Estimating a battery’s State of Charge (SoC) using a multimeter is a relatively simple process, but it’s important to note that this method can only provide a rough estimate, as the actual SoC is affected by a variety of factors, such as temperature, aging, and discharge rate. Here are the methods for estimating for two common types of batteries: lead-acid and lithium-ion batteries:
Lead-acid batteries
1. Preparation: Make sure the battery has been sitting for at least a few hours to allow the voltage to stabilize. If it has just been charged or discharged, the measurement will be inaccurate.
2. Measure the open circuit voltage (OCV): Set the multimeter to DC voltage mode and connect it to the positive and negative terminals of the battery. Read and record the open circuit voltage value.
3. Reference table: Compare the measured open circuit voltage with a standard lead-acid battery OCV-SOC table to get an approximate SoC percentage. For example:
– 12.65V and above ≈ 100% SoC
– 12.45V ≈ 75% SoC
– 12.24V ≈ 50% SoC
– 12.06V ≈ 25% SoC
– 11.89V and below ≈ 0% SoC
Note that these values apply to batteries at complete rest and may vary slightly between brands and models.
If you don’t have a voltmeter or battery monitor installed, you can use a multimeter to estimate the battery’s state of charge (SoC).
First, you want to make sure that no loads are drawing current from the battery and no charging source is putting current into the battery. After checking these, wait at least an hour before taking a measurement so that the voltage stabilizes and the reading is as accurate as possible.
After this, set the multimeter to measure DC voltage in the 0-20 range (a typical range for most multimeters without autoranging) and touch the probes to the battery terminals (the probes will glow red and black to indicate + and -). The voltage readings can be cross-referenced in the graph below to give a rough indication of the battery’s SoC percentage based on your battery chemistry.
Lithium-ion Batteries
1. Preparation: Again, let the battery sit for a while before testing to avoid biasing the results of charging/discharging immediately after measurement.
2. Measure the open circuit voltage (OCV): Use a multimeter to measure the voltage across the battery.
3. Reference table: For lithium-ion batteries, it is usually necessary to refer to the manufacturer’s OCV-SOC curve or table. In general:
– 4.20V ≈ 100% SoC (full charge voltage)
– Around 3.7V ≈ 50% SoC
– 3.0V ≈ 0% SoC or near deep discharge
Important: The OCV-SOC relationship of lithium-ion batteries is non-linear, with more dramatic changes at near full and near empty states, and more gradual changes in the middle area.
Notes
– Temperature compensation: Battery temperature affects voltage readings, so it is best to measure under moderate ambient temperature conditions.
– Battery type specificity: Batteries of different chemistries have different OCV-SOC characteristics, so be sure to use the correct parameters for evaluation.
– Health considerations: Aging batteries may not provide the corresponding capacity even if they display normal voltage, so estimating SoC based on voltage alone has limitations.
Additional notes:
In order to obtain a more accurate SoC evaluation, other technical means can also be combined, such as tracking the amount of power in and out of the battery through the coulomb counter method, or using a professional battery management system (BMS). If you have questions about how to use a multimeter to estimate the SoC of a battery or need a custom cable, you are welcome to email TST CABLES cable engineers at any time to communicate and get free samples.
Also available in:
Arabic 
English 
Japanese 
Russian 
Portuguese (Brazil)