Plug option with stock traction battery

Greetings,

I just got the EV mod from Coastal Tech installed. Everything went great - it's an easy and straightforward mod if one is technically inclined.

My ex-wife lives about 0.7 miles from my apartment. I often will nip over there to get the kids, get the thing they forgot, get the other thing they forgot, etc. At 1.5 miles round trip, it's a little too far to do the whole thing in EV mode. It would be nice if I could "fill" the traction battery at home, and make the whole trip in EV mode, and then top off the battery again.

1) Sure, this wouldn't make my Prius a full EV. But it would enable me to save some gas, and more importantly, cut those initial NOx and CO emissions way down. (The worst emissions of almost any modern, properly pollution controlled ICE are right after it starts, with a cold catalytic converter.) And this mod wouldn't be too useful for commuters. But for my large number of short trips, it would save a lot of money and gas and pollution.

2) No, I'm not looking (right now, at any rate,) to modify the usage parameters of the battery. I'd just want to fill it up to 8 bars (80% SoC, I think.) And I'd want the ICE to kick in at 3 bars as usual.

3) Yes, I realize that this won't make economic sense. I'd probably invest $50-$300 in parts for a charger, 2-30 hours in time to get it to work, in order to save the energy equivalent of less than $1 of gas on a single top-off charge. It's more about being able to do it than making money.

4) The biggie. Conventional NiMh chargers run at constant current, for a fixed time, typically C/5 or C/10, where C is the full cell capacity, and then factor in a 60% inefficiency (so they charge for 8 hours at C/5, etc.) What I'd want to do is use a constant voltage charger, set to the voltage of the cell at the 80% full level. That way:

a) There's be no possibility of overcharging - when the cell reaches that 80% charge level, it would be in electrical equilibrium with the charger, no current would flow, etc.

b) I wouldn't have to worry about the initial state of the traction battery. If, by some chance, it's overcharged (over 80%), the charger would actually discharge the traction battery down to full. And the lower the charge of the traction battery, the more current would go in. Of course, I'd have to build in current limiting as a safety mechanism as well.

c) The one disadvantage of this method is that it would be an exponential charging curve, and would take a lot longer to get to the full state than some other form of smart charging. There's probably ICs out there that monitor the charge state and control charging current, but they probably would get their insides toasted by the 201 Volts of the traction battery.

So, I'm thowing this out to the collective wisdom of the Prius Chat group. I have considerable experience working with power and battery charging circuits. I'm comfortable with circuit design, an oscilloscope, a soldering iron, and modding my car. I will insist on building in many safety features, like isolation relays for both the power input and the traction battery connection.

Any thoughts? Any comments? Any previous experience? Yes, I've read about the mods going on in California and Canada with Li-Ion batteries, Argonne's research, etc. Want to know what else is out there.

Regards,
aeronaut