Hello, There is one scenario I would guard against. I used 4 Headway cells as the 12V aux battery in the 37 Jag. It is hard to say whether my difficulties were specific to just Headway or apply to all lifepo4 cells.
Here's what happened: I charged each of the cells using a single charger and then hooked them in series. After letting them settle I hooked up the 13.8V DC-DC. Everything seemed ok at first, but after a while, the voltages would start to deviate. By a LOT. One cell would be 4.2V and another 2.5V. YIKES! I would re-balance the cells, re-connect, and the same thing would happen. I replaced a few of the cells but it did not help. Eventually one cell would only go to 2.5V even when on the charger and got quite HOT. Since the charging voltage had to go somewhere, it went into heat. This is your perfect fire scenario. Here is my theory. The resting voltage of lithium is 3.333V BUT 13.8V / 4 = 3.45V. This means the cells are constantly under a state of charge past their resting point. Tiny differences in resistance start to add up over hours and days. This causes the cells to deviate wildly (this is just my theory). The solution (in my mind) could be two-fold. 1. Use a BMS to prevent over-voltage/under-voltage if the cells do start to deviate. It should also alert you if your cells are out of whack. 2. A lower constant voltage of 13.3V would keep the cells out of the 'charge' zone and perhaps not 'drive' so much deviation. Since the 12V lithium cells on the market are made up of smaller cells, I would be wary of this scenario and inquire to the manufacturer what they have done to prevent this. Cheers, Marcus On Wed, Feb 13, 2013 at 7:17 PM, Bruce EVangel Parmenter < [email protected]> wrote: > After reading what Bill posted > > http://electric-vehicle-discussion-list.413529.n4.nabble.com/Boeing-787-battery-fire-caused-by-shorted-cell-tp4661251p4661254.html > > I wonder why any other type of Li-ion battery is sold? > It sounds like LiFePO4 is the only way to go, right? > > ... > Now I am curious. I personally have a need to replace a couple of 12V > group 24 deep cycle PbSO4 house batteries. These are not only used for > emergency 12VDC power for an .5 hour @1kW-load, but also start a small > 1kW genset. > > I have tried in the past two 6V traction batteries in series, but had to > put a 12V deep-cycle in series-parallel to be able to start the genset. > > *My question is why not use one 12VDC LiFePO4 battery. My small house > rack can handle one or two group 27, or 24 batteries. > > I was looking at > > http://www.ebay.com/itm/12V-40Ah-LiFePO4-Lithium-Prismatic-Battery-Pack-with-AC-Charger-LP500-/250996143478?pt=Laptop_Batteries&hash=item3a70893576 > > This one comes with a compatible charger. I have seen these in the past, > but would like to know > -are there better ones to consider, > > -has anyone tried these. > > If I did find one, > - should I disconnect the 12VDC recharging leads from the genset as the > voltages will be wrong for LiFePO4, right? > > I request evdl members to post their ideas, corrections, or any other > thoughts on my questions. > > > {brucedp.150m.com} > > -- > http://www.fastmail.fm - Accessible with your email software > or over the web > > _______________________________________________ > 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) > > -- Marcus Reddish *North Valley Systems LLC* Stevensville, Montana 406-360-8628 northvalleyev.com -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130214/3c2fb4dd/attachment.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)
