Battery-Powered Backpack Sprayer Conversion
Step-by-step guide made from Indy Farm Life's YouTube video.
Convert a manual 4-gallon lever-pump backpack sprayer into a battery-powered sprayer using a 12V diaphragm pump and a cordless tool battery adapter kit. The internal pressure cylinder is removed and sealed for gravity feed, while the pump, low-voltage cutoff board, and voltage regulator are mounted onto a plywood base attached to the sprayer frame.
The full step-by-step Spetz Specs for this project (every part, size, diagram and step, free): Battery-Powered Backpack Sprayer Conversion Spetz Specs.
- Difficulty: beginner
Disclaimer
- This project involves wiring a lithium-ion tool battery, operating circuit boards near water and chemicals, and using a propane torch. Follow all manufacturer safety instructions for your battery, tools, and chemicals, and consult a qualified professional if you are unfamiliar with DC electrical wiring.
Safety
- Thoroughly rinse and neutralize any herbicide or chemical residue inside the sprayer tank and pump assembly before cutting or disassembling parts.
- Always use an inline fuse and low-voltage cutoff protection when wiring directly to lithium-ion power tool batteries to prevent short circuits or permanent battery damage.
- Shield open circuit boards and wire connections from water and chemical splashes during filling and operation.
Must know before you build
- Vent the tank lid with a breather hole or valve so the suction pump does not pull a vacuum.
- Test-fit inlet tubing before securing the pump to avoid sharp kinks at the tank bottom spout.
- Set low-voltage cutoff above 15.3V; tool battery packs lack built-in over-discharge protection.
- Seal both cut ports of the internal cylinder with silicone so fluid reliably gravity-feeds.
- House the bare voltage controller inside a plastic box to shield live boards during tank refills.
Tools
- Hacksaw (manual hacksaw)
- Propane torch (handheld propane torch) (optional)
- Cordless tool battery (lithium-ion slide battery pack, DeWalt, XR, 20V Max, 4.0Ah)
- Wire strippers (manual wire stripper and crimper, Irwin Vise-Grip)
- Screwdriver (manual screwdriver)
- Cordless drill (cordless drill/driver, DeWalt, Atomic, 20V Max)
Materials
- Manual backpack sprayer (1, manual pump backpack sprayer, Roundup, 4 gal)
- Silicone sealant (1 tube, silicone caulk)
- Battery adapter and voltage regulator kit (1 kit, Power Wheels battery adapter and PWM speed controller kit, 20V battery adapter with switch, 30A fuse, low-voltage protection board, variable DC voltage regulator, and voltmeter)
- 12V diaphragm pump (1, 12V DC diaphragm water pump, Amariver, 12V, 1.4 GPM)
- Quick-connect lever wire connector (1, 2-pole lever splice connector)
- Plywood board (1 piece, scrap plywood)
- Black paint (1 can, paint, black)
- Conduit clamps (one-hole metal conduit straps)
- Wood screws (1 box, flat-head Phillips wood screws, Hillman, #8 x 3/4 in, zinc plated, 100 pack, #8 x 3/4 in)
- Heat-shrink butt connectors (insulated crimp butt splices)
- Plastic storage box (1, clear hinged plastic container)
- Double-sided mounting tape (1 roll, double-sided adhesive tape, 3M)
- Zip ties (nylon cable ties)
- Clear vinyl tubing (1 piece, flexible clear hose)
- Hose clamps (2, worm-gear hose clamps)
Step by step
Step 1: Modify and install the tank outlet housing
Remove the manual pump linkage and internal pressure cylinder, cut and seal the cylinder base for gravity feed, cut a drainage notch, and reinstall the housing into the tank.
- Disassemble the manual pump linkage and remove the internal cylinder. Unbolt and remove the manual pump lever arm, connecting rod, and bottom plunger assembly from the base of the sprayer. Pull the large internal plastic pressure cylinder out through the bottom opening of the tank.
- Cut the top and bottom off the internal pressure cylinder. Use a hacksaw to cut the upper dome off the pressure cylinder and cut away the lower plunger tube. Keep the middle threaded collar housing intact, making sure to cut below the side outlet spout.
- Tip: Make sure your bottom cut is below the side outlet port where the chemicals exit.
- Common mistake: Cutting above or into the side outlet port on the lower housing.
- Seal the cylinder housing ports with silicone. Remove the screw and washer from both the top and bottom of the cut housing, fill the internal passages and small inlet port completely with silicone sealant, and reinstall the screws and washers so fluid can only exit through the side spout.
- Tip: Remove the washers and screws on both ends before pumping the cavity full of silicone, then reinstall them to lock the sealant in place.
- Cut a drainage notch and reinstall the outlet housing. Cut a vertical notch into the side wall of the plastic housing collar so liquid at the very bottom of the tank can drain into the exit hole. Insert the modified housing with its O-ring gasket back into the bottom of the tank and thread the plastic locking ring around it.
- Tip: Cut a vertical notch in the side of the housing collar so the last inch of fluid at the bottom of the tank can drain completely.
- Common mistake: Forgetting to cut a drainage notch in the housing collar, leaving an inch of undrainable liquid in the bottom of the tank.
