Super Mean Well: The Scooter Fast Charger

Disclaimer: Do not try this unless you are very comfortable with the concepts and dangers of battery charging. I am pursuing this as an experiment, not advocating it for general use. Attempt at your own risk.

A while ago, I had the idea of repurposing large LED power supplies as battery chargers. The premise is that both use the same output characteristic. That is, they will apply a constant current or a constant voltage, whichever is lower. For batteries, the constant current is applied first and supplies most of the charge. Then, when the applied voltage reaches a certain threshold, the voltage remains constant and the current ramps down. If you don't understand how this works, you probably don't want to work with batteries anyway.

Mean Well makes inexpensive 100W LED power supplies (presumably for commercial lighting) under the PLC-100 line. They are roughly the size and weight of laptop power supplies and are available in a range of voltage outputs, up to 48V. They have a constant-current and constant-voltage trim pot that allow adjustment within a limited range around the nominal output. They retail for about $65 and are available from several electronics distributors including Mouser.

I have been using the PLC-100-36 to charge the batteries on the BWD scooter for some time now. The scooter battery pack is lithium iron-phosphate. (Note that these are generally safer than other lithium-ion chemistries, which is why I am more comfortable using a non-standard charger.) It's a 33V, 4.4Ah pack (equivalent to two 36V DeWalt drill batteries). I set the constant current to 2.2A, so it's a C/2 charge, meaning it would take approximately two hours to fully charge the pack.

There is obviously no balancing circuit (though I did try to make one). It's important that the cells be well-balanced before charging and that the voltages be monitored in case one starts to peak early. Cell voltage monitors are widely available, for example this one and this one that also balances. They will not automatically cut off the charger, though, so constant observation is required. (Generally a good idea for battery charging anyway.) For general use, an integrated charger/balancer such as this takes care of all the monitoring and cutoffs. The downside is that for most of these RC chargers, you still need a bulky front-end DC power supply.

Though it is compact, inexpensive, and plugs directly into the wall, the 100W Mean Well supply falls short of the RC chargers in output power. So I was very glad when I came across the new HLG-240 line. This is a 240W version of the Mean Well LED power supply that retails for about $130. It comes in a compact (but dense) aluminum enclosure with wires already attached. Here's the HLG-240-36:

And for scale, here it is next to the scooter:

It's not as light as the 100W version, but it's still something you could see carrying in a backpack. And just like the 100W version, it plugs straight into the wall. This particular model has a voltage trim range of 33.5-38.5V and a current trim range of 3.3-6.7A, a good match for the scooter battery pack. Other models offer other ranges. (Be careful: Only the model with the "A" suffix are adjustable!) And here it is doing its thing:

With a maximum output current of 6.7A, this supply can now charge the scooter pack in about 45 minutes (~1.5C). Whether or not this is practical or necessary, I don't know. I've never had a case where I absolutely needed to charge it in less than an hour. But it's interesting to know that the capability exists for about the same cost and size as a laptop power supply. (Okay, this is a little bigger than a laptop supply.) I still don't recommend it for casual use, since it doesn't implement balancing or voltage monitoring. But if somebody made an small adapter that did these things, I could see it being a good, inexpensive solution to small EV charging.