Dual battery system testing

Before setting out on a long trip, it’s comforting to know that your charging system is working properly. You do not want to be stranded in the outback with two flat batteries! The following tests can be performed with a digital multimeter and in less than an hour.

Multimeter and electrical basics

I assume that have a basic understanding of electrical systems and know how to measure voltage with a multimeter. Here are a few pointers for the novice:

  • Basic multimeters have settings for Direct Current (DC) volts, DC amps, resistance (ohms) and Alternating Current (AC) volts. Many auto electrical systems are 12 VDC. Larger vehicles are 24 V.
  • DC is signed and current flows from positive to negative by convention. When testing a DC circuit, reverse the multimeter leads and the sign will go from positive to negative and vice versa.
  • Voltage is measured in parallel. Example: to measure battery voltage, probe the positive battery terminal with the positive lead (usually red) and the negative with the negative lead (usually black).
  • Current is measured in series. To measure current, you will have to break the circuit and insert the multimeter into the circuit. Example: to measure DC current, connect the positive lead ‘upstream’ and the negative lead ‘downstream’.
  • Voltage is easy to measure. Current is more dangerous. Most multimeters will blow a fuse or ‘burn’ if subject to more than a few hundred milliamps.
  • Always check the multimeter selector switch and leads before testing. Decide what you are going to test and where you are going to place the leads before proceeding. Do not experiment or use trial and error.

Disclaimer: Electrical systems and batteries can be dangerous. I am not responsible for any losses, damages or accidents you may incur by following these instructions. If you don’t understand then do not do it!

Step 1 State of Charge

Measure the voltage and temperature of both batteries. Car and Deep Cycle Battery FAQ provides State of Charge (SoC) tables for different battery chemistries. It is recommended to let a battery rest (no current flow in/out) for two to eight hours before measuring SoC.

Main Aux.
Rested voltage 12.59 12.64
Temperature (degC) 27.2 24.1
SoC 93% 84%
State of Charge (SoC) measurements. The main battery was a wet low maintenance battery and the aux was a flat plate AGM. The aux battery was old and did not charge more than 12.66 V.

Both batteries should be fully charged. Low battery voltage could be mistaken as incorrect charging voltage and/or excessive voltage drops in the cables.

Step 2 Voltages with battery isolator disabled

I installed a switch to disable the battery isolator when the aux battery is not being used. Otherwise proceed to Step 3.


  1. Measure voltage at the alternator and main battery with the engine running at idle, first with no load.
  2. Measure voltages again with the headlights on, high beam (if the main battery is fully charged then something needs to be switched on to load the alternator).
  3. Measure voltage at the aux battery to verify it is isolated from the alternator charging system.

It is difficult to measure accurately the resistance of large diameter battery cables. Resistance can be inferred by measuring voltage drops under load (voltage equals current times resistance). Large voltage drops indicate large currents or large resistances. As a rule of thumb, voltage drops in (short) cables should be less than 0.2 V.

To find the alternator output terminal, look for a thick cable running to the main battery positive terminal. Alternator voltage is measured between the alternator battery terminal and the alternator housing.

Here are some measurements:

No load Load Tolerance
Alternator voltage 14.21 14.17
Charging voltage (battery) 14.11 13.97 13.8-14.8 V
Voltage alt. to main + 0.09 0.19 0.2 V
Voltage alt. body to main – 0.04 0.07 0.2 V
Results of charging system tests with battery isolator disabled, engine warmed up and idling.

The charging voltage was high because the battery was in absorption/float mode. Switching the head lights dropped the voltage only slightly because the alternator was supplying most of the current.

Low charging voltages could be due to:

  • Flat battery (bulk charging, voltage increasing over time).
  • Bad battery.
  • Cable (too small) and connection (e.g. damaged, corroded) resistance.
  • Bad voltage regulator.
  • Inadequate alternator output.

SoC measurements should have identified a flat/faulty battery.

