Author Topic: Battery Capacity  (Read 1013 times)

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Offline dsly

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Battery Capacity
« on: July 08, 2015, 01:07:49 PM »
In the notebook section, there are values posted for battery capacity.   When I went to the Energizer website, the numbers were significantly different.  I don't know why, maybe someone does.
Here's what is posted:
"Average alkaline battery capacities

Size     Watt-hrs
9 Volt 0.56
AAA1.3
AA2.9
C6.0
D13.6

Here's what is from the Energizer website:  (alkaline cells)
9 volt  610 mah     5.49 watt-hours
AAA    1250 mah    1.87  watt-hours
AA      2800 mah    4.2   watt-hours
C         8200 mah    12.3  watt hours
D        21000 mah    31.5 watt-hours

Maybe this is just an improvement in batteries?  Different chemistry?  Any thoughts?
For knowledge, learn one thing per day.  For wisdom, forget one thing per day.

Offline Gene

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Re: Battery Capacity
« Reply #1 on: July 08, 2015, 02:46:22 PM »
There is another factor you have to take into consideration.  Its on the manufacturer datasheet but its something most people wouldn't think about for looking at it.  What?

Battery cell impedance (or in this case because its DC, resistance though I've seen it stated both ways).

The issue is, those capacities stated on the manufacturer's web sites are usually for very low current drain continuously - low enough that the cell resistance doesn't hardly ding you.

The issue is, when you start drawing serious current from the battery, the cell resistance acts like a resistor between your load and the battery and it has a voltage across it and that voltage removes from the voltage the battery can supply at its terminals and at a lower voltage your load usually requires more current to compensate.  Yeah, this is a degenerate situation.  And no they don't always tell you in charts what to expect but I do know the energizer site if you look at the actual battery datasheets (they're on there but you have to hunt for them) they show charts if battery voltage versus drain from light to high current and you can see how much faster the battery dies for drawing higher current.

There are other factors too with battery chemistry affecting this.

Bottom line is, go buy yourself some rechargables.  If you're hankering for 9V, there are now 2 cell li-ion 9v batteries out there for $2-3 each on auction from eBay vendors in the US and they sell the require chargers.  I got 4 batteries and a 4 cell charger for about $17.  The top-end cell voltage is 8.4V (no its not 9V because li-ion cells fully charged are 4.2V so times 2 is 8.4V).  BUT the advantage here is the cells are really about 600mah (which is about on par with an alkaline throw away 9V) AND li-ion cells output voltage drops very little to dead (dead is around 7V where for a 9V alkaline to get the same energy out of it you'd be more around 5V).

3 of these in series will make you close to 100 liters of 20PPM before its time to recharge.  If we're in such dire straights here in the US that you NEED more than 100 liters to save yourself and your family , sorry, game over!

Offline dsly

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Re: Battery Capacity
« Reply #2 on: July 09, 2015, 02:11:15 PM »
Thanks, Gene.  I dug a little deeper, as you suggested, to find the data sheets, and I see what you mean about how things vary with load.
Thanks for the explanation!
For knowledge, learn one thing per day.  For wisdom, forget one thing per day.

Offline fishing4fun2

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Re: Battery Capacity
« Reply #3 on: July 15, 2015, 10:11:48 PM »
Thanks, Gene.  I dug a little deeper, as you suggested, to find the data sheets, and I see what you mean about how things vary with load.
Thanks for the explanation!
Yea when you have a question about power supplies or electrical situations Gene is the guy to ask, he set me straight on my set up as stubborn as i was at not understanding until it sunk in.