Thanks Jamie and Ray, you have given me something to add to my teaching curriculum this is why i have had better luck with flooded batteries, overcharge at a slow rate is not as bad as overcharge in sealed batteries. DArryl
--- On Mon, 12/7/09, R Ray Walters <r...@solarray.com> wrote: > From: R Ray Walters <r...@solarray.com> > Subject: Re: [RE-wrenches] Strings and series of batteries with reverse > return linkup > To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> > Date: Monday, December 7, 2009, 1:59 PM > This is an excellent > point that Jamie is making here. On electric vehicles, we do > see up to 156 vdc series strings, and even with just a > single string at 48 vdc, the separate cells are not being > charged equally.Current is equal, but not the voltage > across each cell. > It's alarming to see some batteries (especially > sealed ones) going way over charge, while others are > undercharged. The controller only sees that the total > voltage is at full charge, and then shuts off. I've > pulled individual batteries out to charge separately. I > think we really need smarter charging systems (similar to Li > ion) that shutoff individual batteries or small groups to > allow others to fully charge without overcharging > others.Just imagine seeing a new expensive sealed > 12 v battery hitting 16.5 v, while others are still below > 13.5v. It'll make you sick to your stomach, hearing it > gas, when it's not supposed to.HIgher charge > currents make the problem worse.Here's a > regulator from Manzanita Micro that can do > it: http://www.manzanitamicro.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=46&category_id=17&option=com_virtuemart&Itemid=64I've > worked with Rich Rudman there for years; he's very > knowledgeable and good to work with. It might be a good > idea, especially on large sealed batteries. > > R. > walters...@solarray.comsolar > Engineer > > > > > > On Dec 7, 2009, at 12:21 PM, James Surrette > wrote: > > Hi Darryl, > > > > To further on > your comments (I hope); > > > One thing > we've noticed that is an often overlooked (potential) > problem. Even with a simple series sting (6, 12, 24 > cells), the individual cell voltages are never identical - > meaning initial charge acceptance is never identical. > When you initially "activate" or > "commission" a bank, assuming 48VDC you begin to > charge at 57.5-60V. The overall voltage is attained > relatively quickly but the individual cell voltages can be > shockingly different (2.2 - 2.7V). The commissioning > is complete when all cell voltages "balance" to > the charging voltage, i.e. 2.4V. As series > voltages increase (120VDC+) or multiple banks are used, the > initial commissioning becomes increasingly more important as > overcoming imbalance within the bank become more difficult. > The most efficient commissioning occurs when low > current is applied (2% of 20hr) until cells balance. > The point to the comment, you might get away without > balancing / commissioning / activating a series string but > is imperative if using multiple parallel banks. > > > > Regards, > > > > Jamie > > >>> Darryl Thayer <daryl_so...@yahoo.com> > 12/5/2009 9:31 PM >>> > This also assumes all the identical batteries are at > identical temperature, and it one is slightly warmer or can > not lose heat as fast, if being charged it will get > warmer. the warmer battery will do what? have a lower > vltage, have higher current and receive more heat, and get > warmer, this is significant. I have never seen a > battery set tha all batteries have the same thermal > conductance to ambient > Darryl > > --- On Sat, 12/5/09, boB Gudgel <b...@midnitesolar.com> > wrote: > > > From: boB Gudgel <b...@midnitesolar.com> > > Subject: Re: [RE-wrenches] Strings and series of > batteries with reverse return linkup > > To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> > > Date: Saturday, December 5, 2009, 3:45 PM > > Antony Tersol wrote: > > > Assume 3 parallel battery stings, with identical > > batteries of internal resistance r. > > > > > > Assume identical wire leads of resistance R. > > > > > > Then when the batteries have the same state of > charge > > and their internal resistances are equal, one can > solve for > > the current in each string. > > > > > > I1 = I3 > > > I2 = I1 r / (r+R) > > > > > > where I2 is charging current thru the middle > string. > > > > > > The amount that the middle current is reduced is > a > > function of the relative sizes of the wire and > battery > > resistances. > > > > > > For R << r, I2 --> I1. > > > For r << R, I2 --> 0. > > > > > > With a buss bar arrangement, I1 = I2 = I3 = V/(2R > + > > r) > > > > One thing this does not take into account though, is > the > > inductance of all the wires brought > > on by the wire lengths and their loop area, which in > a > > usual inverter with ripple current, will also limit > the > > current > > through the system... That will also have > > some undesired consequences with the voltages > > seen at the inverter terminals due to resonances, > etc. > > > > In ~can~ be significant. > > > > boB > > > > > > _______________________________________________ > > List sponsored by Home Power magazine > > > > List Address: RE-wrenches@lists.re-wrenches.org > > > > Options & settings: > > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > > > List-Archive: > > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > > > List rules & etiquette: > > www.re-wrenches.org/etiquette.htm > > > > Check out participant bios: > > www.members.re-wrenches.org > > > > > > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org > > > > > > This email and its attachments have been scanned by > iConnection E-Mail Firewall for viruses, spam, and malicious > content. > > > The information transmitted in this email is intended only > for the entity or person to which it is addressed and may > contain confidential/privileged material. 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