Running a cordless lawn mower with a DC-DC buck converter?

Thread Starter

jjbobay

Joined Oct 20, 2017
8
I have a 24v (20Ah SLA) cordless mower, and the batteries only seem to last 2 seasons, and they require a 12-hour charge. I also have two EGO 56v 5Ah Li-ion batteries for another power tool. The mower with its current batteries runs 45-60 minutes, and the wiring to the battery is no more than 16awg, so I assume the motor takes 20-30A under load. Would it work well to use a 30A DC-DC buck converter I found on Amazon to convert 56v to 24v? Theoretically, the amp-hours would double to around 10Ah at 24v, which would be fine since I have two batteries and they quick charge in 45 minutes. I'm just wondering if the buck converter would handle that kind of a load for a sustained period of time, and would it produce an output a high-power motor like this could use properly?

Sure, I could buy more batteries, buy a mower that goes with the battery, or build my own Li-ion battery pack, but these would be expensive (and in the case of building the battery pack, difficult for me to build a proper charger), and seeing as I already own a battery pack and charger which also works with other tools, a $25 investment would be much better than the other options. Plus, it would make the mower lighter and I should have no slow down, even as the Li-ion pack gets at the very end because the buck converter should minimize any voltage drop until the pack shuts itself down.

Similarly, I have a 14.4v cordless vacuum cleaner, about 10 years old, I purchased at a garage sale, with 12 NiCd D-size batteries which don't hold a charge. Again, I could put $70 into larger NiMH batteries but I wouldn't be able to use the pack for anything else. Could a similar DC-DC buck converter (one with an adjustable voltage output, rated at 20A) with an 18v Ryobi cordless tool battery run the vacuum? This one has a computer-controlled variable-speed motor in it so I would not want to over-voltage it at 18v for fear of frying something. Or would I be better to use some diodes to drop the voltage a couple of volts? Again, I do not have a current rating for this, and not even an Ah rating for the batteries, but I would assume since they're 10-year-old D cell NiCD, the wouldn't exceed 4Ah.

Since DC-DC converters claim to be 90% or more efficient, it seems to me that these conversions would work, but I don't want to purchase them and then fry them, or my tools. I have already used the 18v Ryobi batteries to overvolt my 4-year-old son's power wheels extensively, and have found they can really take a beating. But those are simple brushed motors with somewhat lower power ratings than I'm thinking of here. So am I crazy for wanting to try this, or is it feasible?
 

Alec_t

Joined Sep 17, 2013
12,203
Welcome to AAC!
Would it work well to use a 30A DC-DC buck converter I found on Amazon to convert 56v to 24v?
Probably not, but it depemds on the converter characteristics. The problem is that a motor at start-up draws several times the current that it does at its normal running speed. If the normal current is 20A the start-up current would be way above the 30A converter rating. The converter might get fried or it might refuse to run or it might actually get the motor going.
 

Dodgydave

Joined Jun 22, 2012
10,050
Probably wont be any different, your converter needs 240W, @ 56V input thats 5 amps, which is the capacity of each battery, so 10aH will give you two hours at 100% efficiency.
 

Thread Starter

jjbobay

Joined Oct 20, 2017
8
Welcome to AAC!

Probably not, but it depemds on the converter characteristics. The problem is that a motor at start-up draws several times the current that it does at its normal running speed. If the normal current is 20A the start-up current would be way above the 30A converter rating. The converter might get fried or it might refuse to run or it might actually get the motor going.
I hadn't thought of that.. And I'm purely speculating on the current draw anyways, so I could be way off on that too. And I don't have any device to measure greater than 10A DC unfortunately. :(
 

crutschow

Joined Mar 14, 2008
27,734
You could add a low value shunt resistance in series with the motor to measure the current.
For example, 1 foot of 16AWG copper wire has a resistance of about 4mΩ, giving a voltage drop of 4mV per amp.
You should then be able to determine the approximate current using a multimeter with a 200mV range to measure this voltage drop, which will give a 50A full scale current.
If you are concerned, the fusing current of 16AWG copper wire is 117A.
 

ian field

Joined Oct 27, 2012
6,539
Welcome to AAC!

