Arduino Forum

Using Arduino => General Electronics => Topic started by: jardane on Oct 08, 2014, 04:53 pm

Title: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 08, 2014, 04:53 pm
(http://cdn.shopify.com/s/files/1/0154/4091/products/arduino9vcable1_large.jpg?v=1346831974)

I have 9V to jack (photo above) and I want to splice in a second 9V battery so I the Arduino will be running on 18V. Is there any potential issues with this?

I also want to splice in a switch too, so I can turn the whole device off to save on battery life but that's an afterthought.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 08, 2014, 04:58 pm
It would be over the recommended 12V and just under the max of 20V, and anyway it works on 7V.....

Quote
The board can operate on an external supply of 6 to 20 volts.....  If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: polymorph on Oct 08, 2014, 05:01 pm
Why would you want to do that? The regulator on the Arduino Uno is a linear regulator. All the power, and I mean every bit of it, from the extra 9V regulator will be wasted as heat.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 08, 2014, 05:05 pm
I am going to be running a lot on the  Arduino UNO including a Digital-to-analog converter, an LM3914N-1 bar/dot controler, a micro SD card reader, a transistor that is pulling 5V to power a speaker and several LEDs.

I want to make sure i have enough power for everything.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 08, 2014, 05:11 pm
But putting two batteries in seriese will not give you any more power than from one battery. It all gets wasted in the regulator.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 08, 2014, 05:17 pm
It was recommended by a friend of mine and I thought it sounded odd, so I thought I would ask. So wound one 9V handle all my equipment?
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 08, 2014, 05:22 pm
Well those 9V things are notorious for not holding much energy. It may have enough power, but it probably won't last long, since energy is power x time.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 08, 2014, 05:27 pm
It's for a prop so it's not going to be running 100% of the time and I can make the battery easy access so I can carry spares.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jremington on Oct 08, 2014, 05:32 pm
You can put identical batteries in parallel, then the combination will last about twice as long.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 08, 2014, 05:33 pm

identical batteries


.... but that's the tricky part.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 08, 2014, 05:34 pm

You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't.
They will cross charge and heat up.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 08, 2014, 05:35 pm


You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't.
They will cross charge and heat up.


Surely not if they were truly identical? (Not that they will be......)
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 08, 2014, 05:37 pm
When someone asks for help I assume it is practical help they want, not theoretical help that postulates impossible conditions.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 08, 2014, 05:38 pm
No batteries are ever truly identical even when brand new from the same pack.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 08, 2014, 05:43 pm
Exactly, so stick to just one, assuming it can provide enough power in the first place.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 08, 2014, 05:52 pm
The worst case scenario is I can run some of the devices from a secondary battery that is not connected to the Arduino. I can run the DAC on a secondary battery without issue and I might also run the SD card reader off that as well.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 08, 2014, 05:54 pm
Quote
I can run the DAC on a secondary battery without issue and I might also run the SD card reader off that as well.

You can but I would assume that your DAC requires 5V and your SD card requires 3V3 so you need some regulators as well.

Instead of all this fannying about why don't you just calculate the current you will need?
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 08, 2014, 05:55 pm

Instead of all this fannying about


Will the real DI Grim please stand up.....
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: polymorph on Oct 08, 2014, 07:42 pm
Do you have more room? Can you fit something slightly larger in, maybe something longer? A 6 cell AA pack will last a lot longer than a 9V radio battery.

9V alkalines don't pack much punch, note that the best on this list is 490mAh at 100mA draw but that is for a brand I've never heard of. The other alkalines are more in the range of 300mAh, with capacity going down a lot at 500mA current:
http://www.powerstream.com/9V-Alkaline-tests.htm

Whereas a 1.5V AA alkaline is over 2Ah at 100mA, and still over 1Ah at 500mA:
http://www.powerstream.com/9V-Alkaline-tests.htm

6 cell AA holders:
http://www.amazon.com/Philmore-Battery-Holder-Standard-Connector/dp/B000LFVFU8/ref=sr_1_1?ie=UTF8&qid=1412789999&sr=8-1&keywords=six+aa+battery+holder

http://www.amazon.com/Battery-Holder-6-AA-Long/dp/B00LPXLDF2/ref=sr_1_2?ie=UTF8&qid=1412789999&sr=8-2&keywords=six+aa+battery+holder

http://www.amazon.com/TOOGOO-Battery-Batteries-Holder-Storage/dp/B00H8KVHT8/ref=sr_1_5?ie=UTF8&qid=1412789999&sr=8-5&keywords=six+aa+battery+holder

