CMM1200 starts, but runs very slowly

detour

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I have a CMM1200 that was a warranty replacement for a CMM875. The CMM1200 portion is seven years old. I've replaced a hinge pin on it and nothing else... so far.

This mower has always worked consistently for me. I tend to mow every 2nd or 3rd weekend - usually the front yard one day and the back the next. I always keep it plugged into the charger when not in use. All of a sudden, I went to start it after a slightly longer break (5-6 weeks) and it was running very weakly. I managed to mow for about 15 minutes before the blade wasn't turning fast enough to cut grass. I plugged it in to charge overnight and got the green, fully charged light. But the mower is still running extremely slowly. It sounds like nothing is happening and after a few seconds you can hear (and see) the blades moving very slowly.

Today, I opened it up and took a multimeter to it (as best I could). Each individual battery measures right at 12 volts at rest. In series, they measure 24v at rest. When I pull the handle lever, the motor starts, but it runs extremely slowly. I tried measuring the voltage through the brushes to the motor and it went from 0 to a max of about 5v after 15-20 seconds.

I'm at a loss on what else to diagnose to figure out what might need to be replaced. Any thoughts?
 

detour

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In trying to diagnose a little more, I took the brushes off just to see how worn they are. They are definitely worn down a bit. I'm going to order replacements. But after putting the motor back together, the mower seems to have a little more power. Not as much as it should, but noticeably more than it did when I posted last week.

I also checked the battery voltage a little more closely after reading in several places that the batteries should read higher than 12V at rest. The upper battery looks to be right about 12v on the nose (with an analog meter). The lower battery appears to read a bit higher than 12V. If the new brushes don't solve the problem, I may buy one new battery first. Anything wrong with not replacing them as a pair?
 

Filek

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In trying to diagnose a little more, I took the brushes off just to see how worn they are. They are definitely worn down a bit. I'm going to order replacements. But after putting the motor back together, the mower seems to have a little more power. Not as much as it should, but noticeably more than it did when I posted last week.

I also checked the battery voltage a little more closely after reading in several places that the batteries should read higher than 12V at rest. The upper battery looks to be right about 12v on the nose (with an analog meter). The lower battery appears to read a bit higher than 12V. If the new brushes don't solve the problem, I may buy one new battery first. Anything wrong with not replacing them as a pair?
I hope I am not too late to give you an opinion. It does sound like to me that it is a battery issue.

12 volt lead acid battery should be closer to 12.7-12.9 volts fully charged after resting off of the charger for many hours. If you measure the voltage immediately after charging, you would measure about 13.3 volts.

You could try measuring the voltage of the batteries while charging and they should reach a combined voltage of over 26 volts. If not, then it may be your charger. If they do get up to that voltage and you have such poor performance, then one or both batteries are at the end of their life.

Do not mix old and new batteries! It seems like a good way to save money, but you will handicap yourself. The torque and to some extent the RPM of the lawnmower is essentially determined by your weakest battery. So adding a new battery to an old battery means you won't get all of the benefits of the new battery, and the new battery will die faster. Its just the way batteries work in series.

Let us know how you make out!
 
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