Optimizing pack use versus plug-in configuration

This is a continuation of the discussion starting on page 11 of jdh2550's YAPiP thread. It started with the idea is that owners who keep the stock NiMH pack may want to isolate it after an add-on pack has discharged past a certain point because of the costs associated with using the add-on pack in certain charge-sustaining mode (eg electricity from gasoline costs ~$.35c/kWh, while electricity from the wall cost ~$.12/kWh or less, so from an economic perspective it's advantageous to have the stock pack/system take over after the added plug-in pack has depleted itself).

There are also other considerations, like pack lifespan/cost if it experienced the higher C-rates the stock pack sees. Something like this could even extent to optimizing system design. If someone could use a Winston/stock pack instead of just an A123 pack for half the cost (The Winston pack could supply power at the lower 1C/2C rates and the stock pack could take up the slack), I think that could be worthwhile justification for pursuing this idea.

Here's a start to the discussion.

1.
Q-What is the most likely failure mode of a two pack situation and what kind of problems are we looking at due to failure?

A-

Q-Could having two packs prevent other failure modes?

A-Probably. If the PHEV pack had a cell or few go bad, the battery controller should have something that tells the stock controller/pack to take over if the PHEV pack voltage drops too much.

2.
Q-How much more in the way of hardware/cost would it be?

A-It would be anywhere from additional contactors to who knows what.

Q-How much more in the way of software/time would it require?

A-

3.
Q-What would we use each pack for and why?

A-We would continue to use the stock pack because it's less costly than using the add-on PHEV pack during normal hybrid operation. We may also use the stock NiMH pack in certain situations to avoid degradation of the PHEV pack due to high C rates. The disadvantages of this are retaining the weight/space taken up by the stock pack, more hardware, and designing a more complex PHEV battery controller than the one pEEf/jdh2550 designed/is designing.

 

Someone could bypass the stock pack using pEEf/jdh's method once that's up and running, or via the hybridinterfaces approach, then have a contactor isolate the add-on pack once it's voltage/charge drops too much so that it wouldn't be charged through normal hybrid operation.

Another approach would be to design an ECU that could selectively use both packs depending on the conditions, so that the add-on pack wouldn't see the high charging/discharging rates the normal pack sees on top of being isolated once it's voltage/charge drops to some preset level.

 

I was under the impression the HI pack just initially connected the packs then isolated them once some SOC was reached. Has the designer updated it to selectively use one or both packs depending on the discharge/charge rates?

 

Doh! Totally forgot about that. In that case it's a moot point outside of which pack can better handle higher charge/discharge rates and costs more to use for energy storage.