[dexion]

Well, after some long thinking I decided to upgrade my saturn with lifepo4 cells. My lead pack just barely makes it back (fine for getting there its all downhill) from work so it was either more lead or lifepo4 cells. I am glad (on paper anyhow I went with lifepo4 cells.) I ordered them on monday 10/19 and recieved them friday 10/23rd via fedex freight to my office in detroit. So far about all I have done is unpack them and verify they aren't squishfied. At the moment I am charging them all to 3.5V in attempt to start the pack off as ballanced as I can get. For now I am not going to use any bms solution I will monitor the cells manually like I have done with my TS powered bike. I have several options for a bms in mind if it turns out to be needed. Ah well its only money . I have more of a write up on my blog not too much info so far since they haven't been mounted in the car yet even. Its from a different company here in the usa that I haven't really heard anyone talk with. But after many long conversations with them I decided to take the plunge. I will update the blog (more info on the batteries and why I ordered what I did on the 10/20 post) and here (if anyone is interested here that is) with my experience.




thanks
dex

[ahambone]

Congratulations on going LiFePO4! The most important part of any battery management is the low voltage cutoff (LVC) function. This is where the BMS tells you to stop driving when any individual cell goes below ~2.1 volts. Please be careful monitoring your cells while you use them without a BMS. You are running a big risk of killing one cell or the entire pack.

It can be very easy for you to read a "safe" voltage for your entire pack because many cells are still at ~3.0 volts and one weaker cell drops below 2.0 volts, which can damage the LiFePO4 cell.

Please read everything you can about low-voltage cutoff on lithium cells and monitory your pack very closely. If you are looking for cheaper BMS solutions please investigate either volt blochers or the Goodrum/Fechter BMS kit over at http://www.tppacks.com/products.asp?cat=26 .

(Note: I have no affiliation with the GF BMS or VB BMS. I've built two GF BMS systems from their kits and am satisfied with my testing so far of the units. As I understand it both products work as advertised and can help users from killing cells!)

Cheers,
--Adam

[ahambone]

Udpate: The G/F BMS kits can be bought pre-assembled at this site:

http://rechargeablelithiumpower.com/...alog/index.php

Cheers,
--Adam

[dexion]

I agree dont run them below 2.0 (thats where these are speced 3.65-2.0) but I also WORRY about putting something like those boards or VB's in the environment of a car (dirty, noisey, bumpy, hot, cold etc.) The cells have a three year warranty and aren't very costly to replace after the warranty period as well compared to putting something to manage 43 cells onboard. I just am leary about too many points of failure. I would rather have a battery go belly up rather than have something short on a board every battery is connected to or even have an issue with a VB and have that go up. Sort of the devil you know deal. If I kill a battery there isnt likely to be a fire. I may regret it later I suppose for now I will monitor and test and see what the data points too. Oh, a bit about my commute. I was doing about 200WH a mile with LA i rarely pull over 100amps. I will be driving at about a 50% dod and charging at work so in my case it just may not be needed (like my TS scooter I installed a paktraker just to see that it just wasnt an issue with only a 50% dod run with a charge to full and back again.) I suppose I could pull the paktraker off the bike and put it on 24 cells of the car and watch them but again, its a point of failure. I am sure there are cases a lvc is needed and wanted but if I have a cell that cant handle 50% dod without going below 2.0V I want it to die and be replaced under warranty.

[ahambone]

If you are reading the cell voltages between every drive or every two drives then you will probably be safe enough. I just wanted to make sure you knew the risks. It sounds like you know what's going on.

As for the EV vibration/shock issues for the BMS, there are dozens of circuit boards mounted in every car that withstand road vibration without issue. Just use stranded wire for all of your monitor/balancer connections and package the BMS boards in a rubber-footed box and you should be fine. The same is true with your on-board charger, controller, etc.

Cheers,
--Adam

[dexion]

Heh im so anal about this, every time I stop I read the voltages. During charging I read voltages every hour. If I had a bms I still would, if I had a big lcd screen (that SE wanted to sell me) I still would. Right I would mount everything on shock mounts if I decided to use it. But unless you have "driven" on detroits roads you would not understand how poorly they are maintained. There are many 40mpg roads you cant go over 15-20 mph fo blocks at a time because most of the road isnt there any more. I want to move to hawaii hehe.