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jakob avatar image
jakob asked

Heavy sparking when connecting Multiplus II 48 V to a Lifepo4 battery bank

I have a Multiplus II 48 V that I want to connect to a battery bank of four 100 Ah Lifepo4 batteries in series. I get heavy sparks when connecting the negative cable to the battery bank.

The first time I managed to destroy the connector at the battery cable. Second time I tried the battery BMS was shocked in to sleeping mode.

I assume that the capacitors needs some type of pre-charging but I have not found a good way of solving the problem. I have tried the following:

1) Connect once again after the first spark - Resulted in BMS problem

2) Precharging with a resistor (25W and 33 Ohm) for approx 5 seconds- Still BMS problem

3) Add a switch between the battery bank and the main fuse (before the Multiplus) to avoid sparking. I used the following switch: https://www.bluesea.com/products/6006/m-Series_Mini_On-Off_Battery_Switch_with__Knob_-_Red. It should be capable of 300 Ah but when switching it on it was destroyed.

4) I was told by my retailer to switch the Multiplus on and of a few times before connecting the cables to uncharged to capacitors - Still sparks and BMS problems

Do you have any experiences and can recommend a way of solving my problem? Can I pre-charge the capacitors via a 48 DC power supply?


Multiplus-II
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the-doon avatar image the-doon commented ·

Sounds like you are putting a dead short across the battery by the way you are describing the bang. I’d stop doing what you are doing. What batteries are you using?

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rickp avatar image rickp commented ·

I’ve used the capacitor method on my 24 volt system without issue, but what you’re describing doesn’t sound like capacitor loading, it sounds more like a short. Recheck all wiring and connections from scratch… assume nothing is right and double check everything. Tripping a BMS is normal, but frying a switch isn’t.

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6 Answers
H.W.Oelofse avatar image
H.W.Oelofse answered ·

I use a 15 ohm high powered resistor to initially connect the batteries to the inverter as this will limit the current and charge up the capacitors in the device after a few seconds you can connect the terminal quickly and there will be no sparking. There is a large capacitor bank in the inverter that does indeed behave like a dead short initially, this is also assuming there are no errors with polarity. I see you are using 4 12v lithium batteries in series, have you taken the opportunity to charge them all to the same state of charge before using them in a 48volt system?

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Jack Peters avatar image
Jack Peters answered ·

As others have already suggested, this damage does not sound like capacitor inrush current. The system should be turned off and checked for an internal short in any equipment or incorrect wiring.

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trackman avatar image
trackman answered ·

What was the fix for this? I have the exact same issue. New MultiPlus II 2X120V with 2 12v 100 ah batteries. Very first time attempting a connection and the sparking is fierce.

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JohnC avatar image JohnC ♦ commented ·
@Trackman

This is actually quite normal, 'filling capacitors'. Not to say it can't occur from a fault too, but that's unlikely. Just slipping a live battery cable tab over a terminal bolt can damage the threads too. The spark from a 48V batt can be an underwear-changing experience.

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jetlag avatar image
jetlag answered ·

As already mentioned above, connect the nagative terminal and then pre-charge the capacitors inside the multiplus with a resistor e.g. 10-50 ohm and 5W or more. This kind of pre-charge is almost a must as you can "damage" the terminals caused by the sparks.

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trackman avatar image
trackman answered ·

The instructions seem detailed I am surprised they speak nothing of this procedure. Thanks everyone I'll track down a resistor that might work


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snoobler avatar image
snoobler answered ·

Additionally, the capacitor charge surge can trigger over-current protection as the BMS thinks there's been a short circuit. This can prevent the battery from ever charging the caps.


Be prepared for this if the BMS ever goes into low-voltage protection.

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