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Topic: Arduino monitoring Amp flow from a Car 12V Battery to some drvice (Read 2463 times) previous topic - next topic



I am looking at installing a arduino into my car to look after 2 Power circuits I have installed into my car.

Due to the circuits been on the battery if i forget to turn them off i can flatten my battery fast.

On one circuit it can hold upto a 30 amp load so car fridge or power Inverters and so on

on the other is UHF Radios, GPS, Phone Chargers. mainly basic electronics.

with the high power connection i want to ways the arduino monitors the connections.

The first is on a Timed circuit so it will turn of a relay after a time and that is easy

with that i want a Amp monitor so it watchs how meany amps have been used and when it hits lets say 50amp it kills the circuit. as you all know a car needs amp to kick it over not Volts so if i can keep 100 amps free on the battery on a 150 amp battery my car will start.

So does anyone have a idea on how to do this...

also with the amps monitor i dont know i will need it on the timed circuit as i will only have the car fridge on the 30 minutes after the car is turned off it is only needed if i put a over ride on the arduino to tell it not to use the time circuit and just to keep the circuit on. and if i do that i dont want to kill my battery so arduino is to look after how meany amps the circuit has taken and kill the circuit if it takes to much.




I would just use a VSR (Voltage Sensitive Relay), that's what most 4x4 owners do. Redarc et al make them.

To get cleverer you will have to coulomb count and try to determine the SoC (State of Charge) of the battery, this can be done but frankly I can't see that it's worth the trouble.

Rob Gray aka the GRAYnomad www.robgray.com


I think the poster has confused current and capacity?  You can drain a battery to death at 10A and having a device that trips at 50A isn't going to stop that...

Lead acid batteries are wrecked by over-discharge and the easiest way to detect that is
measuring the voltage - a 12V battery that discharges to 10.5V is on the point of ruin,
for instance.  Tripping at 11V is probably a safer margin.

Standard vehicle batteries are not deep-cycle and deep-cycling them a few dozen times
will reduce the capacity a lot from what I hear.  They are designed to spend most of the
time at full charge and occasionally produce 1000A without buckling the plates.

Deep cycle batteries couldn't sustain such current levels without damage, but can be
deep discharged regularly.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Aurbani, do you know the difference between Ah (amp hour) and A (amps)? And CCA (cold cranking amps)?

A vehicle battery that is rated at 500 cold cranking amps may hold only 40Ah of reserve capacity. 500CCA means that it can put out 500A at sufficient voltage to turn the starter over for 30 seconds, I seem to recall that being 7.2V.

But 40Ah of reserve capacity means that it can only supply 40A for one hour, or 20A for 2 hours, etc. So it can only put out 500A for a very short time before it is exhausted. I think for a car battery it is rated as how long in minutes you can draw 25A until the battery is fully discharged.


Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts


I think there might be a confusion between amps and ampere-hour.
If all you want is to cut power at 10a, just replace the inverter fuse with a 10a fuse. (brings lots of 10a fuse)


Personally I would fit a relay into the 'ON' side of ignition switch circuit, turn key off, it switches everything off.
If you really want to use Arduino to switch 'on' if a fridge needed power (after you monitor temp/time to 'warm up' from specific temp and time to cool back down to get some approximate timing)
Using an ammeter you can work out very roughly how much run time you will get
Use battery voltage monitor as already mentioned to keep outlet 'off' if voltage drops below 11.0 v

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