06-11-2020, 10:09 PM
If the trickle charger has not done its job, you may be suffering sulfation, which is caused by a battery not being fully charged over a period of time. This may be cured by a short term overcharge of about 25% (i.e. 7.5 volts on a 6 volt battery). A lot of large capacity smart chargers have a "de-sulfation" function, which takes the battery slowly up to the overcharged state, but I'm not sure about the small jobs for starter batteries. My experience is on larger installations on narrowboats. If the batteries have been stored in a cold garage that won't have helped, so bring 'em in from the cold and get them charged to 7.5 volts for a few hours. It may not work, but you have nothing to lose.