Thermal runaway

[Ralph Day]

I'd heard the term thermal runaway, but didn't really have an appreciation of it's effects or causes...until now.

An ex-off grid/microFIT owner (like me) has a series/parallel set of batteries (t105 type I think) that had one shorted cell in 2 batteries, one in each string. Time and work contraints prevented any remedial action (too busy, who isn't), and I didn't know what to say. I suggested consolidating to one string of good batteries, but time was the enemy.

Significant other said she could "smell batteries" in the garage. The battery box/room is sealed off of the garage. The owner went in the room and the acid fumes were almost overpowering, the batteries were bubbling to beat the band. He took a temp check of the electrolyte and it was 80C. Remembering your battery eq charging bulletins we know to remove charge source when temp approaches 40C...double that and there's going to be trouble. I think they were lucky to avoid a fire or explosion or both. Close call.

Now I know, if you have a shorted cell/bad battery in a string Get it Out! The culprit may result in the whole pack being overcharged severly.

The owner now has some L-16's (working, free, enough for a 48v pack), and a ton of scrap to deal with. I've offered to help in the swap out feeling somewhat sheepish because of my ignorance.

Lesson learned: deal with bad cells or batteries early!:cry:

Ralph

[Rob Beckers]

Auch! I didn't know batteries could do that. Thanks for the info Ralph!

So this was during charge? Because of the lower voltage (shorted cells) the charge controller kept pounding the batteries?

-RoB-

[Ralph Day]

During charge, overcharge. Solar or utility the chargers pounded out the amps.

Status: Checked the sg of the bank and there were 4 batteries out of 16 that had sg readings we felt made the unit worth trying to work with. Ken already had 2 of the same type bought recently. The sg's of the rest of the original bank varied from 1.280 to 1.130, cells in the same L16 battery, as well as all over the map battery to battery. We deemed any with any cell under 1.140 as toast. There was a lot of gunk came up in the hydrometer when checking them. Black and flakey or grey and sediment-y.

Ken also was given a ups 24v inverter/charger with 8 L16's of unknown pedigree. We checked the L16's and they were from 1.160-1.180. They willl be under charge, then eq, hopefully they won't need a heavy corrective eq but I have my doubts. The pure sine wave 24v invertercharger we hooked up to 4 of the "maybe usable" batteries, and a little smart charger is on the other 2 mu batteries. Everything in a state of flux, with lots of bubbling sounds in the garage.

The inv/charger looks like the idea thing for a cabin or cottage with the right number of panels and charge controller.

Ralph

[Chris Olson]

So this was during charge? Because of the lower voltage (shorted cells) the charge controller kept pounding the batteries?


Rob, that's correct. What happens is that the shorted battery is similar to just removing it and replacing it with a cable. So say you have 4 batteries in series for 24 volt (or whatever). The controllers will still run the string at whatever voltage you have set for absorb (say it's 29 volts). But the remaining three batteries will run at 9.6 volts instead of 7.2.

If a battery goes open, no big deal and the string just goes dead. But when they short internally it's bad.
--
Chris