Dear Ole Mechanic;Lawnmower Battery

Why won’t my riding lawn mower’s battery last as long as my pickup battery?  It seems that I replace the mower battery every two years. 


Dear J.M.: 

Vibration is the biggest of several things that kill riding lawn mower batteries.  Some of the other things that kill them are neglect, heat, and cheap charging systems.  I’ll touch on all of these, but first, the biggie--vibration.

If you compare your mower’s engine mounting to your pickup’s engine mounting, you will notice that the mower’s engine is bolted down solid to the frame.  On the other hand, the pickup’s engine has rubber motor (and transmission) mounts between the engine and the frame.  The mower’s engine transmits all of it’s vibration into the frame, and, since the battery is also mounted to the frame, it gets vibrated real good.  The rubber mounts for the pickup’s engine help keep the vibrations from getting to the frame, and since the battery is mounted indirectly to the frame and not the engine, it doesn’t get much vibration at all.  Now, J.M., you didn’t say if your mower had a one-cylinder engine or a two-cylinder engine, but in either case, fewer cylinders means more vibration.  More cylinders, like the six or eight that you probably have in your pickup, results in  smoother running and less vibration.  Sure the mower manufacturers could put in rubber motor mounts and multi-cylinder engines but that would cost more and the darn things already cost too much!  Back to the vibrations.  Nearly all batteries have positive and negative “plates” in them, but the “plates” are not solid.  They are made of a porous granular material--usually lead alloys--that are loosely pressed together, and then held together by a mesh screen.  This allows the liquid electrolyte to soak in and make more electricity.  The vibration shakes the material out of the mesh, and when it builds up in the bottom of the battery and contacts the positive and negative plates, it shorts them out.  The battery is dead.  At this point, J.M., you might think that you could flush out the battery, charge it, and get another season out of it.   DON’T TRY IT!  The liquid electrolyte solution in the battery is sulfuric acid.  It will eat through clothes, eyes, skin, fingernails and bone.  Nuff said, don’t try it.

Tne second of the other problems is neglect.  As grass and stuff collect on top of the battery, the collect moisture.  The “stuff” and moisture will conduct a little electricity between the two battery connections and will slowly run the battery down--especially while the mower sits unused during our month or two of winter.  If a battery sits in a discharged or dead state long enough, it gets a coating on the plates (sulfates).  Once that happens, it can never really be brought back to a full charge.  It dies.

The heat problem comes from the heat of the engine running.  All to often the battery is located too close to the engine due to the limited space in a mower. The heat evaporates some of the liquid electrolyte out of the battery.  If you don’t or can’t replace the evaporated liquid with distilled or rain water, then the battery will run dry.  It dies.

Lastly, the cheap charging system.  Most of today’s mowers rely on a magnet and a coil of wire that inside the flywheel to make a small generator to charge the battery.  This setup does not put out much electricity to charge the battery, but it does keep putting it out all of the time the engine is running.  This can lead to one of two problems.  If you have a small yard, then the small amount of electricity does not fully charge the battery, and it slowly dies.  If you have a big yard, then you may wind up over charging the battery, and that causes the liquid electrolyte to boil out of the battery, and it dies.  Sure the mower manufacturers could put in a better charging system, but that costs more.  As I said before, the darn things already cost too much.

Is there anything you can do to extend the life of the battery?  Yes, when you get done mowing, clean off the top of the battery.  This can be as simple as using a whisk broom to brush off the top, or it can be as complicated as using a shop vac to clean it off.  Also, once in a while (say monthly), check the liquid level in the battery--if you can.  Some batteries are actually sealed, and they can not be checked.  When checking the liquid level, follow the warnings on top of the battery, and look down in the holes.  If you see ridges, then that is the top of the plates, and you need to add distilled or rain water.  I don’t recommend using well water, but it would be better than running the battery dry.  The last thing you could do is get a small battery maintainer and hook it up to the battery if the mower will be setting unused a month or more.

Now, J.M., if you do all of the things that I recommended, you may--just may--get the battery to last three mowing seasons instead of two.

Good luck!

Herr Professor Nuzanbolts