Battery Vehicle Society

Hi Bill,
You have basically hit the nail on the head and answered it all yourself. these cells are intrinsically stable with no self discharge and store very well. If you keep the cells in their happy mediums and not abused/corroded, they will outlive us.

If I'm not wildly mistaken, Greg of Gregs garage has called it a day after "talking BMS" all these years. However Peter "retepsnikrep" should be around.

I've often heard the forums to query are the model helicopter guys. They know how to abuse very flammable Li-Poly's hard.

You are wildly mistaken, Both myself and Peter are still around. I've been running my Matiz for a year now, testing the bms and Peter and I are slowly working on a new slave board setup based on the pic 12F1822 which has an internal voltage reference, see the bms thread for details.

To answer wattsin's question, I am not aware of any catastrophic cell failures which weren't caused by an external factor, shorts, loose connection or overcharging. The main danger appears to be the electrolyte which is flammable. Anything that causes the cell to get too hot can set the electrolyte on fire.

But I am aware of quite a few non-catastrophic cell failures on packs that weren't being mistreated. Typical failure is very high internal resistance in the cell. This can happen at any time on a cell that is otherwise working fine, with no warning that a cell is on its way out, one minute it is working fine and the next minute it is dead. Failure rate can be as high as 2% or 1 in 50 cells. I recommend buying some spare cells when making a pack, just in case.

Hi Greg, welcome back!!!

I've been reading and updating about the A123, 20AH pouch cells. There has been a faulty spot welder on the production line which has been causing the odd cell to short out internally. I've seen one of these cells had literally burst open a plastic block they were cast in AFTER they were bottom balanced and thoroughly tested, (EVTV). They were being charged at the time. He has also bought an A123 module, pre-used, with two bad cells.

This spot welder issue and disgusting batch control has cost A123, Fisker and many others huge sums of money. Enough to wipe out A123's grant off the Government.

This batch control issue could cost them their ISO rating. Seems some companies like to boast ISO ratings merely to encourage sales, not protect their asses.

I guess once bad cells are pulled. The pack should do well. Mars Rover is a fine example of thoroughly tested cells before use. However, in this case they have a different electrolyte for the cold.

I've not really been following that story, my guess is that the cells where getting to hot internally during the welding process, which is why you shouldn't try soldering foil cell tabs. I stay away from the pouch cells myself, but Peter has used them.

The large (up to 16ah) cylindrical Headway cells look promising. They have screw terminal connections, great for building packs in small volume and have higher C ratings than prismatics. Even though they are cylindrical, when packaged up you get better kwh capacity per cubic meter than many prismatics. Another advantage is more surface area for cooling. Tesla has stated that one of the reasons they chose to use cylindrical cells was the increased surface area which allows them to maintain cell temperature throughout the pack. It is more difficult to get decent cooling around prismatic and pouch cells.

My A123's are mounted into 4mm Correx. They are 7.33mm thick and fit into every second slot I cut. The folded edges of the cells actually grip the sides solid into a firm box.

This allows air, however, these 20C babies don't get hot unless very badly abused or faulty internal welding.

I've already done the maths on built volumes and densities. I failed to obtain EnerDels due to becoming vapourware. These have the edge over all LiFePo4's

I'd post some piccies but don't know how on here.

Just post a link to pictures if you have them elsewhere on the web, sounds like an interesting setup.

I've used 50 x A123 pouch cells assembled in a block for two years.
I soldered the tabs together, they work very well in my phev application.

ASFAIK as Greg has mentioned cells only catch fire if wildly abused or some other wiring short sets them on fire.

As Greg's vehicle was destroyed we will never know what caused that event

I have just made a 12S A123 pack with some spare cells for my Cycle battery I riveted the tabs together on this occasion as a test. Personally I think 3 holes in each tab and some small aluminium washers M3-4 nuts and bolts is a good solution. Perhaps with a smigin of Noalox.

Hi Peter,
I've read your A123/Honda write up a bit back. Good job!

You are right. A good solid low resistance connection will not heat the tabs. That could be a cause of cell death due to overheating of the electrolyte. It's also a waste of energy.

It seems temperature is the key to cell safety. Measuring Voltages only for the Goldilocks zone as you know, is fraught with false paths.

My Chinese exporter submitted me some photo's from another (un-named) buyers efforts. He used brass screws on the tabs and bits of pcb to ensure screws never touch each other. Then he clamped the lot together with alloy floor plate. Wires everywhere. Not what I'd do, especially for automotive use because of vibration issues.

EVTV has a good yak about A123's on this Fridays show. Also built a new pack then auto calibrates/tests it with a cracking little machine. It's well worth a look.

Lox res:-
http://media3.ev-tv.me/news051812 - iPhone.mov'

720p:-
http://media3.ev-tv.me/news051812-1280.mov

I spoke with Cedric Lynch a while back about how he was connecting lipo pouch cells (i think they where Kokam's). He used small bolts and nuts
to clamp the tabs together, 8 in each tab. He had tried soldering the tabs but found it ruined the cells. Seems that a delayed reaction would occur inside the cells and they would start to swell after a while. But then they are a different chemistry than the A123 cells.

I've never spoken to Cedric. Interesting fella. I'd love a bike like his.

Kokams aren't LiPo's. They're LiMgSo......... whateva's. But yes, if the electrolyte gets too hot, kiss them goodbye.
I liked the EnerDel cells because they had huge tabs like elephant ears or cuttlefish wings if you wish.

A123 use a laser welder for the tabs on the battery block but it rendered the pack unserviceable. They must of thought it was very clever of them at the time.

Mine have the tabs folded opposite ways. Each cell alternate to the last. The insulators being PVC sheet, 4mm thick, 160mm long and 10mm high. They have four 1/4" holes to thread the 6mm threaded tie bars through. Of course, I pre-glue two insulator pieces, one each side of the cell and fold the tabs over the insulators to make an inverted "U" shape.

So assembly means sliding the cells into the Correx box. When done, slide in the four threaded studs. Two ends are two tapped blocks for the studs and drilled clearance holes for the side that takes nuts. These latter two sit high and threaded to take a connection bolt. All the clamp ends are 3/8" thick alloy (T6) to spread the load across tab widths.

When assembled raunch it up tight.
Correx is glued into a box with a glue gun. So everything can be sealed up after testing.
Old fashioned Evo-Stik would of been used to glue more Correx with the cells running horizontally to make it fully stiff and strong.