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Heads up warning regarding the new Kelly OBC + DC/DC-converter.

1727 Views 6 Replies 3 Participants Last post by  riwe

I'm finishing a 30kwh battery pack for a leaf+openinverter. I use Audi e-tron modules, 11v nom, 240ah. Controlled by an Ennoid LV BMS (up to 150V). The setup is 36s.
We decided to test the new Kelly OBC + DC/DC for charging the pack and supplying 12v DC. I talked to Kelly about using three chargers in parallel and got a green light from Kelly to do so.
Rectangle Font Gas Cable Electronics accessory

The Ennoid BMS has the option to use a separate charging relay that is triggered by the BMS when the charger starts. The relay is also used to disconnect the charger if a cell voltage or temp reaches a configured threshold.

I used a littlefuse rated for 900V DC and 50A cont. as a charging relay.

The first thing I noticed was that the chargers have a very high inrush current on the DC-side. There is no soft start or pre-charge done when the charger is connected. Kelly instructed me that DC should be connected before AC but this made no difference to the inrush. The inrush current welded my littlefuse charge relay on the second charge test.


I did some measurements and the charger has approx. 600uf capacitors on the DC-side and there is a 4Mohm resistor between + and – to drain the caps when the charger is disconnected. There is no internal relay that disconnects the charger from the battery so there is a potential problem with phantom drain if connected at al time.

After a few tests, and flashes, the charger gave up and now when it is connected it just switches on/off in a loop:

The second charger was DOA and I triggered both the 230V fuse and the earth leakage circuit breaker when connected to AC.

I never got to testing the DC/DC-part.

I’ve informed Kelly about every step and recommended a redesign and some quality testing before shipping out the products. I like the idea about having charger and DC/DC in the same unit and if done correctly it would be a very nice product.

I’d advice anyone who’s thinking about buying this charger to first contact Kelly and make sure that they sorted out their problems before ordering.




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The relay in your BMS is likely a dry contact to control the charger as a logic signal or to run the coil on a battery contactor, not as a charge current interrupter.

The inrush current on the DC side is irrelevant because the charger is a hardwired connection in most circumstances though a battery contactor, not your puny BMS relay. This is why they said connect DC before AC. Nothing stopping you from precharging those caps to protect your contactor.

The notion of 4M ohms being a phantom drain on the battery at 100 microamps is ludicrous.

Part of the problem may be Kelly's, but most that I see is bad application understanding & design on your side of the fence.
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I see. So that is an external charge contactor, not a relay onboard the BMS, which is what it sounded like.

That said, you (the BMS) should remotely be turning the charger on and off before actuating or deactuating the contactor. Breaking the contactor while current is flowing should be a last resort, not normal operation. A good way to fry the charger output is to suddenly drop the load.

Kelly should also have a bleed or protection circuit on their output filter caps. A softstart on the charge current is also a nice to do. This isn't their first rodeo, so all of it is, frankly, surprising.
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