Batteries


I finished testing all 30 of the used A123 cells. They were all about 2.18ah with a 35 amp load. I’ll retest a few of the cells at 10 amps to see how much of a cut Peukert is taking. The fan was helpful in cooling the cells down. As the cells warmed up the voltage would noticeably rise on each cell.

Here is the A123 cell charging fixture without any cells loaded.

Here is how I was discharging each A123 cell at 35 amps. That blue plastic clamp made this mundane testing go by more quickly.

Here is a charging fixture that I made today to charge 10 A123 cells at the same time. I machined slots into the wood to keep the copper plates from rotating. There is also a 1/2 inch wide slot under each cell to hold them in place. The power supply is set to 3.70v. Each cell can reach its finishing voltage separately from the others due to a one ohm resistor feeding each cell. The other important thing the resistors allow is that a drained cell can be put into the fixture next to a fully charged one. The charging takes a little longer with the resistors, but it gets them all fully charged equally. Then I can load test them one at a time to see what their capacity really is with a 50 amp load. There are currently 30 cells awaiting testing. A group of 10 cells in parallel will eventually find its way into a module that will replace a nicad cell in the #1 truck. A complete pack will only take up the driver’s side half of the battery box. The trick will be to make a BMS board that will fit on top of the closely spaced cell posts like the nicads have.

Here are a couple of links for Thundersky cell load tests. 1, 2 .

Here is a paper doll of the 160 ah Thundersky BMS board that I’m developing. Someday I’ll fab a board using the Toner Transfer process. Always a helpful process before paying for a batch.

I ordered 30 used A123 cells this morning. Should be here in a week. I’ve sketched up a diagram of how I can charge so many cells at the same time. I think strapping the whole thing to a 2×4 will work nicely. From the testing I did on Ray’s pack of A123’s, I think they will work great for the scooter and hopefully for the truck. This could be CNC’d and then a lid screwed on. The posts are close together. I don’t know how I’ll get a BMS board across those posts.

Here is a concept sketch of how the A123 module will duplicate the volume of a flooded nicad.

Here is an idea Mike Swift gave me for housing 10 parallel A123 cells in a similar shape to how the nicads are made. It’s tough to see all of the detail on the back of this receipt.

One of the most sobering tests performed by a guy making his own BMS is here. His experiment showed that using the wrong parts can in deed cause a fire. Several designs out there use these same parts.

In another post, this poor fellow had his whole car burn to the ground. What a shame. He did a lot of work on it.

Here is a Prius fire story using A123’s. A bad connection was suppose to be the culprit of that disaster.

This past Saturday I went to the EAA meeting expecting to plug in and charge.
Didn’t happen. The GFCI had tripped on the Dolphin. Different outlets and a few
resets were fruitless. Vehicle still ran fine, but the GFCI would fail.

Here are the steps I took:

-Plugging in at home. GFCI still failed.
-Swapped the GFCI out with another unit. GFCI still failed.
-Visually inspected inside Dolphin. Nothing unusual.
-Isolated ground on AC line with 2 prong outlet adapter. GFCI did not fail.
Charger still works.
-Removed J2, the largest connector on the Dolphin. GFCI did not fail!
-Reconnected J2. GFCI failed again.
-Started truck. Measured ISO in Dolcom. Was -130 points below pack voltage. Not
good. Oh look, the pack has all of 4 bolts holding the lid on and it rained for
2 weeks….
-Took the lid off. No puddles of water found. But had a high humidity feel/smell
to the pack. This backs up the -130 count found on ISO in Dolcom.
-Hung the 12″ fan from the bed and let it blow on the cells all night.
-Next morning GFCI stopped failing. Charges fine on 120vac and 240vac.
-ISO at -100 counts below pack voltage, but pack much dryer feeling/smelling.

Time to go ground fault hunting in the pack again.

Tonight’s test drive showed that the pack has improved. It held it’s voltage with all 251 cells in circuit, up above 310v for about 3 miles. And then declined from there. Much better, but not as good as a complete pack deep discharge. The 14 cells have not had the benefit of a major overcharge. So I’ll try to get to that this weekend.

So the next morning the pack was still soft. The voltage sagged right away. So I electrically removed about 14 cells from the pack so the charger could keep the current level up around 1-1.5 amps even at full voltage. This will overcharge the nicads and help equalize them. Flooded cells are the only ones you can get away with this. So the charger was programmed to stay on an extra 5 hours after full pack voltage, 400v, was reached. The pack was charged like this all week.

A couple of days ago, I noticed that the resting pack voltage after the charger shut off during the night, was very low. About 306 volts. This concerned me. The first day the resting pack voltage in the morning was 325v. But today it occurred to me that I may have finally gotten what I was trying to achieve. The pack’s cells are now all at the same state of charge. So the cell to cell voltage is closer. With 237 cells in circuit (14 removed), at 306 volts, that’s 1.29v each cell. That’s a bit low. Usually the cells are about 1.35v to 1.4v after sitting for hours. I am guessing that the cells hit the knee in the curve as a group, and the voltage starts to go down as the cells are overcharged. This is some fairly serious overcharging. But it shows that the cells now are more equalized.

Midweek I reduced the overcharge time from 5 hours to 2 hours.

This weekend I’ll reattach the 14 cells that were taken out of circuit and see how the whole pack is acting. Although the 14 cells did not get the huge overcharge, the pack should show improvement. I could add some overcharge to the 14 cells before closing the pack up just to try and bring them into the same state as the rest of the pack. I’ll actually make sure I do that.

The bottom line is that with 3 different versions of the same cell, they are getting worn out as exhibited by their impedance being high. This allows the pack voltage to rise artificially high and make the charger think the pack is full when in fact many of the cells are not fully charged, resulting in a sagging pack.

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