Michael, One reason that batteries differ in their charging voltage in series charging is that one battery may reach full a little before others, so you need to continue charging at a much reduced charging rate (less than 0.03C) to avoid gassing out sealed batteries (losing water) to equalize them. Since this was the first charge, you can find they are aat slightly different SoC, so they need to be pulled together more. A battery Balanced can help there too. Then there is the issue of temperature: If one battery finishes charging a little earlier than another, that battery will start warming up (the equalization produces gas and heat - as long as the charging is below the limit where the battery can recombine the gasses, all charging energy will be converted to heat. When a battery is warmer, it has lower internal resistance and the charging voltage goes down. This can even lead to thermal runaway if you charge a battery to a fixed voltage. There probably are other reasons, such as slight differences in the concentration of the electrolyte, but I am no expert there.
I have found that batteries can show different voltages while charging and still behave pretty much the same, you just need to keep an eye on pushing them closer together with every charge to avoid one drifting away from the herd and getting killed early. Success, Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.info Skype: cor_van_de_water Tel: +1 408 383 7626 -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Michael K Johnson Sent: Monday, February 03, 2014 3:25 AM To: Electric Vehicle Discussion List Subject: Re: [EVDL] dissimilar voltages charging series string of AGMbatteries On Sun, Feb 2, 2014 at 7:36 PM, Michael Ross <[email protected]> wrote: > Just brainstorming, your meter is probably not better than ±0.01. You can't > be sure that 13.32 in't 13.33, and 13.36 isn't 13.35. If you are going to > make money and time decisions based on this you should take multiple > readings and be convinced of a stable state. The readings were relatively consistent (the same across multiple readings), though I know not absolutely accurate (adding up the four battery voltage measurements is consistently off by .1-.2% from measuring the chain all together in a single reading, and I have not tested the multimeter against a reference voltage to know how close it is). > Anyway it sounds like a tiny difference to me. To me too. It was the larger difference while charging that made me curious and thus seek the wisdom of the crowd regarding a physical model for that difference. On Sun, Feb 2, 2014 at 9:38 PM, Lee Hart <[email protected]> wrote: > That's why I generally keep these parts in stock. See > http://www.sunrise-ev.com/LeesEVs.htm#zenerlamp I wish the EVDL page on them linked to your page so I would have seen that before ordering. Oh, well. :( On Sun, Feb 2, 2014 at 5:05 PM, EVDL Administrator <[email protected]> wrote: > Connect the 4 batteries in parallel and leave them that way for as long as > you can - at least a week or two. This may not totally equalize the state > of charge among them, but it will at least tend to equalize the VOLTAGE. There might be just enough time for me to do that before mowing season, at which point I should be able to put the charging regulators on. ☺ Thanks everyone! _______________________________________________ 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)
