RLS- I have 7 of these bulbs. (3.5 pairs)
http://www.dx.com/p/t10-1-9w-6500k-1...8#.VAOcE5j4CY0
Slava- just to quantify, your truck sends 2 amps of power to your battery. So if you have a standard 100 amp hour battery, and it's down to 50% power, it would take 25 hours for your truck to re-charge your battery. So while it is quite slow, I disagree with how Madman said it.
As for how to test. Your battery should be at around 12.6 to 12.7 volts when fully charged. Charging it will show around 13.5 to 14 volts. So here's how to test with a multimeter:
- Unplug from any charger- like your trailer's 120 volt plug in. Leave it unplugged for at least an hour. Turn on one light for at least an hour. The reason for this step- your battery might show higher than it really is if you have just finished charging it.
- Plug in your truck, and start your truck.
- Use your multimeter at your trailer's battery. If it is showing over 13 volts, your truck is charging it. If it's under, then you are right that it isn't charging.