Step 2: Wire the test circuit and fit the mounting board
Connect the battery harness, low-voltage cutoff board, voltage regulator, and 12V pump on the bench, then test-fit the painted plywood base onto the sprayer frame.
- Wire the battery harness, cutoff board, and voltage controller on the bench. Connect the positive (fused) and negative leads from the switched battery adapter to the input terminals of the low-voltage protection board. Run the positive and negative output wires from the protection board into the input terminals of the variable voltage controller, and connect the voltmeter and 12V pump leads to the controller's output terminals.
- Tip: Wire the digital voltmeter in parallel with the output terminals on the voltage regulator so you can read the exact voltage being sent to the 12V pump.
- Test-fit the painted plywood mounting board on the sprayer frame. Cut a scrap piece of plywood to match the footprint of the sprayer's bottom metal stand and paint it black. Place the board on top of the tubular metal frame underneath the tank to plan component layout and conduit clamp positions.
Step 3: Mount the components, plumbing, and enclosure to the base
Fasten the plywood base to the sprayer frame with conduit clamps, mount the electrical components and protective enclosure, crimp the wiring, and attach the pump hoses.
- Mount the battery adapter and low-voltage cutoff board and crimp leads. Fasten the yellow battery adapter dock and the clear low-voltage protection box to the front of the plywood base using #8 x 3/4-inch wood screws. Crimp and heat-shrink butt connectors onto the power leads between the battery dock, fuse holder, and low-voltage protection board.
- Tip: Use heat-shrink butt connectors rather than standard twist or clip connectors for permanent, weather-resistant wire splices.
- Mount the voltage controller inside a plastic box and wire the pump. Drill small individual wire pass-through holes in the front wall of the hinged plastic storage container so each lead is held in place while allowing the lid to close. Screw the container to the plywood board with #8 x 3/4-inch wood screws, stick the voltage controller inside using 3M double-sided tape, and route the input and output wires through the holes. Zip-tie the voltmeter in place and connect the controller output leads to the 12V pump using a two-wire lever quick-connect.
- Tip: Use a lever-style quick-connect splice on the pump power leads so you can easily swap out the pump if it ever fails.
- Connect the hoses and mount the 12V pump to the base. Position the 12V pump on the right side of the plywood base, attach the clear vinyl inlet tubing between the tank's bottom outlet spout and the pump inlet barb first to set a smooth, kink-free curve, and secure both ends with worm-gear hose clamps. Fasten the pump's four mounting feet to the plywood base using #8 x 3/4-inch wood screws, then clamp the black spray wand hose onto the right-hand outlet barb of the pump.
- Tip: Hook up the inlet hose between the tank spout and pump before screwing the pump down to the board to ensure a smooth, sweeping curve with no kinks.
- Tip: Orient the pump so the outlet port faces the side you normally carry the spray wand with.
- Common mistake: Screwing the pump to the base board before connecting the short inlet hose, resulting in a tight bend or kink.
- Clamp the plywood base to the sprayer frame. Flip the sprayer up and secure the underside of the plywood board to the tubular metal stand using one-hole conduit clamps and #8 x 3/4-inch wood screws. Clamping the base to the frame is easiest to do before making the final inlet hose connection to the tank.
- Tip: Drill individual holes in the side of the plastic enclosure box for each wire to keep the leads organized and held in place.
- Tip: Leave the plastic enclosure slightly vented during operation to allow airflow, as the voltage controller can get warm inside a fully closed box.
Step 4: Configure the low-voltage cutoff board
Attach the 20V battery pack, power on the circuit, and set the low-voltage protection cutoff threshold to 15.9V.
- Set the low-voltage disconnect threshold to 15.9V. Slide a charged 20V battery onto the adapter and flip the power switch on. Double-tap the left button on the low-voltage cutoff board until the display blinks, then adjust the cutoff threshold to 15.9V so the circuit shuts off before over-discharging the battery.
- Tip: Set the low-voltage cutoff slightly conservative (around 15.9V for a 20V DeWalt battery, which normally cuts off around 15.3V) for extra battery protection.
- Verify the reconnect voltage differential setting. Press the right button on the low-voltage protection module to check the differential value (2.0V), which requires a replacement battery to be at least 2V above the cutoff voltage before the circuit will turn back on.
Step 5: Prime the pump, calibrate voltage, and test spray
Fill the tank with water, check for leaks, adjust the regulator to 12V while priming the pump, and test spray with the backpack.
- Fill the tank with water, prime the pump, and calibrate to 12V under load. Close the plastic lid over the voltage regulator, pour a gallon of fresh water into the tank, and inspect the bottom fitting and hose connections for leaks. Turn on the power switch, squeeze the wand trigger to prime the pump, and adjust the voltage regulator dial so the voltmeter reads 12V while actively spraying.
- Tip: Keep the plastic enclosure lid closed while filling the tank to prevent water from splashing onto the open circuit board.
- Tip: Adjust the voltage controller knob so the voltmeter reads 12V while actively spraying under load.
- Watch out: This is an open circuit board inside the regulator box: do not let it get wet when filling the tank.
- Test-spray the backpack sprayer until empty. Put on the backpack sprayer and test-spray the water, verifying that the demand switch on the 12V diaphragm pump automatically cycles the motor on when the wand trigger is pulled and shuts off when released.