Packs, Chargers, Power Supplies and more – Power Supplies

With the FMA PL6  chosen as the charger I ran right into the realization that I could never use the full capability without some increased power supply capacity…  My standard rig at the field was a low end, unlabeled supply that I got surplus at a hamfest.  It had never failed to deliver but I also don’t think it was designed for more than about 20A at ~12-13VDC.  In the shop I have a bit better solution where I have a 52A 12-15 VDC supply available.

However…  there is a limit on the charger in that the input side of the charger can only accept 40A maximum.  That translates to a max of ~500W of charging power on 12VDC.  In order to get near the full 1000W you would need something closer to 25VDC at 40A.  Of course no charger is fully 100% efficient so the real numbers are 15% or so less but the basic idea holds.  So what am I to do?  My first thought was to start hauling my 52A bench supply to the field but that has two drawbacks.  One, is I would be limited to the 12V max of something less than 500W.  Not terrible, but having the max power available at the field could be even more important that back in the shop where I have (figuratively speaking) all day to charge batteries.  So, off in search of a way to get 25V or so at 40A.

First, I started looking for commercially available, purpose built supplies to do the job and quickly realized that such supplies are both unusual and very expensive!  I really didn’t want to spend 2-3 times as much on the power supply as I did on the charger.  Keep looking… and then I happened to come across high amperage 12V supplies for what I thought were very reasonable prices.  $60 each.  They were converted from computer (Dell server) power supplies and could supply 57A at about 12-13V.  Ok, that’s much more reasonable… maybe I could just use one of those and get along with slightly less than 500W of charge power… at least for now.  But then I recalled I had seen information mentioning using two identical supplies in series to get twice the voltage… Now we’re talking!!

Based on that vague remembrance I bought two of these supplies.  Both together were cheaper than I had paid for the bench supply a few years ago and each was more capable.  After a modicum of research I started experimenting and found that I could get a good working 25V supply at 57A by wiring the two supplies in series.  My first attempt resulted in one of the supplies shutting down… but with a little more research I found that by isolating the ground I could avoid the second supply from seeing a short when both were plugged into the same AC source.

I believe the power supplies likely include bonding the DC output ground terminal to the AC ground.  This means that when the two supplies are plugged into a common outlet and are sharing the same AC ground they are also sharing a common DC ground.  The result is that when the positive lead of supply A is plugged into the negative side of supply B… it is essentially creating a dead short.  Luckily, these supplies are very sophisticated and include the ability to clamp down the output in a case such as this so no damage was done.

Once the grounds were isolated from one another the supplies powered up and supplied a nice steady 25VDC.  I created a carrying case from a Styrofoam cooler and built a power panel in order to have a single 24V output as well as the availability of either 12VDC output if desired.  It is well protected in the cooler (which opens up when in use for better cooling) both from the elements as well as vibration and “bumping” around in the trailer.  I’ll update this post with a picture or two in the next couple days.

This whole rig worked great on my bench, so I took it to the field and after 3 flights on the CZ Cub decided to do a nice 1 hour parallel charge.  I fired up my low cost 900W generator, plugged in my new power supplies and… nothing!!  Further testing back at the shop confirmed that the whole rig worked but just not with my generator.  I’ll go into that in the next post on the subject detailing my search for a new generator.

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