question

Question about charging my soon to be LiFePo4 batteries to 80% each day

I am going to buy a bit more battery than I will need. Firstly because it isn't sunny every day and secondly so I can charge them to 80% SOC and still only bring them down to 50% SOC for the added charge cycles that gains. (I owned one of the first Nissan Leaf's to arrive in the US)

My question: is there a way to only charge them to 80% like there was on the Leaf Charging system, but with my Victron's? I will have the same question for Magnum regarding shore power charging.

-Bill

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You won't be able to set the specific SOC target to 80%, but you will certainly be able to set the charged Voltage lower to equate to around 80%.

I presume you are after a lower SOC to prolong the life of the battery? If so, then lowering the maximum battery Voltage is the best thing to do to preserve the integrity of the dielectric separator of the cells. The Leaf is doing just this, but displaying SOC rather than Voltage which would probably be confusing for drivers.

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In my system I have programmedt both the absorption and the float voltage to 4,05V/cell (in my package with 7 cell in series that means 28,35V)

And I have programmed automatic equalitation to 4,1V/cell (28,7V) every 10 day to balance the cells.

Edit: These are NOT LiFePO4 cells. My cells are litium-ion manganese oxid cells with a nominal voltage of 3,75V and are supposed to charge to 4,10V/cell.

I was just enphasing a point With the example With my battery and configuration: That it is possible to set a lower voltage in the daily charging, and use the Equalize from time to time to top up until reaching the recomended voltage for balancing (depending on the BMS type and settings)

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If these are LiFePO4 cells - even the LiFeYPO4 variety of Winston/ThunderSky - I believe you are pretty much decimating their lifetime. Way too high cell voltage for LiFePO4.

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petajoule ·

No no!! My cells come from an Electric vehicle and have a nominell voltage of 3,75V/cell and are normally charged to 4,10V/cell.

So no damaging With my configuration.

Of course everyone must be sure to configure acording to the battery cells/spesification from their own battery cells. I am not saying that People with LiFePO4 cells (nominal voltage 3,2V/cell) can adopt my settings directly!!!!

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Thanks! I never thought of that. But it makes complete sense. Once I upgrade to the Lithiums I will have to figure out what that voltage would be. :) -Bill

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Personally, I do not think charging the LiFePO4(!) to only 80% will have any significant effect on lifetime prolongation. If you do not overcharge them (and yes - to be on the safe side, 95% SOC might do), you can prolong your battery life by not having deep discharge cycles.

If you keep your battery ping-pong 95% <-> 65% all will be equally well as if you did 80% <-> 50% - maybe better with the additional advantage that you simply have more reserve capacity for the really long unforeseen sunless days.

Comparisons with the Nissan Leaf battery are not 100% in order, as this is a completely different battery chemistry (https://en.wikipedia.org/wiki/Lithium_ion_manganese_oxide_battery)

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The issue is actually more nuanced and complex than that. Have a read through this, paricularly the section on Memory Effects around 2/3 down the page which asserts: [my emphasis]

“These latter aspects have proved to be of key significance when considering the longer term performance of LiFePO4 batteries in house bank applications, because incomplete charge cycles are common when relying on renewable energy sources and shallow discharge cycles are also frequently experienced. These have the potential to render battery banks near unusable after as little as 2-3 years in regular service in the absence of memory-releasing cycles. Ineffective memory-releasing cycles are very common in DIY installations where the charging process is not properly controlled and/or configured incorrectly by fear of overcharging or due to widespread mythologies.

An absence of memory-release cycles caused by ineffective charging allows the voltage bump caused by the memory effect to grow over time. If the absorption voltage and/or the absorption time are insufficient to overcome it, the charging process gradually terminates earlier and earlier. This has a compounding effect as memory-writing begins to occur at lower and lower values of SOC over time and the available capacity of the battery can disappear almost completely without any loss of lithium or chemical degradation as such. Recovering battery banks in this state can be challenging and require many memory-release charging cycles using high absorption voltages, followed by deep discharge.

For these reasons, LiFePO4 batteries should be charged properly whenever the opportunity arises, so the effects of unavoidable previous partial cycles can be wiped out while it is still relatively easy to do so. This calls for a robust absorption voltage and a charging strategy providing adequate charge absorption. Anything else falling short of this will eventually result in significant performance and capacity issues.

I highly recommend reading the rest of that article and all other articles on that site pertaining to lithium batteries. Source:nordkyndesign website

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excellent article! Highly recommended.

For a memory effect to appear, an incomplete charge cycle followed by a rest period and a discharge must have taken place earlier (memory-writing cycle).

I have to study this further (and look up the research paper), but from this it would appear that actually partially charging the battery bank (and then letting it rest some time before discharge) would actually be counter-poductive.

I have to admit this is the first time I hear about such a potential pronounced memory effect in LiFePO4 cells. Lesson learned: never listen to the distributor only:

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