Need a little guidance... I am building a plant sprayer, and have a 12VDC, 7A motor; I'd like to power this from my Makita 18V batteries.

Will one of these buck converters work acceptably?

https://www.amazon.com/gp/product/B08KZPXK63
https://www.amazon.com/gp/product/B076MTWCYS
https://www.amazon.com/gp/product/B078Q1624B

I am thinking the sealed one, given it is a sprayer and will be outside.

Long ago, I also purchased this PWM motor speed controller... could it be used after the voltage step down?
https://www.amazon.com/gp/product/B00DVGGWC0

Thanks for your feedback and ideas!

 

You Makita Batteries may catch on Fire trying to provide ~7-Amps.
You might get away with running 3 or 4 of them in Parallel, but not with just 1-Battery.

 

You Makita Batteries may catch on Fire trying to provide ~7-Amps.
You might get away with running 3 or 4 of them in Parallel, but not with just 1-Battery.

Can you explain how you determined that? Is that based on discharge rate of the battery. Have you used 18v tools? So you are saying I can run a 6-1/2" circular saw off a single 18v battery, or a large rotary drill such as https://www.makitatools.com/products/details/XRH01ZWX, but not a pump to spray?

Let's look at the math.

This video shows a current test on a 18 V Ryobi, a lower grade tool. Just trying to hold the drill chuck by hand, he's seeing 20 A (360 W).

This article says most cordless drills have a power range of 500-1300 W (28-72 A).

This news release from Makita mentions the saw 'developing' (whatever that means) 680 W (37 A).

Makita is making 17" lawn mowers that use 2 batteries.

I know that if I'm using an impact near continuously (to the point of overheating), I'll go through a 5Ah battery in maybe 15-30 mins. Let's assume 30 mins - that would be a 10 A@18 V = 180 W.

While heavy, the pump I have is rated at 12 V / 7 A. That would be 96 W.

It never crossed my mind as an issue.

Based on this, do you or anyone think it is a practical issue?

 

In the past I've used 18V Dewalt products daily for roughly 7-years straight
in a Commercial / Industrial setting.
This is not a particularly important point in making the following estimations........

A Pump creating Pressure usually runs continuously at Full-Load,
not for 3 or 4 seconds like a Trim-Saw or a Drill.

Have you ever used a Battery-Powered Lawn-Mower ?
First it only makes a ~17" wide Cut, and then depends on Inertia to actually cut anything,
and with all that "Limp-Wristedness" it has to use 2-Batteries in parallel.
Ever seen a real Lawn-Service guy using one ??,
It's nothing but a gimmick.
The same thing goes for a Battery-Powered Leaf-Blower, they are a bad joke.

If you're really dead-set on making this work .........
What you will need are some serious Batteries,
like the ones that are used for Big-Quad-Copters / Drones,
which are actually designed to operate continuously with extremely high Current-Demands.
They start-out at around ~$200.oo each for the size you'll need.
A Sealed-Lead-Acid (SLA) Battery might also work,
but they are quite heavy in the size that you will need,
and take forever to charge completely.

Or, get a Pump that pulls half the Current, and use 2 Makita Batteries.

The Advertised ratings for all these Battery-Powered devices
happened in some Ad-Copywriters dream one night,
they are in no way related to realistic, measured numbers that you can expect to repeat,
especially when subjected to Continuous-Duty-Service.
The set-up you are describing might last a week in a Commercial-Usage setting.

However, if you are using this Pump in a very limited, short Duty-Cycle scenario,
such as 2-seconds On, 10-seconds-Off, (repeat),
you might get away with it, that is, under occasional "Home-Owner" type usage.
But then you have the repetitive Motor-Start-Up-Current to consider as well.

And remember, that you are also going to loose ~10 to ~20% of your Power
in the Voltage-Step-Down process,
and, you have to have a proper Socket to receive the Battery(s),
and, an Aluminum-Box to house the Electronics.

It's always better to eliminate as many sources of Power-Loss as possible,
by matching the Batteries to the Load, or,
matching the Load to the Batteries that You already have.

Also, Battery-Life-Expectancy goes down in direct relation to how hard you beat on them.

Gasoline-Powered setups are available, but they're really expensive.

 

Thanks for your response. Agreed that marketing of these number can and is typically misleading.

The intent here is to not to buy yet another a battery that gets used 4x/yr in a 3 month period, then sits around for 9 months to decay - and to simplify things like charging, etc for my wife that may use the sprayer. The sockets for the batteries are about $15 on Amazon... enclosure... eventually would be nice, though the pump is not weather rated, and the voltage converter is.

FWIW, I find runtime on the 18V sawzalls to be very short (maybe 5-15 mins) and it is not uncommon I will burn through an entire battery (maybe a 90% duty cycle) in that period for a 5 Ah battery. I've also run batteries straight through with similar times with things like running a drill in hammer mode, etc. Let's say the sawzall drains the battery in 15 mins (the long end), to me, that would imply that it's seeing 20 A near continuous at 18 V, or 360 W... whereas the sprayer pump should pull 7Ax12V or 96W.

Now, looking at the batteries, on video of someone disassembling a BL1830 showed that it contains 18650 cells, which have the following specs:

• 18650 Cell Features and Technical Specifications
• Nominal Voltage: 3.6V
• Nominal Capacity: 2,850 mAh
• Minimum Discharge Voltage: 3V
• Maximum Discharge current: 1C
• Charging Voltage: 4.2V (maximum)
• Charging current: 0.5C
• Charging Time: 3 hours (approx)
• Charging Method: CC and CV
• Cell Weight: 48g (approx)
• Cell Dimension: 18.4mm (dia) and 65mm (height)

If I'm understanding correctly, max discharge current for one cell is 2.85 A.

Based on it being an 18 V battery with 10 cells, I assume it has 2 sets of (5 cells in series) in parallel.

Does all this sound right, and that it would have a max discharge rate of 2x 2.85 A or 5.7 A @ 18V, or 102.6 W?

Now there are certainly some losses with the voltage conversion, but wouldn't 2 batteries at max discharge in parallel give us 205 W, with some losses of 20% assumed, that would still be more far more than the power pulled by the pump??

If I bail and go for a dedicated deep cycle battery, how do I know how small I can go?

FWIW - My battery powered leaf blower is an EGO backpack style, and it's not quite a commercial gas blower, but isn't far behind. Only gripe is the continuous run time is not that long, but comes with tons of benefits like a fraction of the noise.

 

Battery Capacity aside ...........
I would just go with your Speed-Controller,
https://www.amazon.com/gp/product/B00DVGGWC0
and then not turn it all the way up for extended periods of time.

The Motor can withstand ~50% over Voltage for short periods,
but will over-heat and burn-up if run continuously over ~12-Volts.

It should be fairly easy to add a resistor to the Control-Pot to
limit the Average-Power that the Motor will see.

If you don't need Speed-Control just set the Control and take-off the Knob,
then put a blob of glue on the Shaft to keep it there.