Hi Guys
BMS's - the problem is the load that they put on the cells
Not so much as in stealing capacity but as in what happens if you go away on holiday and your machine is left alone
The past problems have been
The BMS draws down one cell - to failure - over a month or so
When you charge your pack the BMS has died - so you end up charging the entire pack at too high a voltage - That is when people have had fires
If you can get the OEM BMS to work then you can ignore my worries but the aftermarket units at the start caused more problems than they fixed
If you decide NOT to use a BMS then you need to
Balance your cells - either Top Balance or bottom balance
This consists of charging your cells above the usual voltage (or below) and then connecting them in parallel
You need to go above the usual voltage as the voltage/charge slope is too shallow below that point
That gets them all in the same place at either the to (or bottom)
You then decide the voltage that you are going to go to - for the Volt cells I decided 4.05v
So a Volt 2 kWhr pack which is 3P12S - 36 cells arranged 3 parallel and 12 in series
That means that I would charge to 12 x 4.05 v = 48.6 Volts
I would discharge to 3.5 v = 42 v
I'm actually using 84S (6 off 2kWhr packs and 2 off 1kWhr packs) - so I charge to 340 V and discharge to 294 V
This does NOT get me the whole available capacity - but it does give me some margin at each end of the voltage
I was going to make myself a charger but in the end I bought a BRUSA - A guy on the site was selling them - I think $400US - which can be set to switch off when it gets to the right voltage
The other thing that I do is do a check to see if all 84 of my cells are at the same voltage - with a Volt pack you can modify a cheap "Cellog 8- to connect to the Volt BMS wires
Well worth reading this thread
http://www.diyelectriccar.com/forums/showthread.php/2012-chevy-volt-battery-93101.html
I checked my cells were all the same - then charged them - then used them for six months - then checked again
Another six months - checked again
Always within 13mV of each other
And I am BAAD to my battery - my controller is set to 1200 amps and when I'm on the track I use all of that
From what I have seen if a cell is going to die (I killed some Headway cells - and some just died) then the BMS won't rescue it all it will do is tell you that it's dead
And you DO need to know that it is dead before you kill your whole battery!
IMHO the Batt Bridge will tell me when a cell has died - and then I will have to react - but a BMS would do exactly the same
When I was using my Headways I used my Cycle Analyst to measure how much I had taken out of the battery in Amp Hours - that was my fuel gauge
The Chevy Volt cells have a higher voltage/charge curve than the aftermarket cells so I simply take my foot off the throttle and look at the voltage to see how much I have got left in "the tank"