12VDC light s get dim than brighten ???

bad house battery ?
OR
the Dc converter is on its death bed

If batteries more than 36 month old change battery first.

If batteries have not been checked for water in 6 months, be careful dry cell rupture does not occur. wear goggles have water hose available to remove splash.

If it continues change the dc converter

Also clean the battery terminals.

First thought also is the AC to DC convertor is tired and wants to retire to the Villages in Flordia.

Does the generator come on when the air conditioner comes on?
If they go dim after only 15 minutes then batteries are perhaps weak or there is a huge load somewhere, or less likely the battery charging charging system. You would need to measure the voltage when charging to know, should be greater than 13.6 and hopefully over 14 volts when charging.

If there is an invertor (changing 12 volts DC to 120 AC) then turn it off and see how fast things drain down.

Converters will go into storage/float mode after your batteries charge to a certain level, then the 14 charging volts drops back to float mode with effective 13.2v. Depending on how many lights and other things you have on, the battery power can drop back to 13 or less and then your lights may dim. Now it gets a bit weird, you can turn one light off and the power stabilizes or something turns on like a furnace or ac and the converter comes out of storage mode and back into charge again at 14.4 volts. Once it reaches a certain charge it goes into normal/absorption mode at 13.6 out and if there is not a lot of draw or a certain amount of time passes, it goes back to float and then the cycle starts again. I have a fob on my converter that lets me override any of the above modes if necessary.

-Gramps- wrote:
Converters will go into storage/float mode after your batteries charge to a certain level, then the 14 charging volts drops back to float mode with effective 13.2v. Depending on how many lights and other things you have on, the battery power can drop back to 13 or less and then your lights may dim. Now it gets a bit weird, you can turn one light off and the power stabilizes or something turns on like a furnace or ac and the converter comes out of storage mode and back into charge again at 14.4 volts. Once it reaches a certain charge it goes into normal/absorption mode at 13.6 out and if there is not a lot of draw or a certain amount of time passes, it goes back to float and then the cycle starts again. I have a fob on my converter that lets me override any of the above modes if necessary.

X2. What needs to be done is a meter place on the house batts. Get 12V or so on them, and then watch as the lights dim. See what voltage they get to. Then kick on the A/C when it all gets bright and see if the converter kicks in. I'm curious exactly what you mean by "dim" though.

If the batts get down to 10V there is a problem and the converter is weak.... Or yucky terminals.

Converter when AC Power is available should be supplying DC System power demands ....and charging battery

If battery is going bad then converter is having to do double duty vs just DC System Demands.
(Once battery is fully charged it should not be discharging with converter in service)

SO need to measure battery voltage and each cell specific gravity.
Replace bad/damaged battery

THEN check converter DC Output.
With battery disconnected and just converter on (no DC loads) the converter output should be 13.2VDC ......if lower then converter is failing

Gramps, your description is exactly what its doing. Now...why? Thanks all.