AC power + battery backup for portable tankless gas water heater

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This post I'm writing today will deal with tankless gas heaters. Not just any heaters, but the ones that are battery powered, such as this one:

Typical battery-powered tankless gas heater, bottom view.

As you can see on the right corner of this picture, a portable tankless gas heater uses two D-type batteries to ignite gas and keep the water valve open. Compared to more commonly sold mains-powered units, this one has an advantage of being able to heat water during power outage. There's one disadvantage, though: depending on how often you use it, you'll have to replace batteries sometimes as often as every 2 or 3 months! Being a lazy bum just as I am, one day I decided to overcome this disadvantage and make my heater work from mains, retaining the advantage of autonomy from power grid. How?

Today I will show you how to make your tankless gas water heater autonomous from mains electricity (that is, it will work even during power outages).

Pros: you won't have to constantly change batteries.
Cons: none! What are we waiting for, let's get started!

Our "patient"

Given:

old tankless gas heater powered by two D-type batteries like those:

your adept hands.

Gas heater modifications

Since I live in a rented apartment, I did not want to modify the heater in any visible or irreversible way. Bearing that in mind, all improvements I made were minimally invasive. That's what you'll need to do:

Buy two D to AA adapters from China:

Craft two fake batteries from two pieces of wire and battery adapters. They will supply power to internal circuitry instead of conventional batteries. To make them, solder a wire to the backside of + and - contacts inside the adapter like this:

That's what this trick is supposed to yield:
Plug the modified adapters into the battery compartment. The output of our uninterruptible power supply will be connected to them, eventually.
Stretch the wire from some dry corner where you'll plant your electronics to the gas heater. I used a twisted pair, and I advise you do the same to avoid crosstalk.
Connect the adapters with the twisted pair. This connection will be splashed on, that's why I connected the wires submarine-style: smeared some epoxy resin there the wires were twisted together and applied a heat shrink on top while it was drying out. That's the easiest way I found to get a waterproof connection.

This completes the modifications part. Let's proceed to the schematic of our mini-UPS.

Schematic and assembly

The schematic is quite simple (as every schematic I made, ever 😀). For the main power source I used an 1117 3.3V voltage regulator, and 2 cellphone batteries wired in series for backup. There batteries are charged from two 5V PSUs, and buck converter converts their combined voltage to 3.3V.

Here's what you'll need to assemble a thing like that (you can get it all from your local electric/electronics store or from China, the choice is yours):

For AC part of the schematic (left to right, top to bottom) - AC switch, two PG9 cable glands, two Wago connectors (222 series, 2 holes), four quick disconnect crimp terminals, 0.5A fuse and fuse holder. You can find some of it in electric supply stores, some in electronics stores.

For DC part - LM1117-3.3 voltage regulator, PCB mount screw terminal, two PTC fuses rated 0.5A, SPDT relay with 5V coil, perfboard, two 5V power supply units (can be yanked out of USB chargers).

For backup part: electrolytic capacitor rated at least 6.3V with capacity >1000uF, two lithium charger boards (mine are based on TP4056), two cellphone batteries (that's what I used because I had those lying around, but if you're ordering them online or buying offline, why not get yourself some regular 18650 lithium batteries instead?), buck DC-DC converter with 3.3V and at least 1A output.

A project box where you'll cram all stuff I mentioned above.
All sorts of electronic trifles like electrolytic and ceramic capacitors, wire-to-board connectors etc...

When you've got it all, you may begin assembly:

Everything you'll require in one picture for completeness' sake.

Cut and/or drill apertures in the box for cable glands, switch and fuse holder. Mount them all. To make those holes, I found conical drill bit and very sharp hobby knife handy.
Connect Wago 222 to power switch. Mains voltage is to be connected to them - the ability to simply join the wires with your bare hands will come in handy when you'll be installing this box to some remote dusty corner. No crimping, screwing, etc...
Let's prepare the batteries. They can be conveniently arranged as two modules consisting of battery + charging circuit + PTC fuse:
If your batteries, like mine, came with spring-loaded contacts, you can try to avoid making them ugly after soldering wires directly to terminals, and make a male connectors from a piece of a copper bus for them:
Finish assembling AC part of our mini-UPS.
Connect the remaining wires to the battery modules, then hot glue the charging circuit to each module:
Solder power supplies to battery modules. Prepare the connectors to connect batteries and power supplies to perfboard.
Assemble the switching and regulating parts of the circuit on the perfboard.
The only thing left now is to arrange everything neatly inside the box. If you used connectors, it'll be easy:
Hot-glue everything to the bottom of the box. Test that it works (mine didn't work after assembly, turns out I connected 5V and GND to a single spot. Pay attention to details when soldering!).
Switch the circuit breakers off (Safety first!11).
I grabbed mains voltage from a nearby wall socket and carefully hid the wire behind the door frame. You can think of something more clever/visually appealing.
Hook up mini-UPS and switch circuit breakers on. When the power on, the batteries should start charging (red light) and eventually reach full charge (blue light).
Congratulations! You are now a proud owner of a tankless gas water heater that works come rain or shine, and does not require changing the batteries.

Side view: