MPP Solar LiFePO4 system improvements.

[Csimmer]

Just finished an upgrade to my system to optimise things a little.
The goals I had were:

1. Prevent the BMS from shutting the inverter down when it goes to high disconnect. Voltage spike caused by BMS shutting down high currents tripped the inverter occasionally.
2. Charge lithiums to SOC not voltage so I can stop charging at say 80% SOC whilst charging at 100% voltage.
3. Automatically turn on and off second 5kw inverter to save on standby power use.
4. Flexibly share panels between two systems (this was done A while ago)

How I did it

1a) I put in a supercap bank in between the inverter/charge and the BMS. This allows the caps to absorb the excess charge when the BMS goes to high disconnect. I put power diodes in to allow power into the caps quickly but discharge goes via a large resistor to keep the current down to an acceptable level. Super Caps
1b) I also put a contactor in to turn off the PV to the chargers
1c) I installed a large TVS diode across the MPPs (probably not required)

2) I am using the relay on a victron BMV-700 to control the PV contactors which can be set to any SOC or voltage.

3) I paralleled the on/off switch on the slave MPP unit to a relay which is turned on timer relay which in turn is turned on by a current sensing relay which monitors the AC current coming out of the inverters. So you can turn on the second MPP unit when a certain level of current is reached. Mud map of electrical box
electrical box
4) I install DPDT switches to allow me to put panels between my house or shed system PV panel transfer box

 

[Flip]

Please explain more regarding the Super Caps. I just upgraded to a 24V system with Lifepo4 cells and an Overkill Solar BMS. The first night the BMS went into undervoltage protect and prevented the charger from charging the batteries the next morning when I woke up. Today the BMS went into over voltage protect and shut my inverter down resulting in zero house power.
Steve at Overkill gave me a go around on the undervoltage protect using the app, but I have to be home when it happens. I still don't know how to get around the over voltage protect minus flipping the breaker for panels, but again, I have to be home when it happens.
Thanks in advance!

 

[Csimmer]

I don't know if supercaps will he

Please explain more regarding the Super Caps. I just upgraded to a 24V system with Lifepo4 cells and an Overkill Solar BMS. The first night the BMS went into undervoltage protect and prevented the charger from charging the batteries the next morning when I woke up. Today the BMS went into over voltage protect and shut my inverter down resulting in zero house power.
Steve at Overkill gave me a go around on the undervoltage protect using the app, but I have to be home when it happens. I still don't know how to get around the over voltage protect minus flipping the breaker for panels, but again, I have to be home when it happens.
Thanks in advance!

If your BMS is shutting the power down to your inverter I can't see how super caps can help. The only benefit you could get by putting a bank of super caps in parallel with your batteries is that they will smooth out the power so that short duration pulses of charge current or loads are smoothed by the caps. My situation was the BMS would stop the charging current but the inverters could still pull power out of the batteries. My super caps are there just to smooth out the shock the BMS produces when the high charging current is cut suddenly, they also generally help smooth out the power surges in either direction. My supercaps are not installed in the typical way which is directly parallel with the batteries, mine are in parallel with the chargers/inverters so they are on the 'other' side of the BMS.

 

[Flip]

Your initial post said

Prevent the BMS from shutting the inverter down when it goes to high disconnect.

So I thought your solution would help my situation. I think I understand why you say the Super Caps won't help, but I was really hoping. While Lifepo4 doesn't have the maintenance and short lifespan that comes with a lead acid, they certainly have their own set of headaches.

 

[Csimmer]

Your initial post said


So I thought your solution would help my situation. I think I understand why you say the Super Caps won't help, but I was really hoping. While Lifepo4 doesn't have the maintenance and short lifespan that comes with a lead acid, they certainly have their own set of headaches.

Totally different animal.