I have a 1988 300E car started fine this morning in garage she drives 15 miles stops for about 5 minutes at a store then calls me car will not start I get there and battery appears to be low? light come on but dim when I try starter..... jump car and it starts ok drive home about 15 miles and car starts ok now? and appears ok as she has used it now 3 or 4 more times and it starts all the time now so appears its charging ok? any ideas on this? Thanks