The AA Car Battery Solar Charger is a perfectly good solar panel for ensuring that your car battery does not run down if the vehicle is not used often enough. Its one weakness, however, is the cable coming out of the solar panel, which is prone to breaking, in part because the gauge of the internal copper wire is so thin. Worse still, it is all too easy for a wire break to go unnoticed, since the cable's insulation can remain intact, and the blue flashing LED on the solar panel will continue to flash irrespective of whether the car battery is being charged or not. Consequently, if a wire break does occur inside the cable, there is a good chance you will be unaware of the problem and one day when you try to start your car - it won't! I encountered this very problem...and this is how I fixed it.
Using a multimeter, you will need to measure voltage and, ideally, the current to verify all is working as it should. You should get results that come close to the specifications found on the back panel.
To measure voltage, connect the multimeter leads to each terminal of the SAE connector (see picture) that is wired to the solar panel. Expose the solar panel to some sunlight and you should see the voltage rise to a maximum of approximately 21V (this is the open-circuit voltage (Voc)). If you do not see any voltage, you most likely have a break in the wire between the SAE connector and the solar panel (the solar panel itself, its internal diode, and the SAE connector are unlikely to be the cause). Sometimes you can confirm that the wire is the problem by moving it around. This may cause voltage spikes on the multimeter as the copper ends of the wire break inadvertently touch each other.
One quick option to measure the current from the solar panel is to simply connect up the multimeter to each terminal of the solar panel's SAE connector in the same way as it is done for the voltage measurement. Obviously, you will need to ensure that you are using the correct current-measuring ports on your multimeter which are often different from the ones used for measuring voltage. The maximum amount of current you should see when the solar panel is exposed to full sunlight should be close to the Short-Circuit Current (Isc) of 171 mA.
However, the best way to measure the current is to test the solar panel in situ while it is attached to the car battery. In this way, you can not only measure the current going from the solar panel to the car battery, but also verify that the blocking diode inside the solar panel is blocking any return current from the battery when the solar panel is not generating electricity i.e at night. To do this, you will need to insert the multimeter in series somewhere within the circuit between the solar panel and the car battery. The easiest place to do this is at the connection between the two SAE connectors as shown in the image below.
The AA Solar Charger, when placed in full sun (through a windscreen/window) and connected to a car battery, should generate electrical current in the region of 137 mA. This is the current at maximum power (Vmp) or when the solar panel is 'under load' ie. attached to the battery.
When the solar panel is turn faced down, stopping electrical generation and simulating night time, you should not see any current flow as the blocking diode should block any current flowing back from the battery. Without the blocking diode, the solar panel would otherwise act as a big resistor at night and you would continue to see some current flowing but this time in the opposite direction, i.e. from the battery.
Replacing a broken cable on the AA Solar Charger
As discussed above the cable exiting the AA solar panel is prone to breaking internally. My recommendation is to replace it completely with one that is more robust. There are several options to choose from on Amazon and eBay.
In my case, the break in the wire was far enough away from the solar panel that I opted to cut the wires upstream of the break so that I could continue to use part of the original wiring for a new connection. To this old wire I soldered the new wire with its SAE connector at the other end.
You may ask why I didn't just solder the new cable directly into the circuit board and eliminate the old wiring completely. The reason is that the copper wire on the new cable was too thick to pass through the soldering holes in the circuit board. In addition, the replacement cable's overall girth (copper and insulation together) as well as its lack of flexibility made it impossible to incorporate it within the housing of the solar panel as had been the case with the old wiring.
In order to protect the remaining length of old wiring, and the connection with the new wire, from future stress, the old wire and connection had to be housed within some sort of protective compartment.
Therefore, the length of old wire attached to the circuit board was passed through a newly drilled 7 mm hole in the back panel. Using two of the existing screw holes that secure the back panel to the front solar panel, and M2.5 x 20 mm machine screws, I secured a small 3D-printed box to the back panel containing the wire connection and the remaining length of old cable. The box was also designed to incorporate an M12 x 1.5 cable gland, which would grip the new cable tightly to prevent any stress falling on to the old wiring.
Extra: Changing the diode for a Schottky diode
Although not essential, since I had the solar panel opened up, I decided to replace the standard blocking diode (1N4001 ? - the label had mostly gone) that comes with the solar panel with a more efficient Schottky diode (1N5819). Schottky diodes function just as well as 1N4001 diodes when it comes to blocking contra-flow current, but where they are superior is that they have less of a voltage drop when current is flowing in the desired direction (from solar panel to battery):
500 - 800 mV
160 - 460 mV
200 - 300mV
This means that less energy is wasted as the solar panel sends electricity to the car battery. If you want to know how to check the voltage drop across a diode, Fluke, the multimeter experts, have a good guide here.