- Power factor corrected
- Weather-proof for mounting under the hood
- Isolated to work with a series pack of batteries
- 6-8 amps charging capacity
- low power enough to put all 12 chargers on one 15 amp circuit (possibly 20 amp circuit)
- somewhat affordable
I decided to go back to the drawing board and see what I could do with the Belktronix charger. In concept, the charging system was great. It's a nice power-factor-corrected series charger that dumps out a solid 8 amps and it's sealed so that bugs and dirt don't get in.
Since I've moved to the Synkromotive motor controller, there aren't any more mid-pack taps off the battery pack, so the batteries tend to drift out of balance more slowly. Arguably, the resistive shunts with the Belktronix charging system should be able to handle this quite well.
The problem in the past has been the charge detector. Without going into all the details of operation, the OVP lines from all the BatMon modules would pulse and cause the FET inside the charge detector to turn on and off. Since this pulsing was frequent, the FET was often in the switching region and easily overheated.
The FET inside the charge detector determines if the large 3-ohm 180-watt series resistor is inserted in the charging circuit. If the FET is on, then the 3-ohm resistor is shorted and the batteries get a full 8 amps, which is great for fast charging. This brings up a problem when the BatMon boards detect a "full" condition on the batteries. The small 3-ohm shunt resistors start to burn up if they have to sustain the 8 amp current flow. Thus, the charge detector needs to turn off the FET to insert the large series 3-ohm resistor to limit the current. If you turn off the FET too early, it takes forever to charge your batteries and the large 3-ohm series resistor gets really hot. Turn off the FET too late and all your shunt resistors burn up, causing massive smoke and potential fires in your EV. (ugh)
Here's a picture of the inside of the blown charge detector. The FET is clearly blown in half with burn marks all the way around it. Note the melted region on the case cover in the lower right.
The FET in the charge detector is a very good one with a very low turn-on resistance. If we can limit the amount of FET switching and turn it on/off hard to keep it out of the linear region, it should stay relatively cool, even with 8 amps flowing through it. So, I put on my EE hat and designed a completely new circuit to put inside the charge detector box.
Here's the charge detector box with the new circuit inside of it. The circuit (shown at the end of this post) uses the comparators and flip-flop inside a 555 timer (Radio Shack special!) to drive the FET with hysteresis on the input to limit switching.
Here's my old Radio Shack 555 timer handbook. It's a crying shame they don't sell these anymore. I guess it doesn't make a profit and people just aren't into dinking around with 555 timers anymore. As you can see (click to enlarge picture), this tiny chip contains two comparators, an RS-flop and an output driver. This is just what we need to observe the pulsing OVP signals from the Batmon boards to determine if we should turn the FET on and off to short the large 3-ohm series charge resistor.
I tend to go overboard with these things. This is my kitchen table with all the soldering/test equipment on it.
Here is the simple circuit I put inside the charge detector box to control the FET that shorts out the large 3-ohm series charging resistor. Note that this lacks a few safety features that would prevent a blow-up if the user hooked things up in reverse, so this is not a product-worthy circuit. It's simple enough and you can get most of the parts (except the FET) from Radio Shack. If you click to enlarge the picture, you can see the theory of operation and get more details.
I'm going to try this out for a few days and see if I can charge faster without blowing up the FET. The potentiometer still probably needs adjustment to properly set the duty-cycle detection on the OVP pair, but this is a good start. If this works, I'll have fast charging without setting the small shunt resistors on fire. Here's to hope...