Hello, I would like to limit the charge of my LifePO4 batteries to only 80 percent to extend their life. Is there any way to tell the mppt from a victron smartshunt to only charge to 80 percent. Thanks
asked
Can I make a Victron MPPT limit charge to just 80 percent?
Does limiting to 80% SOC really extend the life of any LiFeP04 unless the quality was cr_p in the first place and is there any real benefit? I have my doubts. Yes I have read all the theory.
Our 4 cell 300Ah Sinopoly has survived 8 years of full-time travel in our motorhome charged via solar up to 50A and c75A alternator output. It also starts the 3.9l TD engine.
Typical SOC is never static for long. It is being used. Pointless purchase if not.
Dawn 82% - Midday 100% - Dusk 95% -repeat. Lowest ever in mid winter was 39% from memory. 20% SOC noisy alarm and 12.1V LV cutoff never triggered
The annual 30A constant load capacity test last November still delivered >4000Wh = >300Ah. How can we possibly "extend life" or ever measure it?
No BMS involved or cell balancing necessary to date. Yes I occasionally check.
Charged by all sources at 14.1V. Once the battery terminals reach that 14.1V it is full and "absorption" for any time adds a few Wh at most. "Float" at 13.4V maintains 99-100% SOC until the sun sets.
Lowest ever in mid winter was 39% from memory.
So basically (during everyday usage) you limited the used capacity to 61% from the total 100% battery capacity.
And if this is typical:
Dawn 82% - Midday 100% - Dusk 95% -repeat.
It means even lower usage.
Minimum ever SOC of 39% also means that during everyday usage you stayed well above the dangerous zone when discharging.
Charged by all sources at 14.1V
This is also relatively conservative (if cells would be equal, it would mean a max voltage of 3.525V).
I limit nothing. We just use the battery. Typically 800Wh dusk to dawn. Lack of sunshine or driving next day will deplete it a little more.
Satellite TV, Android boxes, phones, notebooks, tablets, internet, 740W, toaster, 750W Instant Pot, 255l Digital Inverter 230V fridge freezer - 300W inverter fed, water pump, engine starting etc. Solar energy keeps it happy most days.
The maximum cell voltage difference measured at 14.1V in 8 years is 0.04V and experimenting to balance all cells to within .005V takes a few seconds at 5A and achieves no measurable change in cell capacity. i.e. a waste of time achieving nothing in reality apart from as I have mentioned in the past - psychological satisfaction.
Pushing cell voltages to 3.6V for our battery pack takes seconds at 50A and achieves nothing.
I limit nothing. We just use the battery. Typically 800Wh dusk to dawn. Lack of sunshine or driving next day will deplete it a little more.
When you only use 0.8kWh from a 4kWh battery, you are limiting the usage (indirectly, by the way your battery is sized compared with your consumption needs).
Of course that in such conditions the battery degradation will be reduced.
in normal use you can just limite the voltage to limit the soc, but with node red you can stop the mppt at a soc level
what battery do you used?
some batterys need to be full to have corect balancing cells !
The battery is actually a DIY battery I made from individual cells and a bms. I do not think the balancing would be a problem for me because I can lower the balance voltage. Thanks how would I set that up with node red, do I need a raspberry pi ?
I would like to limit the charge of my LifePO4 batteries to only 80 percent to extend their life
If this is the objective, ignore SOC. Always use voltage.
If you would not have a BMS, you could stop the charging somewhere in the range of 3.40V-3.45V (cell voltage). No point in going with a lower limit than this. This also would require to manually perform a bottom balancing of your cells before putting them in use.
But since you have a BMS which performs cell balancing, you can't stop the charging that low. Lowering the balancing voltage bellow 3.40V is wrong, because there is no point to "balance" in the flat section of the charging curve. You must charge the cells in the 3.50V-3.55V zone in order to perform cell balancing.
Never let any cell go above 3.60V when charging.
Also, if extending battery life is the main goal, stop the discharge above 3.10V (cell voltage). Even above 3.15V.
