Much discussion centers around “correct” voltage (V) to charge lithium iron phosphate (LFP) batteries.
Spoiler alert: there is no correct V for all situations because optimal V is completely dependent on charge rate. RV charging is further complicated because there are three potential power sources, the tow vehicle, the sun, or shore power. Each can have different charge rate and should ideally have a different target V.
Charge rate is reported as fractions of C best illustrated through examples:
A 100Ah battery charged to 100% in one hour is
1C charge rate (current = 100A)
A 100Ah battery charged to 100% in 10 hours that is a
0.1C charge rate (current = 10A)
A 100Ah battery charged full over 20 hours is
0.05C, 5A.
These charge/discharge curves were compiled for 4-y old Winston cells but they are widely applicable to LFP. (I hope the person who compiled them is OK with my reposting, I hope it posts, it didn't appear in the version to be viewed before posting):
[IMG]file:///C:/Users/dharr/AppData/Local/Temp/msohtmlclip1/01/clip_image002.jpg[/IMG]
Inverter shutting off under load — northernarizona-windandsun (solar-electric.com) about 5th post down I believe.
The V on the Y-axis is for an individual cell; multiply that by 4 to calculate typical 12V battery equivalence.
The critical takeaway is that charging at 0.2C up to 3.4V/cell will result in under 70% full whereas charging at 0.05C will yield above 95% capacity at 3.4V.
As State of Charge (SOC) nears 100% at slow charge rates, it takes very little incremental charging to make the cell V rise faster and faster. This “shoulder” phenomenon is why it is important to not push these cells too far. In a multi-cell battery, the balancer has to keep all the cells at close to the same V. As one cell approaches 100% SOC, its V begins to rise very rapidly and the balancer has to work harder and harder to keep that cell from “running away” and potentially reaching damaging V. There is definitely a risk to pushing cells too far. If you’ve ever witnessed this by watching cell voltages, it is impressive how fast it happens. At slow charge rates, this runaway can occur at voltages normally considered completely safe, i.e. under 3.5V/cell or 14V/battery.
Most important though, there is no reason to push to high V because there is very little capacity gained by doing so. Similarly, you can see that discharging cells below 3.1V (12.4V battery) doesn’t yield a lot of capacity for the rapidly increasing risk of low-V cell damage, depending on discharge rate of course.
YOUR solar charge rate is dependent on
YOUR solar collection (the current your system generates) and
YOUR storage capacity.
YOU have to determine the best charge target (boost or absorb voltage) for
YOUR system. There is no best absorb V for all systems.
Final note, depending on the charge rate of your solar setup, you may well want a different target V for your solar charge controller as for your shore power converter because your solar system will likely provide a significantly slower charge rate than shore power and should have a correspondingly lower target V.
I am presenting info lots of folks already know but I wanted to provide a basis for people who are designing their system or pondering ideal absorb/boost V targets. The curves above are the most helpful resource I have stumbled upon. I hope they help others.
Linear Mode