Enclosed with a power switch:
http://www.amazon.com/Spring-6x1-5V-Battery-Holder-Storage/dp/B00HG8EMM6/ref=sr_1_4?ie=UTF8&qid=1412789999&sr=8-4&keywords=six+aa+battery+holder

Powerjack for Arduino already installed:
http://www.amazon.com/Beautyforall-Battery-Storage-Holder-Arduino/dp/B00L0642SE/ref=sr_1_6?ie=UTF8&qid=1412789999&sr=8-6&keywords=six+aa+battery+holder


Or use an Ultralife Lithium 9V battery.
http://www.batterymart.com/p-ultralife-9v-lithium-battery.html

(http://www.batterymart.com/images/u9vl-charts_lg.gif)
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: staticsan on Oct 09, 2014, 04:05 am


You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't.
They will cross charge and heat up.


That would not be putting two batteries in parallel, but in series and then shorting them. Two batteries in parallel is both positive terminals together and both negative terminals together. They can then supply 9V to the circuit for twice as long as a single battery.

And they only need to be the same capacity (Ah) and voltage; they don't need to be "identical". The same brand bought at the same time would be enough.

Wade.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 09, 2014, 04:58 am



You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't.
They will cross charge and heat up.


That would not be putting two batteries in parallel, but in series and then shorting them.


I'm 106% sure that Grumpy_Mike knows what it means to put batteries in parallel, and what the risks are.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: staticsan on Oct 09, 2014, 05:29 am




No you can't.
They will cross charge and heat up.


That would not be putting two batteries in parallel, but in series and then shorting them.


I'm 106% sure that Grumpy_Mike knows what it means to put batteries in parallel, and what the risks are.


Well, I don't know Grump_Mike since I'm new here but I do know using batteries in parallel is widespread, common and safe. But I don't want to start an argument, either.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: KenF on Oct 09, 2014, 05:55 am
You would be better off using one of those batteries to power the uno and the other to provide power to the ancillaries that you're connecting.  But these things really do have poor performance for anything other than very low power consumption over a long time.  So for multimeters and fire alarms they're fine.  Anything else, they're a waste of time.

You're better off with a pack of c or d cells.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: KenF on Oct 09, 2014, 06:08 am

(http://www.batterymart.com/images/u9vl-charts_lg.gif)

That's better!
Am I the only one with a small screen :)
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: polymorph on Oct 09, 2014, 06:58 am
Oops, sorry. I linked that from an external source and had only a few minutes to post all that.

I can type fairly quickly, but, well...
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 09, 2014, 09:39 am
Quote
Well, I don't know Grump_Mike since I'm new here but I do know using batteries in parallel is widespread, common and safe. But I don't want to start an argument, either.

If you know that then you know wrong.

Connecting battries in parallel is a bad thing and can lead to all sorts of problems.

Yes it is done and it knackers the batteries, depending on what sort they are the damage done can be large or small.

When you need extra current or capacity and have to connecting batteries in parallel it is common to connect them together through diodes to prevent problems. But basically they cross charge causing excess heat and loss of capacity and with certain types of battery damage to the battery.

Battries in seriese are fine and that is common, that increases the voltage not the capacity.
Title: Oh really!
Post by: Paul__B on Oct 09, 2014, 09:41 am
It's not rocket science.
(http://i.ebayimg.com/00/s/MTAwMVgxMDAx/z/3MUAAOSwDN1UM161/$_12.JPG) (http://www.ebay.com.au/itm/1PCS-9V-DIY-Battery-Plastic-Holder-Box-Case-6XAA-6xAA-6-AA-Wire-27cm-Plug-E0Xc-/311122060833)


If on the other hand, you want rocket science:
(http://i.ebayimg.com/00/s/NTAwWDUwMA==/z/WaoAAOxyemBR6OnY/$T2eC16N,!)8E9s4l6cB,BR6OnYbRsQ~~60_12.JPG) (http://www.ebay.com.au/itm/Battery-Storage-Case-Box-Holder-for-2-x-18650-Black-with-6-Wire-Leads-BA-/400742478409)

Use two 18650 rechargeables.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: polymorph on Oct 09, 2014, 02:52 pm
Grumpy_Mike is correct. Although you may find people using batteries in parallel, it is a bad idea without isolating them with diodes.

