[milton1205]

Here is a little more info on our battery failures:

1) The two batteries that failed were bought in 2007 and out of the same batch number. They did not come in the 2007 kit of parts.

2) We only used these batteries in competition. (During the build season we use 2006 or earlier batteries.)

3) We use previous years' chargers to charge all batteries.

4) We measured all of our 2007/2008 batteries' impedance under load and found that they were mostly <10 mOhms ( which meets the current spec). The exceptions were the two that failed (about 5 Ohms) and one 2007 battery that was 12 mOhms. Testing was done on fully charged batteries. All of the batteries read >13V after charging and recovered to at least 12.5V after the load test (except one of the failed batteries which read 10.83V).

5) We removed the plastic cover to expose each cell's rubber cap. In the failed batteries these were "dimpled" indicating the battery was probably over heated at some point. The failed batteries also exhibited either a reddish residue or a trace of yellow liquid.

6) The batteries show no indications of heavy impact, and none were dropped or handled by the cables. Also in both of our on-field failures there were no impacts that caused the failure.

7) Our current robot does not draw significantly more current than any previous year's electrical system, and battery voltage after a match is >12.5V. We can probably run two or three matches on a battery before seeing a performance drop, but we have not tried this with any of the 2007/2008 batteries.

Summarizing:
One of the failures was a single cell failure as indicated by the voltage ( and the red discharge near one of the cells). The second failure is probably related to a loss of electrolyte.

At this point we are investigating the chargers, specifically, the charged voltage threshold and the trickle current over long periods. We are also (re)-implementing the battery tracking records ( time in, time out, match used etc.) as we were a little lax this year.

One more note:
In buying replacement batteries, the country of (manufacturing) origin has changed from Vietnam to China. We intend to characterize the new batteries this weekend.

We would appreciate any thoughts you have.

Thanks
Mitch Milton
Team 1126 - Electrical sub-team