I beg to differ with you Cor. Trying to "float" a LiFePO4 cell at 3.45V will definitely overcharge it! The absolute highest voltage a LiFePO4 cell should be held at is 3.40V and 3.38V would be best.
On Thu, Feb 14, 2013 at 1:37 PM, Cor van de Water <[email protected]> wrote: > I would definitely *not* lower the charging voltage > because that simply means that your are running your car > with an almost depleted battery.... > 3.45V float charge per cell sounds perfectly OK, be it a little low > for the normal charging, but acceptable as a longer time float level. > Then simply add a BMS that keeps each cell at no more than 3.45V > and there should be no issues. > (Since the charging current can be excessively large in SLI application, > many Li-Ion 12V replacement batteries come without BMS. > I say that is bad practice - you will have to either influence the > alternator to deliver no more than the allowed voltage, or your > battery should disconnect from the charging voltage if it gets too high > and cell(s) go outside safe values. > > BTW, 4.2V sounds way too high for this chemistry. > > Cor van de Water > Chief Scientist > Proxim Wireless Corporation http://www.proxim.com > Email: [email protected] Private: http://www.cvandewater.com > Skype: cor_van_de_water XoIP: +31877841130 > Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203 > > > > -----Original Message----- > From: [email protected] on behalf of Marcus Reddish > Sent: Thu 2/14/2013 8:36 PM > To: Electric Vehicle Discussion List > Subject: Re: [EVDL] 12V LiFePO4 batteries > > 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) > _______________________________________________ > 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) > -- David D. Nelson http://evalbum.com/1328 http://www.levforum.com Nokia Lumia 920 Windows Phone 8 _______________________________________________ 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)