Probably not, but it depemds on the converter characteristics. The problem is that a motor at start-up draws several times the current that it does at its normal running speed. If the normal current is 20A the start-up current would be way above the 30A converter rating. The converter might get fried or it might refuse to run or it might actually get the motor going.
Not just motor start current - there's also longer duration stall current if anything jams the mower.

Unless the module is short protected, some sort of current trip will need to be added.
 

tcmtech

Joined Nov 4, 2013
2,867
Given you have SLA batteries it might be easier to just get or make a 28 - 29 VDC charger that puts out 5 - 10 amps and make your charging process for them far faster.
 

Thread Starter

jjbobay

Joined Oct 20, 2017
8
You could add a low value shunt resistance in series with the motor to measure the current.
For example, 1 foot of 16AWG copper wire has a resistance of about 4mΩ, giving a voltage drop of 4mV per amp.
You should then be able to determine the approximate current using a multimeter with a 200mV range to measure this voltage drop, which will give a 50A full scale current.
If you are concerned, the fusing current of 16AWG copper wire is 117A.
Thank you for the tip, I just tried it with both 16 and 18 gauge stranded copper wire (1ft of wire connecting between the 2 series 12v batteries). The results were much higher than I had anticipated. With 16awg (4 milliohms per foot), on startup I saw 500mv voltage drop = 125 amps, no load = 31 amps, and under fairly heavy load = 61 amps. I'm surprised since the 20Ah batteries last 40-60 minutes, I was expecting much lower. Tested with a medium-quality old Radio Shack meter and a cheap Harbor Freight one, both read the same. So at no load, 125mv / 4milliohms=31.25a, correct?
It seems to me if it is drawing 30-60a, the batteries should only run 20-40 minutes instead of 40-60, especially since SLA batteries are rated at 20hr discharge rate, and will be significantly lower Ah rating at high load such as these. Oh, and I found the internal wiring is 12awg stranded instead of the 16awg I had previously speculated.
 

Thread Starter

jjbobay

Joined Oct 20, 2017
8
I also tried my 10a max multimeter with the cordless vacuum, and, much to my chagrin, it measured all the way up to 21a (startup) and about 15a running. So in both cases, I had underestimated the motor current, which means a larger DC-DC converter, or sticking with the original batteries.
 

crutschow

Joined Mar 14, 2008
27,734
Those measured currents do seem high.
Can you try a solid copper wire (such as from romex type cable) to do the measurement?

Make sure you do the voltage measurement directly to the bare wire, after the point is is attached to the load and source.
 

Thread Starter

jjbobay

Joined Oct 20, 2017
8
Those measured currents do seem high.
Can you try a solid copper wire (such as from romex type cable) to do the measurement?

Make sure you do the voltage measurement directly to the bare wire, after the point is is attached to the load and source.
I will try that, I read about different diameter strands having slightly different resistances. And that's a good point, too, I did not measure from the actual wire, but from the battery terminal, which is not copper. I will re-test and see if I get more reasonable results. I'm glad you also feel that is a bit high. Thank you for the tip!
 

Thread Starter

jjbobay

Joined Oct 20, 2017
8
I measured the mower's voltage drop with 4' of 14awg solid wire this time, clamping the voltmeter to the wire after the connection. Using i=v/r, and using milivolts and miliohms, the math was simple. 10.1 miliohms for 4 feet. Running idle showed a 150mv drop /10 = 15a. Under some load, 250mv = 25a, and start was hard to determine since the sampling rate of the meter is only a few Hz, but I saw a reading of 750mv, 75a. Still a tad bit more than expected (and hoped) but now my 50 minute runtime on 20Ah batteries and 12awg internal wiring makes sense.
It sounds like quite a task for a DC-DC converter to handle, even if I could find a 30a one. It looks like I'd need 40 or 50 amps at least, maybe with a decent-sized capacitor to help with that starting load.
It sounds like it's going to be best to keep using the SLA batteries. I did wire a 'center tap' to allow me to use a 12v smart charger to charge each battery individually, and has proven to be a viable option.
Crutschow, thank you for all of your help!
 

