Measuring Battery State of Charge

The meter shown above would be part of every off-grid solar photovoltaic (PV) system if it existed, but unfortunately it does not. Making an accurate State of Charge (SOC) determination is not a simple task. The purpose of this article is to demonstrate an easy way to make fairly accurate SOC determinations using a digital voltmeter.

Battery state of charge (SOC) at any given moment is perhaps the most important PV system statistic you can have. Not only will it help you decide when to conserve power in order to avoid a shortage, but more importantly it will help you avoid damage to your battery bank and prolong its life.

To determine the SOC, three things must be considered; battery temperature, battery voltage, and the amount of current flow (charging or discharging). The results of this procedure will only be accurate if your batteries are at about 70 degrees Fahrenheit, plus or minus about 5 degrees.

At any given time, your batteries are either charging, discharging, or at rest. To make SOC testing as easy as possible, this procedure includes the ability to test during any of these conditions. During the day, when sunlight hits the panels, use the “Charging” procedure. At night, use the “Discharging” procedure. If no load is connected, and no charge current is flowing, you can use the “At Rest” procedure.

Test Conditions:

For greatest accuracy when using the “Charging” procedure, a significant charge current must be flowing. This happens when your solar panel array is in full sunlight, during peak sun hours. A small PV array produces about 10 to 30 Amperes of charge current, applied to a battery bank of perhaps 500ah capacity or less. A larger PV array might generate 35 to 65 Amperes of charge current, applied to a larger battery bank. Both are examples of significant charge current flow.

For greatest accuracy when using the “Discharging” procedure, the load must be significant when considering the capacity of the battery bank. You can easily create the appropriate load by using incandescent light bulbs. For example:

A 50-Watt light bulb is a significant load for a 100 to 200 amp/hour battery bank.
A 75-Watt light bulb is a significant load for a 200 to 400 amp/hour battery bank.
A 100-Watt light bulb is a significant load for a 400 to 600 amp/hour battery bank.
For a battery bank larger than 600 amp/hours, use two 75-Watt light bulbs.

SOC determination when discharge current is flowing:

After applying a significant load, measure the voltage at the positive and negative battery terminals. Find the voltage nearest to the actual voltmeter reading in the “Discharging” column in the chart at the end of this article. Follow that row to the left-most column where the SOC percentage is indicated. For example, if the DVM voltage reading is 12.26 volts, then the battery has about 70% of its charge remaining.

% of Charge - - - - Charging - - - - At Rest - - - - Discharging

70 - - - - - - - - - - - - 13.30 - - - - - - - 12.36 - - - - - - 12.25 (Volts)

SOC determination when charge current is flowing:

Measure battery voltage while significant charge current is flowing. Look at the “Charging” column at the end of this article, and find the voltage nearest to the actual reading in that column. Follow that row to the left-most column where the SOC percentage is indicated. For example, if the voltage reading is 13.47 volts, then the battery is charged to about 80% of its capacity.

% of Charge - - - - Charging - - - At Rest - - - Discharging

80 - - - - - - - - - - 13.45 (Volts) - - -12.46 - - - - - - 12.30

If the actual voltage exceeds 14.75 volts, the battery is fully charged, and your charge controller may be attempting to apply an equalizing charge. To explain an equalizing charge is beyond the scope of this article, but is included here to let you know that it does not indicate a problem.

SOC determination when batteries are “At Rest”:

If the batteries are neither being charged, nor is there any load connected, compare the actual voltage reading with the “At Rest” column in the chart below. Batteries are at rest at night, when the sun isn’t shining, and no significant load is connected. Batteries can also be at rest when the array current (charge current) exactly equals the current required by the load (discharge current). For best results, the batteries should be at rest for several hours before taking a voltage reading.

You might experience a voltage reading far in excess of the fully charged voltage reading listed on the chart. This is due to an effect known as “Surface Charge”. This surface charge will dissipate after the batteries have been at rest for several hours, and you’ll be able to make a fairly accurate SOC determination. Because of nuances of “At Rest” battery voltage readings, you’re likely to achieve more accurate results when using the “Charge” or “Discharge” procedures.

Due to variables, such as the type of batteries and their condition, don’t expect 100% accuracy. However, the approximate SOC value will be helpful in determining when conservation measures are needed. Keep the chart and a good DVM near your battery bank, and check the SOC often.

State of Charge (SOC) Chart:

% of Charge - - - - Charging - - - At Rest - - - Discharging

100 - - - - - - - - - - - - 14.75 - - - - - - - - 12.70 - - - - - - 12.50
90 - - - - - - - - - - - - - 13.75 - - - - - - - - 12.58 - - - - - - 12.40
80 - - - - - - - - - - - - - 13.45 - - - - - - - - 12.46 - - - - - - 12.30
70 - - - - - - - - - - - - - 13.30 - - - - - - - - 12.36 - - - - - - 12.25
60 - - - - - - - - - - - - - 13.20 - - - - - - - - 12.28 - - - - - - 12.15
50 - - - - - - - - - - - - - 13.10 - - - - - - - - 12.20 - - - - - - 12.00
40 - - - - - - - - - - - - - 12.95 - - - - - - - - 12.12 - - - - - - 11.90
30 - - - - - - - - - - - - - 12.75 - - - - - - - - 12.02 - - - - - - 11.70
20 - - - - - - - - - - - - - 12.55 - - - - - - - - 11.88 - - - - - - 11.50
10 - - - - - - - - - - - - - 12.25 - - - - - - - - 11.72 - - - - - - 11.25


Tip: When batteries are deeply discharged it’s important to recharge them within 24 hours in order to prevent permanent damage.

Disclaimers:

This procedure and the accompanying chart applies to lead-acid batteries, and may not be accurate for other battery types. While the results of these tests can be helpful, they are only approximations. Due to many variables, do not expect a high degree of accuracy. If you need greater accuracy, consider a Tri-Metric Battery monitor. A good-quality hydrometer can be used if your batteries are not the sealed type.

Working with lead-acid batteries can be dangerous, use extreme caution.

John