Thirty-one and a half hours and still not fully charged. Slow battery chargers are definitely that but then again if the instruction estimates that 20 to 25 hours puts the battery at full charge or at so many volts what would make a battery after 31 hours still not be fully charged?
Below examines a few of the things that could be the problem.
One thing that can put a drain on the charger is the light under the hood. If you are going to charge the battery without removing it from the vehicle any lights that come on when the hood is raised could make the charge time longer.
To help solve the problem the light bulbs can be removed until the vehicle is charged up and then put back in. If the battery is taken out of the vehicle to be charged elsewhere removing the light bulbs shouldn't matter.
If the battery charger appears to be working and it's exceeded the charge time estimated in the instructions its possible the battery is bad and may have to be replaced.
Another possibility is that the battery charger is wrong for the vehicle or it may be one that maintains the battery. Some battery chargers aren't made for large vehicles but they may maintain the battery in a large vehicle.
This means the charger will keep the battery at full charge it won't fully charge a dead or near dead battery. A charger that maintains a battery would be good for a vehicle that isn't ran enough to keep it at full charge or for a vehicle, which sits for a length of time where it can run down.
The charger would typically have to stay connected to the battery if the vehicle isn't ran enough or sits for lengthy periods of time. Chargers that maintain batteries might actually work to fully charge a large vehicle battery that is dead or near dead but it would probably take a long time a lot longer than thirty-one hours, in many instances it would be more than several days.
1.5 amps for a battery charger are normally not going to work well for larger vehicles but it should work for small ones or batteries for small equipment such as lawn mowers. 1.5 amps will usually maintain larger vehicle batteries.
Before buying a battery charger make sure it is one that will work with what you will be charging. If it is a battery charger that maintains batteries on large vehicles it will say so on the box or in the description for the product if it is being purchased online.