Internal Resistance checker (draft step)

Projects General Guidance
1

Hi

I am working on a battery internal resistance checker.

For my RC car i use 4 x AAA Nimh batteries non packed.
I want to group the batteries with same (or close) internal resistance.
Actually, i calculate it manually from the reading of Vopen, Vload and iload.
With the famous formula Ri=(Vopen-Vload)/iload

The Arduino can do this in a very smart way.

R_LOAD is 0.5 Ohm (5W)
The battery is 1.5v (Max)
Yes, i want to drain about 3A for an accurate check.

Question:
Can i do this : 3A to the analog input (A1)?


(this pic is not from me but i am ok with it, except for the battery voltage, mine is 1.5v max)

2

cmak-fr:
Question:
Can i do this : 3A to the analog input (A1)?

Er... not quite sure what you think you mean there.

You can measure the voltage at pin 0 and at pin 1, the difference will be the voltage across your 0.5ohm resistor when the MOSFET is turned on.

Yours,
TonyWilk

3

Thank you for reply
ok...
Does the mosfet RLZ44N be a good choice?
Will it add some resistance to the circuit. If so, will it be a better choice for the logical switch?

4

If you are worried about the transistor's resistance, use a relay.

.

5

The IRLZ44N will be fine.

When it is conducting around 3 Amps there should be very little voltage across it (less than 80mV I'd guess)

Yours,
TonyWilk

6

larryd:
If you are worried about the transistor's resistance, use a relay.

Er, no I don't think so.

Managing a lower on resistance than that MOSFET will not be a run-of-the-mill relay.

Yours,
TonyWilk

7

Hi,
Welcome to the Forum.

Don't forget to connect gnd of the controller to gnd/-ve of the battery.
Make the controller gnd connection at the battery terminal so load current doesn't cause any unnecessary volt-drops.

Tom... :slight_smile:

8

TonyWilk:
Er, no I don't think so.

Managing a lower on resistance than that MOSFET will not be a run-of-the-mill relay.

Yours,
TonyWilk

I didn't say a relay would be better than a MOSFET? :wink:

9

thanks TomGeorge, i will do what you said.

Will a a IRFZ44N do the trick as well as a IRLZ44N ?
(Its easier to shop for me)

10

cmak-fr:
thanks TomGeorge, i will do what you said.

Will a a IRFZ44N do the trick as well as a IRLZ44N ?
(Its easier to shop for me)

Unforunately no, you need the IRLZ44N, as it is what is called Logic Level MOSFET.

The IRFZ44N needs more than 5V gate to source voltage to completely switch ON.
The IRLZ44N will completely switch ON with 5V gate to source voltage.

In your case a good low Ron is necessary.

Tom... :slight_smile:

11

Ok

The specs of the product i can buy shows
RDS(on)
0.025 ohm

This seems good.

Thank you a lot to all repliers
Help is very appreciated

To be followed next step

12

The specs of the product i can buy shows

What product?

13

larryd:
What product?

My Crystal Ball says the IRLZ44N, RDS(on) 0.025 ohm max. at VGS= 5V, ID=25A (pulse)

... but who knows what cmak-fr was really referring to.

Yours,
TonyWilk

P.S. yeh, sadly, still had a window open with the datasheet in it. :smiley:

14

I have an issue with residual current after fet have switched off the load

A1 from previous picture is useless.
Here is the loop

Read A0 If V=0 -- assume there is no AAA bat plugged -- display some message If V>0 -- assume a bat has been plugged -- switch D7 high -- read A0 for loaded voltage -- switch D7 back low -- calculate internal resistance ![|500x339](http://nsa39.casimages.com/img/2018/03/03/180303115756750586.png)

The issue is that when the user (me) unplugs the AAA bat, the voltage reading on A0 does not go back to 0. It remains present, changing around the AAA bat voltage value.

Does it come from the mosfet i am using, should i add some load somewhere ?

15

Why don't you enable the internal pull up on the analogue pin,
and assume no battery is connected if the A/D value is e.g. >1000.
Leo..

16

Leo,
I did use the input pullup feature. this does not the trick.
When the AAA battery is disconnected, the A/D value stay quite the same (around 250).

Maybe this would be a clue
I use a battery holder for the AAA bat. (clean, no prob with it)
When i remove the bat, reading A0 gives around 250 (should be 0)
When i touch with my fingers both of the terminals of the empty bat holder, the reading goes to 0.

A workaround does the trick but i dont like it because this discharge the AAA bat if connected: adding a resistor (tested with 10kOhm) between +bat and ground. It works but the AAA bat will discharge and i must not do it.

17

Hi,
Can you post a picture of your project please, so we can see your component layout?

This is not the sort of project to do on a protoboard.

Do you have a DMM?

Tom.. :slight_smile:

18

You should be getting an A/D value of 1023 without battery and when the mosfet is 'off' (D7 LOW).
Maybe you're measuring too close after the time that the mosfet has turned off.

Post a real picture of your setup, and the code.
Read the forum rules before you do.
Leo..

19

Sorry A/D reading is >1000 as you said
But then, analogRead gives wrong voltage value
I seems that there is no way to disable pullup with in loop after it has been enabled

Analog0 (with no pullup) 'see' some current when Fet has been activated. This current disapear when fet is back on even with no AAA bat plugged.

i Will post pictures and code

20

Hi,

And can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?
Including your power supply, and arduino connections.

Thanks.. Tom.. :slight_smile: