Figured out that contacts are more prone to oxidizing in the case of DC 12V E27 lamps

I'm from Ukraine, so normal power is a rare thing. Most of us forced to experiment with DIY crazy shit in the scale of our budgets. So I have a 12V battery with a charger and I have found a DC 12V lamp to save some power on AC-DC 12V-220V converting. Sitting with a light is better than sitting in the dark.

But I start noticing that lamp is emitting light much more dimly than before. It happened gradually through 1.5-2 years, so the first idea was to blame the lamp itself. Shitty lamp working almost constantly for 2 years might started dying. I checked voltage. 13V from the PSU with battery. Ok. Replaced the lamp (luckily I had an unused one from the same set as the dim lamp) and... the same shitty dim light. But I wasn't completely sure that it was brighter 2 years ago. I could easily imagining things. Returned the previous 2 years old lamp. The light became much brighter. Almost as I remembered it.

Cleaned the contacts. All is nice now.

It looks like DC low voltage/amperage doesn't work fine with E27. Oxidizing is more significant in comparison to AC 220V.

I hope my story is dull enough for this community :)