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My headlights function as desired, however, if they are left on without the engine running, they start to dim after a few minutes as if the battery has been drained even though the battery has been fully charged. I would think on a full battery that they might last more than five minutes. Does anyone know if this is normal on a 6-volt system?
Thanks in advance for any information.

They should stay reasonably bright for a lot more than 5 minutes, I've occasionally left mine on for 20-30 minutes. You may have high resistance at some point, perhaps the switch or ground.

Thanks. I'll check those.

Hi. I finally got around to to checking the impedance and found that the headlights show 1.3 ohms and the tail lights show 0.8 ohms. I think 1.3 ohms may be a bit high. What do you think? Thanks.

Hello. Here is what I did for most of the day today on my headlight issue and it did not solve the problem. Since I am a novice at this I would like for those of you who are to kindly inform me of what I could have done in addition. I would greatly appreciate it.
First, I took my ohmmeter and checked the resistance from the headlight reflectors to a head bolt and received 1.3 to 1.5 ohms. The headlight reflectors actually go to ground. I then dismantled the entire assembly on both sides. I then removed the paint on the underneath of the headlight casings where they meet the pedestals. I also removed the paint on the top of the pedestal where the casings rest. I then checked the resistance in the wiring within the headlight sockets to the ends of those wires and received 0.5 ohms for all three lines. That was for both headlight assemblies. I then checked the resistance from the light switch to the end of those wires where the headlight wires plug in and received 0.5 ohms on each and every one. I then reassembled the headlight casings and attached them back onto the pedestals. I checked the resistance again from the headlight reflectors to the engine head bolts and received 0.5 ohms on both sides. After plugging in the light wiring to the switch lines, I turned the lights on and started timing. The lights dimmed after 5.5 minutes. I truly do not know what else to do except suspect that my Optima battery has been sitting for so long it cannot hold a charge under that type of load condition for too long. However, I do tend to doubt that theory. But, as I said earlier, I am a novice and I probably missed something. If anyone has any additional information on procedures that I might try, it would be greatly appreciated.
Thanks.

Batteries go bad. They have two purposes, 1..hold 6/7 volts measured post to post, the second function is to discharge to maintain a drain whether big or small until the battery charge is depleted. A battery can run the lights when the car is off, and the battery is capable of producing 4 to 500 amps, ie: starter. A battery can have 1/2 of its potential fail....run lights for a short time VS., not crank the starter. Your battery needs to be fully charged and checked for sufficient voltage eg., 6 Volt might be has high as 7 volts on a fully charged battery. The next test is checking to see if the battery can withstand a major electrical draw by using a carbon pile (battery tester) that can discharge high amps 400/500 amps for 20 to 40 seconds. If the battery can put out that amperage and return to the static 6/7 volt indication showing the battery is still charged then in all likely hood the battery has both abilities to hold a 6/7 volt charge and discharge high amp's like cranking a starter.