Thread Starter

jjbobay

Joined Oct 20, 2017
8
Now, onto the vacuum cleaner.. At a local hamfest, I picked up 3 50 amp bridge rectifiers which drop the voltage 1.2v. That seemed high until I realized it's going through 2 diodes in each direction, total of 4. So it seems as if I could try stacking 3 of them with my 18v Ryobi battery (20v open circuit) and get 14.4v. I've never used a diode specifically to drop voltage, and especially not at fairly high current (I measured about 20a startup current). I guess I'll be wasting 24 watts per rectifier, correct (1.2v * 20a)? Does anyone see a problem with doing this?
 

tcmtech

Joined Nov 4, 2013
2,867
Unfortunately given all the runaround required to make non standard stock batteries work with your assorted high load stuff it just seems simpler and likely easier and cheaper to rebuild the old packs and be done with it.

Here's a great place that carries high quality battery cells of assorted sizes and chemistries that I use fairly often when I need to rebuild a nonstandard battery pack for something on the cheap.

http://www.all-battery.com/index.aspx
 

Thread Starter

jjbobay

Joined Oct 20, 2017
8
Update- I determined the lawn mower just took too much current, the converter would cost more than the mower is worth, so I bought a new 56v EGO 21" mower and love it. Problem solved!

The vacuum cleaner, however, has new life. I purchased a pair of 50a bridge rectifiers at a local hamfest, eventually got time to wire it up to drop the voltage from 18v (20v open circuit voltage) of my Ryobi One+ battery. The rectifier spec says it drops 1.2v per side, so I figured 2 in series would drop 4.8v. In reality, it only drops 1-2v total and gets extremely hot during use (I estimate about 15-20a of running power, so it was dissipating 40-60w), so I decided to try running the 14.4v vacuum directly on the 18v battery. It is a variable-speed logic-controlled motor so I figured the controller would either compensate for the extra voltage, or burn up. I wasn't going to spend $80 to build a new 12 D-size NiMH battery pack anyways. Basically, it ramps right up to normal motor speed and seems to work perfectly. Apparently the controller diverts only the power it wants to the motors. I have only run it for about 4 minutes at a time, at which point the 14awg stranded wire from the battery to the input terminals heats up warmly. So at least some of the extra power is being dissipated as heat, but the vacuum seems to work great. I tore apart a Ryobi charger that quit working, mounted it onto the vacuum, ran a short wire, and it works great so far! The drill batteries have an overcurrent shutdown (which I have found occasionally kicks in when overvolting kid's Power Wheels) so even though the vacuum is likely pulling 15-20a from a 4Ah battery, it seems to be fine with it. The batteries have taken some serious abuse in the Power Wheels with no noticeable loss of capacity. So it looks like everything is happy, and the solution was simpler than expected.

I appreciate everyone's suggestions and input on measuring current. Hope this helps someone else modify something.

Oh, and I have been running my 5-year-old's Power Wheels and Peg Perego ride-on toys for 1.5 years on the 18v Ryobi batteries. Cheaper, faster charging, and much more fun! But those are simple circuits- motor, switch, batteries. Not much to go wrong other than very warm wires, or a burned up motor (which I haven't had yet).
 

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MisterBill2

Joined Jan 23, 2018
9,278
What I have tried, and found that it did not work very well, was powering a 12 volt tool from the car battery using some #12 extension cord stock. The operation was pitiful. And dragging around a car battery and using a short#10 cord was not worth the pain. So now I use 2-stroke engine yard tools and they last a long time and I never need to charge the battery packs. Sure they are smelly and noisy but a lot more powerful. All is trade-offs!
 
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