Building a low-power solar/wind buck-boost converter

Question

Using a 18V(max) solar panel, I'd like to charge a 12V lead-acid battery. I'm working from the basic inverting buck-boost circuit. My main question is, being a relatively low-power circuit, can I connect the output directly to the appropriate battery terminals, or would I need to re-invert the polarity before doing so?

It'll be monitored / driven by an Arduino. This is the inspiration: Tim Nolan's MPPT Solar Charger.

A future expansion I'm planning for this is to add a (rectified) wind turbine input as well. Obviously this would need a separate buck-boost circuit, but the question there would be: how safe would it be to connect the output of the wind buck-boost to the battery, in parallel with the solar buck-boost? I'd assume that if my software is written right, the output voltage will be the same from either side so there should be minimal problems. Please correct me if I'm wrong ... but that's a future consideration.

My significant parts list:
• Arduino for the brains/PWM
• IR2104 MOSFET driver
• IRF540 MOSFETs
• MAX4080S Current sense
• Resistor divider
• Schottky diode for the buck-boost section
• 100uH inductor for the buck-boost section

Edit: I like Olin's simple answer, but it's not the answer to my question, as it doesn't give me any guidance with my project. Any other takers?

Answer 1

You may not need a power supply at all, just a diode between the solar panel and the battery. Lead-acid batteries can be float-charged for long periods of time without damage. If the maximum current the panel puts out at about 13.6 V doesn't exceed the float charge current of your battery, then there is no need for any converter between the panel and the battery.

If you're worried about efficiency, save the time, effort, and money that would go into the switching power supply and get a bigger solar panel instead. Sunlight is free, so efficiency is more relevant measured in effort or dollars per Watt than percentage of energy conversion. Get a solar panel just big enough to deliver the maximum float current at 13.8 V, then put only a Schottky diode between the panel and the battery.

Answer 2

As a starting point, and for future expansion such as your proposed wind turbine generator, I would separate the project into two sections:

1. Power input conditioning - this would consist of a buck/boost converter for each input (solar, wind, exercise bike etc.) where the output is set at a fixed voltage, say 24V DC. These outputs can be connected in parallel via schottky diodes to a single large storage capacitor (10,000 uF or above).

2. Battery charging - the 24V DC from the big capacitor could then be used as input power to a standard battery charger circuit, which would limit both the voltage and current going to the battery based on it's maximum charging current, maximum voltage, and maximum trickle (float) charge current when the voltage hits maximum. Lots of chips out there for that, or you could buy an off-the-shelf solar charge unit for this.

My main point is that one should keep input and output separate, makes things much easier to design, and to monitor correct operation when in use.

You could use the same battery charger circuit directly from the solar panel, of course, but for future expansion with other power sources, I think this might be the way to go.