Notes on converting a manual sprayer to battery power

A friend of mine had a manual sprayer, and we got the idea of converting it to battery power. Read this mini-review to see how it turned out.

As usual, a reminder: to keep up with new posts on the blog, I recommend following me on Instagram, where I'll post notifications about all new posts and maybe write about something interesting too - link to account

The idea came about for two reasons: we had an old sprayer, IIRC with an 11 L tank, a battery from a Makita tool, and a desire to make life easier.

We started looking into ways to do it and found there were two approaches:
1. A sealed tank, pressurized with an air pump to deliver liquid under pressure.
2. An unsealed tank with a water pump that pumps liquid from the tank to the nozzle.

The first option is better in some ways, but it has a few limitations. For example, you have to monitor the pressure so the tank doesn't turn into a balloon, and all the connections have to be airtight, including those under pressure.
The second option is simpler, since purpose-built pumps are available, so that's the one we decided on.

First, I should apologize: I hadn't planned to write a review at the outset, so I'll be showing you the already assembled device.

We bought a pump, a hose, and a few other parts, got the tank out, and started modifying it.


Here's the unit almost fully assembled. I'll use it to explain how everything works.


The main component is a pump bought through prom.ua. It turned out we could pick it up locally, and at the time it was also cheaper than from other sellers - link


It turned out to be a specialized sprayer pump, which is a plus. It also has thermal protection (according to the specs) and a pressure relief valve that shuts off the motor if the pressure gets too high. That proved really handy.
The inlet is on the left, the outlet on the right, and the power wires are underneath. It's all very simple. The pump has rubber mounting feet to keep vibration from being transferred to the tank.


The manufacturer's stated specifications. I didn't check them; it was enough for us that the pump worked well and drew relatively little current, since its current draw determined the type of power converter we needed.
The pressure protection also meant we didn't need a current limiter in the converter, which I'd planned to use as a safeguard.


We ran into a couple of minor issues with the outlet. First, the pressure there is fairly high; second, we wanted to be able to remove the hose and nozzle.
To solve this, we simply cut the original tank fitting off, drilled it out a little, then glued it onto the pump outlet. Once the glue had set, we pressure-tested it and were completely satisfied with the result.


We bought the hose at Nova Liniya. There was a wide selection: soft and rigid. We chose one in between, since the rigid hose was awkward to use and the soft one could kink.
We also bought a couple of elbows, choosing ones that the hose would fit inside. There's no pressure at that point, so this arrangement works fine.

The hose from the elbow goes to the pump, while the elbow itself is screwed into the tank and serves as an adapter, making the connection neat and secure. To secure the elbow (which has a threaded section), we drilled a hole, made a cut in it, bent the plastic out slightly to create something like a single-turn thread, and screwed the elbow in.


Inside the tank is a second piece of hose, also fitted tightly. There's an elbow on its end too, but without a thread. We initially put a filter in it to catch larger debris, but later removed it. The elbow stayed as a weight to keep the hose at the bottom.


Electronics.
There are several options:
1. Power it from a 12 V battery—the simplest option. If you want to adjust the speed, add a PWM controller.
2. Power it from an 18 V battery, but a step-down converter is needed.

My friend has a lot of 18 V batteries, so we decided to go with a step-down converter, which would also regulate the pump's power.

We also needed protection against over-discharging the battery. Adding it to the battery itself would have been inconvenient, so we decided to take a different approach: install a low-battery alarm that would beep when the battery was nearly discharged. To get ahead of myself, we set it to about 16 V, or 3.2 V per cell, so we could finish the job while knowing the battery was about to run out.

All the electronics are installed in an electrical junction box that cost 35 UAH.


Now, a little about the electronics.
Low-battery alarm units are available for different voltages, either as kits or assembled. Obviously, it would have been easier for me to buy a kit, but I had to modify it anyway, because it turned out that the 24 V version wouldn't let me set the threshold to 16 V. If you're using a 12 V battery, just buy an alarm rated for the same voltage.


The board has two thresholds: one at which it starts beeping and another at which it stops, to prevent the battery from being over-discharged.
Both thresholds need to be changed, and one of the Zener diodes also needs to be shorted; otherwise, the board won't work.
To change the thresholds, select values for resistors R1 and R5, and short one of the Zener diodes, D2 or D7 (assuming the board is rated for 24 V and the battery is 18 V).


R1 and R5 can be 150–180 kΩ; I think I ended up using 180 kΩ.



Converter We got one with displays for the battery and output voltages, but it turned out there was no point in that—you can use any converter with a suitable current rating.
Connect the battery to the input and the pump to the output. Couldn't be simpler.


I added a diode at the input to protect against reverse polarity. To do that, I cut the power trace and mounted the diode assembly "dead bug" style.


That left the question of power adjustment. The board's built-in adjustment lets you vary the voltage over a wide range, which we didn't need at all, so I had to tweak it a little.
I desoldered the board's original trimmer and connected a 20 kΩ potentiometer in its place.


I also connected a 10 kΩ resistor in parallel with it, and a 20 kΩ resistor in series with that combination (if I remember correctly).
The result was an adjustment range of about 8–13 V, making it smoother and more convenient to use.


A list of the assorted small parts we used for the modification:


For connecting the battery, it would be best to find an old cordless drill and remove its battery holder. We didn't have that option, so we improvised with "whatever was at hand"—and even built in a physical safeguard against connecting it the wrong way.


We connected it and did a few tests. Here are some photos from assembly and testing. The only thing we forgot was to install a switch; it isn't there in the earlier photos.


And that's about it. We didn't make a special mount for the battery; my friend said a small bag would do just fine. The battery can go in the bag, which will be attached to the tank.


A short video demonstrating how it works and switches to auto-lock mode when the water flow is shut off at the spray gun handle.
For the automatic shutoff to work properly, there must be no fluid leaks in the system downstream of the pump.


We could leave it there, but I think it's worth mentioning that we did run into some problems. It turned out that if you spray for a relatively long time (say, 5–7 liters in one go), the pump shuts off. I suspect it overheats, but I don't have enough data to be sure. In any case, if the pump stops pumping for some reason and just hums, you can simply let it "rest".

That's about all from me. I hope this was useful. If you have any questions, ask away and I'll do my best to help.