Saturday, June 14, 2014

Repairing my Craftman electric lawn mower

One of the things that really irritate me, is the noise and the smell of a gasoline lawn mower.
The tranquility  of a beautiful place is always destroyed when a gasoline lawn mower is turned on.
It's incredible how much pollution and noise one small contraption like that can generate.
Very very annoying.

So it was not a choice when I bought my electric lawn mower.
For many years I used a wired version, but admittedly, is not really "user friendly".
You need to drag the cable that always entangle itself somewhere.

Because of that, and the fact that I will NEVER buy a gasoline lawn mower, 4 years ago I bought a battery operated lawn mower.
Let me state that I'm not advertising or sponsoring anything, but this is what I choose 4 years ago, a Craftsman 48V cordless lawn mower.
Reading the reviews of the brand/model I bought, I noticed that many people complained about the quality.
Well, I have to say that I am amazed by the fact that for 4 years, I had NOT a single problem !
No gasoline to buy, no oil, no maintenance at all. Just pick it up when I had to mow the lawn and put it under charge after.
And most importantly, not pollution and excessive noise ! For me but also for neighbors.

Not a single problem.

Until recently.
It was an expected problem actually. The batteries died.
I repeat, I'm amazed they lasted for 4 years !

It was time to put the little beast in maintenance.

The machine is very very simple and well designed, the only problem was removing the protective cap to access to the batteries and motor.
When finally I was able to open it, I was able to see the batteries.

Let me say that they were HOT ! Very HOT. Even the cover was hot.
Probably few more days and they could catch fire. So I was lucky to catch them on time.
Why I decided finally to check them out ? Well, because really the machine was not working anymore.
I knew the battery were worn out for quite a time, probably for about a year.
But the machine was still working, for less time than when was new, but still enough to have the work done.

This time it was totally dead.

Here what I found when I opened the lawn mower.

Seeing the batteries, I was able to trace down some spare parts, so I ordered 4 new flame retardant Apex batteries compatible with the NPX-35TFR.
Very important !! When buying spare parts, BE ALWAYS SURE to have an identical equivalent or better.
A flame retardant battery is not an option here. Non flame retardant batteries probably can easily catch fire!

I removed the old ones, and replaced them with the new ones.

Even if they were not fully charged, there was enough juice to have a small run.
The machine turned back like new !

Actually almost.
Now it seems to have developed a overheating problem.
The machine is running for a 2-3 minutes before the protection cut off the power to the motor.
At this time is necessary to troubleshoot the machine.

Troubleshooting the beast


  • the first time the machine is turned ON, it runs for at least 2-3 minutes.
    After that time of "work"  (cutting grass) the protection intervene.
  • it needs at least 2-3 minutes to cool down before the protection can be reset
  • after that it can work for other 1-2 minutes ("cutting grass") before the protection goes off again
  • the protection box is very warm
  • running the motor without load (not cutting grass) it last little bit longer but in the end the protection goes off
  • the motor is "cold" at touch, is not overheated


The machine is very simple.
There are 3 components:
  1. The batteries
  2. The control box/wiring
  3. The motor


The batteries now are new, so the problem can be located in the control box/wiring or the motor.

Control box/wiring

Because I felt the control box hot at touch, I decided to took it apart, but  I did not found any problem.
The heat simply was the over current through the switch and fuse.
Actually the control box is very simple. There is a mechanical bipolar switch and a 20A trip fuse.
I didn't find any visible damage and the fuse seems OK.

The control box opened

The main switch

The main switch opened, like new

The 20A fuse

Thus the most probable reason of the problem is that the motor when under load, uses more than 20A !
Since it does the same also without the load, but using more time before the problem shows up, something must be wrong with the motor itself.

This also can explain why the original batteries were so worn out and melted.
Probably the problem is  going on for long time. The old batteries were simply unable to provide more than 20A so the fuse never tripped.
Now with new batteries the fuse can trip because they can provide more than 20A.


The third component to examine, is the motor.
Possible causes of the problem:
  • dirt/dust in the motor
  • axes support damaged
Only taking apart the motor can say something.
To remove the motor is necessary to remove all the cabling and the battery holder.

Then the blade and fan needs to be removed

At this point is possible to remove the motor unscrewing the 4 bolt.

 The motor

Of course is a good occasion to wash the body, up and down.

The motor is a 48V 15A Johnson Electric, model 724_04025.
Taking apart the motor didn't show any particular problem or damage or dust.
It is a brush motor and the two carbon contacts are almost new, making a good contact.
The bearing rings are sealed and rotate with no particular problem.

At this point the next step is to put it together the lawn mower, sharp and balance the blade and then do some measurement to see how much current actually the motor is drawing during the normal use.


After putting together the lawn mower, I connected a clamp amperometer to the main cable coming out from the batteries and started to do some measurements.

How I measured the current. Removed the protective dome and connected a clamp Amperometer (40 A range) to the main positive cable.
The Amperometer is set for DC current of course.

Here some some results (approximated):

  • Motor without fan and blade : 1.6 A
  • Motor with fan and blade (original) - no load : 7 A
  • Motor with fan and blade - mild cutting : 8 - 9 A
  • Motor with fan and blade - heavy cutting (thick grass) : 13-15 A
  • Motor with fan and blade - heavy cutting (thick grass) peak : 19 A

During the tests the fuse tripped many times and all the times the maximum current measured was less than 15 A but prolonged.

I made the assumption the fuse is defective.
I bypassed the fuse and continued to cut grass measuring the current. Almost all the time the current never went over 15 A.
After the work I checked the temperature of the motor and the batteries.
Just little bit warm. Of course the batteries were depleted.

Final considerations

The empiric tests showed the possible cause of the problem in the fuse.
The nominal current of the motor is 15 A and in normal mild cut was below that value (8-9 A).
The fuse started to trip after few minutes with a drawing  of 12-13 A, once bypassed the fuse, the motor didn't show any overheating as well as the batteries.

Another  possible cause of the problem is the blade.
I never checked it before, now tearing down the machine and putting it back together, I noticed to have much less vibrations.
So it is possible the blade was not well attached in the past, causing more vibrations and thus more current.

I ordered a new blade and I need to find a new 20A protection fuse and this should solve the problem since I don't want to buy the entire control just because the fuse (it is an option at least).


  • I received a new blade and a the original tie for the batteries.
    With the new blade the vibrations are almost disappeared, and it seems the machine behave better.
    I was able to have the entire front yard mowed with 3/4 charge of the battery, basically like when it was new.
    I ordered a new 20A fuse and I plan to add an Amper meter just for fun.

A note about batteries recycling

Maybe someone is curious to know what I do with the old batteries, especially with batteries like the ones I removed from the lawn mower.
Well, are years I use a recycling service company for the batteries : Battery Solution
I usually buy a "iRecycle  kit" for household.
The last one I bought was the "bucket", big enough to hold my battery need for almost an year.
Of course now I have to order a new iRecycle kit since the bucket is full :)

I can not stress enough the NEED to recycle batteries instead than trash them away !
From the button batteries to the sealed batteries, it is IMPORTANT to recycle them.
Don't throw them away !


Other interesting links

Looking for information about my lawn mower I found also things like: