12 volt battery status for a new guy

Greetings All
Forgive the 'new' guy question.
I am looking to have a battery indicator display for a 12 volt battery.
Saw this item, on eBay and Amazon.
What else will I need to interface and work with my Arduono Mega?

Photo attached

The module you linked to will work entirely 100% on it’s own. It has absolutely nothing to do with Arduino, and it doesn’t make any sense writing “Arduino” in the title - it’s the seller hoping for extra views because people search for that term.

So, it has a single, 2-pin connector that you connect to your battery. Nothing else.

// Per.

Do you want to monitor the battery's voltage using the Mega? You just need a couple of resistors and an analog pin. Look up "voltage divider". However, is anything charging this battery when connected to the Mega? Is it in a vehicle, or connected to a solar panel? If so, some extra protection for the Mega would be a good idea. In vehicle, you can get damaging voltage spikes, and with solar panels, the voltage can rise to double the battery's voltage, or more, if you were to accidentally disconnect the battery while the panel was working.

Thanks for the replies
Yes, I would like to use the Mega and will checking AGM battery voltage that is being charged via solar.

The solar cell/battery combo is about 100 yards from my house. Long term, I would like to see these readings in my house, probable via WIFI.

Hope this helps and thanks for any aid that you can get a new member going.


How big is the panel? What power is it rated for? In full sun, disconnected from the battery, what voltage does it produce? Measure it with your multimeter. (Don't say "I haven't got a multimeter", you know what the reply will be!)

Then work out what resistor values will reduce that voltage to below 5V when used as a voltage divider. Aim for the total of the two resistors to be around 10K to 20K. Post your calculations and we will check them for you.

The problem with WiFi is that it is not very low power, for battery powered circuits. If you want the Arduino to respond to web page requests at any time, so you can view the charge level, a WiFi enabled Arduino is going to consume around 80mA continuously. That could be more than your panel produces on less sunny days, certainly at night! One strategy is to have the Arduino spend most of its time in deep sleep, to reduce power consumption. It could wake every 15 mins, read the charge level and transmit this to a web server on your local network or on the internet. You can then view the charge level at any time. But this all makes the project more complex, so think very carefully, especially if this is one of your first projects.

If you are sure it must be WiFi, then I would recommend not using the Mega. Get an esp8266 based board such as Wemos Mini and use that instead of the Mega. This makes things considerably simpler. Note that a Wemos cannot be powered by 12V directly, so a dc-dc converter is a good idea, adjusted to output 3.3V that the Wemos uses. This is much more efficient than a voltage regulator.

I have been able to trickle charge a 12V AGM battery from a solar panel (/w night blocking diode) at 0.02C (e.g. 200mA on a 10AHr battery) without needing a charge controller, but I do use a switchmode power supply (OKI-78SR-5) to convert the battery voltage to 5V for the controller.

I am setting up to try a Raspberry Pi Zero like this, but it will eat a lot more power and need a larger battery to do the same trickle charge rate (1A on a 50AHr battery). The Pi Zero and AVR combo use about 170mA which is 4.25AHr. The trickle charge will (on a good day) last for about 5Hr yielding 5AHr of charging if I can find a spot that has sun for that long. That may not be enough charge from what I have seen, so I will have the Pi Zero halt at midnight or poor sun to be safe.

Pi Zero W is a superb little bit of kit. But it draws even more current than an esp8266, and, more importantly, takes around 1 minute to boot before it can do anything useful, vs 5 seconds for an esp. Horses for courses.

The R-Pi Zero (I don't have a W yet) is a good pill, but it is slow at booting and a little power hungry. I have better luck with Python on it than I do with C++, so I push the complex stuff to the R-Pi and do the simple time critical stuff on an AVR.

Thanks for all the comments and ideas.
Yes, I actually have 2 voltmeters. No. I haven’t put my tongue on the battery terminals…yet. Ha.
Looks like wifi isn’t the way to go.
My solar cell is 100 watts into a 100ah battery, power a few 12 volt LEDs. More than anything, I was looking at this project more as a learning g tool than anything. Great ideas and thoughts.


You should use a battery charging module to prevent overcharging, which will ruin the battery. They are pretty cheap.