Combining chargers in parallel

[danielrlee]

I'm trying to combine two chargers in parallel and am experiencing some strange behaviour.

The first charger is actually a pair of Meanwell HLG-240H-48As, permanently paralleled, each set to 49.8V (12S lipo @ 4.15V/cell), running at their maximum amperage of 10A (5A per supply). Using them like this, they perform exactly as expected, charging at a steady 10A, tapering off once they hit 49.8V.

The second charger is an EMC-1000, also set to 49.8V, limited to 18A. Again, on its own it performs as it should.

Running this lot together though doesn't behave as I'd expected them to. Connected with their outputs paralleled, but not hooked up to a battery, they power up to show an immediate 50.4V. Why would this be? I know the EMC-1000 is sensing the voltage from the Meanwells and I can hear the relay switching. I imagine the EMC-1000 is supplying the excess voltage, something to do with it being a charger rather than just a power supply, but why exactly?

Connecting the paralleled chargers to the battery gives an immediate output current of around 20A (I would have expected 28A) and within the space of 10 seconds or so, tapers down to 18A, even though final voltage hasn't been reached.

What exactly is going on here?

 

[flyinbrick]

The first thing that comes to mind is that two "dumb" chargers in parallel will not have the same exact output voltage. Even a slight difference will cause a current loop, possibly damaging one or both chargers.

 

[danielrlee]

The first thing that comes to mind is that two "dumb" chargers in parallel will not have the same exact output voltage. Even a slight difference will cause a current loop, possibly damaging one or both chargers.

When you say "dumb chargers", are you referring to the Meanwells? Although they won't be matched EXACTLY with the same voltage, they'll be sharing the load equally through the CC phase of the charge and only then differing slightly during the CV phase.

Anyway, they're not causing a problem running in parallel, so that's why I'm more inclined to put this behaviour down to the EMC-1000.

 

[flyinbrick]

Here is what I'm imagining is happening. Take two plain 'ol power supplies (non-cc) and hook up positive to positive, negative to negative. If the voltages are not exactly the same (pretty much impossible), you will have a current of (v1-v2)/R where R is the total resistance of the wires (ohms law). This means that When R is really small (as the wire should be), the current can be high despite v1 being *close* to v2 and current will be going into one of the power supplies. This is a hunch I have - can't answer your question without doing some circuit analysis.

 

[danielrlee]

Here is what I'm imagining is happening. Take two plain 'ol power supplies (non-cc) and hook up positive to positive, negative to negative. If the voltages are not exactly the same (pretty much impossible), you will have a current of (v1-v2)/R where R is the total resistance of the wires (ohms law). This means that When R is really small (as the wire should be), the current can be high despite v1 being *close* to v2 and current will be going into one of the power supplies. This is a hunch I have - can't answer your question without doing some circuit analysis.

Okay, but these ARE all CC power supplies, so that shouldn't happen, right?

 

[flyinbrick]

Not necessarily, the fact that they are CC limited just means that the current will not "run away" and be limited which is good. The problems you have *may* be due to the fact that most power supplies are not designed to sink current. This is related to trying to parallel two batteries together for higher capacity. In theory, you can do it, but have to be careful. If the battery voltages differ enough, then the moment you connect the batteries together, the higher voltage battery will flow current into the other battery, technically charging it. The problem is that if there isn't a resistor in-between, the current is un-controlled and may be too high. That's why you will find ppl recommending the use of a diode or some sort of active device to prevent that.

 

[danielrlee]

Not necessarily, the fact that they are CC limited just means that the current will not "run away" and be limited which is good. The problems you have *may* be due to the fact that most power supplies are not designed to sink current. This is related to trying to parallel two batteries together for higher capacity. In theory, you can do it, but have to be careful. If the battery voltages differ enough, then the moment you connect the batteries together, the higher voltage battery will flow current into the other battery, technically charging it. The problem is that if there isn't a resistor in-between, the current is un-controlled and may be too high. That's why you will find ppl recommending the use of a diode or some sort of active device to prevent that.

Thanks for taking the time to help and provide a further explanation in regards to your theory.

When I get a moment at some point this weekend, I intend to hook up a watt meter to each individual output and investigate further. I'm not convinced that what you suspect is necessarily what is going on in this instance, but rather the Meanwells are causing the EMC charger to "go high" with the voltage it supplies. After all, a reading of 50.4V surely makes no sense at this point.

 

[danielrlee]

Having investigated the issue a little further, it turns out that both a combination of my own misunderstanding and my own screw-up has resulted in me causing the problem in the first instance. I thought I should update this thread for completeness' sake.

I had believed that the Meanwells only taper off their current once the full charge voltage has been reached, as with a dedicated charger. This turns out not to be the case; the current starts to reduce a little before. This explains why I wasn't seeing an expected 28A output, even though full charge voltage had not been reached.

The other cause only became apparent once I decided to re-check the voltage of the EMC-1000. Somehow, I'd managed to alter it to, yes you guessed it, 50.4V. Having now set it back to 49.8V, all is as it should be and I'm now charging at around 1400W with my paralleled 'mega charger'. I'm left feeling like a bit of a dick, but at least it's all resolved now.

 

[cwah]

So paralleling 2 charger is good? I want to do the same to increase output

 

[danielrlee]

So paralleling 2 charger is good? I want to do the same to increase output

It shouldn't a problem as long as the chargers are current limited and not just dumb power supplies. There are some exceptions however, eg. all my RC chargers pause to check the pack throughout the charge and paralleling with another charger just confuses this testing process.

Which chargers are you looking to combine?

 

[cwah]

I haven't decided yet, but I'd like the ability to sometime have an extra fast charger when I am travelling in hand! :