Can I use two 9V batteries in series to power an Arduino Uno?

Also, you will be burning off lots of energy on the linear regulator.

The Arduino takes 100mA under worst case scenario at 5V. Given the principle of linear regulators they will be pulling 100mA out of the batteries, and burn off that 13V of excessive voltage. This means 1.3W of heat dissipation and ~30% efficiency.

The AMS1117 on the Arduino is indeed rated for 100mA, but the SOT-223 package cannot really handle the 1.3W heat dissipation. Your board, running off 18V, can get hot real quick, and its life will be reduced.

Although replacing the AMS1117 chip is easy and cheap, but having a part burning off that much energy is not a good design.

If you have some other part of the circuitry that requires the 18V power, you should either drive the Arduino using only 1 instead of 2 of the batteries, or use an switch-mode DC-DC converter to derive the 5V voltage out of the 18V in a more efficient (usually 90+ percent) way.

I feel like this stopped being about my question a long time ago.

jardane:
I feel like this stopped being about my question a long time ago.

Welcome to the forum!

Paul__B:
It's not rocket science.

If on the other hand, you want rocket science:
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Use two 18650 rechargeables.

The two 18650's packs quite a punch!

I have 5 of those wired in parallel in a phone charger pack, and during wiring I accidentally shorted it out for a split second, and it blew out some of my solder joints immediately and heated up the wires to the point that I dropped them. I used lead-free solder that contained silver, which should be the lower resistance type already. The batteries are fully charged before this incident so maybe that is the reason why it bit so hard.

Energizer has an application note on the subject. They recommend using "fresh" batteries if you're going to put them in parallel to minimize the chance of problems. And if you can't be trusted to do that to use Schottky diodes. They also warn about shorting the batteries if you install one backwards.

http://data.energizer.com/design_hints/pages/dhints_seriesparallel.html

technix:
The two 18650's packs quite a punch!

I did not choose to refer to it as "rocket science" without excellent reason!

technix:
I have 5 of those wired in parallel in a phone charger pack

So you are saying that is actually a commercial product, and they are wired in parallel? Or this is your own concoction?

jardane:
I feel like this stopped being about my question a long time ago.

But the fact is, you were answered (a long time ago). Your original idea was totally impractical, and you have been given the proper recommendation - six alkaline AA cells, or if you really want performance, the two 18650s. That's not so bad. And you now know more about batteries. :smiley:

Darn I better tear down all those solar powered sites that I have put in the last 20 years as the batteries are all in parallel. Oops there goes the car battery.

Yes I will give the fact that parallel does have issues once a cell fails or starts to fail, but so does series.
So unless we can run all things on simple one cell batteries we will be prone to the eventual doom.

If space is an issue ro weight is an issue two nines in parallel will do for increased run time.
If space or weight is not an issue then use the battery trays listed several time through out this post.
Or run on the 7.2 or 9V RC hobby batteries.

It hasn't been said, but putting rechargeables in parallel is entirely different than putting non-rechargeables in parallel.

spicetraders:
Darn I better tear down all those solar powered sites that I have put in the last 20 years as the batteries are all in parallel.

Yep designed by a moron, who knew nothing about batteries and considered a design a success if it didn't immediately blow up.

Having said that Lead Acid batteries do suffer the least form being abused in this way because the charging requirements are quite simple. However, the batteries will degrade faster if you connect them directly in parallel than if you do it properly. Also the efficiency of the storage of the hard won solar power is less than it could be.

Grumpy_Mike:
When someone asks for help I assume it is practical help they want, not theoretical help that postulates impossible conditions.

You are right he did ask for practical help, not negative help

[quote author=Grumpy_Mike link=topic=271346.msg1916136#msg1916136 date=1412978139

Having said that Lead Acid batteries do suffer the least form being abused in this way because the charging requirements are quite simple. However, the batteries will degrade faster if you connect them directly in parallel than if you do it properly. Also the efficiency of the storage of the hard won solar power is less than it could be.
[/quote]

Who said they were lead acid?
And I guess 10 years of heating up has been why the desert is hot here.

Can someone close this thread so people stop fighting?

Sometimes positive help is negative help.

As for paralleled cells, I've seen a lot of battery packs with rechargeable lithium ion or lead acid cells in parallel. But those are rechargeables. Different situation.

spicetraders:

Grumpy_Mike:
When someone asks for help I assume it is practical help they want, not theoretical help that postulates impossible conditions.

You are right he did ask for practical help, not negative help

And he was given exactly that ... before he started telling us we were wrong.

polymorph:
As for paralleled cells, I've seen a lot of battery packs with rechargeable lithium ion or lead acid cells in parallel. But those are rechargeables. Different situation.

Disagree. "Rechargeables" are almost certain to be at "different states of discharge" because they all have slightly different capacities, leading to (from the Energiser notes posted earlier).

Batteries connected in parallel should be at the same state of discharge. If batteries at different states of discharge are installed into a device using a parallel battery configuration, the battery with the higher voltage will charge the battery with lower voltage until voltage equilibrium is reached in the system. This charging could lead to leakage, elevated temperature, or other damage to the lower voltage cell.

It might be acceptable to put batteries in parallel when you want very high current and you're not expecting a long lifespan (rockets, helicopters, etc. Stuff that lasts 20 minutes...) In this case the current being drown will drown the charge being passed between batteries.

If your aim is to get "longer life" though, you're doing it very, very wrong.

You can disagree all you want. I work with 18650 LiIon rechargeable packs every day that have three cells in parallel, four of those in series. They are assembled at the factory. I've also taken apart a few high capacity laptop power packs that put 2 and 3 cells in parallel.

Say two non-rechargeable alkaline cells are connected with slightly different capacity/voltage: one cell pushes current into the other. A bad thing, which shortens the life of the cell having current forced into it, and using up charge from the higher voltage cell. Since the one cell is damaged, it shortens its life, shortening the life of both cells. So you don't get double the capacity.

Two rechargeable LiIon cells in parallel, one has slightly less charge and therefore voltage. The more charged cell merely recharges the one with less charge, thereby balancing the cells. Neither has its life shortened. Since the voltage of a LiIon cell correlates pretty well with state of charge, charge balancing naturally occurs when recharging and discharging.

Please don't give me ridiculous examples of paralleling a dead and a fully charged cell, or a new cell with an old worn out cell. I think everyone here knows you never mix batteries in different states of charge and/or lifecycle, even in series.

AWOL:

jardane:
I feel like this stopped being about my question a long time ago.

Welcome to the forum!

Indeed, that is how forums are.

You got an answer:

  • no, it is not a good idea to use two 9V batteries, neither in series nor parallel
  • There is no reason to feed your Arduino 18V
  • If space allows, it is better to use a pack of 6 AAs

Be happy that you got a good answer, then lean back and enjoy, as the thread takes off in whatever direction it chooses.
That's just the way it is.

:%

Peter_I:
Be happy that you got a good answer, then lean back and enjoy, as the thread takes off in whatever direction it chooses.
That's just the way it is

Well said....

polymorph:
one cell pushes current into the other. A bad thing, which shortens the life of the cell having current forced into it,

What makes you think it is a bad thing or shortens the life of the cell? Most alkaline cells are at least partially rechargeable, some are indeed sold as such.

In fact, to actually charge a partially discharged alkaline, you need a substantially higher voltage.