[AlexanderB]

Originally Posted by RedDevil

I like the idea of a plugin hybrid. So much so that I want to do that too.
No doubt you've seen what Planetaire has done to his Prius.

Yes, I did. = Even if I don't get nearly the range, I do want something like that. Turns out the Prius is an excellent candidate for modding too, lots of info online.

Originally Posted by RedDevil

Unlike the Prius my Insight won't be able to do prolonged 85 km/h runs in EV mode; it will restart the engine within 3 km no matter what. It will reduce fuel consumption considerably, increase boost etc. It will be more fun to drive.

Ah, too bad. Well, I don't "need" 85 km/h (it would be neat to do my work commute full EV if the pack would support it, but I don't "need" to, I mostly want to do the first/last section of the trip, where I get stuck in traffic, to be plug-in electric, instead of idling my engine for no reason.. If theres enough spare capacity to boost the fuel efficiency while on longer trips the freeway, thats a nice bonus for me. The first battery pack I'll add "should" have a range of 10-20 km to satisfy most of my EV needs, if I can get it to 40+ I could do the work commute full EV.

Originally Posted by RedDevil

Be cautious on the 18650 Lithium cell route. There is a big fire hazard if you overcharge or overheat them; they should not be combined in large packs unless very closely monitored and very well ventilated.

I know, I've worked with lithium batteries before. They'll be mounted in a sturdy, well-ventilated fashion, get a BMS, and some cooling fans.
I won't discharge them too hard, I guess somewhere between 1 and 2 amps per cell, shouldn't be a problem.

Originally Posted by RedDevil

Several DIY PHEV hybrids have gone up in flames already.

Oh, thats no good. I should add a fire supression system to the list, and some way to quick connect/disconnect the pack, so I can charge it outside the car.

Originally Posted by RedDevil

(LiFePO4 cells are slightly heavier for the same capacity but much safer. Though even those need careful monitoring.)

Li-ion is already safer than lipo, and I've never had a problem with those either. The problem is that good batteries are expensive new, and almost none are sold second hand..

Used 18650 batteries are easy to source from "dead" laptop packs (where the BMS in the pack is either broken or 1 or 2 cells out of 6 are dead and as a result, it has almost 0 usable capacity in a laptop, and is considered "dead".
Even if you count a generous portion of the cells as dead, you probably still end up well below $2/cell foor good quality but slightly worn cells. Who cares if they're "only" 2000~2300 mAh from their original 2500~2800, if they're quality cells from an OEM pack, they'll still last a pretty long time before they become so bad they have to be replaced.

Originally Posted by RedDevil

OTOH they can be had very cheaply, like these: USD $ 18.99 - ICR 5000mAh 18650 Li-ion Rechargeable Battery(10pcs), Free Shipping On All Gadgets!
I'm toying with the idea to buy a bunch of those for a proof-of-concept test pack.

Yeah, well.. no.
1: 5000 mAh 18650 cells don't exist, they just put a very optimistic label on a crap battery.
2: cheap 18650 batteries are usually pretty crap quality, high internal resistance, low capacity, short lifespan, or all of the above.
3: for the same (or lower) price, you can get the good cells out of "dead" laptop packs, they'll be high-quality cells, even if they're a little worn, and you have to sort the dead ones out from the good.
4: I should talk to the local recycling company to see if they can provide me with free laptop batteries. (They probably won't, but who knows.) Buying them overseas from ebay seems like a silly thing to do, even if they're cheap enough to be worth the effort.