Quote:
Originally Posted by crx_rogus
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.
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The 120S pack reads like it is connected in parallel to the second 120S pack ... with + to + ... and - to - ...
Think of it like this ... the 120 cell series pack has the IMA system ask for ~100 Amps of Assist... all cells in series may not have equal voltage but they all share equal current... that means that as long as the 120 cell series pack is being discharged at ~100 Amps ... then every single cell and subpack is also being discharged at ~100 Amps... if your secondary battery is connecting its + terminal to the + terminal of the OEM battery and the - of your secondary battery is connecting to the - terminal of the OEM battery than you are connecting in parallel... If both of those parrallel subpacks have equal Voltage , equal SoC, equal Internal Impedance / resistance ... than 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.
Quote:
Originally Posted by crx_rogus
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?
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no.
Once you connect the + to + and - to - of each battery together you have connected them in parallel and you could see ~50 Amps or more trying to go through your wiring from the secondary battery... any time the system asks for ~100 Amps of assist.
Unless you have some type of current control to limit the current flow.
The lower amount of current ... would only be the case if the secondary battery pack was not connected while the IMA system was running in the car.... every time you start the car or had it running the secondary battery would have to be disconnected.
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.
Quote:
Originally Posted by crx_rogus
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!
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You are correct a balancing system would be a benefit .. I have tested several packs that are out of balance.
The issue is how does that balancing system ... and connections react when the IMA system tries to operate for Assist and regen.... and what is the worst case scenario to build / prepare for.
Quote:
Originally Posted by crx_rogus
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.
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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... if a NiMH cell vents it electrolyte it can also experience a similar effect to the PbA that lost its electrolyte ( water ).
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.