There are a lot of bad ideas on the internet. Heck, there are a lot of bad ideas in some books!

Better just to use higher capacity batteries.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: MarkT on Oct 09, 2014, 10:42 pm


You can put identical batteries in parallel, then the combination will last about twice as long.

No you can't.
They will cross charge and heat up.


Not if they are identical, from the same batch, but its better to get a bigger battery.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 09, 2014, 10:56 pm

Not if they are identical, from the same batch, but its better to get a bigger battery.

Yes, a fundamental law of physics is that nothing is identical.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: staticsan on Oct 10, 2014, 05:08 am

Quote
Well, I don't know Grump_Mike since I'm new here but I do know using batteries in parallel is widespread, common and safe. But I don't want to start an argument, either.

If you know that then you know wrong.


Hey, could you chill a bit? This is an unpleasant welcome to the Arduino forums, even for someone who has been involved in both web-based forums and hobby electronics for many years, and even if you are absolutely certain of your facts.

I'm going to try to not let you scare me away from here.

Wade.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 10, 2014, 08:15 am
Quote
I'm going to try to not let you scare me away from here.

Good. No attempt to do so was intended.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Paul__B on Oct 10, 2014, 11:13 am

I'm going to try to not let you scare me away from here.

Well, you can't say you weren't warned!   :D

(The name tends to give it away.)
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: fungus on Oct 10, 2014, 11:37 am


I'm 106% sure that Grumpy_Mike knows what it means to put batteries in parallel, and what the risks are.


Well, I don't know Grump_Mike since I'm new here but I do know using batteries in parallel is widespread, common and safe. But I don't want to start an argument, either.


I'm with grumpy: You're wrong. Batteries in parallel is a very bad idea and will absolutely not make them last longer (in fact they'll probably last a lot less then a single battery).

Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: staticsan on Oct 10, 2014, 11:55 am


I'm going to try to not let you scare me away from here.

Well, you can't say you weren't warned!   :D

(The name tends to give it away.)


*sigh*

This is unfortunate. As was the fact I needed to say the quoted line above. I am comfortable being wrong. I am not comfortable being confronted with a dogmatic stance about it. I hope the forum is normally nicer to new people.

I stand by my original comment that two batteries in parallel are sometimes a viable solution. Just not a preferred one.

Wade.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: AWOL on Oct 10, 2014, 12:12 pm
Besides which, two 9V batteries would have about the same volume as four AA cells, which be more preferable.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 10, 2014, 12:18 pm
Quote
two batteries in parallel are sometimes a viable solution


But isn't it true that if one is of lower voltage, and that is almost certain to be the case, then the higher one will try to charge the lower one. Without some electronics in place to prevent that, that can't be right, since not all batteries are re-chargeable. So to me- and I'm a mere civil engineer who doesn't profess to have any more electrical know how than my 18yo daughter- viable means it must have some electronics in place.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: fungus on Oct 10, 2014, 12:26 pm

This is unfortunate. As was the fact I needed to say the quoted line above. I am comfortable being wrong. I am not comfortable being confronted with a dogmatic stance about it. I hope the forum is normally nicer to new people.


If you come in here saying "2+2=7" then people telling you're wrong isn't 'dogma', it's because you're 100%, flat-out wrong. The sum of 2+2 is not a subject open for debate.

If you're convinced you're right, fine, but come back with some experimental data that shows how you're right and we're wrong.

We too are comfortable being wrong. Being wrong is how we learn. All we ask for is evidence...
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Paul__B on Oct 10, 2014, 01:30 pm

But isn't it true that if one is of lower voltage, and that is almost certain to be the case, then the higher one will try to charge the lower one.

Mike - not without experience we understand - has a problem with things in parallel - LEDs and now, batteries.

I am sure people do from time to time successfully use batteries in parallel, though I cannot think of any such example myself.  It would seem reasonable if the batteries are from the same batch as they would tend to closely equalise their discharge.

A battery has an internal voltage and an internal resistance, what happens is that with discharge, progressively less area of electrode material becomes available and the internal resistance progressively increases so that the battery which is more replete will have a lower internal resistance and will provide more of the current, discharging faster until it matches the other.  This would seem to equalise out the discharge between the batteries.

