Quote:
Originally Posted by IamIan
.... if the IMA system ever asks for its peak of ~100 Amps of Assist... you could end up with ~50+ Amps being asked to come from the second battery pack and travel over the wire you have connecting between each of the 20 subpacks. ...
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If you think of each battery pack in R/C battery pack terms as 120S (120 cells in series, none in parallel), the heavy duty end cables and the balancing harness connector from one 120S pack would be connected to those of the other 120S pack in a way that would balance each pack's cells via connected pairs rather than cause each to instantly burst into flames (correct polarity in other words). As the two packs would sit with vehicle parked and off, those paired cells (subpacks in our case) would be balancing each other at an equalized voltage. Each pair would have an unavoidable voltage difference from the other pairs (which I was hoping the mass of zener diodes home charging arrangement would help address, but I understand now why it wouldn't be reliable enough), but the goal would be to have cumulative subpack imbalance issues cut in half to insignificance via the linked pairs.
If there's a serious voltage difference between two paired subpacks for whatever reason, under full and sustained assist, a significant current >15A could flow across, but wouldn't that require a radically degraded cell?
But it is all on the assumption that voltage equalization between pairs will significantly help reduce charge capacity degradation, based on great apparent emphasis with lipo and li-ion R/C packs on maintaining cell balancing to keep them from thermal runaway. In the R/C world I assumed NiMH packs don't tend to have balancing connector harnesses because of their relatively low-end nature vs. lithium ion or polymer packs... perhaps not worth it from a cost perspective. But 120 NiMH cells in a singe string being expected to stay balanced with such high charge and discharge currents?!?! I was figuring they could use all the help they could get, besides sharing the load with another string of 120, and assumed that would help!
Quote:
Originally Posted by IamIan
Also the Voltage seen on a battery terminals fluctuates away from the electrochemical potential of the battery the faster / higher the current flow... if you charge with 10 amps the voltage goes up... more than it should just from ohmic forces ... if you take the 10 amps of charging off... the voltage begins to fall back down as the chemical energy inside balances out and catches up.... some of the voltage change is from ohmic forces... but not all of it.
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Does that voltage change (+ under charging, - under discharging) increase or decrease with cell degradation? If it increases, sort of like a lead-acid without enough water, a balancing harness arrangement would help prevent/delay an over-volt condition with the bad cell under full sustained charging and prevent/delay excessive discharging (and perhaps the voltage reversal condition you mention) under sustained full assist.
Regards,
Roger
