Wednesday, October 26, 2011

Resolving Overheating Issues

I have started driving on the highway in 4th or even 5th gear instead of 3rd. This reduces the engine noise and transmission vibration so much that now I only hear the wind and other cars when I am cruising down the road at 70mph.  The only problem with driving at a lower RPM on the electric motor is now my previous fixes to the controller overheating issue are no longer sufficient. 

The controller has a fancy circuit which at low RPM acts like a DC transformer.  It takes a mere 100Amps from the battery and delivers 400 amps to the DC motor.

"Young lady, in this house we obey the laws of thermodynamics!" -- Homer Simpson

I assure you, no conservation of energy violations are going on here.  Just a practical application of Maxwell's equations being put to good use.

The DC transformer-like feature allows the motor to still provide plenty of torque without putting excessive strain on the battery pack. I assume this is also why I am getting extended range lately. 64-miles on one charge is my new distance record.  

I found an utterly massive heat-sink on eBay for a mere $64 delivered. Weighing in at 17 lbs, this aluminum behemoth would probably keep itself at a constant 74 degrees.  

After swapping out the 4 lb heat-sink for this much larger one, I have yet to hear one complaint out of the motor controller regarding temperature issues. With this improvement, the next weak link in my electric truck is the massive 11 inch diameter, 178 lb Kostov DC motor itself. It will get pretty warm after 20 minutes of extreme, hard driving.  I now consider the overheating issue resolved.

Battery Charger 2.0

I beefed up the wiring in my home-made battery charger and also added better capacitors that have a lower ESR (series resistance that robs power, making things get hot).  Before I made these changes, the charger could deliver up to 18 amps at 120 Volts and charge up the battery pack in 12-15 hours. Now it draws 30 amps at 240 volts and will charge up the battery pack in 5-7 hours.
Now I can come home after working late, plug in for an hour to add a quick 8 miles to the range of the truck, run a few errands in the evening, drive out to see a movie, come back home at 10PM, plug in all night and by 5AM, the truck has a full charge, ready to take me the 40 miles back and forth to work. Nice!

People of the world! Why aren’t we all driving electric cars? They are efficient, quiet, non-polluting, cheap to operate/maintain and super fun to drive.

Automakers of the World! Why aren’t you mass producing electric cars. Quit focusing on $45K - $100K custom cars and start cranking out the electrics as if you’re making Model T’s. If you build them we will buy them.

Oil Companies of the World! Your days in the fuel business are numbered. The only oil you’ll be selling will be for making molded, plastic seats.

OK, enough of the soap box!  Where was I?  Oh yeah!  Battery chargers.  

My capacitive battery charger is still not perfected but it's a whole lot better.  I have not as of yet added any overcharge protection because overcharging lead acid batteries occasionally, brings all cells back to the same level and equal playing field.  By using a capacitive, pulsating DC charger, the batteries are also desulfated and reconditioned.
Most of the time I can schedule it into my day to plug in and manually unplug before I leave, but yesterday was a weird exception where I plugged in later in the evening.  By the time I went to bed 4 hours later, the battery pack was only 70% charged.  I didn't want to leave it on all night because 10 hours would overcharge things too much, heating the batteries unnecessarily and waste power.  I plugged in to 120 volts for the rest of the night.  The battery pack ended up not getting a good enough charge and I almost didn't make it back home after driving 18 miles on the freeway with a 20mph headwind.  

I still have some work to do before I will be satisfied with the charger, but it's not bad for a home-made charger. I am amazed at how simple the capacitive charging circuit really is.  2 components:  A capacitor (made of several caps actually) and a bridge rectifier.  It cost me about $50 for eight large surplus 50uF 440VAC capacitors at NPS and a couple bucks to my good man in Hong Kong for the 50 Amp bridge rectifier.  I am also using the hell-freezer heat-sink for the rectifier as well.  

Update:  10/28/2011  Don't expect cheap semiconductors from Hong Kong to last more than a few days.  The 50Amp 1000 Volt bridge rectifier went off like a firecracker.  I am suspecting the 1000 Volt component was counterfeit and was more likely a 100 Volt one.  The 2nd one I bought as a backup only lasted about 30 seconds before it started making crackling noises and shorting out internally.  I replaced it with  my original, non-counterfeit rectifier.  It runs slightly warmer and less spectacularly.  

The down side to the capacitive charge circuit is the power factor is not very good.  With a power factor of 1, it would be possible at 240 Volts and 30 Amps to draw 7200 Watts.  My circuit, with it's power factor of only 0.33, pulls nearly 30 amps at the beginning of the charge cycle but only delivers 2500 watts.  A far cry from 7200 watts the circuit is capable of providing.  Hmmmm.
Remember, even though I am pulling 7200 Volt-Amps from the power outlet, I am only paying for 2500 watts.  Even my solar panels only have to cough up 2500 watts.  We must be vigilant against the trickery of 
power factor.