Michael Ross wrote:
I think y'all are getting off track covering Marcus comment. He was wondering why BB'g can't be done with the cells in parallel. My answer was basically me BS'ing, because I haven't really figured it out yet myself.
I think you can bottom balance by simply connecting all the cells in parallel, as long as you discharge them low enough to insure that they are all at (approximately) 0% state of charge. That would be around 2.75v.
You don't want to go higher, or the spread in their individual SOCs will be too large. You don't want to go lower, or you risk damaging them from excessively deep discharge.
Bottom balancing reduces the need to monitor when the first cell goes dead. Ideally, the whole pack goes dead at once, which stops you from driving (your controller will probably shut down before you murder them all).
But, it doesn't do anything to prevent overcharging. Some cells will reach "full" before others. Without a BMS, you're depending on your charger to be smart enough to stop before the first cells to reach "full" get damaged while the rest of them are coming up.
If one insists on having no BMS, I think something like my Batt-Bridge is essential. See http://www.sunrise-ev.com/LeesEVs.htm#battbridge It's basically an "idiot" light, like an ICE's "check engine" light. It just tells you that *some* cell somewhere in the pack is at a noticeably different voltage than the rest. You won't know which one; but at least you will be WARNED to stop and check it out before you murder the pack!
I think the general advantage of having no BMS by way of BB'ing is...wait for it - not having a BMS.
Well, you *will* have a BMS no matter what. *Something* has to prevent overcharging and excessively deep discharges. Either it's automated (done for you), or manual (you monitor it yourself). If neither of these is done, then you're likely to murder your pack well before its promised life.
There is a certain appeal to having a pack that will last nearly forever and requires no hardware management at all.
This can be achieved, in fact. Use a pack of Edison (nickel-iron) cells. He made these cells in the 1920's, and examples can be found that are still working today! Just charge and drive. The only maintenance is to add water as needed.
I probably won't achieve that with my salvage pack of mongrel cells, but I think it may be possible with good quality new cells. It is elegant (if it works). What I learn from this I may apply to building a pack like that.
You can get by with used, surplus, or mismatched cells as long as you have a BMS (manual or automatic). A BMS lets you get a little more life out of them, and avoid further degradation caused by balancing issues. I have used mismatched lead-acid packs well beyond their normal end-of-life by monitoring and balancing them (both manually and automatically). This extends their life, and lowers your cost per mile.
Perhaps the biggest advantage of a BMS is that it lets you *see* what is happening. Ignorance is bliss. It's easy for people to happily believe their pack is perfectly balanced as long as they never measure anything. But when they do, they often discover surprising voltage extremes, particularly while driving or charging!
For your experiments, at least set up a way to check the individual cell voltages. It can be as simple as a set of switches and a meter, as shown on my Balancer web page. Experiment! Learn for yourself what's true, and what is just talk.
-- ICEs have the same problem as lightbulbs. Why innovate and make better ones when the current ones burn out often enough to keep you in business? -- Hunter Cressall -- Lee Hart's EV projects are at http://www.sunrise-ev.com/projects.htm _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