The intrinsic voltage may change according to changes in the concentration of various reactants, which is why rechargeables in general hold their voltage better as the electrolyte is not a consumable; its concentration does not change until the end of the discharge (if then).  In order to charge a battery (which is to some extent possible even with "non-rechargeable" cells) you require a significantly higher voltage to reverse the electrochemical reaction, so parallel cells with different degrees of discharge are not particularly likely to charge or be charged by the other but will simply float to the higher voltage.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 10, 2014, 02:15 pm

I am comfortable being wrong.

Good glad to know you are comfortable.

Quote
I stand by my original comment that two batteries in parallel are sometimes a viable solution.

Glad to know you are comfortable about this.

However this is a beginners forum, and while you might be comfortable about this bit of "wisdom" you do have a responsibility to people who are just learning not to mislead them.
As you say you are new. All the answers given here are pear reviewed, that is there will be many members checking for the correctness of any statement. If they spot a wrong one then it is the ethos of this forum to inform the member of this.

Then we can have a debate and put forward further arguments. All I see from you is that you say:-
Quote
I stand by my original comment


This is not a matter of opinion, it is a matter of fact. If you have some special circumstances where you think your original comment has some validity then state them. If it is just your opinion then in terms of scientific debate you have not got a leg to stand on.

You are welcome here to ask any question you want.
Quote
I hope the forum is normally nicer to new people.

Yes it is, no one is ever given a hard time for asking the simplest of questions, and if they don't understand the answer they only have to ask for a more detailed explanation.

However if you want to give advice, it has to be correct. This is not a "blind leading the blind" forum that you see so much of on the net. This is a solid advice forum.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 10, 2014, 02:23 pm
Quote
All the answers given here are pear reviewed


.... which is maybe why some answers are pear-shaped....  :P
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 10, 2014, 03:06 pm
Spelling!
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: technix on Oct 10, 2014, 07:00 pm
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.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 10, 2014, 07:25 pm
I feel like this stopped being about my question a long time ago.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: AWOL on Oct 10, 2014, 07:28 pm

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

Welcome to the forum!
Title: Re: Oh really!
Post by: technix on Oct 10, 2014, 07:57 pm

It's not rocket science.
(http://i.ebayimg.com/00/s/MTAwMVgxMDAx/z/3MUAAOSwDN1UM161/$_12.JPG) (http://www.ebay.com.au/itm/1PCS-9V-DIY-Battery-Plastic-Holder-Box-Case-6XAA-6xAA-6-AA-Wire-27cm-Plug-E0Xc-/311122060833)


If on the other hand, you want rocket science:
(http://i.ebayimg.com/00/s/NTAwWDUwMA==/z/WaoAAOxyemBR6OnY/$T2eC16N,!)8E9s4l6cB,BR6OnYbRsQ~~60_12.JPG) (http://www.ebay.com.au/itm/Battery-Storage-Case-Box-Holder-for-2-x-18650-Black-with-6-Wire-Leads-BA-/400742478409)

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.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jboyton on Oct 10, 2014, 08:56 pm
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
Title: Re: Oh really!
Post by: Paul__B on Oct 10, 2014, 11:07 pm

The two 18650's packs quite a punch!

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


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?


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.   :D
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: spicetraders on Oct 10, 2014, 11:26 pm
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.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: polymorph on Oct 10, 2014, 11:45 pm
It hasn't been said, but putting rechargeables in parallel is entirely different than putting non-rechargeables in parallel.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Grumpy_Mike on Oct 10, 2014, 11:55 pm

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. 
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: spicetraders on Oct 11, 2014, 12:08 am

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
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: spicetraders on Oct 11, 2014, 12:10 am
[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.

Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: jardane on Oct 11, 2014, 12:36 am
Can someone close this thread so people stop fighting?
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: polymorph on Oct 11, 2014, 05:08 pm
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.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: fungus on Oct 12, 2014, 10:49 am


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.

Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: fungus on Oct 12, 2014, 10:54 am

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).

Quote

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.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: polymorph on Oct 12, 2014, 03:41 pm
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.
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Peter_I on Oct 13, 2014, 12:06 pm


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.

:%
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: JimboZA on Oct 13, 2014, 12:15 pm

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....
Title: Re: Can I use two 9V batteries in series to power an Arduino Uno?
Post by: Paul__B on Oct 13, 2014, 03:19 pm

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.