge battery to 100% SOC (100 Ah as per your example). The link I supplied explains this. Do you have any other source that says different?

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