Quote:
Originally Posted by IamIan
... the Total Current of ~100 Amps would be divided equally between both of them and they would each be discharging at a ~50 Amp rate... and your connecting wire will see ~50 Amps... if your secondary battery ever has a higher SoC... or a higher Voltage ... or lower Ohms from resistance or impedance... your secondary pack could end up seeing more than ~50 Amps out of the ~100 Amps the IMA system was asking for.... or if it has a lower voltage , SoC , higher Ohms ... it might see less % of the ~100 Amps of Current.
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I'm still not getting why every single balance cable would see such assist current vs. the 100A -rated outer main cables that see the full ~144V potential, but you cover other needs for hefty balance cables well later.
Quote:
Originally Posted by IamIan
...As long as the secondary battery pack is never connected while the IMA system is in use... And as long as each 6 cell subpack is only connected to another 6 cell subpack then ... the worst case would be dV of 8.4 - 6.0 = ~2.4V dV subpack to subpack... with as little as NREL tested minimum 120 cell pack of 0.36 Ohms for all 120 cells... 0.36 / 120 = 0.003 * 6 cells in series = 0.018 per stick one OEM +1 Secondary = 2 sticks whose Ohms add = ~0.036 Ohms minimum ... for up to a worst case of ~66 Amps... without the IMA system operating... with the IMA system operating you can also be subject to a % of the ~100 Amps as discussed above.
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That drives home the loads balancing cables could easily see. I was hoping keeping cells paired together would prevent such dV rises, but equal no-load cell voltages =/= equal cell health or SOC levels.
Quote:
Originally Posted by IamIan
The artificial Voltage rise from non-ohmic forces has to do with the chemical reactions inside ... and that the rate of chemical reactions is slower than the rate of current the electronics can apply... as a cell degrades it's capacity drops so it voltage rise with SoC increases... while its voltage rise due to Ohmic forces reduces ... because NiMh cell fail as a short circuit with nearly 0 Ohms ... unlike PbA or Li that fail open circuit with lots of Ohms...
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That could be enough to cancel my balancing harness idea altogether, since that happening in a cell would force a permanently excessive charge in the rest of its subpack and a permanently reduced charge in the paired subpack. It also explains how stringing 120 cells won't fail catastrophically with failing cells getting via increased ohms most of the ~144V charge voltage all by themselves; they just short themselves out of the way of the others.
Quote:
Originally Posted by IamIan
...An easy analogy is that of cooking a turkey ... the chemical reactions only happen so fast... if you increase the temperature / rate of energy flow ... you are more likely to burn the outside of the turkey while the inside is still frozen... chemical reactions are just slower than the rate at which energy itself moves... in the form of heat or in the form of electricity.... the electrical current will use the path of least resistance inside the cell... and those molecules along that path of least resistance will get pushed with allot more energy than they can use for their chemical reactions ... the energy will defuse out into the rest of the cell... but it just takes time for those other chemical reactions to catch up.
.. as the rest of the cell catches up the artificial non-ohmic Voltage change levels out.
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Thanks for the very clear explanation/analogy. To avoid possibly driving you bonkers I've ordered
Batteries in a Portable World to help me have a hint of a clue regarding best use and maintenance of common battery types (I use NiMH and deep-cycle PbA every day already and LiPo occasionally).
Regards,
Roger
