question

peterfromswe avatar image
peterfromswe asked

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 ControllersBMV Battery MonitorLithium Batterysolar
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1 Answer
adev avatar image
adev answered ·

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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kai avatar image kai ♦ commented ·

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.

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peterfromswe avatar image peterfromswe kai ♦ commented ·

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).

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kai avatar image kai ♦ peterfromswe commented ·

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.

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peterfromswe avatar image peterfromswe kai ♦ commented ·

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).

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kai avatar image kai ♦ peterfromswe commented ·

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.

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peterfromswe avatar image peterfromswe kai ♦ commented ·

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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kai avatar image kai ♦ peterfromswe commented ·

There's any number of references, but this one gets to the point pretty directly:

http://nordkyndesign.com/practical-characteristics-of-lithium-iron-phosphate-battery-cells/

Have a look at section called "Typical Cell Operating Limits", a few paras down from the table. That site in general has a wealth of info on LiFePos in the marine environment, its a good reference site.

It goes back to the physics/chemistry of the batteries. Higher voltage = higher SOC. The more energy is stored in the battery, the higher the terminal voltage is, and vice versa.

Like I said, I think the answers on gwl could have been better worded. Note how the answer (deliberately or not) avoided a directly addressing the question. Readers are left filling in the gaps and can come to conclusions like getting to 100% SOC at lower voltages.

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peterfromswe avatar image peterfromswe kai ♦ commented ·

Thank you for the informative link.

Section/Excerpt below comes from that and clearly proves my point:

The Relation Between End-of-Charge Voltage and State of Charge

LFP cells simply don’t really charge at voltages up to 3.3V and then fully charge already at 3.4V and upwards. The transition is so abrupt that claiming to control the charging process by adjusting the voltage is purely and simply bound to fail.


Thank you anyway.

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adev avatar image adev peterfromswe commented ·

having read through that link in it’s entirety (very interesting and informative, thanks) I’m now inclined to agree with @PeterFromSwe on this...

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adev avatar image adev peterfromswe commented ·

also from the same: ”...The recommended upper SOC limit is 90% however, not 100%, and charging to 100% SOC in this context means absorbing the cells at 3.65V until the residual current is C/30. Anything short of this will not – by definition – achieve 100% SOC.

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