Disconnect batteries and Multiplus goes nuts...sort of

[pda1]

Here's an odd situation that happened today.

I shut down my system (24v- 2kW) so I could remove an unnecessary temperature wire on my two Battleborn "in-series" batteries. Note- I shut down each component using battery switches beginning at the input to the SCC, then shut down the Mulitplus 24-2000 then turned off the battery switch.

Then, I removed the temperature sensor wire from one of the batteries (it's not needed).

Next, reconnected the batteries.....
- Turned the battery switch on.....

- Turned the inverter switch on (not the switch on the multiplus- I have a switch that disconnects the positive cable feeding into the multi....

- Turned all switches "on" (MPPT/PV input)

- Turned on the AC breakers connected to the Multi....

THEN BLAMMO!!!! (sort of)

The Multi switched to CHARGER mode and the BMV-712 said 50A are going into the batteries.

I shut the whole system down again, checked the batteries voltage (more or less 0 volts) so I assume the BMS kicked in and shutdown the batteries.

Next, I put 12v on each (disconnected) battery and put them in series again. Battery voltage went back to a normal 26.5.

Reconnected everything and had no further problems. Multiplus was running fine.

So, here's the question- what caused this and how do I avoid this in the future?

'Glad to be alive....

 

[sunshine_eggo]

Here's an odd situation that happened today.

I shut down my system (24v- 2kW) so I could remove an unnecessary temperature wire on my two Battleborn "in-series" batteries. Note- I shut down each component using battery switches beginning at the input to the SCC, then shut down the Mulitplus 24-2000 then turned off the battery switch.

Then, I removed the temperature sensor wire from one of the batteries (it's not needed).

Next, reconnected the batteries.....
- Turned the battery switch on.....

Where's your pre-charge resistor?

- Turned the inverter switch on (not the switch on the multiplus- I have a switch that disconnects the positive cable feeding into the multi....

Where's your pre-charge resistor? Either this or the prior operation triggered BMS short circuit protection.

- Turned all switches "on" (MPPT/PV input)

The MPPT likely pre-charged the inverter caps after the batteries triggered short circuit protection. The Victron MPPT can run on PV only.

- Turned on the AC breakers connected to the Multi....

THEN BLAMMO!!!! (sort of)

The Multi switched to CHARGER mode and the BMV-712 said 50A are going into the batteries.

I shut the whole system down again, checked the batteries voltage (more or less 0 volts) so I assume the BMS kicked in and shutdown the batteries.

Next, I put 12v on each (disconnected) battery and put them in series again. Battery voltage went back to a normal 26.5.

Reconnected everything and had no further problems. Multiplus was running fine.

So, here's the question- what caused this and how do I avoid this in the future?

Install a pre-charge resistor circuit to charge the MP caps before connecting direct to the battery.

Or

1) Assuming you have available PV, Turn on PV/MPPT to supply voltage to the MP terminals while the MP is in the OFF position. This will charge the caps.
2) connect battery to MP.

 

[pda1]

Pre-charge resistor? What's that?

 

[sunshine_eggo]

Search results for query: pre-charge circuit

diysolarforum.com

 

[pda1]

I saw a video by Will about pre-charging an inverter. He recommended a 25w 30 ohm resister.

Is that the correct wattage and Ohm value? My Multiplus is 24-2000. Geez, I hope I haven't wrecked the M+. So far the inverter's working fine. No idea about the charging functionality.

It looked like he just connected the resister to the NEGATIVE terminal of the battery and inverter.

 

[sunshine_eggo]

99.999% sure it's fine. Basically, it's like you connected the battery for a microsecond and then disconnected and proceeded to bring up the rest of the system. Had you simply disconnected and reconnected the battery, it likely would have resumed normal operation as that would reset the BMS short circuit protection.

Doesn't matter which side, but (+) is preferred, so you don't break the DC ground.

A #2 pencil sharpened at both ends gives you about 6Ω. Works fine.

25W, 30Ω should be fine. It's not an exact thing. A wide range of resistance/power ratings will work.

 

[pda1]

Thanks. Your comments are always helpful.

 

[sunshine_eggo]

Thanks. Your comments are usually helpful.

Revised... There are those who would vigorously disagree with "always"...

 

[Supervstech]

Revised... There are those who would vigorously disagree with "always"...

Nah... always applies, just not to EVERYONE!

 

[pda1]

The world exists in absolutes.

Variables are for those attempting to talk their way out of the trouble and consequences they got themselves into.

 

[sunshine_eggo]

The world exists in absolutes.

Variables are for those attempting to talk their way out of the trouble and consequences they got themselves into.

I'm guessing you're not married...

 

[pda1]

That's a good one....

Still married- 25 years . My wife and I are a very good match.

 

[sunshine_eggo]

That's a good one....

Still married- 25 years . My wife and I are a very good match.

On second thought... A wife's world does consist purely of absolutes... absolutely whatever she wants.

 

[pda1]

The charging resister- can it be one of those wirewound types- like this one;

Wirewound resister

 

[sunshine_eggo]

The charging resister- can it be one of those wirewound types- like this one;

Wirewound resister

More than adequate. Don't overthink it.

 

[Supervstech]

The charging resister- can it be one of those wirewound types- like this one;

Wirewound resister

Sure, or a pencil, or a lightbulb, or a roll of wire… pretty much anything to slow the electron flow…