Voltage drop when measuring voltage with Load/Current on battery

Hello! So this is my first post, Im new to the Arduino world but keep learning.

I can´t figure out whats making an voltage drop (on the arduino measuring) when Im putting load on my battery, but not on my multimeter?

First things first, my project:

It´s pretty simple, I made a regulator to regulate the water temp in a isolated bucket, with a 12v car battery (100Ah), one DS18B20+ sensor and a 75w heatingcable.

When the temp is below setpoint, a 5v relay gives power to the heating cable.

And to make sure Im not draining the battery (Below 12v) I measure the voltage on the battery with a voltage devider. ( See attached schematics )

The problem is:

when there is no load (relay open) the measurment from the arduino gives me the exact voltage as my referens multimeter.
But when there is load ( 6.25A / relay closed ) the measurement on the arduino drops almost 1V compared to the referens multimeter (measuring directly on the battery), that is still stable at the same Voltages?

Im guessing that the referens multimeter gives me the exact Voltage on the battery and not the arduino Analog input measuring?

What am Im missing? Is it not possible to get a correct meauring from the battery with load?

(excuse my bad drawing)

Regards

Did you connect ground of the Nano directly to the battery(-) terminal.
Did you connect the 30k resistor directly to the battery(+) terminal.

Sharing ground and supply with the heating element wiring could cause this volt drop.
Post a real picture.

How did you power the Nano.
Leo..

The Arduino uses a voltage as a reference. That is default the Vcc of the microcontroller which is 5V. That 5V could change.

You can use the internal reference. It is about 1.1, but it can be 1.0 to 1.2. You have to put the actual voltage in the sketch.
You could replace the 10k with 22k, and the 30k with 330k. Then you can measure up to 18V.

It looks OK to me... Try putting your multimeter directly across the Arduino's ground and the Arduino's analog input.

You might be getting some kind of glitch when the relay switches-on. Are you reading the voltage once, after switching or continuously in a loop.

Otherwise, a wrong-reading could be a fluctuation/change in your reference voltage (Vcc) or a software bug. (But a drop in the reference would cause an increase in the reading, and there's no way for the reference to be high-enough to cause such an extreme error.)

Your wires are probably getting hot too if there's 1V dropped across them. Are you using wires rated at 7A?

Hi,
Make sure you have your connections like this.

Tom... :slight_smile:

okey, so to be sure that it was not just the resistans in the cables from the battery to the terminal connection points , i tried with bigger cables...

From the battery to Load 4.0 mm2 cables and from the Battery to measuring point of the arduino 2.5 mm2 cables.
The Voltages drop actually decreased which is good, but there is something seriously wrong with my wiring, as I today have burnt 3(!) arduino Nanos!

They are dead when using VIN for power... but works with USB connection? Therefore Im guessing the regulators gone?

I tried to do a real schematics, could someone explain why Im burning all my arduinos? :frowning:

Wawa:
Did you connect ground of the Nano directly to the battery(-) terminal.
Did you connect the 30k resistor directly to the battery(+) terminal.

Sharing ground and supply with the heating element wiring could cause this volt drop.
Post a real picture.

How did you power the Nano.
Leo..

Yes, i Tried to connect the Nano Directly to the Battery terminals.
The Voltage drop decreased but somehow burning my Arduino Nano after running a while with the load.

So I tried with a seperate powersource for the arduino (9V) battery on VIN. But the problem remains, Still smoking my arduinos when with load =/

A Nano on 12volt (or 9volt) can't power anything heavy, like a 80mA relay coil.
Try a 5volt cellphone charger on the USB socket.
Leo..

Wawa:
A Nano on 12volt (or 9volt) can't power anything heavy, like a 80mA relay coil.
Try a 5volt cellphone charger on the USB socket.
Leo..

Oh... so maybe its that simple? Because the relay is drawing 100mA...
Does it help to change the regulator to a 7805? (And get rid of the one mounted on the nano)

or maybe I should change to a 12V relay and power it from the battery? But do I need a 12v regulator then for the Relay? The battery can be anything from 11.9 to 13V, and I assume the Relay coil wants 12v?

