typically with a new installation some energy is taken by the battery balancing circuits, this may decrease after some time. It also depends on the BMS code, how they allow for the efficiency of the charge/discharge cycle, and the accuracy and balance of the current measuring shunt.
If the shunt is designed to handle large currents, some do not have the resolution to track small currents as accurately : so if you are only passing a few amps through a 500A shunt, then expect SOC errors to occur. With Lithium, the main indicators you need to watch are max and min cell voltages at full and low charge. If the difference is high, then the battery is getting out of balance, and needs to spend more time at full charge. SOC should be regarded as a rough indicator of how much charge is left. Ideally this should reset to 100% on full charge, every 2-3 days.
Charging to 100% SoC every 2-3 days (to reset SoC counter and allow cell balancing) is much more often than normally recommended, which is usually every 2-4 weeks.
On my system (48V 700Ah with REC-BMS and Quattro 48/10000) reported SoC at 54V (float voltage) drops by about 1% per week, I run a charge to 100% every 2 weeks which stops when the cells are balanced to within 10mV (usually takes an hour or so).
What are you quoting the SOC figure from, the BYD BMS or the Multiplus or do you have a BMV or SmartShunt battery monitor. This is key info to provide advice.
OK, so 2-3 weeks is ok, as long as the system is well balanced.
An off grid system with plenty of PV capacity will usually reach the Soc reset voltage every day unless cloudy.
It could do, but it shouldn’t. Mine charges up with solar before lunchtime every day to 54V (99% SoC) but only does a full charge to 57.3V (reset to 100% SoC and cell balance) every two weeks.
Ok, so my system resets the SOC every time the voltage exceeds a certain limit, can happen daily or not for a few weeks. My main criteria for health of system is the min cell voltage in the morning, and max delta voltage of the cells at that time. If one cell goes very low in the morning and is first to reach 3.55V, then there’s a problem with that bank.
so basically, the system sits at 100% all the time because it is provided shore/generator power. and the Victron Multiplus only inverts when the shore power is lost to keep the communications site up.
This is the second system we have installed onsite, and I don’t have any issues with the first installation. (Second system has been running for around 6 months losing around 1% a day.
The SOC must be false, because if the SOC after 30-40 days hits 50% SOC, I attend the site, turn the system off and on. and the SOC comes straight back to 100%
I am reading the SOC from the Multiplus ii via IP log in.
The only differnce between the 2 sites is
Site 1 - (working fine) has the negative cabling on Circuit Breakers
Site 2 (losing 1% a day) has the negative running to a bus bar (not a smart shunt) purely via 1x neg cable out of the Victron - to a 4way bus bar that them then individual cable run to the 3 batteries
both units were sent from the same supplier, but different configuration for installation.
All of the programming site of things is identical.
The SOC in the multiplus is an amphour counter so if the batteries are full and being kept this way it should not see any current flowing into the Multiplus from the batteries. There is very little you can do with the Multiplus battery monitor, no zero current calibration like on a SmartShunt and the battery monitor itself is less accurate than a SmartShunt but for a system without DC chargers or DC loads it usually suffices. Consider adding a SmartShunt.
