# Arduino Forum

## Using Arduino => General Electronics => Topic started by: Max_Maker on Jan 02, 2013, 02:58 pm

Title: I have a "newbie" question about battery current
Post by: Max_Maker on Jan 02, 2013, 02:58 pm
Hi everyone. I'm thinking about connect arduino to a 12 V DC battery car, but i have before to find out two things:

- An electronic device, when it's connected to a battery, only gets the amp required or the battery gives more and the device breaks?

- A voltage stabilizer is good or i need one with current limit?

The thing is that i want to connect to a battery car delicated electronic (12 V stabilized, no much current) and a lipo battery charger (11-15 V, a lot of current) and i don´t know if i can use a little regulator for the first thing or it must be higher because the battery.
All this it's because a battery have 13,8 V peaks, etc.

To sum up, i mainly want to know if a device "gets the amp required or the battery gives more and the device breaks" and if an stabilizer heat up although it require a little current.

Thanks!
Title: Re: I have a "newbie" question about battery current
Post by: fungus on Jan 02, 2013, 03:21 pm

Hi everyone. I'm thinking about connect arduino to a 12 V DC battery car, but i have before to find out two things:

- An electronic device, when it's connected to a battery, only gets the amp required or the battery gives more and the device breaks?

The amount of current that flows will obey Ohm's law.

It can't give more than that without breaking the fundamental laws of physics.

Title: Re: I have a "newbie" question about battery current
Post by: Max_Maker on Jan 02, 2013, 04:22 pm
Estaba hablando de que si da más amperaje del que el aparato electrónico soporta.
I = V / R, aplicando al circuito si la intensidad interna de la batería es baja, podrá dar más amperios. Seguramente te estarás refiriendo a la hora de calcular la I que requiere una resistencia, como el aparato, y es I (requerida) = V (batería) / R (aparato), but i ask, get sure and don't break anything.
Title: Re: I have a "newbie" question about battery current
Post by: Grumpy_Mike on Jan 02, 2013, 04:28 pm
Quote
An electronic device, when it's connected to a battery, only gets the amp required or the battery gives more and the device breaks?

It depends on the device you are powering. If the device is designed to be powered with 12V then that is correct. However some devices are dependent on being driven at the correct current not voltage. For example an LED needs to be driven at a fixed current not a fixed voltage.

Quote
A voltage stabilizer is good or i need one with current limit?

A voltage stabiliser is needed if the voltage you have is too large for the device you have. A stabiliser with a current limit is only useful for protection if something goes wrong. Once the protection kicks in it no longer has the same voltage output.
Title: Re: I have a "newbie" question about battery current
Post by: kf2qd on Jan 02, 2013, 05:02 pm
If the current was controlled by the supply then each and every one of us would have a serious problem in our homes. The power company supplies MEGAWATTS and we use KILOWATTS, if the device using the power was not the limiting factor on our usage our houses would have all burned down yers ago.
Title: Re: I have a "newbie" question about battery current
Post by: rvasque on Jan 02, 2013, 05:21 pm
If your load is using up 100 milliAmps (mA), then it will only draw 100 mA even if your power supply (in this case, a battery) is capable of supplying a lot more.
Title: Re: I have a "newbie" question about battery current
Post by: fungus on Jan 02, 2013, 05:23 pm

Estaba hablando de que si da más amperaje del que el aparato electrónico soporta.

If you know the volts and the resistance then you know the current that will pass.

Is the current too much? That depends only the device, it has nothing to do with the power supply.

eg. If you have a 5V, 1000000000000000000000000000 amp power supply connected to a 100 Ohm resistor then only 0.05 amps will flow. No more, no less.

You seem to know Ohm's law, but you don't yet believe it.

I = V / R, aplicando al circuito si la intensidad interna de la batería es baja, podrá dar más amperios.

No. If the voltage is lower, the amps will be lower.

Title: Re: I have a "newbie" question about battery current
Post by: Max_Maker on Jan 03, 2013, 11:46 am
Ups, intensidad interna no, ¡resistencia interna!, se me ha ido la mano. Lo trabajo con las baterías Lipo, que pueden entregar mas corriente cuantos más "C" tengan, y estos dependen de su resistencia interna.

Thanks everyone for the replies, yes, was obvious:

If the current was controlled by the supply then each and every one of us would have a serious problem in our homes. The power company supplies MEGAWATTS and we use KILOWATTS, if the device using the power was not the limiting factor on our usage our houses would have all burned down yers ago.

But i want to ensure that. Electronics are so expensive... =)

Greetings.
Title: Re: I have a "newbie" question about battery current
Post by: AWOL on Jan 03, 2013, 11:56 am
Quote
Electronics are so expensive

No, happily, they're about as ludicrously cheap as I can ever remember them.
Title: Re: I have a "newbie" question about battery current
Post by: nickgammon on Jan 03, 2013, 12:15 pm
Yeah, I bought 500 x zero-ohm resistors for \$4.

(http://www.gammon.com.au/images/Zero_Ohm_Resistors.jpg)

Can't do much better than that! ;)
Title: Re: I have a "newbie" question about battery current
Post by: retrolefty on Jan 03, 2013, 05:34 pm

Quote
Electronics are so expensive

No, happily, they're about as ludicrously cheap as I can ever remember them.

Correct, but I guess it depends on the users definition of 'electronics'. Apple iPhones are expensive but raw electronic components have never been cheaper in my lifetime as they are now, plus the internet opens up a worldwide seller/buyer connection making component selection and purchases easier then ever.

Bottom line, there has never been a better time to enjoy electronics as a hobby.

Lefty
Title: Re: I have a "newbie" question about battery current
Post by: retrolefty on Jan 03, 2013, 05:46 pm

Yeah, I bought 500 x zero-ohm resistors for \$4.

(http://www.gammon.com.au/images/Zero_Ohm_Resistors.jpg)

Can't do much better than that! ;)

Interesting. Usually these are used instead of simple bus wire runs when having to use auto insertion machinery, not for hobby use. But I did a quick search on 28 gauge tinned copper bus wire and couldn't find any cheaper per foot (assuming a 2.5" resister length) then your purchase price! Of course you are limited to the pre-cut lengths of the 0 ohm resistors, where with bus wire you can use any length you require to bridge two points.

So what was your reasoning for purchasing those, just couldn't pass up a bargain?

Lefty
Title: Re: I have a "newbie" question about battery current
Post by: JimboZA on Jan 03, 2013, 05:51 pm
[zen]If a resistor has no resistance is it still a resistor?[/zen]
Title: Re: I have a "newbie" question about battery current
Post by: retrolefty on Jan 03, 2013, 06:09 pm

[zen]If a resistor has no resistance is it still a resistor?[/zen]

Of course it is, if for no other reasons then the lead wires have a finite resistance. Zero ohms is just a figure of speech, so zen that.  :D
Title: Re: I have a "newbie" question about battery current
Post by: Max_Maker on Jan 05, 2013, 04:03 am

Quote
Electronics are so expensive

No, happily, they're about as ludicrously cheap as I can ever remember them.

I wish but I am afraid I have to disagree with you. :)
Like Retrolefty said, it depends on the device. I want to connect a FPV system to my car battery, and that doesn't have the resistors price, haha. Also, the Arduino to control A/V and power supply switches.

Greetings.
Title: Re: I have a "newbie" question about battery current
Post by: SirNickity on Jan 05, 2013, 04:51 am
I was informed long ago that a car battery is anything but 12v.  You say "it has 13.8v peaks" but it can be A LOT higher than that.  For a beginner, there isn't really an easy answer here.  Using LM7809 or equivalent regulators is wasteful of battery power (excess voltage converts to heat) and still needs to be regulated by the Arduino to 5v, wasting more power still.  Also, the electrical environment in a car is very unreliable and dirty, so unless you're using a stout regulator, it may not survive very long.

