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Old 06-19-2009, 05:02 PM   #7 (permalink)
Join Date: Feb 2009
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Some more thinking out loud about controller logic...

i guess it would still work of the two battery voltages are the same - you can program the micro to just limit the current from the large pack when it increases above 70A.

The controller would start out 100% on so that both battery packs are at equal voltage. (if they start out equal, there should be no current flow).

The two packs will supply their own respective currents as power is drawn (maybe proportionally to internal resistance?), but the voltage of the two packs will remain be equal as long as the current is less than 70A on the large pack.

The controller will limit large pack current if it increases above 70A forcing the small pack to supply the rest.

it seems like the small pack will drain faster until its resting state voltage is lower than the large pack - i guess it'll be charged by the large pack at this point?

it seems like an involved problem to work out in my head, but i guess it comes down to voltages. Keep the voltages on the respective packs within their respective limits and all is good. I can't say how performance would be... I think unless you have a significantly high voltage difference between the packs, you'll end up sharing load between the packs.

Now i'm thinking about the 70A figure... with my 144V pack, cruising at 45mph requires 50-70A and 65mph requires 80-100A. Lets say you need 70A from your battery system at cruise - if all 70A comes from your large pack, you won't be able to charge your small pack. If you need 100A at cruise, 70A will come from the large pack, and 30 from the small (assuming everything is setup to allow for this).

Are you intending to charge the small pack while cruising or just while standing still? or are you typically traveling at speeds that allow for it? I imagine your current draw is a little less, but its gotta be in that ball park...

anyways, just more thoughts...
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