Probing around the battery leads with the engine running and the alternator loaded, you might be able to pinpoint voltage losses.

Also check that the alternator drive belt is not slipping. Increase engine speed and the alternator output should also increase. Perhaps some vehicles have undersized alternators.

If the battery is good, the alternator drive belt and pulleys are OK and the voltage at the battery terminals is low then the alternator and/or voltage regulator is probably bad. You may see a red charge warning light on your vehicle’s dash.

Finally, to confirm isolation, I measured 12.64 V at the aux battery with the engine running and the isolator disabled, the same as measured before starting the engine.

Step 3 Voltages with battery isolator on

The previous testing can be extended to include the aux battery. Switch the isolator on and connect a load to the aux battery.

My aux battery is in the back of the vehicle and long multimeter jumper wires were required to measure voltages between the main and aux batteries. There is negligible current flowing during a voltage measurement and small diameter wires are suitable.

Alt. Main Aux Drop Tolerance
Charging voltage 14.04 13.84 13.38 13.8-14.8
Voltage alt. to
main +
0.15 <0.2
Voltage alt. body
to main –
0.06 <0.2
Voltage main +
to aux +
0.26 <0.5
Voltage main –
to aux –
0.23 <0.5
Charging system test results with battery isolator enabled, 7 A load on aux battery, engine warmed up and idling.

As per the design, 0.5 V drops are allowed in the long cables between the main and aux battery. Note that the alternator chassis return voltage drop (0.06 V) was less than half of the voltage drop in the alternator cable (0.15 V). The aux to main negative voltage drop was relatively large (0.23 V) because I installed a long cable rather than a chassis return.

7.2 A is a small load and voltage losses will increase with current. For example, the total cable voltage drop at twice the current = 2 × (0.26 + 0.23) = 0.98 V at 14.4 A. The alternator current could also be larger than the load current if the aux battery is also accepting some charge. I have measured charging currents in a subsequent post.

Step 4 Isolator cut-out

Having completed alternator testing, switch off the engine and observe the isolator disconnect the aux battery.

My isolator is designed to disconnect the aux battery when the main battery voltage drops below 12.8 V and after a delay of 60 seconds:

  1. Switch the engine off and immediately measure both the main and aux battery voltages. Then switch the parking lights on to draw the voltage down quickly.
  2. Monitor the main voltage and observe the isolator disconnect. On my isolator, a blue LED goes out and there is an audible click from the relays.
  3. Remeasure main and aux voltages after about 5 minutes and then switch the parking lights off.
Main Aux Remarks
Initial voltage 13.62 13.45 Falling.
Cut-out voltage 12.70 12.78 Cut-out voltage 12.8 V.
Isolated voltage 12.35 12.75 Load on main battery.
Results of isolator cut-out tests. The main and aux battery will show different voltages when isolated.

Step 5 Isolation from the auxiliary battery side

I use a solar panel to charge my aux battery when camping and the vehicle is parked. I have a single-sense isolator and there should be no charging voltage at the main battery when the solar panel is charging the aux battery.

Main Aux
Voltage rested 12.62 12.60
Voltage aux charging 12.62 13.70
Results of isolation test with aux battery charging. The main battery was isolated and its voltage did not change. The isolator was enabled but it did not connect the aux battery to the main battery.

Diode troubleshooting

Automotive alternators have diodes to rectify AC power to DC. Diode failure is either an open or a short circuit and will result in unstable voltages and reduced output. Better methods for detecting diode failures are oscilloscope measurement and/or load testing.

My cheap multimeter is useless for measuring alternator AC ripple voltage. The DC setting did give a useful indication however. When measuring DC charging voltage directly on my new alternator I observed variations of 0.01 to 0.02 V. With my old alternator, the voltage was fluctuating wildly over a range of about 10 V. I assume there was a diode problem. I didn’t have a high current ammeter at the time.


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