The correct thing to do here is have a robust filtering stage to try and stabilize the 9 to 18v input, then use an efficient switching regulator to output a clean, regulated 5v directly to the Arduino's power bus rather than using the onboard DC input.  This isn't quite trivial, and I've been working on a design off and on over the last year to do just that.  I have it completed, I just need to have the PCBs made and run it through a series of tests.

Anyway, good luck with your project and be careful.  As with many things in electronics, the problem is often much more complex than you thought.  Makes for a steep and frustrating learning curve, but it gets fun when you start to understand it all.
Title: Re: I have a "newbie" question about battery current
Post by: retrolefty on Jan 05, 2013, 06:23 am
Quote
I was informed long ago that a car battery is anything but 12v.  You say "it has 13.8v peaks" but it can be A LOT higher than that.

Well to be clear it's not the battery causing those over 12.6vdc evnets but rather the alternator/regulator charging system of the car. When running the battery is not supplying any current but rather is just charging and the alternator/regulator is what is powering the auto.

Lefty
Title: Re: I have a "newbie" question about battery current
Post by: dhenry on Jan 05, 2013, 02:14 pm
Quote
To sum up

Put a regulator there and call it a day.
Title: Re: I have a "newbie" question about battery current
Post by: dc42 on Jan 05, 2013, 02:35 pm

Yeah, I bought 500 x zero-ohm resistors for \$4.

I purchased some of these http://uk.farnell.com/panasonic/erj6gey0r00v/resistor-0805-0r-5-0-125w/dp/2057661RL (http://uk.farnell.com/panasonic/erj6gey0r00v/resistor-0805-0r-5-0-125w/dp/2057661RL) recently. They are described as 0 ohm +/-5%. The ones I received had a resistance of at least 0.001 ohms, so I was thinking of sending them back for being out of tolerance.  ;)
Title: Re: I have a "newbie" question about battery current
Post by: clionetech on Jan 05, 2013, 05:23 pm

Arduino board is a active element, ohm's law apply but little bit tricky in calculating power consumption. For Arduino board case, using linear regulator, it only draws current it needed (around 40mA without external shield connected) for a range of input voltage.

For converting 12V input to 5V VDD, power distribution is:
A) Total input power 12V x 40mA = 480mW
B) Dissipation on voltage regulator (12V-5V) x 40mA = 280mW
C) Dissipation on ATmega MCU and peripheral components 5V x 40mA = 200mW

If you add a led shield, current consumption goes high (>40mA) and power dissipation in both B and C part will increase proportionally. But, if input voltage increase (>12), only power dissipation on voltage regulator B increase.

Let's go back to your questions:
- An electronic device, when it's connected to a battery, only gets the amp required or the battery gives more and the device breaks?
Ans: It only gets the amp it required, for linearly regulated circuitry.

- A voltage stabilizer is good or i need one with current limit?
Ans: For the case of car battery, voltage pulse can up to 14V with running engine, stabilizer is recommended. From my experience, some voltage regulators cannot sustain such a high voltage drop, let's say from 13V to 5V. And high power dissipation across it (as in B above) will create heat and burn the regulator ultimately.
Title: Re: I have a "newbie" question about battery current
Post by: SirNickity on Jan 08, 2013, 11:40 pm
Keep in mind that a simple linear regulator may lack sufficient protection to be directly connected to an automotive power source.  What you "should" see and what really goes on are not the same.  Alternators are not ideal sources, and the spark plugs and lots of inductive stuff generally do not help matters.  You will have people tell you "my LM7805 has been running for months without a problem", and it may continue to do so.  But counting on that is foolhardy, IMO.  At least chuck a diode, inductor, and cap on the input.  :-)