Testing a small 12V solar power system

I have a dual-battery system in my vehicle and solar panels for camping. This article describes how to test a small 12 V solar power system.

The system I tested includes two 120 W solar panels in parallel, a Pulse Width Modulation (PWM) charge controller and 225 Ah wet deep-cycle battery. I have no experience with and no procedures for Maximum Power Point Tracking (MPPT) controllers.

I assume that have a a basic understanding of electrical systems and know how to measure voltage with a multimeter. I have outlined multimeter and electrical basics for the novice in a preceding post.

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 Wait for a clear, sunny day

The system should be tested at maximum current, with the solar array in direct sunlight and facing the sun.

Step 2 Measure State of Charge

The solar charge controller must not limit the current flow during testing. There are two different ways to load the system:

  • Start with a full charged battery and connect a load slightly greater than the solar current.
  • Use a lightly discharged battery, around 80 per cent State of Charge (SoC), as the load.

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. I tested with a lightly discharged battery:

Rested voltage 12.19
Temperature 20.6
SoC 83%
State of Charge (SoC) measurements.

Step 3 Measure charging voltages and current

A multimeter and ammeter are required. Most multimeters are not rated for high and continuous DC currents. I have previously used cheap analog DC ammeters. My new solar charge controller measures current.

Several voltage measurements are made at various points in the circuit:

  1. Solar panel voltage, controller input, controller output and battery voltage.
  2. Voltage drops in the positive line.
  3. Voltage drops in the negative line.
  4. Solar panel open-circuit voltage (i.e. panel disconnected from the controller).

Solar current should be constant. I recorded current three times during testing and calculated the average.

You will need to pop open the waterproof cover on the back of a solar panel to access the terminals. For parallel solar arrays, it is only necessary to measure voltages at one panel. A long jumper lead is required for measuring voltage drops from the panel. Any thin, insulated cable will do since there is no current flowing during a voltage measurement.

To safely disconnect the panels before testing open-circuit voltage, they can be turned face-down on the ground or covered with any opaque material.

The following table details measurements for my system. Average solar current was 11.6 A.

Measurement Total volts Pos. volts
Neg. volts Per cent drop Design
Solar panel 14.64  c. 15
Controller in 13.32
Controller out 13.19
Battery 12.84
Panel to ctrlr. drop 1.30 0.65 0.65 -9% 0.9-1.3
Controller drop 0.14 0.13 0.01 -1%
Ctrlr to battery drop 0.36 0.31 0.05 -3% -2%
Panel to battery drop 1.78 1.09 0.69 -12%
Solar panel open-circuit 20.6
Solar power measurements at average 11.6 A solar current. Morningstar Prostar-30M controller. Note that voltages are additive. Voltage drops in the positive and negative leads are added together to give total voltage drop. Differences between terminal voltages are equal to the sum of voltage drops between terminals (e.g. 14.64 – 12.84 = 1.30 + 0.14 + 0.36 = 1.78).

First, compare voltages versus design. The battery cable was 6 AWG (= 13.3 mm2) yet the voltage drop from controller to battery exceeded two per cent. The battery was not full and charging voltage drops to what the battery can accept during bulk charging. Overall, 12 per cent of the solar array voltage was lost and mostly in the solar cable. I have 15 m of 6 mm2 cable.

Second, compare solar panel measurements versus rated values. Current was 13 per cent below expected 6.7 x 2 = 13.4 A. Rarely have I seen more than 12 A from these Chinese panels, they perform like 110 W panels. For comparison, my old 54 W Kyocera panel frequently produces rated current and often a bit more. Japanese panels are better but way too expensive.

Rating Measured
Solar irradiation (W/m2 1000
Temperature (degC) 25 c. 40
Maximum power (W) 120 × 2
Voltage at max power (V) 18 14.6
Current at max power (A) 6.7 × 2 11.6
Open-circuit voltage (V) 22 20.6
Specifications for my Chinese solar panels. I have two panels connected in series so power and current are multiplied by two.

Measured solar panel voltage was less than rated because hot solar cells output lower voltages. The open-circuit voltage suggests cell temperatures were about 40 degC, which is reasonable for a solar panel in direct sunlight on a warm day. Secondly, PWM controllers operate near battery voltage and not at the maximum power point.

Resist the temptation to multiply amps by volts because the result will disappoint (e.g. 14.64 x 11.6 = 170 W). Amps are more important for small 12 V systems with PWM charge controllers.

Low charging current could happen if the solar panels are shaded or the sun hidden by clouds. Reasons for low charging voltage at the battery include: low panel voltage and current, excessive voltage drops (e.g. small cables, bad connections), heavy load on battery and a flat or bad battery. By checking voltages at several points in the system, one can isolate the source of problems. To troubleshoot controller performance, compare results with previous tests and/or retest the system with a different controller

Step 4 Check regulated voltage and current

When the battery is nearly full, the solar controller should start cutting back the solar current. A green LED on my controller starts flashing and there is a slight buzzing noise in PWM mode.

A fully-charged battery (Step 2 above) is advantageous here because you will not have to wait long for the controller to start regulating voltage and current.

The following table shows regulated voltages from my testing. Current decreased gradually during absorption (PWM) and was about 0.5 A in float mode.

Mode Measured
Expected Volts
(25 degC)
PWM 14.1 14.15
Float 13.4 13.70
Regulation voltages for Morningstar Prostar-30M controller set for flooded battery. Regulated voltage is temperature compensated (> 25 degC).

Wrong charging voltages could result from an incorrect battery selection (if selectable on the controller) or a bad charge controller. Under- or over-charging will reduce battery life and should be corrected.


3 Responses to Testing a small 12V solar power system

  1. I’m not sure where you’re getting your information, but
    great topic. I needs to spend some time learning much more or understanding more.
    Thanks for magnificent info I was looking
    for this information for my mission.

  2. Johnette says:

    You have got, many valid points! I appreciate you making this
    kind of post available, the rest of the website is also well done.
    Have a great day. Cheers!

  3. Jamie Brent says:

    That is one of the best blogs ive read with good informative infomation on deep cycle batteries.
    Charging rates rates do vary and never be 100% accurate see more info at http://www.leisurebatteries4less.co.uk

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