# Resistance measurement across reversed batteries using multimeter

If you take two similar capacity batteries and put them in contact in a reversed configuration (anode-anode or cathode-cathode) in series, the resistance measured by a digital multimeter across the batteries is ~100 ohms, but if you exchange the leads of the multimeter the reading shows 0 ohms. Why is there a polarity issue for resistance measurement by the multimeter ?

I confirmed this with two independent multimeters. Can someone please clarify ? Thanks.

• One of the batteries is slightly different from the other, probably because of manufacturing tolerances. Commented Aug 19, 2016 at 2:54

An Ohmmeter measures resistance by applying a current to its probes, and measuring the resulting voltage. As a result, in any circuit which generates its own voltage, you will have the potential for errors.

In the case of 2 back to back batteries, that's a good try. However, any slight difference between the voltage of the batteries (and there will be, battery voltage changes with discharge state, discharge history and temperature) will give a non-zero voltage.

When measured one way, this voltage will add to the resistance*current voltage, and so artificially increase the apparent resistance reading. When measured the other way, it will subtract, and may even give a negative voltage. A meter will not have been designed to interpret a negative reading, and so will probably read zero.

The only reliable way to measure the internal resistance of a battery is to measure changes in terminal voltage when you change the terminal current, either by changing the load, or using an AC excitation signal.

When using a meter to measure resistance, a different reading for either polarity is a good indication that 'something is going on', and neither reading should be trusted. Badly behaving circuits could include (not an exhaustive list) a battery, a diode or other semiconductor, a big capacitor with some residual charge on it, or junctions with thermo-electric voltages being developed.

With experience, you can use the fact that a DMM on ohms is a current source with a voltage measurement, and use it to investigate components other than resistors. The rate at which the reading changes when measuring the 'resistance' of a large capacitor can give you some indication of its value. Be aware that different ranges will use different currents, and an auto-ranging meter will quite happily switch ranges when you don't expect it.

Any Ohmmeter measures resistance by applying a voltage between its probes, and measuring the resulting current (or perhaps applying a current and measuring the resulting voltage). As a result, you CANNOT measure the internal resistance of a battery or other power source using an Ohmmeter as the battery's own voltage will upset the meter's reading.

To measure the internal resistance of a battery, you need to make several measurements. First, measure the open-circuit (no load) voltage of the battery, then connect a load, and measure the load current and the resulting battery voltage (preferably simulatneously, with two meters.)

Attempting to measure the internal resistance of a battery with an Ohmmeter (multimeter, DVM) may damage the meter.

• My question referred to the observed "polarity". A measured value of the resistance should not change when the leads of the multimeter are interchanged. Commented Aug 19, 2016 at 3:05
• Any measurement of a battery's resistance with an ohmmeter is meaningless. For your "measurement", the meter probably is not designed to show a negative value when measuring resistance, so showed "0" rather than "-100" for your second reading. Commented Aug 19, 2016 at 3:56