Sacrificial cell - the easiest BMS there is ????
Just want to bounce around an idea here.
It's based on these general "impressions" which I have gained from my 2 year peek into battery technology:
Impression A) People want more range than what the currently available batteries can provide.
Impression B) Batteries last much longer if they are being cycled in many shallow cycles, rather than frequent deep cycles.
Impression C) Batteries get damaged by heat caused by a variety of "mechanisms". The worst damage is probably done by heating up excessively from the inside out, particularly by exceeding maximal charge and/or discharge rates, and over-charging and over-discharging.
Impression D) You need high voltage batteries to keep cable sizes and resistive losses low in the overall system. More volts generally mean more "Ooomph" and more speed. Unfortunately, this also makes it very dangerous.
Impression E) To get more Ooomph you need many cells and that means a complex and expensive Battery Management System (BMS).
Impression F) Batteries generally like it to be used to no more of 50% of their capacity, for both peak current draw and total Ah drain.
Impression E) There is always one cell that bites the dust first - depending on circumstances, it may then take the others down with it...
Now to the conclusions from these impressions:
1) Combining A) + B) +F) ===> In order to get good battery-lifetime range, the available trip range needs to be reduced even further - by about 50% ! Not a popular idea, I'm quite certain...
2) Because it is always one cell that goes down first, you might as well make sure you know exactly which cell this will be. If you can be certain that this cell will be the one that gets over-charged or over-discharged before all others and that it suffers more heat damage than all other cells, then you only need to monitor this single cell! That's the idea. Quite simple.
The proposed solution (possibly hogwash!):
Place a sacrificial cell into the battery string, monitor only this cell. The operating conditions for all other cells should then always be within the parameters conducive to maximising the lifetime energy delivery of the battery.
Here is an example to illustrate the idea:
Imagine a 40s 40Ah battery.
Instead of monitoring every cell, place just one 25Ah cell into the series and monitor this cell constantly.
When it dies (and die it will!), replace it with another 25Ah cell, again and again and again, until the pack has deteriorated down to maybe 30Ah.
Once the pack capacity has deteriorated significantly, despite the very gentle treatment it is receiving, you could either get a new pack or use an even lower spec sacrificial cell to keep it going.
Needless to say, older cells of the same product line might be perfect sacrificial cells. Down-cycling at it's finest!
What say you?
It sounds like an idea, it's probably a way to preserve a good pack, but..
-wouldn't you have to limit the max amp/amph draw to what that 25ah cell can deliver? (loss of "ooomph"/range)
-wouldn't there be a possibility of reverse charging, just like with a dead cell?
-if another cell is dying due to any reason, you still wouldn't know I think.
that's why I was thinking.. isn't there a device that can do what a good BMS does (among other stuff): measure each cell, and report in any way?
basically a volt-meter with 40 probes (or whatever, i'd be happy to wire them to each cell) and a display or usb port for read-out.
we'd just have to check once in a while..
does anyone know about such a device?
battery surgery every 10,000km sounds like a pita.
I'm hoping my 40AHA Sky Energy cells have enough capacity to cover my normal commute at less than 80% DOD.
With 42 cells, the stock Vectrix controller should limit charge and discharge to acceptable levels, with the EVWorks BMS as back up.
At 60km per charge and the manufacturers claim of > 2000 cycles, I'm looking at 120,000 km. but I think that cycle life refers to 0.3C "standard discharge"... 30,000km wil pay for the cost of the pack compared to my alternative transport. I guess we are alpha testers here :)
I think you are right in the points in your first post.
Basically, current battery technology is not yet upto consumer demand/expectation.
For EV's to become mainstream, either they need to get better or ICE cars become less viable.
Sacrificial cell BMS might be the way to go for backyard tinkerers, but an EV consumer won't tolerate less range,less performance and higher maintenance costs for the sake of battery longevity.
if the sacraficial cell is to be used to limit discharge in Ah (and possibly peak in A), there is another way to do it.
for the vectrix, changing the measured capacity to say 20Ah would achieve the same thing, although you wouldn't have to tear into the pack every 10'000km.