Originally Posted by dladd
Why do you say guaranteed? I won't run my cells low. I have an EVDisplay amp counter, and I know how much is left in my cells. My BMS also warns if a cell gets low. When they get low, I'll recharge.
Fwiw, I have not run out of fuel in my gas car in over 20 years. Why would I now do it with my electric car?
I don't have the EV experience of you or many others here, but I do have 6 months of daily driving 20-40 miles per day (with a 50 mile range), and have a BMS that top balances. I'm not worried, and I will not run my cells low. I'll stop and call a tow truck first.
These Lithium cells are SOOO consistent and predictable, it would be hard to be caught off guard on mileage/state of charge, assuming you are using a good amp counter (which is required instrumentation, imo).
anyway... I don't think top vs. bottom is really that big a deal as long as you have built in protection on the other side, but I do thing that balancing is! Top or bottom, but pick one and do it. And make sure you can monitor the other end of the charge cycle. If you are using a shunting BMS (I am) then obviously you need to top balance.
Well, if you drive with a ragged bottom with varying capacities you could easily run the low cell into reversal with out knowing you have done so and it would be done quickly. I have done this. I have about 20 miles range and if I drive 18 miles I will be in range of ruining a cell if I continue to drive further if I have one or more that are discharged more than the others. If you keep your cells balanced on the bottom it won't ever be an issue and the little extra you get at the top is so little that you should not even bother. So bottom balancing is the SAFE way to balance and just allow the top to be a bit ragged. A very safe and simple thing to do. Having voltage monitoring is fine but with lithium the voltages can't give you an accurate of measure of capacity remaining. They are not like lead acid at all.
I say guaranteed because you will at some point drive your car to the lower limits. Yes, you could have a BMS and rely upon them to do their job. If however one module fails the whole system has actually failed.
I see that one is waiting to get his BMS REPAIRED. OUCH. A failed BMS? My point exactly.