Need to charge the weak battery in my Denali GMC, and all I have is this. Safe or not?
http://www.amazon.com/Schumacher-SEM-1562A-Speed-Charge-Maintainer/dp/B0009IBJAS
It will work fine as long as you switch it to the 12 volt setting. It will take it some time to charge your truck battery though.
Believe it or not, I have charged a 100Ah car battery with a 12V, 0,5amp wall wart. The battery was at 10-11 volts and brought it to 13 volts in 8-10 days.
Ok, not very practical, but I was curious if it was possible.
One thing you need to be careful with. If the battery voltage is pretty low, it will draw more current (amps) from the charger than the charger is capable of putting out. If you have a good charger it will have a shutoff switch when things get too hot, but even still its not good for the charger and is minor fire risk. If your charger has a meter on it and the meter is not pegged at the maximum amperage then you are okay.