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Originally Posted by IamIan
... each of those 6 cell sub-packs might not react exactly the same to the same dV... one might provide ~40 Amps... the other ~60 Amps... etc... for the OEM series connection it doesn't matter because the net out of that parallel connection is still the same ~100 amps it is asking for from the series connection.
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I understand about series vs. parallel (I've monkeyed with electronics since I was wee), but I didn't think there would be that big a dV across a pair of subpacks that had just spent at least the last several hours or so balancing each other in a no-load situation. Now I know.
Quote:
Originally Posted by IamIan
... as the failed cell's OHMs go up but the series connection forces the same AMPs of current through it the dV and heat from that failed cell rise drastically... and bad things can happen.
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That's exactly why I assumed long strings of battery cells of any type are a bad idea, not realizing NiMHs ever so conveniently short themselves out of the way.
Quote:
Originally Posted by IamIan
... The expensive Li packs for EVs and such often use a BMS that will disconnect a bad cell from the series and bypass it ... so that the series current does not keep going through the rising OHMs of the bad cell... or they sound an alarm , or turn off the system.
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and thus probably avoid having a $40k+ car do the same thing Sony and Dell laptop computers did awhile back... have their lithium-based battery packs burst into bright, smoky "thermal runaway" states taking the value of whatever they're in with them.
Quote:
Originally Posted by IamIan
... NiMH is different ... sense it fails toward 0 Ohms a failed cell weakens the whole pack... but does not cause the major problems a failed PbA or Li cell can... but NiMH still benefits from a BMS as well.
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It would be nice for the Insight to have something besides a way to gradually de-EV upon noticing subpack thermal issues, something to help counteract any developing out-of-balance issues.
Quote:
Originally Posted by IamIan
Now a separate issue from cell failure ... is cell reversal.
Cell reversal happens when the cell has not failed ... but has reached it 0% SoC before the rest of the series connected battery pack has... because the rest of the series connected battery pack will continue to force equal amounts of AMPs through it ... you begin to charge that cell in reverse ... so the voltage of the cell reverses ... the + terminal becomes a - terminal ... etc... reversed charging of a cell has allot of OHMs so it produces allot of heat and dV... it also is very bad for the battery cell, no matter what chemistry it is... voltage reversals should be avoided ... bad things can happen.
Some chemistries are more or less tollerant of cell reversal than others... I have pulled NiMH cells into voltage reversal several times... it damages them and reduces their capacity ... but it isn't an instant death sentence ...
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Hmmm... NiMH cell failure = shorting itself out of the way, yet cell reversal can lead can eventually lead to exciting things possibly involving release of a cell's magic smoke (does it eventually short itself, or keep its radical overvolt condition?)... One issue with Insight 120S packs then is, I'm guessing, overcharging leads to relatively benign cell failure while chronic discharging getting out of hand can lead to catastrophic cell reversal.
I'm also getting the impression that the Tesla Roadster among approaching others must have some astoundingly sophisticated BMS hardware and software, with a significant cost of the vehicle development just going into BMS development. A Roadster stranded with a flaming battery pack would produce unproductive
PR, especially in the courts. My multicharger alone has all sorts of warnings about choosing the correct Li
x setting stemming from the 0.1V cell potential difference between LiIo and LiPo.
Quote:
Originally Posted by IamIan
No problem at all... Batteries are one of my hobbies... a NiMH is still my personal favorite for most applications... some of the newer Li have been able to match or beat NiMH for cranking power... and Li has better energy per L or energy per kg ... but the tolerance , durability, and cost of NiMH still have many benefits over Li.... that might change eventually... but for now... what battery type best meets an applications need still depends on what those needs for the application are... PbA for instance ... with it's low $ per Wh ... beats the crap out of Li for stationary applications where weight or volume don't matter much.
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PbA works well for my home's hobby-grade solar-recharged 12V subsystem (powering the 3 D->C cycle slow refreshing of the spare pack's subpacks effortlessly), and has reliably and safely and without stink (except near the recharging racks) moved countless tons of paper, ink and anything else imaginable for at least a couple decades at work, but getting folks to remember to recharge the battery, not the drive terminals, on some of the smaller equipment is always a challenge.
Quote:
Originally Posted by IamIan
There is tons you can find out there on batteries ... so enjoy.  |
I've been gradually reading V2G-101, learning how V2G-enabled versions of cars like the (apparently truly heading towards real production) Chevy Volt (and current Tesla Roadster) will (/can) earn their owners / leasees up to about half their new cars' monthly payments back via buffer service micro-contracts with the local grid utility. But first, we need a smart grid. Flywheel, hydraulic and pneumatic -based hybrids also appear viable. There is indeed alot of really fascinating stuff going on out there, mostly off-topic for this thread.
Regards,
Roger
