Convert battery powered string lights to AC/USB?

Hi folks!

Lets get it out of the way: the easy answer to my question is going to be "buy new lights." But that's the safe and lazy way out!

There are three sets of holiday string lights in my home right now that run off battery packs. Two of them use 2 AA's and one uses 3. Typically the Eneloops I put in them last about a day of continuous use, or a few days of use while we're home. This is a PITA. I don't want to keep swapping these batteries out.

My ideal solution would be to convert these lights to a hardwired source, but I don't have the first clue how to do it. Any tips for where to start?

How long is a ‘day of continual use’? How many hours?

eneloop AA battery capacity is about 1800mAh, (going from memory) so 18 hours to run the batteries flat would mean about 100 mA of continuous current draw. 9 hours until dead would be about 200mA load current.

3 eneloop AA batteries in series will deliver about 3.9 volts. Feeding a full 5 volts from a strong USB charger could over current the LEDs on the lighting string. If the LED string is actually rated for use with three fresh alkaline AA batteries (circa 4.8 volts) then powering it with a direct feed from USB 5 volts could work nicely.

Just wire the +5 volts from USB cord to + terminal in battery housing, and ‘black/ground’ wire from USB charger cable to - battery case contact.

Somewhere I have seen ‘dummy’ AA sized battery ‘shells’ for sale, with wire leads to do exactly this, power the device externally.

The two strings that utilize only two AA eneloop batteries may also work out nicely. One string battery contacts would be connected in series to the other, then the paired strings powered from a single 5 volt USB power pack.

Commercial products like this are sold as Battery eliminators

BigClivedotcom on youtube has a video on converting them to run off off 5V adapters.https://www.youtube.com/watch?v=CQcRLq4yq6E

In short, determine what current (I) the string draws, insert resistors (R) to drop the difference in voltage (U) between 5V and the battery voltage, according to Ohms law (U= R x I), while also making sure not to overheat the resistors.

Resistors are a pretty ugly way to do it IMO.

Some like this:
https://www.ebay.com.au/itm/AC-100-240V-...m-/221399192313

But yeah 5V supplies are common. The 3xAA unit would likely run just fine directly off 5V (think 3 x 1.66V batteries)

Or something like this if you can solder - guessing probably nto given the question
http://www.ebay.com.au/itm/DC-DC-Step-Down-Buck-Mini-Module-3-3v-5v-6v-9v-12v-Adjustable-Voltage-Regulator/112426265937?hash=item1a2d20ad51:g:fVwAAOSwH3NXmsyx

If you have a spare 12V or 5V or 9V or whatever V adapter laying around, then a nice way to dial in the exact voltage that the batteries were giving, is to purchase a very cheap DC-to-DC voltage converter.

Tony recently got a couple of those for his BlueGigaEmpeg project.
Very cheap from Asia on eBay (like maybe, $1/each), somewhat more from more local retailers.

But also remember that the strings are designed to run off batteries and those drop as they discharge. Rechargeables less so than alkalines (or the ancient zinc-chloride ones).
So the strings will run OK for a span of voltage. I run a few 3x AA ones straight off LiIon 18650's (not constantly on) which are about 3.7V for most of the cycle - when I start noticing the LEDs getting dimmer it's time to recharge) .
Slightly less bright than with fresh alkalines, but brighter than the alkalines in the later part of their life.

Nice thing is that with With either an adjustable buck converter, as well as resistors, you can tweak how bright you want the string.

Rummaging around a second hand shop or your box of old power adapters may reveal some that directly output the desired DC voltage.

3 volt output (or thereabouts) will substitute for two AA batteries.

4.5 volt to 5 volt (give or take) adapter would power the 3*AA light string.