Monday, August 30, 2010

More battery testing

I ordered 12 new cells for the upgrade. The plan was to replace the two cells that have had bad sag since day one and 10 new cells. I decided this would be a great time to do another capacity test for comparison against my original cells.

When I got the original cells I was told the factory capacity tests were all above 160, if I recall they were mostly around 165Ah. I charged a group of four new cells the same way I did the old cells. That is, not long after switching from CC stage into CV stage I remove charge and begin testing. The old cells were drained at ~130 amps until 2.5v and the worst cells yielded 145Ah, most around 150Ah. The new cells I only drained down to 2.8v and the reason is that at 2.8v they had produced 183.5Ah!

This raised a few thoughts of course and along with the fact they have improved the cells, I'm lead to believe that Sky Energy (CALB) who sells an equal size and weight 180Ah cell for more money is most likely selling the exact same battery. Unfortunately I don't have the 180Ah cells to test, perhaps they are getting 190-200Ah. I believe the batteries might be coming with more capacity also so that after time, they are still considered to be at their rated capacity or higher.


Anonymous said...

Hi there,

I just recently found your blog and started reading it from the start, a very interesting read i must say, amazing work on the mechanical and electric/electronic parts. I plan on doing some kind of conversion myself someday, but not in the near future. I have been googling around for projects such as yours, and found some quite interesting, including the one on

I don't know if you are aware of that one, basically it is an older guy doing a conversion of a porsche speedster and a Mini Cooper, using the same batteries as yours. The guy has a video blog, and tends to talk a lot and drag on, but he touched the subject of BMS some time ago, and seems to have had some quite heated discussions on the ev forums regarding those systems. Basically what he says is that you only need to monitor individual voltages, and maybe temperatures of the batteries, and should not top balance them as everyone thinks, with shunt circuits and what not. He is quite stubborn in his beliefs, but actually that makes sense, as he showed a nice example with wooden bars of different length representing the different capacities of the batteries (it seems they all have some tolerance). Maybe you can check it up yourself if you are interested, but when you charge them and balance them at some maximum voltage, and then start discharging, the weakest cells in the pack will be completely discharged while the others still have some capacity left, meaning that their voltage will start dropping rapidly, while the rest are still holding on. At this point, in extreme cases, the weaker cells will be reversed by the higher potential of the surrounding cells, and this is really the worst thing that can happen to a cell, worse than overcharging and probably worse than shorting it. Maybe that is why two of your cells died (ok, i remember that you shorted one, this might have helped), as he put it, the weaker ones get eaten by the stronger ones. What he suggests as the right way to do it, is balance them once at the bottom, perhaps 2.5V per cell, individually discharging them to get to that exact level, and THEN charging them up to the capacity of the older cells (i believe you measured 150 Ah on some of them). As you noticed, they are normally pretty close in performance, so you only need to balance them on the bottom like once every 6 months or so, and you can remove your shunt balancers. It is very helpful that you can monitor each sell voltage individually. These things seem to be dangerous (not speaking against your modules, but in general), and many batteries seem have burned because those modules got hot, malfunctioned, and shorted the two terminals. And if something like that happens, there is no easy way to disconnect them quickly.

If you do that, your new cells will be performing not to their full potential, but to the potential of the older cells. And that should be fine, otherwise if you try to charge all the batteries up to the Ah rating of the new ones, you can run into problems when pushing the car to the limit on its range. But at least you will know that all the cells are in their safe range.

Hope I don't come across like some smart-ass who just saw a video and is trying to teach you something, or criticize your work. I really admire your project, and am only trying to give friendly advice.

P.S. Btw, i did some battery testing for my bachelor thesis, although for some electronic components and not for EVs, and also read some things on Lithium iron phosphate cells, although i didn't actually use them, so I believe the above makes sense, from what i have seen.

Brian said...

There are positive and negative sides to both approaches.

If you don't use 80%+ of your capacity, like I do, I think the bottom balance approach would work. He doesn't take internal resistance into account and I've found that the batteries come out of balance faster than one thinks, especially as they age.

The point he makes about all your batteries will be drained at the same time also means you could be dead on the side of the road where as a top balance approach gives you the option to push that lowest battery a little bit and make it home if it comes down to it.

Monitoring all cells and using low voltage indicators allows the driver to make the decision. Not a good idea for a production vehicle, but in my case I prefer it this way. :)