Sunday, June 21, 2009

A New Hope

After the utter failure of the Soneil and the Joule chargers, I did some more research on individual battery chargers. Ideally I was looking for the following:

  • 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 did some more searching and found the BatteryMinder 12248, the Xantrex Trucharge 10, and the Waeco PerfectCharge IU1012. The Waeco units seemed really good but were from Germany with no distributors in the USA. The Xantrex and BatteryMinder units were $150 apiece (ouch!) and didn't meet all the criteria above.

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...

1 comment:

martinwinlow said...

Martin from the UK, here. You may recall I was having issues with my Charge Detector too.

I have gone with the G2 charger upgrade option and am waiting for it to arrive. In the end, my 20kWh pack really needs the extra charge rate to allow it to charge fully within a half-reasonable time (8 hours) and allows all the charging to be done overnight during the 'off peek' time which costs 1/3 of daytime peak rate electrcity (off-peek = 6p/kWh in the UK).

Meanwhile, I too have modified the G1.5 system I have by adding a simple opto + transistor inverted amp to the OVP output (from the IsoBatMon) which allows it to switch off a relay that was latched on by a push button at charge commencement. So after an initial 'bulk charge' the relay switches in the big resistor (20 ohm in my case) and reduces the currant from around 10A down to around 1A.

This is low enough for the LionMon's 4 ohm by-pass resistor to cope with when the cell reaches the 3.85V OVP threshold allowing for cell balancing after the bulk charge phase. Further, even when all the cells are up to 3.85V, they can sit there happily indefinately until you switch of the charger. Just wastes a bit of juice. I'll email you a schematic of the circuit if you are interested. Just let me know.

I'm probably going to use a 556 as a dual flip-flop to reduce the frequency of my DIY tacho sensor which is a top-dead-centre sensor out of an ICE with a home made clamp on shaft collar with a couple of magnets embedded in it.

Meanwhile my van is getting me 20 miles to work and back quite happily - and saving me £4 ($US6.40) every time. Only another 10 years to go before I break even! MW