Put a voltmeter across the battery terminals. With car off, it should be 12V, and 13.5V or so when running.

Another place to check is the brake fluid in (both sides of) the reservoir. I had a rusted through brake line that lost fluid, and the ASR light came on. I would have expected a different light, but it did get me looking, at least! (Do you have any new puddles under the car?)

Gerry