Thursday, May 13, 2010

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.

1 comment:

  1. Shane,

    I found you becuase of the BLDC arduino based stuff you have on the site but then I noted these attempts at a CC/CV charger that is both reasonable in cost and portable and resilient?

    When you have time, take a look at the Meanwell S-350-48 model, not the SP (without a current limiter front end as they only implement hiccup mode)... these S-350 can be had for about 45.00 with shipping from a variety of suppliers in the US, china and elsewheres...

    Though they are a tad wider than yours... I find they will fit perfectly in the space between the rack and the rear tire, a largish handlebar bag, backpack, trunk, panniers, etc...

    We have been hacking these into CC/CV Lithium Polymer, SLA, LiFePo4 and other chemistry chargers for nearly a year now over on and the process is fairly simple and straight forward...

    With a few mods:
    1.) Replace thermistor with resistor (variable if you want to fine tune the fan speed as it will remain on with this mod, which is what you want for 2 reasons - if the fan is on it runs cooler and if the fan comes on while charging the TL494 bus drops which messes with the error amplifiers and actually increases the current by about an amp.

    2.) Decide what power (watts) you want to run at - I have a modified S-350-48 which I've modified into a 15 or 18S (depending on a 3pdt switch setup) LiPo charger which pushes 460w (yep, not kidding)...

    Even more impressive - if you configure R33 correctly after determining the power level... you will end up with a Constant Power -> CC -> CV charger which is about 30% faster to the 90% charged point.

    There is tons more detail on Endless Sphere - search for CC/CV Mod, Meanwell S-350-48... etc.

    Oh and once the fan on always mod is done, and everything is tweaked... we have solutions for HVC/LVC and balancing in place of kit form (schematics available and artwork freely) and the modified meanwell will indicate end of charge when the fan turns off and current flow drops to Zero...

    As an example... I just charged a 12S4P 20AH LiPo pack from 3.3v per cell to 4.16v per cell at 20A continuous from a simple pair of these set to the same voltage and in parallel... took 33m to 90% and another 20 minutes or so to taper to C/20 where I ended it.

    All cells within .06 mv (well within spec).

    I would love to help with the Arduino based motor controller - ebikes are my forte but scooters (go peds and such) are cool too... I was just thinking we could use some of the controllers I have here (spares and brokens, lots of em) as external Power Section they have everything needed... just need to tap the right lines to drive the Fets gate drives...

    The stock FETs are P75N75F with a max rating of 75v and 80A continuous but the RDS(on) sucks - I normally replace the stock fets with IRFB4110 for 4 mohm rdson max, 100V and 120A!

    The controllers are available in 6, 9, 12, 18 and 24 fet (I don't have any 24 or 18 left but lots of 9 and 12s are available).

    I use them on my ebikes (after I build them up, fix the voltage regulator for 24S - 100v 88 nominal) and reprogram the primary, and phase currents... tweak the rest of the settings as needed for things like regen braking and so on and finally... optimize for a given motor.

    I can get these in Sensored and Sensorless for reference, but then all we need to do is knit in a pin header for an Arduino and you have not only a shield but a ready to go, enclosed and very capable (4-5kw) gate drive and power section! Just add arduino, drop me a pm on ES and I'll send ya one for playing around or a few if that suits you (I have some with burnt out MCU also for protoshields.

    mwkeefer on