Saab 9-3: Battery Charge Warning Despite Healthy Alternator And Battery

My Saab recently started to throw a warning when first starting the car: To clear the warning, I had to turn the ignition off and on again - the warning didn't fire the second time. The warning itself is triggered by the battery's voltage dipping too low under the demand of the starter. Just occasionally this led to a failed start and I'd need to jump the car to be able to get it started. Normally, this would point towards a knackered battery or the alternator failing to charge the battery properly. However, I replaced the alternator relatively recently, and the battery in the car was only about 4 months old (the one the preceded it was knackered though). This post describes troubleshooting and fixing the issue. It should also apply to the Vauxhall Vectra (and the basic theory applies to most cars). The Stopgap - A Trickle Charger I've been feeling overloaded for much of this year, so I didn't really want to have to deal with this as well. So, like the mature adult that I am, I tried…