I have 2 x 100ah LiFePo4 batteries (2 years old approx) and use an Enerdrive ePRO monitor for my caravan. I also have 3 x solar panels to charge.
For the last 3 or 4 weeks I have had van parked up and have turned off all appliances (fridge), we have since had continuous full sun most days which previously that would give 100% soc, via the solar panels, by the end of the day. And was at 100% when I last checked weeks ago (According to the ePro monitor)
When I checked van yesterday the monitor showed 25% soc. I then connected 240v mains but the monitor has topped out at 28% after several hours on charge.
So my questions are
1. do LiFePo4 batteries loose soc with no use?
2. Is it possible the monitor is not in sync with battery soc? If so why
Unlikely same fault with both solar and mains chargers, and ditto for three batteries
Possible that the battery monitor has lost its synchronisation. Have you checked the settings, and checked that there are no fuses blown between the shunt and the meter
Lithium batteries prefer long term storage at 50 to 70% SOC so if going to be stored for months then better to fully disable the chargers and lift the negative lead from the battery. Check voltage a week later and then bi monthly
I'd suggest disconnecting both batteries and check that voltages are both the same.
Let sit for a couple of hours, or maybe overnight and check each voltage with a good digital meter. There are voltage-charge curves available for lithium but the error will be fairly large
-- Edited by Tony LEE on Monday 15th of March 2021 03:53:49 PM
Check what net current/voltage input/output you are getting on the monitor to see that there is no large current output from the batteries. If you have connected the 240v charger and are confident that the batteries are at 100%, reset the monitor to 100% and check the monitor for the next few days to see how the SOC changes. Also check that the monitor has been connected in circuit correctly.
Answers
1. very little
2. I would have thought only if it is faulty
3. Check the voltage. If it's around 12.8 I'd suggest something is wrong. If fully charged it would be a lot higher.
I understand it's not a good idea to leave lithiums charging when not being used. I'm with Tony in checking voltage of each battery but disagree that error is large. If you measure to 2 decimal places and check the curve it should be reasonably accurate.
For what it's worth I have an isolator on both house and start batteries and flick them both when we get home. Good for months
This is the Techies Corner of the forum, so now for my long winded answer, and the reason why.
The true state of charge on a LiFePO4 battery is measured by the voltage of each cell, "During The Charge Process", not the standby or discharge voltage and by progressively subtracting the accumulated discharge current from the known fully charged capacity.
Unlike chemical lead acid batteries, where a physical change takes place, with Lithium we are simply forcing the crystalline Lithium structure to accept and store excess ions.
Hence the name "Lithium Ion".
There are many varieties of Lithium Ion cells made by adding other elements and compounds to the lithium, but all varieties of lithium cells are known as "Lithium Ion".
LiFePo4 is Lithium (Li) Iron (Fe) Phosphate (PO4).
The nature of Lithium cells means they cannot be successfully charged as a battery.
What? I hear you say!
Yes, that's right, they can be discharged quite fine as a battery of cells in series, but they do not "charge" very well in series.
Charging is done by a Battery Management System (BMS) "per cell" because even identical cells will discharge and recharge at different rates.
During charging, once an individual cell's voltage reaches the charge voltage, all charge current flow through that cell stops and it therefore ceases to pass charge current on to another cell in series.
Charging starts at 3.1V per cell:-
3.2V gets you to 8%
3.3V to 30%
3.4V to 98%
3.5V to 99%
3.6V to 100%
3.7V to 100%
Voltages over 4.2V per cell will likely overheat your cells and cause irreparable damage.
The industry recognised charge voltage to reach 100% is 3.65V per cell.
Most BMSs simply divide the input charge voltage by 4 and feed that to each cell. For a 4 cell battery that means a charge voltage of 14.6V.
Unfortunately, the designs of BMSs vary and usually the less than perfect efficiency of the division circuitry consumes some of the charge voltage.
Most manufacturers therefore recommend a charge voltage of 14.8V for a 4 cell "battery".
However, as you can see from my table, 3.4 volts per cell (13.6V) gets you to 98% or a measly 2% difference in capacity compared with 3.65V per cell (14.6V).
On resting, over 12 to 24 hours, a fully charged 4 cell battery will settle from 14.6V to somewhere between 13.4 and 13.6 volts.
Not allowing for a settling voltage, a freshly charged lithium cell (3.65V) will measure a sharp starting drop, then the discharge voltage curve is near flat:-
3.25V at 5% discharge
3.20V at 40% discharge
3.15V at 80% discharge
3.08V at 95% discharge
Discharging past 95% sees a very steep discharge voltage curve,
2.90V at 98% discharge
2.60V at 99% discharge
Most BMS have a low voltage disconnect at 11.8V (2.95V per cell) to protect the cell and provide a minimum of around 2,000 recharge cycles at maximum DoD.
So, in answer to your original question, because of the settling voltage and the near flat discharge voltages, Lithium battery "state of charge" monitoring systems measure the voltage of the battery when charging and then start at 100% when 14.6Volts is reached. Thereafter, the monitoring system attempts to keep track of the 'amps in' and the 'amps out' by running all power through a shunt in order to tell you where you're at with remaining capacity.
Unfortunately some things like compressor fridges are usually directly connected to the battery and may not run through the shunt of the monitoring system.
This results in your systems getting totally out of wack within a week or so until you fully recharge the battery and then reset the monitor.
