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?
Two 9V batteries in Parallel
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- stinkbutt
- Posts: 593
- Joined: Wed Feb 17, 2010 2:40 am
Re: Two 9V batteries in Parallel
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.
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.
- adafruit_support_bill
- Posts: 88097
- Joined: Sat Feb 07, 2009 10:11 am
Re: Two 9V batteries in Parallel
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
- rizla420
- Posts: 332
- Joined: Thu Feb 05, 2009 9:03 pm
Re: Two 9V batteries in Parallel
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?
Maybe another option is a 9v rechargeable nimh battery. Any suggestions there?
- adafruit_support_bill
- Posts: 88097
- Joined: Sat Feb 07, 2009 10:11 am
Re: Two 9V batteries in Parallel
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.
- zener
- Posts: 4567
- Joined: Sat Feb 21, 2009 2:38 am
Re: Two 9V batteries in Parallel
AA are the best. The NiMH AA are nice.
-
- Posts: 17
- Joined: Sun Dec 05, 2010 10:47 pm
Re: Two 9V batteries in Parallel
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.
D batteries can last up to 10 times longer than A batteries.
- stinkbutt
- Posts: 593
- Joined: Wed Feb 17, 2010 2:40 am
Re: Two 9V batteries in Parallel
'Cuz he needs 9V. That's a pound and a half of batteries.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?
-
- Posts: 17
- Joined: Sun Dec 05, 2010 10:47 pm
Re: Two 9V batteries in Parallel
Here's a handy list from Wikipedia on batteries:
Looks like if weight is an issue then the NiMH AA (like Zener said) are the way to go.
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)
- zener
- Posts: 4567
- Joined: Sat Feb 21, 2009 2:38 am
Re: Two 9V batteries in Parallel
AA has better energy density than D, and AA has more mWh per $ than D, way more. More bang for the buck.
Please be positive and constructive with your questions and comments.