Battery Questions

Hi,

I’ve been thinking about starting a project with Arduino for some time. It’ll be my first Arduino project and also my first electronics project of this kind.

My questions:

  • Is it safe to power the Duemilanove with 12v at 10A or will this need to be down-rated?
  • Will a lead-acid battery always produce a constant voltage/current or will this decline as the charge is used?

Many thanks,
-Josh.

The on-board regulator will take care of the 12v → 5V conversion, no problem.

a lead-acid battery’s voltage will vary, but it will never get below a useful operating voltage for the arduino. I think an SLA starts taking damage when it drops to the 10V range, and this is still more than enough for sane operation of the arduino.

For example, a vehicle power connection is a nominal 12V battery. You’ll see 14V+ when the engine is running, down to 12V when the engine is off, and even less if something (e.g. the arduino you forgot to unplug) has drained your battery to the point it won’t crank the engine.

-j

Thanks for your reply.

As it happens, I’m going to use a 12v battery to power 2x cigarette lighter sockets as well as the Arduino.

Doesn’t the Arduino overheat if you run it off 12V for too long? (I have no idea how long too long is.)

I’ve got in my notes that some people use a voltage regulator to drop the 12V down to 9V first, then run the 9V into the Arduino. I even have a part number:
9V Voltage Regulator (drop 12V to 9V, then connect to Arduino) - Digi-Key#: 497-1447-5-ND Manuf: L7809ABV - $0.94 each

Good luck!

It does get hot; “too hot” depends on operating conditions. Is it mounted in a box? In a vehicle? In a box in a vehicle? Will it run for long periods of time? If any of these are true, you need to think about heat dissipation.

Using a 9V regulator ahead of the 5V (on board) regulator just spreads out the heat load. If, for example, both were mounted inside aplastic enclosure, there is no long-term difference that going straight to the 5V regulator.

For vehicle mounted applications, I mount the 7805 directly to an aluminum enclosure or sufficiently ventilated heat sink. A more efficient switching supply would probably be a better solution.

-j

the LM7805 is a great regulator…

I wouldn’t even bother with the 7809, just take the +V and regulate it at 5V. Determining if you need a heatsink or not depends mainly on the output current draw, and also the Voltage difference from the regulated value.

just remember that with that regulator you may draw up to 1.5A WITH a proper heatsink…

My personal expierience is that :

250mA - slightly warm to the touch
500mA - 600mA - really warm **Heatsink considerable
700mA - Def. need heatsink

***one last thing about the 7805:

If you connect the center swiper on a variable resistor to one of the fixed wires, connect one side to the center pin on the regullator (GND) and the other to GND, you change the regulators reference to ground, and can therefore change the ouput voltage to a variable state. This is helpful when you really need to tweak on sensitive things.

Whether the onboard regulator can handle it or not, it strikes me as slightly safer to use an external one since it’s of trivial cost compared to your arduino.

Though this does assume an external regulator would fail in some vaguely protective manner. Do they?

it strikes me as slightly safer to use an external one since it’s of trivial cost compared to your arduino.

The only regulator I can recall having fail, failed so that Vin was passed to Vout unregulated. It just so happens it was even mounted externally like you suggest (for heat dissipation reasons; vin was 24V). It killed US$400 worth of sensors anyway.

The reason to mount a regulator externally is to help you get rid of heat, if necessary.

-j