question

robtor avatar image

Limit SOC for longer LiFePO lifetime

Is it possible in some way to limit the maximum SOC of my Pylontech US2000 Batteries wit the Cerbo GX?
The reason for my idea is to longer the lifetime and healt of my batteries by not charging them to 100% in the summer, because I have a very large battery storage, more than 20kWh, and thatswhy it would be enough to charge them to 80% at least.


MPPT - Solar Charge ControllerLithium Batterybattery chargingcerbo
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@robtor

If you have a gx device and set it up the Pylontech with DVCC it will dictate to the system how to take care of itself. So short answer - no.

Most Pylontech batteries are destroyed by the banks being undersized for inverters so are discharged faster than recommended, or a re charged faster than recommended. Stored at a higher than recommended temperature environment as well.

That being said you can lower your charged voltage settings. But again the programming between the batteries and Cerbo usually will override this. I may be mistaken but Pylontech top balance so need to be charged up occasionally as well anyway.

Most manufacturers don't allow their batteries to fully charge through bms controll anyway, even though your battery says 100%, the cells are actually capable of taking more. I am sure Pylontech have done the same they have been in the business a long time.

Thanks for your reply, maybe you're true and if pylontech says they're charged 100% they aren't instead.


3 Answers
Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Please note that the 'batteries live longer at 80% SOC' is relevant for 'regular' li-ion batteries, and not for LiFePO4 batteries, these work at much lower cell-voltages so aren't affected like li-ion at 4,2v (or even 4,35v nowadays)

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cpt-pat avatar image
cpt-pat answered ·

Setting a charge cutoff tail current, which most Victron products support, will accomplish charging to less than 100% SOC, and most importantly, prevent overcharging. LiFePO4 cells can be quickly damaged by overcharging. Overcharging is common in DIY implementations where users fail to understand the interaction between charge voltage and charge current. Many of those DIYers only monitor charge voltage. Any charge voltage over 3.4 volts per cell (13.6 volts for a 4 series-cell battery) will eventually overcharge a LiFePO4 battery. Some BMSs will prevent overcharging by tracking SOC and terminating charging at a SOC threshold (through coulomb counting), but most don't and only monitor cell voltages.

This article has excellent detailed advice: https://nordkyndesign.com/charging-marine-lithium-battery-banks/

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pmikep avatar image
pmikep answered ·

I don't know anything about Pylontech batteries. But I know that Battle Born wants their batteries charged to at least 14.4 V (100% SOC) periidically so that their internal BMS's can kick in and balance cells.

I have heard many ppl say that 80% SOC gives longer life for Lithium batteries. But as Daniël Boekel (Victron Energy Staff) said above, this 80% number seems to come from papers on Li-ion batteries. Not from studies on LiFePO4.

Battle Born's warranty is 10 years, without regard to SOC. Whereas I have a pair of prismatic-based batteries whose warranty coverage varies with SOC. (Primarily DOD.)

So if BB isn't worried about SOC top, neither am I. I am happy to have my MPPT charge up my batteries early to 100% SOC and then run in Float (98% SOC) for the rest of the day.

As an aside, I have heard that the rate of charge current can affect life, a lower charge being better than a high one. But again, I think that BB tests their batteries for longevity by cycling between a high discharge and then their max charge current (0.5C). If mine last longer than 10 years because I'm charging at only 0.1C, they will outlive me. (And remember, they don't "fail," per se. They simply lose capacity over time.)

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