After over a year of effort t I have finally built a boost circuit to charge an EV battery pack from a lower voltage.  It’s been frustrating, expensive, and slow but now we’ve got something that works– not great, but a place to start.  It has been an exercise in patience, applying systematic processes and electronics gear acquisition.

A boost circuit uses an inductor (basically, a coil of copper wire) and a power MOSFET (an electronic switch) to increase the voltage of the electricity that enters the circuit.  Whenever the current flowing through the inductor decreases, the voltage of the electricity coming out the other side increases temporarily.  By turning the MOSFET on and off quickly, the voltage can be regulated to something higher than the input.  This is important because most motors/generators need to spin very fast to generate enough voltage to charge EV battery packs.  This is often impractical to do mechanically:  a pedal powered generator needs to be pedaled faster, a wind generator needs to have more wind, a solar panel needs more light.

I’d hoped initially that I could just throw together something I found on the web and off I’d go, but of course it’s never that easy.  I scoured the web and selected a Linear Technology chip to be the brain of the operation.  Linear provides a decent modeling tool (LTSpice) and a huge datasheet.  My initial enthusiasm was slowly eroded with each hurdle.  First, Linear only provides the chip in TSSOP format rather than hobbyist friendly DIP.  This is an itty bitty chip that was impossible to solder with my $30 Home Depot soldering iron.  Of course, the next natural step was to buy Ben dinner and several margaritas later, Ben had soldered a few LTC samples to some TSSOP to DIP converters he had lying around.  I fiddled with LTSpice until I built a circuit that worked, at least in the simulator.  The final hurdle with the simulator was that the LTC datasheet had an error, sizing the startup delay capacitor thousands of times too large so that the chip took several minutes to startup.  I never waited long enough for it to start working, so I thought it was broken.  I built the circuit on my protoboard, hooked it up to a 12v battery, and POOF!  the protoboard melted.   I bought another protoboard from Jameco, and to prevent a repeat of this incident, a Mastech power supply with a current limit.  Ben and I analyzed the circuit from every angle on a couple of Saturday afternoons with no luck– it would work for about 5 seconds, and then the LTC chips always fried themselves.  One more dinner and some homebrew later, Mike professional stepped through the circuit methodically and couldn’t figure out what was wrong.  Instead, he sold me his Tekronix oscilloscope and wished me luck.

Since Ben and Mike were stumped, I appealed, on advice of the AVR Freaks forum, to Linear themselves.

Amazingly, they sent out two application engineers.

Not surprisingly, the engineers were more interested in my collection of electric guitars than my circuit.  After examing a few guitars, they took one look at the protoboard on the kitchen table and said “that’s never going to work.”  That was the last I heard of them.

I took a few more stabs at making the circuit work.  One evening while I tried to solder a Linear LTC to a converter board, the $30 soldering iron burned two of the $7 chips and $15 adapters to a crisp and I ran out into the garage in a fit of rage and smashed it with a hammer.  I gave up on the LTC chip.

I took a break from the circuit until Mike came over one day and handed me a rough schematic of the boost circuit in his battle bot battery charger.  It outputted 70v and was nothing more than a 555 timer, an inductor, and a few other common parts.  I built the circuit in Electronics Workbench and it didn’t work.  I’d just started a new job 30 miles away in downtown San Jose, so each day for two weeks I read on the train about circuit theory and how boost circuits work, and built the circuit, piece by piece, in Electronics Workbench with Mike and Ben’s help over email.  It was a fun and exciting time and then came building the circuit on a protoboard.  Once again, I broke the problem down into lots of smaller problems and tested each piece before integrating it into the whole.  First, just the timer circuit, then, just the MOSFET switching, then, the drivers for the MOSFET gates.  This process was slow but much more predictable– each portion took one or two days and integrating usually took just a few hours for each part.

After this string of good luck, something had to bring balance.  I was tired of the 555 timer chip, so I used my one good piece of lab equipment– a fancy HP function generator– to generate the fast on/off signals for the MOSFET.  Well, such a sensitive piece of equipment is not meant to drive a crude thing like a power switch, and while I was microwaving dinner in the kitchen I left it hooked up and running and something blew up inside it.  The oscilloscope seemed to wilt at this development and the trace– the little line that runs across the screen– expanded until it was as fat as the big toe on my foot.  I twisted the knobs but it too, was broken.

Mike and I spent two evenings troubleshooting both pieces of equipment and figured out what was wrong with each– but it was still frustrating.

Still, within a month, I had the whole circuit working.

Then I tried to solder it together using my Wen soldering gun (the sort of thing you might use for plumbing) but this was like trying to play a piano with my feet.

I caved into Mike’s suggestion and purchased a professional Metcal soldering iron.   After this arrived, building the circuit onto a board went quickly and smoothly.   I had to modify the size of the timing capacitor for the circuit to work at higher power.

Finally, last night, I tested the circuit.  It outputs 28 volts for input voltages over 10v.  Success!  Without tuning it at all,  the efficiency was between 25-60% from 10v to 24v.  These efficiency numbers aren’t very good, but they’re better than the 0% efficiency of the previous setup with the generator hooked up to the batteries directly and with further tuning, my calculations lead me to believe that efficiencies of 80% could be a realistic goal.

Now I’ll go and start that tuning…

Tags: Serial Hybrid Drive
Posted in Project | No Comments »