Sunday, June 3, 2012

My worst cell

I was curious to see how much my cells have degraded so I took the car out for a test drive. I forgot how much I missed this car until I dropped the top and drove through the hills. What a blast!

I drove 65 miles and brought it home using 137 Ah. At this point my check engine light was coming up with only about 1C draw so I new at least one cell was getting low. I saw that my voltages were fairly well scattered around which was expected considering how scattered the self discharge seems to be.

Below are the photos of each cells voltage starting from the back of the car and working forward. There is one cell at 2.7v. This cell is basically dead and my worst cell. I'll probably yank or replace at least it. I have two more cells at 2.9 which means there is maybe another 5Ah in them tops. Most of the remainder of the pack is around 3.1. I'm guessing there is at most 10Ah in these cells. On the last two pictures you'll see sequentially 10 cells which are all 3.25v. These are the additional cells I added about 1.5 years go which have almost no miles on them compared to the others. They are still happy and have at least 20-25Ah left in them I'm guessing.

So this means that my worst cell is still giving me just over 85% capacity after four years. If I hadn't over charged some cells slightly, and over discharged multiple times, and never went over the 3C rating, I would be a bit upset. Considering the abuse I'm guessing this is right on par with what's to be expected.

So now I'm debating what to do. Ultimately I'm thinking I'd like to go with a much higher pack, say 350v using 100Ah CALB cells. These cells are now rated at 4-5C with 10 second bursts of 8C! This would require multiple changes. First and obviously the pack needs to be completely replaced which isn't cheap. Second the motor controller would need to be replaced with something like the Netgain or Soliton1 which allows high input voltage but you can regulate the motor side voltage to keep it safe. This also equates to even lower cell side amperage draws which should be easier on the cells. I'd also need to replace the charger. No hard feeling here though. The Thundersky charger is twice the size of say the Manzanita Micro charger for the same output (did I mention is weighs twice as much too?).

My other thought is to slowly replace the TS 160Ah cells as they wear out with CALB 180Ah cells. Physically they are identical so no mods would be required. These cells are also 4-5C rated so they can handle the amps my modified Curtis is demanding. I've got some thinking to do.

Either way, before I get back on the road any time soon, the AC needs to be redone. I'll need to engineer the mounting bracket (again) and machine some parts for the pulley. It's a shame it wasn't a direct bolt on replacement from the ADC 9".

It's suppose to cool off next week, so despite no AC, I think I'll drive it to work and see how the batteries hold up before investing any further time.


Troy said...

Good to see you still working on this project. Not sure where you mounted the AC compressor, but have you considered a drive shaft Driven one? this is common on Hot Rods that want AC but want to keep the engine area free of AC components.

Brian said...

I'm still using the stock compressor. It's just to the side of the motor. I'll post some pictures soon.