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dktr asked

Changing maximum charge voltage

Dear all, I’ve just bought a LiFePO4 battery for my van solar system and I want to make sure it lasts as long as possible. One of the main ways of doing this, if I understand correctly, is to have it stay in the range of 25-75% charge. On the lower end, I have a Battery Protect which I will simply configure to cut-off at 25%. My question is thus; how do I find out which voltage is equal to 100% and which voltage is 0%? Secondly, how do I configure my charge controller so that it stops charging at whichever voltage is equal to 75%? If it’s of any help, I attach both the battery specs from the manual and the options I have on the charge controller. Forgive me if this is basic, I tried researching but to no avail, mainly because I don’t really know what to look for. Many thanks.

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battery chargingLithium Batteryvoltage
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solardude answered ·

Look for LiFePo4 voltage curves. All drop in LFP batteries use almost identical curves. I like charging to 3.5V/cell, or 14.0V if trying to get max life. Anything under that isn’t necessarily extending its life, but will leave a lot of power unused.

The bottom cutoff should be around 3.0V-3.1V/cell or 12.0V-12.4V. If you have plenty of battery for your application, 12.4V will extend it’s life a bit more.

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dktr avatar image
dktr answered ·

Hi Solardude, thanks for getting back to me. So in terms of the numbers you’ve mentioned, 3.5V and 3.0V, what does that equate to in terms of percentages? And how do I input this into the charge controller, is it the absorption voltage that needs to be changed? Many thanks.

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klim8skeptic avatar image klim8skeptic ♦ commented ·

Hi dk,

" So in terms of the numbers you’ve mentioned, 3.5V and 3.0V, what does that equate to in terms of percentages? " These are simply safe charge and discharge voltages for your cell. As a % wise it is simply too hard to guess without a (dialled in) battery SOC meter of some kind. A lithium cell will read 3.0-3.2v between 10-90% charge.

Given the specs from your battery manufacturers, it would seem safe to apply the charging parameters from the Victron mppt. https://community.victronenergy.com/storage/attachments/6515-6856f4a3-88f5-4ef6-afe3-adf06cf52f73.png

Note the charge limit of 14.6 -+ 0.2v.

Note the DICSHARGE limit of 10v.

Note the STANDARD charge limit of 20a.

Note the ABSORB charge cut-out of 2a.

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solardude avatar image solardude commented ·

12.0V is approx 10% SOC

12.4V approx 80% SOC

14.0V is approx 95% SOC

However this is slightly dependent on amperage when discharging and charging. I would make absorb settings 14.0V, 20A and absorb time 1 minute. Lifepo4 doesn’t need absorb time at that voltage/SOC.

Battery recomends up to 14.6V, but to make it last longer 14.0V is plenty

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