# Will this work for voltage step-down for my project?

Thread Starter

#### rossn

Joined May 20, 2021
3
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!

Last edited:

#### LowQCab

Joined Nov 6, 2012
1,914
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.

Thread Starter

#### rossn

Joined May 20, 2021
3
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?

#### LowQCab

Joined Nov 6, 2012
1,914
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. Thread Starter #### rossn Joined May 20, 2021 3 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.

#### LowQCab

Joined Nov 6, 2012
1,914
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.