Hi Guys, Thanks for all the feedback, really interesting.
I have a 10 year old battery pack, I got it second hand out of an old conversion. The balancers do a great job of keeping the pack balanced, I think they can move around 10A. Most of the time they can keep the charge in control, it's only in the last 20% where I guess the charger is doing constant voltage and I guess is pushing more current than the balancers can move. Or the batteries are reaching their limit and no amount of balancing is going to add more charge to certain cells. Basically my pack has a couple of outlier cells that charge really slowly (as in their voltage doesn't change) and a couple of cells that charge really quickly (the voltage rises up and over 4V quite quickly). I've found that looking at the cell voltages gives a fairly accurate representation of the SOC. I've logged each cell every few hours, particularly after a charge or a drive, over a week or two and everything was fine until recently. I measure down to the millivolt. Is that accurate enough? And our temperature here is so stable, particularly in the underground garage where I keep the car that I can't imagine the internal temp of the battery is really that different to ambient but I don't know much about all this. Will go and do some research. It's like the Curtis SOC has missed a chunk of Ah going into the pack. I guess I'll have to draw the pack down using a load to get the charger and the Curtis on the same wavelength again. I looked at single cell charger/discharger but the ones I saw do max 2A and the maths of 25 160Ah cells meant I was facing two months of slowly discharging cells! I've gotten a lot conflicting advice about the max charge voltage of a Thundersky cell. Bill said to not let them go above 3.8V while the Thundersky page - http://www.thunderstruck-ev.com/Manuals/Thundersky%20Product%20Manual.pdf - says 4.25V. I tend to stop the charging when cells get over 4V. Is that causing damage to the battery? I was planning on building my own charge monitoring system using an arduino to monitor individual cell voltages and if any one cell goes above say 3.8V then shut off charging for a period of time to let the balancers work. And then shut off the entire charge once the pack reached a fixed voltage - say 88V. But you guys are saying that shutting of charge at 88V would not give me an accurate reflection of a fully charged pack. I'll look at the Ah meters you linked to. Many thanks, Matthew 07966 806 727 On 8 August 2017 at 08:00, Bill Dube via EV <firstname.lastname@example.org> wrote: > In LFP, temperature influences voltage more than SOC in the region of 20% > to 80% SOC. The voltage changes just a tiny amount between 20% and 80% SOC. > > Basically, if the voltage changes it is more likely to be due to a change > in temperature, than a change in SOC. > > Indeed, you could measure the temperature of each cell, along with the > voltage, to tease out the SOC, but you need the average internal > temperature of the cell, rather than the external temperature of the cell, > and you need a very accurate measure of voltage. It is way simpler and > cheaper to count amps. > > > > On 8/7/2017 2:31 PM, paul dove via EV wrote: > >> Sure you can. Voltage is the only way to discern SOC. >> You just can't measure accurately under load. You have to measure open >> circuit voltage. >> www.mdpi.com/1996-1073/9/11/900/pdf >> >> >> >> From: Willie via EV <email@example.com> >> To: Matthew Quitter via EV <firstname.lastname@example.org> >> Sent: Sunday, August 6, 2017 9:13 PM >> Subject: Re: [EVDL] Curtis 1238-6501 State of Charge >> >> >> On 08/06/2017 07:38 PM, Matthew Quitter via EV wrote: >> >>> Hi Guys, >>> >>> My 1238-6501 Curtis controller gives very different SOC % for similar >>> voltages. >>> >>> For example I've recorded a pack voltage of 82.2 and had the Curtis show >>> 22%, 31%, 43%. >>> >>> The pack is made up of 25 Thundersky 160Ah batteries. >>> >> With LFP cells, you can't accurately infer SOC from voltages in the >> middle of the range. The voltage/SOC curve is too flat. >> _______________________________________________ >> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub >> http://lists.evdl.org/listinfo.cgi/ev-evdl.org >> Read EVAngel's EV News at http://evdl.org/evln/ >> Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group >> /NEDRA) >> >> >> >> -------------- next part -------------- >> An HTML attachment was scrubbed... >> URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/ >> 20170807/6b534a2b/attachment.htm> >> _______________________________________________ >> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub >> http://lists.evdl.org/listinfo.cgi/ev-evdl.org >> Read EVAngel's EV News at http://evdl.org/evln/ >> Please discuss EV drag racing at 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 > Read EVAngel's EV News at http://evdl.org/evln/ > Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group > /NEDRA) > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20170808/182dcaa4/attachment.htm> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Read EVAngel's EV News at http://evdl.org/evln/ Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)