Regards

sparwen:
Does it help to change the regulator to a 7805? (And get rid of the one mounted on the nano)

or maybe I should change to a 12V relay and power it from the battery?

But do I need a 12v regulator then for the Relay? The battery can be anything from 11.9 to 13V, and I assume the Relay coil wants 12v?

Regards

Yes, but 130mA (relay+Nano) will dissipate 13 - 5 = 8 * 0.13 = ~1watt in the 7805.
It could get to 50C above ambient.
Use a small heatsink, and don't forget the caps on input and output.
No need to get rid of the regulator on the Nano. Just connect 5volt from the 7805 to the 5volt pin of the Nano.

A 12volt automotive relay and NPN transistor with 330ohm base resistor and kickback diode across the coil could also solve the problem.

No. A 12volt relay will probably work ok between 8 and 18volt.
Leo..

Hi,
Where did the Solar charger come from? ? ?


Please can you post some pictures of your project, so we can see your component layout?
Do you have PV panels connected?

Can you completely disconnect the solar controller and with a DMM, measure;

  • Resistance between Solar Panel + and Battery + terminals
  • Resistance between Battery + and Load + terminals
  • Resistance between Solar Panel - and Battery - terminals
  • Resistance between Battery - and Load - terminals

Your solar controller has a LOW Battery switch off facility.
It does this by disconnecting the LOAD from the battery.

MOST controllers disconnect the negative of the load from the negative of the battery.

In your circuit, if that happens, the load will not be disconnected because you have -Battery connected to -ve load through your circuit.

You have an earth loop through your circuit, a part of that circuit is carrying load current.

Please some pictures.

Thanks... Tom... :slight_smile:

Hi,
Please try this mod to your wiring, keep load current out of the gnd circuit of your controller.

Can you tell us the part number and make of the relay please?

Tom... :slight_smile:

Hi,
When your problem occurs, can you measure the voltage AT the potential divider resistors as in the diagram below please?
voltmeasurement.jpg
And between the gnd and A3 terminals of the Arduino.

Tom... :slight_smile:

Wawa:
Yes, but 130mA (relay+Nano) will dissipate 13 - 5 = 8 * 0.13 = ~1watt in the 7805.
It could get to 50C above ambient.
Use a small heatsink, and don't forget the caps on input and output.
No need to get rid of the regulator on the Nano. Just connect 5volt from the 7805 to the 5volt pin of the Nano.

A 12volt automotive relay and NPN transistor with 330ohm base resistor and kickback diode across the coil could also solve the problem.

No. A 12volt relay will probably work ok between 8 and 18volt.
Leo..

Please update me? what is your calculation? 13-5 = 8 * 0.12 ?
The specs on the 7805 says 1A output, so i guess that drawing 150mA wouldn´t be a problem? Or is it anything else i need to think of?

Okey, I thougt that the onboard regulator still would draw power if it still was mounted/and defect?

Yeah Im going for a 12V relay ( Specs: 12v 400Ω 30mA ) and with a heatsink on the 7805. But how do I know how big resistor to use, you say 330Ω?

Regards

TomGeorge:
Hi,
Please try this mod to your wiring, keep load current out of the gnd circuit of your controller.

Can you tell us the part number and make of the relay please?

Tom... :slight_smile:

Thanks for the help Tom!

Sure, here are som pics of the project, not sure do if the help?

The controller is a BSV20A some chinese controller order from AliExpress =)
The solarpanel isn´t connected at the moment... but will in the future.

Sure I will update you with the results tomorrow when i done the measurements ( only have half the projekt here at home ).

The Relay is JS1a-B-5V-F, but im planing to change it to a Finder 36.11.9.012.4011 and draw the power from the battery instead. Im also adding a 7805 with a heatsink instead of the monted regulator (I burned 3 of them yesterday...

So look at the new schematics, what do you think? Does it look correct, Im going to disconnect the - Load / Hetingcable as you said.

Regards