Test charging system


#1

Can I use a hand-held multitester instrument (that has DC & AC settings) to check the charging system on my John Deere lawn tractor?



The battery died completely and won’t take a charge. I’m trying to track down the problem and want to make sure the battery is being recharged when the engine’s running.

Thanks.


#2

You sure can do that. Set the meter to read DC volts and see what it shows across the battery while the engine is running. You should see around 13 volts or more with the engine running. More RPMs should increase the voltage slightly.

There shouldn’t be a current drain problem while the engine is off but if you see the voltage drop below 12 volts after time check for that.


#3

Thanks. If I want to see if the charging system is working, can I disconnect the battery while engine is running and hook the meter up to the battery leads to see what’s coming back to the battery? Or does what you advised above do the same thing? Does any reading above 12 volts mean that the battery is being charged?


#4

Do not disconnect the battery while it’s running.
Simply connect the VOM across the battery terminals and check the voltage on the DC scale.

The statement about anything above 12 volts meaning the batt. is being charged is incorrect.
A 12 volt battery has 6 cells; each with 2.1 volts if the battery is good, for a total of 12.6.

What you’re looking for is around 13.2 to 14 with the engine running, depending on the battery charge, any lighting on, etc.


#5

can I disconnect the battery while engine is running and …

No. Not only is that going not tell you anything about the battery or charging system, it can damage it.


#6

Connect a battery charger to your battery to determine if it will take a charge. Charge it and then try to start your mower. It is possible that your battery has failed. If the battery will take a charge, then you can try to determine if your mower’s charging system is at fault.


#7

First check the voltage at the battery with the engine off. It should be between 12.4 and 12.8, depending on the condition of the battery. If it’s less than 12.0 volts, charge or replace the battery first. Then, with the voltmeter still connected to the battery, start the engine. The voltage should slowly begin to climb above 13 volts as the battery begins to charge. If you start the engine and the voltage stays steady or begins to fall, the charging system is not working. You can load the system by turning on the lights as you watch the meter.


#8

Safety. If you work near the battery or on it, at least wear eye protection. Goggles at least. When a battery pops, it will spray acid all over.