Voltage divider arduino 36v

I have an arduino and I would like to measure the voltage on the engine battery 12vdc and the backup battery 12vdc and the trolling motor battery bank 36vdc. The 2 12v batteries are in parallel. I planed on powering the arduino from the house battery, and monitoring all 3 banks of batteries. Also I will make sure the grounds are shared in each bank. The arduino will controll a relay to feed power to the voltage divider network to monitor voltage, then open the relay as to not discharge the battery all the time.

I have found some info on which resistor values, capacitor and diodes to use on 12vdc batteries but I would like for advice on the 36vdc setup. It will be charging at approx 14.4v per battery or 43.2 volts.

Just act like it is 50Volt so scale it down 1:10

GND ---[10K] -----(analogRead)-----[100K]-------- +36Volt

Than the highest analogRead() will indicate 4.32 volts or raw around 860 when 43.2 volts occur

36V will show up as 3.6Volt or raw around 720

That will give enough room to notice power drops.

On my schematic I posted I was using a 22K resistor on the + to a 2K going to ground with a capacitor of 1uF and a diode to ground then measure the analog input on the Arduino. Similar to what you suggested.

Sometimes I see complicated systems with many components to filter out all kinds of noise, other times a simple 2 resistor voltage dividers.

Do you think the capacitors and diodes are un-necessary for a typical automotive style environment?

Thanks,
Steve

robtillaart:
Just act like it is 50Volt so scale it down 1:10

GND ---[10K] -----(analogRead)-----[100K]-------- +36Volt

Than the highest analogRead() will indicate 4.32 volts or raw around 860 when 43.2 volts occur

36V will show up as 3.6Volt or raw around 720

That will give enough room to notice power drops.

That's actually an 11:1 divider, so 36V will show up as 3.273V

You could use 82k:10k for a 9.2:1 divider, takes 36V to 3.913V, 43.2V to 4.696V

If you add 100nF capacitor to the analog pin to ground, you can scale up the resistor values a lot, so
820k:100k can be used which drains less power (not that is likely to matter with big accumulators).

A little safety tip - split the high-side resistor into two, use one at the battery and one at the Arduino end then you reduce any risk of accidentally shorting raw 36V to the low voltage circuitry. Here that
would mean 39k+39k : 10k. The resistor at the battery end protects the sensing cable should it get shorted too.

On my schematic I posted I was using a 22K resistor on the + to a 2K going to ground with a capacitor of 1uF and a diode to ground then measure the analog input on the Arduino. Similar to what you suggested.

That should work. Note that the voltage ratio is 2K/(22K+2K) but may want to calibrate your software (measuring the actual voltage with a multimeter) anyway for resistor tolerance and power supply (voltage reference) tolerance.

Do you think the capacitors and diodes are un-necessary for a typical automotive style environment?

You can leave-out the 1K series resistor.

The capacitor will give you some smoothing/filtering so you may not need it depending on the stability of your readings.

I "like" the protection diodes where there's an "unknown" alternator/generator and possibly inductive loads on the batteries. However, the ATmega chip already has some "small" built-in protection diodes and with your chosen resistor values, you probably won't get enough current to fry those built-in diodes. So the diodes might not be necessary.

Perfect. Thanks for the double check.

-Steve