My question is: when does the charger change from absorption to bulk charging and how domI ensure that the battery is not trickle charged to death. I have contacted REC BMS and had quick replies, but I am not able to resolve my issue.
l run a REC BMS, Cerbo GX and Multiplus II 48/3000/35 on a 16S 300Ah LiFePO4 battery pack on my electric boat with all the components connected via CAN. The BMS controls the charger/inverter via DVCC and I share the current and temperature sense from the REC BMS.
I started using the default settings from the BMS and I adjusted some paramters afterward, like capacity, max charge and discharge current.
The parameter: cell end of charge voltage is set to 3.58V which is good
The parameter: end of charge hysteresis per cell is 0.25V which would be good (3.58-0.25=3.33V) BUT somehow REC BMS factors this 0.25V with 0.2 ( see latest REC Q BMS manual, which means that the battery floats at 3.58-(0.25*0.2)=3.53V per cell … which is in my opinion REALLY BAD?! for LiFePO4 cells…
The reaction from REC BMS:
So why does Victron wants these cells to be trickle charged to such a high voltage…
To resolve the high trickle charge voltage I changed the parameter: end of charge hysteresis per cell to 0.9V instead of 0.25V. Because 3.58-0.9*0.2=3.4V which I would think is a better float voltage (should even use a lower voltage). I have done this in concert with REC BMS support.
However, now my charger stays in float and chargers the battery to 54.4V (3.4v/cell) even when the battery voltage is lower then 54.4V when the charger is switched of.
I wonder when does the BMS decide to charge the battery to 57.3V (3.58V/cell) again? Is that the case when the battery voltage drops below the end of charge voltage-hysteresis which is in my case 3.58V-0.9V= 2.68V?
The parameter: end of charge SOC hysteresis (default 5%) does not appear in the settings tab when using the REC BMS WIFI module. Not sure what this does.
What i have seen is that my battery was at 53V (below the float voltage) with the charger off and at 83%. The DVCC CVT voltage (charge voltage) is still 54.4V (3.4v) and not 57.3 (3.58v)
So how do I charge my battery to 100% full when I want that before going on a boat trip, how do I ensure that the battery is not trickle charged to death?
I could reset the hysteresis to 0.25V again and this will probably recharge to 3.58V when the battery voltage is below the 3.58v-0.25V, and switch of the charger after charging is finished, but this will not work when I connect the solar array in the near future. So I assume in this case that the bms commands to bulk charge the battery voltage is below the hysteresis voltage, but i am unsure if this is how it works.
Why the 0.2 factor on the hysteresis? The BMS charge method would be OK if the factor was not used. The battery would then float at 3.33V instead of 3.58V and the bulk charge would start again when the battery drops below 53.3V (3.33V/cell). The SOC end of charge hysteresis of 5% would ensure that the battery only recharges when the SOC is below 95%!
THIS WOULD BE PERFECT I THINK, but the 0.2x factor ruins the whole charge algorithm and the 5% SOC hysteresis can not be set anymore, and perhaps also not used in the algorithm anymore.
I am the edge case as some people in this forum would call it, but I think the BMS is totally useless to me for charging the battery when the BMS will destroy the expensive battery by trickle charging it to death.
I wonder what
- other users experience is,
- whether some of my findings and ideas are actually correct.
- hot to get a working system, ideally as described above
