I am working on a project where I am switching between a solar panel and a 20 volt, 5 amp AC to DC wall adapter to charge a 12 volt lithium iron phosphate battery. I am using a Morningstar Duo solar charge controller to safely charge the battery. However, when I connect the wall adapter to the controller it draws up to 7 amps (when it is only rated 5 amps) and causes the supply's voltage to drop.

Now it appears that the PCB board on the battery is damaged.

Anyone have any insight on what might be causing the problem or how to safely limit the current being drawn.

Thanks a lot everyone who takes the time,


Link to charge controller:



closed as off-topic by brhans, Bence Kaulics, Daniel Grillo, Voltage Spike, Ricardo Jul 27 '16 at 19:54

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the repair of consumer electronics, appliances, or other devices must involve specific troubleshooting steps and demonstrate a good understanding of the underlying design of the device being repaired. See also: Is asking on how to fix a faulty circuit on topic?" – brhans, Bence Kaulics, Daniel Grillo, Voltage Spike, Ricardo
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  • \$\begingroup\$ So it is broken. Don't use it. \$\endgroup\$ – Eugene Sh. Jul 27 '16 at 14:42
  • \$\begingroup\$ How did you connect everything? That controller can take up to 30 V on the input so even if you connected a power supply where a PV panel would go, that per say would not break it. Most likely you did something wrong. \$\endgroup\$ – winny Jul 27 '16 at 15:34
  • \$\begingroup\$ Get a new one, see electronics.stackexchange.com/help/on-topic for future questions. \$\endgroup\$ – Voltage Spike Jul 27 '16 at 19:46

The Morningstar SunSaver Duo is designed for use with solar panels only, and is not specified for operation with an AC/DC power supply. It is a PWM solar charge controller.

A PWM charge controller regulates battery voltage by switching the connection between the solar panel and battery on and off with a variable duty cycle. The SunSaver Duo applies PWM at 10Hz or 300Hz, which is too low to effectively smooth out current pulses with an inductor. Peak current must therefore be limited by the solar panel itself.

To use a power supply you need some way to limit the peak current. The simplest way to do this is just insert a resistor in series. You can calculate the required value using Ohm's Law:-

$$Resistance = \frac{Voltage}{Current}$$

For example if your PSU puts out 20V then \$\dfrac{20V-12V}{5A} = 1.6\Omega\$.

$$Power = Volts \times Amps$$

With the resistor dropping 8V at 5A it will dissipate \$8 \times 5 = 40W\$. You will need a large high power resistor, and it will get hot!


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