Hi my MPPT went through bulk and absorption cycles today while disconnected from the batteries. I am trying to determine if this is normal behaviour. I would have thought that once the batteries were reconnected bulk charging would resume.
System set up
- Two 12.8V lithium smart in series feeding 24/3000 Multi-plus operating off grid
- 1.2kW solar PV fed through a 100/50 MPPT
- Battery protect between the MPPT and the batteries
- VE Bus BMS charge disconnect wired to battery protect to prevent charging due to high cell voltage or low temperature.
- Raspberry PI using Venus SW is logging data to the VRM.
Temperature dropped to -2C overnight for the first time but warmed up to a high of +14C by 2pm
The site is 200+ km away but looking at the VRM data, I noticed the MPPT battery current remained zero throughout the morning presumably due to low temperature shutdown.
I have the MPPT absorption voltage = 28.4V and fixed absorption period of 15min.
You can see in the chart that the MPPT battery voltage reached 28.4 at 7:12am and stayed there for 15min, then dropped at 7:27am. The MPPT battery current was zero during this time. (Lower right corner of the chart below)
Here is a zoomed view of the absorption cycle:
It seems the MPPT completed its absorption cycle at 7:27am and then switched to float, without actually feeding any current to the batteries.
At 13:04 the batteries had warmed up enough for the BMS to trigger the battery protect to close the circuit and you can see the MPPT battery current begin flow, but the MPPT remained in float.
Is this normal behaviour in this situation? I would have thought that once the batteries were reconnected, bulk charging would resume.