Two 9V batteries in Parallel

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rizla420
 
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Two 9V batteries in Parallel

Post by rizla420 »

Looking for some general advice on something. I'd like to know whether anyone has encountered two 9V batteries in parallel with a 2.1mm barrel jack. I'd like to increase the battery life of my project by using two 9v batteries in parallel. I've done a few searches but cant find anythign off the shelf.

I know i can hack soemthing together to do this, but i was hoping for a prebought solution.

Another thing I was wondering about was whether there are any single 9V batter holder with a 2.1mm barrel jack that have some sort of battery life indicator.

Any ideas?

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stinkbutt
 
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Re: Two 9V batteries in Parallel

Post by stinkbutt »

2 9V batteries in parallel will generate the same voltage, so assuming your project is not already running up into the current limit of the single 9V battery, (that is to say, assuming the internal resistance of the battery is small compared to the impedance of the load) then yes, chances are you'll pull half the current from each.

Putting batteries in parallel like that, however, is a little tricky, particularly with 9V batteries. If one runs down to a lower voltage before the other you're liable to get one trying to back-charge the other. That's bad. This problem's exacerbated by the fact that 9V batteries are for BANNED, for the most part. They're less reliable and have much less capacity than AA and AAA batteries.

Which suggests an alternative: Why not just use 6 AAA or 6 AA batteries instead? Remember, under the hood a 9V is nothing more than 6 AAAA (yes, quadruple A's, that's pretty much what they're used for, populating 9V's,) batteries in a can. Once you start talking about two 9V's you're approaching the footprint of 6 AAA's. You'll enjoy exactly the same capacity as a single 9V, more if you use 6 AA's.

As for battery life indicators, that's a little tricky. For that you'd need to measure the voltage across the terminals and determine from that how far along the discharge cycle it is, and for anyone but the manufacturer, I'm not sure if it's worth your while to test and test and test the voltage vs. mAh curve. And even once you've got that you'd need to plot out V vs. mAh remaining, you'd need to do some ADC to convert it to a number of some sort.

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adafruit_support_bill
 
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Re: Two 9V batteries in Parallel

Post by adafruit_support_bill »

As Stinkbutt says, 9v batteries are far from the ideal power source. The Adafruit 6xAA holder is a good option for extended run-time. http://www.adafruit.com/index.php?main_ ... cts_id=248

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rizla420
 
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Re: Two 9V batteries in Parallel

Post by rizla420 »

Thanks for the input guys, as i read a little more I realized that a lot of people said 9V were shitboxes. So i'll look into the 6 AA batteries. I really wish there was some way i could know the battery life remaining, but alas certain things cant be reasonably accomplished.

Maybe another option is a 9v rechargeable nimh battery. Any suggestions there?

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adafruit_support_bill
 
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Re: Two 9V batteries in Parallel

Post by adafruit_support_bill »

I used those for testing cardiac telemetry systems about 6-8 years ago. They may have improved since then, but they seemed to have about half the life of the alkaline versions.

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zener
 
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Re: Two 9V batteries in Parallel

Post by zener »

AA are the best. The NiMH AA are nice.

parkham
 
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Re: Two 9V batteries in Parallel

Post by parkham »

I know this might seem crazy, but if one wanted a battery pack that would last a long time, why not use Alkaline D batteries in a series to produce the desired voltage?

D batteries can last up to 10 times longer than A batteries.

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stinkbutt
 
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Re: Two 9V batteries in Parallel

Post by stinkbutt »

parkham wrote:I know this might seem crazy, but if one wanted a battery pack that would last a long time, why not use Alkaline D batteries in a series to produce the desired voltage?
'Cuz he needs 9V. That's a pound and a half of batteries.

parkham
 
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Re: Two 9V batteries in Parallel

Post by parkham »

Here's a handy list from Wikipedia on batteries:

Code: Select all


from http://en.wikipedia.org/wiki/List_of_battery_sizes

Numbers listed are in mAh

AAA--540-1200 (alkaline best)

AA--1100-2900 (NiMH best)

C--3800-8000 (alkaline best)

D--2200-12000 (NiMH best)

9--volt400-1200 (Lithium best)

Lantern--10500-26000 (alkaline best)
Looks like if weight is an issue then the NiMH AA (like Zener said) are the way to go.

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zener
 
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Re: Two 9V batteries in Parallel

Post by zener »

AA has better energy density than D, and AA has more mWh per $ than D, way more. More bang for the buck.

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