Table of contents
Abstract
I converted an AA powered holiday candle to run on old 9v batteries.
Project
Every time I replace the 9v battery in my smoke detectors, I end up with 3 more slightly-used but still somewhat usable batteries. It seems wasteful to just recycle them, and I don't really have anything else that can use them.
For example, this used battery still has over 9v - I think that means it's still about 70% full? 9.6v is new.
At the same time, every holiday season I burn through a whole lot of AA batteries in my LED candles. I did some scouting to see if there are any holiday lights that run on 9v batteries, but it looks like they all have standardized on using AA batteries instead - which makes sense, as AAs are generally cheaper and last longer).
So I thought I'd try running an LED candle on a 9v battery, to see if that's a feasible way to use up the old batteries while enjoying the ambiance of the season.
I had 3 candles that were already in rough shape, so I could do some experimenting without upsetting others in the household who might not want me breaking their stuff. And I still had a couple of DC-DC step-down modules I've been curious to try. I also still had several 9v connectors I had salvaged over time from old electronics.
The size of the 9v battery looks promising, compared to the 2AA compartment. So I cut out the plastic bits, removed the rusty joining plate at the one end, and added testing wires to the connections.
Using the power supply set to 3.3v (close enough to what 2 new AA batteries add up to), I see the candle still works now that the corroded contacts aren't in the way.
So I went to the next step - to see if the step-down converter can do something useful with the 9v battery.
I soldered the 9v connector wires to the input of the voltage converter, and added some test wires to the output - with my trusty voltmeter keeping an eye on things. The converter I used is adjustable, so I had to make sure the voltage was what I wanted. It wasn't - it started by outputting 8.3v, which would burn out the candle. Luckily, just turning the adjustment screw allowed me to get a nice respectable 3.29v. Does it bother anyone that I didn't persist to get exactly 3.30v? 
So now I could solder the wires onto the candle to see if it worked before fitting it all in. It did! woohoo!
But alas... it looks like the 9v battery is slightly too thick!
So I pulled out my chainsaw! and put it to the side so I could get to the small battery operated little rotary tool behind it.
It's a bit hard to see, but I just added a hole to allow most of the 9v battery to be pushed up into the hollow area inside of the candle - there's LOTS of space in there. Careful with the wires though.
That worked, and also allows for plenty of space for the conversion module. I did have to use different wires on the module - the original was not flexible enough to put up with the bendy-squishy fitment.
And viola! Check it out!
The cool thing is that it does work nicely and gives a nice bright light as if there are new AA batteries in there the whole time. And when the battery runs out, it just stops.
Unfortunately I only seem to get 2 or 3 days from each used battery - I guess that's to be expected because 1) 9v batteries have less capacity than 2 AAs, 2) the 9v batteries I'm using are already down to maybe 40-70% full (some are better than others), and 3) the step-down module adds some extra losses. And I guess 4) the on/off switch is the original one and is past the module - so the module is always connected to the battery, which probably drains it a bit even when the candle itself is off.
Given that there is a lot of extra space inside of the candle, I suppose I could tie 2 or 3 of the 9v batteries together in parallel to get a week of runtime.
But a few days of bonus ambience candle light from batteries that were already destined to be recycled is not bad at all.
Happy Holidays to all! and may 2026 be full of happiness and fun projects for all!
-Nico
