a battery that is able to start the engine in less than absolutely perfect conditions must be considered OK.
It is not so that a battery which starts OK is a good battery.
All that proves is that its internal resistance is low enough and that it holds enough charge for the starting.
The battery capacity could be 1/3 of the new capacity.
I've just replaced a £400 Concorde battery, after 6 years, due to this. It "worked" just fine.
If I get a start off a weak battery I always stay at idle and watch the ammeter. It should show an initial high rate of charge which after a few minutes should start to reduce. If the charge rate stays very high then you have battery and/or charging issues
