power supply problem

Thread Starter


Joined Aug 21, 2007
One of our Cushman carts used to get around the plant came in with a work order saying they didn't think it was charging right. The carts have onboard battery chargers. Can anybody recommend a proceedure for investigating this problem without an occiloscope but with just a vom?
I have a couple questions. If you put the test leads on the batteries when the charger is operating what would that tell you compared to disconnecting the charger from the batteries and then putting the leads on the charger? I understand the basic components of a power supply-at least the older ones-the transformer, the rectifier(usually diodes), the filter or part that smooths out the voltage ripple. To check diodes would you need to disconnect them from the circuit? Thanks for any help! Leon


Joined Jan 28, 2005
One technique you can try is to obtain a very low valued, high wattage resistor, for example 0.1 ohms at 5 to 10 watts. Put the resistor in series with the connection to the positive battery terminal and then set the voltmeter to measure volts across this resistor. Ohms law will allow you to calculate from the known resistance and the measured voltage what the current that is being delivered to the battery is when placed in charge mode.



Joined Sep 11, 2007
You might want to check the batteries rather than checking the circuit board.

batteries usually fail to completely charge, due to over usage, and over charging conditions.

check the amp out put of the battery, the problem could be fixed by just replacing the batteries


Joined May 19, 2004
if you know the nominal battery voltage (say 24vdc), read the voltage on the battery terminals, this will give you an idea of the amount of charge/discharge. then read the voltage again on the terminals with the charger plugged in. the charging voltage should be 15 - 20% above the nominal battery voltage.

also, make sure to check all connections and look for possible fuses.


Joined May 7, 2007
I agree that you probably have an old battery that has been deep-cycled too many times.

The charging circuit should have a "charge" and "float" cycle. Assuming you have a lead-acid battery the nominal for a 12V battery is around 12.6V (at rest/open circuit) under a load it will be lower. The charging voltage will as mentioned above 10% - 20% higher. The float voltage will be around 13.5V (but this is dependent on specifec lead-acid type).

Old batteries will have a higher internal resistance so you can also measure that to give you an idea of the state of the battery.


Joined Aug 30, 2007
I used to work on some of these. What I did first was to check battery voltage with charger off ( we had 36 volt buggies six 6 volt batteries ) should read battery voltage, 24 or 36. Then plug in charger and voltage should go up. Ours had a battery sensing circuit, charger wont turn on untill it's connected to battery. You can hear a relay inside the charger clicking on when it energizes, that relay switched the line voltage (120v ) to the primary of the charger. If that's ok but the charger still doesn't work look at the relay contacts, I've seen them burned bad enough that the 120v won't make it to the transformer. If that's ok check that you have a voltage at the secondary of the transfoemer. If that's good look at the rectifiers, they're usually mounted on a sheet of aluminum for a heat sink. After that ther's a fuse mounted under a plastic dome, sometimes it's visable,sometomes not. Some have a circuit breaker instead of a fuse. It's all simple trouble shooting, start at the beginning and work towards the output...
Hope this helps..