Stringing 36v chargers in serial

[flashedarling]

I am having trouble picking a good 72v charger to test my open BMS on. I may just take pgt400's advice and string together 2 36v chargers to make a 72v charger. If both are identical with the same current and voltage limits it seems like it should work. My question is would they share voltage equally so they both hit cutoff voltage at the same time or do I risk uneven charger voltages, having one hit cutoff before the other and having too much current forced through it? Or does having too much current from the other charger in CV mode even matter?

The alternative is I only have 1 Open BMS board could I hook Charger A- to Charger - and cell 1 on the BMS, A+ and Charger B- to cell 12 and finally B+ to cell 24? Would that work?

 

[flip_normal]

For a 24 cell pack you need to charge at 3.67V * 24 = 88V. I'd use 2 of 48V (nominal) PSU's adjusted down to 44V each wired in series. You can get a 500W version from ebay for about 50 bucks each (+ shipping) for a 1KW charger. They output constant power so that the output voltage of each will rise to the set point, then the current will taper off to zero as the pack reaches full capacity. At the beginning of the charge the current could be 15 amps or more. For a low C rate pack you could use lower powered PSUs. I’ve got a couple spare of the 48V 500W ones (tested and working under load) if you want to do a deal.

The current through the two chargers will always be the same as they are wired in series.

At about 3 mins into this video you can see jozzer using two of the ebay PSU’s to charge his 72V lifebatt packs.

 

[swbluto]

If the chargers are grounded(Having something referenced to a common ground), I don't think it'll work as the output voltage would be in reference to that ground, not the previous voltage in series. However, If they have some form of isolation(Such as with a transformer or opto-isolation) on the supply side, it seems like it should. But this question needs one of the EEs on the forums to answer accurately.

 

[pgt400]

I've been using (3) of these 24v 5 amp units for my 6 AGMS. One across every 2 batterys. They are 3 stage smart chargers, fan cooled, small, light and only $46 each. Isolated output. The chargers are in series with 4 leads going to the batterys, 0, 24, 48 and 72 volt. They always finish charging within minutes of each other.


http://www.batteryspec.com/cgi-bin/cart.cgi?action=link&product=121

 

[fechter]

It should work OK as long as they are isolated. If they are not isolated, you can isolate one by cutting a connection somewhere. It is OK if one charger is not isolated as long as the other one is.

 

[flashedarling]

I'm not sure I understand, how does one isolate a charger? Does it mean severing the connection to the ground plug on the input side?

 

[fechter]

It varies depending on model. I try to leave the ground pin connected to the hot side stuff (transformer frame, heatsink etc.) and cut the trace that connects the output side on the circuit board.

The cheezy way is to just snip the ground pin off the plug on one of them. Don't charge in the rain....

If one charger is still grounded and two are in series, you would still have some degree of protection aginst electric shock.