Solar car battery charger and power supply with MCU interface

There is a range of interesting Lipo and Li battery chargers for various purposes with varying interface types.

I need a power supply module that can charge a car battery when energy from the solarpanel is available and provide readings on charge levels and power on the battery for the mcu. Furthermore it should provide stable high quality 3,3V and 5V and 12V output at 2A-5A for usage in other modules.

I cant find that module. Have you seen something that might help me out? :wall:

I can’t help with suggestions, but I don’t think you will find what you want in one device - or if you do, you won’t like the price.

From my research, there are really two levels of solar chargers available. The first is the ones that are intended to support multi-battery setups in the most efficient manner, and prolong battery life, as they’re intended to run off-grid houses. The second are much cheaper and designed for single or small multiple battery systems, and aren’t intended to run large capacity panels. Neither of these modes really require 3v3 or 5v outputs, so I suspect you’ll be looking for your own solution for that part of the system. The first type of solar controller may have data output capabilities to help inform you of charge state and current flow, but the second certainly won’t.

You may find you will need to design your own controller here that has what you need. There are numerous charge controller chips that you could look at. At the lower end of the scale you’re talking about, there are things like the LT3652 (see example board [url][/url]) that you could integrate into a design; or you could just design in the management features you need in the charge lines of a cheaper controller - that could be as simple as an ADC line connected to a voltage divider on output/battery voltage and input/solar voltage will give you information about what is going on.

Good luck!

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I’d suggest the easiest option is to use an off the shelf solar charge controller that gives out 12V and then make a board that has the 3.3V, 5V and 12V you need. That is going to be your cheapest option.

If this is only needed for a few small projects, you could use an off the shelf controller as above and then use a number of those little cheap DC-DC buck converters you find on eBay. Fitted into a box they would provide a working solution at low cost. It all depends on the quantity you are looking for.


I suppose a UPS type system may be more of a starter

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There should be some good choices available in the recreational vehicle market. There’s a wealth of material under rv solar battery charger system.


I have a setup with one of these running in my backyard on a small 10W panel. [url][/url] It runs reasonably well, for the price…

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Would it make sense to use this, and put a voltage divider circuit on the battery poles and feed it into an analog input pin on the MCU? I need to be able to monitor the power left on the battery…

Or might I be able to find a “protected voltage divider” module, I am not completely happy about putting that circuit together, would like something tested…

I like how the item description mentions the unit’s “thunder protection”

And “Keep children and incapable persons away from the controller”

For some reason I doubt there’s enough protection there. But let the ISIS folks keep thinking it’s so :wink:

I have a voltage divider on an analog in pin on one of mine. But I haven’t used it in any level of real monitoring. You could do the same on the solar panel output line as well to get an idea of solar production.

What sizes are you using?

Do you mean what size resistors? I decided I wanted to protect my Oxygen module from a closed-circuit failure of the charge unit, so direct to the solar panel voltage which from memory was 21v max, so I specced my resistors based on that (which reduces the resolution of an in-spec, sub 15v reading). They’re just standard 1/4w thru-hole resistors on perfboard, and I just used something in the 10k area so I didn’t waste power.

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@ Brett - thanks. Will try that. How will the voltage reflect the status of the battery. Roughly?

yes, it will roughly reflect the voltage of the battery :slight_smile:

I’ve just found this calculator that is good at helping show the % tolerance impact on overall results [url][/url].

Just remember that the voltage you measure, in time of charging, will be up to something like 14.8v for a standard 12v battery. What you measure is the voltage on the pin, at the ratio of your resistors. Say we have a 10k and 2k resistor divider. That means at 14.8v input, you get ~2.47v, or close to…
14.8v = 2.47v
14.0v = 2.33v
13.0v = 2.17v
12.0v = 2.00v
11.5v = 1.92v

So the expected working range is going to be based on your battery chemistry, but in general you want to be above 11.8v for a standard gel-cell SLA or lead-based chemistry. You’re sampling under load as well, so that’s not a true indicator, but there’s no way around that in the scenario we’re talking about.

Edit: the state of charge (SOC) of a battery under no load is roughly shown here [url][/url]. Again remember you’re testing under load, and also possibly influenced by charging, so you can’t just use those voltages as absolutes. In all honesty, “fuel gauge” type chips exist to be more accurate about this kind of estimation process, and may be worth considering if you want anything more detailed. I don’t know your full application, but I would just look to use the SOC as a “low cutoff” voltage in your circuit - in fact, if you use the “output” terminals of the charge controller I posted the link to, it will handle that for you and cut off the output if it reaches it’s pre-set point (unfortunately, for $12 you don’t get to override those kinds of settings)