How to load a lithium lifepo4 battery to only 80%

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flux asked Jul 11, '23

How to load a lithium lifepo4 battery to only 80%

Hey,

Im having a 12v 100ah lithium lifepo4 battery and a 100/20 MPPT device.

Im trying to set the MPPT controller to load my battery not to 100% but to 80% or 90%.

 I know there is an absorption voltage, float voltage and in expert mode even more voltages (equalization and bulk voltage) that can be set to go from one loading phase to another. Unfortunately the manual is no help for me. 

While loading most of the time the controller is in BULK mode, then goes over to ABSORBTION mode and when full it goes to FLOAT mode. 

Could someone explain these voltages and how to set them to load only 90% of the battery? The absorption mode is not clear to me, the other seem more logical (but would be awesome if someone would explain them too).  

Here's the datasheet for my battery

Current settings are:  

Absorption voltage: 14.20V 

Float voltage: 13.50V 

Expert mode deactivated  

Thanks in advance!

MPPT SmartSolar Lithium Battery voltage

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Modifications Space

Node Red Space

Answer 1 · 2023-07-11T18:16:53Z

Kevin Windrem answered · Jul 11, 2023 at 06:16 PM

You could look at the battery's specifications and see if they provide a SOC vs voltage curve and use that to set the absorption voltage.

Note that lithium batteries use the absorption phase for cell balancing and if you don't reach the specified voltage, the battery won't balance. You would need to find a way to perform a cell balance once in a while to insure the battery remains healthy. So I don't recommend stopping charging early. Just let it go to 100%.

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flux commented · Jul 11, 2023 at 06:26 PM

Hey thanks for your advice. I provided the datasheet of my battery above. Looks like it is more complicated than I thought

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joe-horton commented · Jul 15, 2023 at 06:16 PM

Given that you just let the balancing complete and the battery is at 100% then can you lower the 13.5 float to a lower number so battery is held at this lower SOC via shore or solar power and as the battery discharges it will be held at the lower voltage until time for the next balancing. If this is true, where do you change the float voltage, ie what is really in control? Or do you change it in all 4 that I find in my system: MPPT, Lynx smart BMS parameters, Multiplus thru ve config3, and Lynx Smart BMS thru DVCC menu?

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Kevin Windrem joe-horton commented · Jul 15, 2023 at 08:33 PM

There is no master. You'd need to adjust each charging source individually.

A better approach would be to use Node Red to monitor the SOC and reduce the DVCC max charging current to zero. That will affect all devices the GX device can control. Orion Chargers would still need to be managed separately, maybe via a relay to turn them off.

Note that LFP batteries don't have the same risk of thermal runaway as do other chemistries (like those used in some cars). So it is far less likely for charging above 80% will risk fire or even shorten battery life. There's a reason Victron hasn't implemented SOC limited charging and this is why.

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joe-horton Kevin Windrem commented · Jul 17, 2023 at 01:43 AM

What is "Node Red"? I'm in a VanCamper where the ambient temp in the van can get real warm say at a trailhead left in the full sun or parked for days between trips at home. Goal is not to have a 100% SOC in a >100F van because that does shorten the battery life. At home it would be nice to just plug it in and leave all on and at 100% SOC. After a trip, you apparantly want to park in the sun so van, with a little MaxAir fan ventilation, will dry out all the way thru the layers of insulation.

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