Test Your EV Charger's Output Voltage with a Multimeter

Why this happens

A charger that has failed may produce no output, a low voltage or an unstable one, and the symptoms can look like a battery problem. Measuring the open-circuit output at the plug and comparing it with the label helps decide whether the fault is in the charger or the vehicle. Some chargers only switch their output on when they detect a battery, which is itself useful to know.

What you need

Step by step

1. 1

   Read the charger label

   Find the output rating on the charger label, for example a full-charge voltage such as 54.6 V for a 48 V NMC pack and an output current in amps. Note the pin or polarity diagram if one is printed. If there is no clear pin information in the label or manual, do not guess by probing random pins; ask the dealer instead.

2. 2

   Inspect before testing

   With the charger unplugged, look over the whole charger: mains cable, plug, case, vents and output lead. Do not test a charger with cracked casing, damaged insulation, burn marks or a rattle of loose parts inside. Make sure the work surface is dry and non-conductive, and remove rings and watches.

3. 3

   Set up the multimeter

   Plug the probe leads into the meter's COM and V sockets, not the amps socket. Select DC volts with a range comfortably above the charger's rated output, such as the 200 V DC range on a manual meter. Check the meter works by measuring a known source, such as a fresh AA cell, before using it on the charger.

4. 4

   Power the charger with the output disconnected

   Lay the charger flat with the output plug resting on the dry surface, not connected to the vehicle. Plug the charger into a properly earthed wall socket and switch on. Keep the output plug's pins pointing away from you and from anything metal. Note what the indicator light shows; many chargers show green or a standby light when no battery is connected.

5. 5

   Measure at the output plug

   Holding only the insulated parts of the probes, touch the red probe to the positive pin and the black probe to the negative pin of the output plug, as shown in the manual. Keep the tips apart and do not let them slip into contact with each other or other pins. Read the voltage and remove the probes. Switch off and unplug the charger from the wall when done.

   Tip: If the pins are recessed, ask the dealer for the correct test method rather than forcing probes or using bare wire.

6. 6

   Interpret the reading

   A reading close to the label's output voltage means the charger is producing voltage with no load. Zero or a very low reading may be normal for chargers that only start when they detect a battery; check the manual. A reading well above or below the label, or one that jumps around, suggests a charger fault. Remember that a correct open-circuit voltage does not prove the charger can deliver its rated current.

7. 7

   Decide on the next step

   If the charger reads correctly but the vehicle still will not charge, the problem may be the charging port, the vehicle's wiring or the battery's BMS, which is a service centre job. If the charger reads wrong, replace it with the manufacturer's approved charger for your battery. Do not repair or adjust the charger yourself.

How to tell it worked

You know whether the charger produces roughly its rated output voltage at the plug, which tells you whether to look at the charger or the vehicle side of a charging problem.

Questions