(Internal Resistance) CR123A vs 2 series 2 parralell AA's

[Cavelightchris]

Are CR123A batteries allot better that AA's when it comes to having a lower internal resistance when it's under load?

I wonder which one would have a lower internal resistance:

1 CR123A battery(3 volt)

vs

Take 2 sets of Alkaline AA's wired in series, and then wire the two packs together in parallel, that should give you 3 volts.


I would like to know this too but it's not as important to me:
If the CR123A still had a lower internal resistance, I wonder how many more pairs of AA's you would have to attach in parallel before they either tied, or the AA's finally won.


How hard would it be to do this test?
Please if somebody knows how and also wants to know; feel free to do it for me If anyone is willing to do this test for me I will award them with 4 really cool avatars for this forum, ha ha. Thats all I got.

 

[Mr Happy]

A crude way to test this would be a flash amp test.

With a meter on its 10 amp (maybe 20 amp) range, connect the meter probes briefly to the CR123A in question and see what the maximum amp reading is.

Then put two alkaline AA's in series and do the same test on them. Double the amp reading to account (approximately) for having two sets in parallel.

Whichever gives the largest flash amp reading has the lowest internal resistance.

 

[SilverFox]

Hello Cavelightchris,

Two alkaline cells in series have approximately the same internal resistance as one CR123 cell.

Tom

 

[Cavelightchris]

Well I have learned that when you wire up AA's in parallel you get a lower internal resistance, so doubling and approximate wouldn't be an accurate test.
But thanx for the info.

could I buy 2 double AA(series) holders at circuit city for $2, wire them up in parallel and still do that test on them?

To give a rough idea, I wonder what the comparison between these 3 would be:

CR123A vs AA vs C vs D

Because isn't a C battery supposed to be the equivalent of 3 AA's, and a D is the equiv of 4 AA's?

Can someone do that test for me; I can't find my digimeter (I think someone stole it), I don't know how to do a flashtest, and I don't even own a CR123A battery, ha ha.

 

[Mr Happy]

Well I have learned that when you wire up AA's in parallel you get a lower internal resistance

Yes, that's right. Also, and by corollary, 2 AA's in parallel will be able to deliver twice the current of 1 AA by itself. So if you double the current from 1 AA you get approximately the current from 2 AAs.

so doubling and approximate wouldn't be an accurate test.

It's approximate because the current is limited a bit by the resistance of the meter leads and internal shunt. But it wouldn't be far wrong.

 

[Cavelightchris]

O well thanx Silverfox, that was part of what I was looking for, but now the question is, how much better would it be to parallel wire on another 2 AA's in series.

I thought of a cool experiment that would give a real world example of how much better it would be.

Run a LED flashlight on 2 AA's in series till it shuts off and time how long it took.

Now double that number to get a prediction of what will happen if you parallel wire on 2 more AA's in series.

Lets say the light was on for 1 hour, so your guessing with another 2 AA's added it will burn for 2 hours.

I heard since wiring up more AA's in parallel gives you a lower internal resistance that it should last even longer than 2 hours, the question is how much longer?

What if you found out for instance that wiring up 6 AA's in parallel was really efficient and worth doing, then that's the way I would want to design my caving pack. I am always carrying around 12 batteries with me anyhow.

 

[Mr Happy]

I heard since wiring up more AA's in parallel gives you a lower internal resistance that it should last even longer than 2 hours, the question is how much long?

Only a teeny bit longer. As alkaline cells drain, their voltage decreases until it is no longer enough to run the light at decent brightness. Almost all of the voltage decrease over time is due to the chemical energy being used up, and very little of it is due to internal resistance. For the most part, how long a light runs is proportional to how much energy is stored in the battery. If you double the battery size by paralleling up you double the energy and double the run time. The physics of energy balances ensures that you can't really do better than this.

 

[Cavelightchris]

Thank you very much, I understand it allot better now.

Would the lower internal resistance just contribute to a higher voltage then?

Someone said when a 1.5 Alkaline AA is under a large load it is only 1.2 volts.
Would adding another AA in parallel give you less of a voltage drop?

If so, then couldn't it be possible that since the new setup is, lets say running at 1.3 volts now, that the light wouldn't even last for the whole predicted 2 hours since the LED was being driven harder?
I mean the voltage increase would make the LED even brighter wouldn't it, well I know it does when using this chip.

 

[Mr Happy]

If so, then couldn't it be possible that since the new setup is, lets say running at 1.3 volts now, that the light wouldn't even last for the whole predicted 2 hours since the LED was being driven harder?

Ah well, if there were no other things to take into account that might be true, but there are other things to consider that affect the result.

Firstly, the voltage of an AA alkaline cell is only 1.5 V (actually it's 1.6 V) when it is brand new. As soon as you begin to use it the voltage starts decreasing until it is exhausted, when the voltage may be below 1.0 V.

Secondly, alkaline cells do not like heavy loads at all. The classic example of this is in digital cameras where alkalines may only give 30 pictures before dying. Heavy loads cause alkalines to have a much lower delivered capacity than light loads.

Therefore when you parallel up alkaline cells on a heavy load, the load on each individual cell is lessened and it therefore will last better in spite of driving the lamp more brightly.

Alkaline cells also last better if they get intermittent uses with a rest between. Therefore if you had a pack with 12 AA's in it, and you could rotate between the AA's two at a time to give the others a rest, you would probably get the best run time out of all of them. But this would be complicated and it is dubious if it would be worth the bother.

 

[MorePower]

Only a teeny bit longer. As alkaline cells drain, their voltage decreases until it is no longer enough to run the light at decent brightness. Almost all of the voltage decrease over time is due to the chemical energy being used up, and very little of it is due to internal resistance. For the most part, how long a light runs is proportional to how much energy is stored in the battery. If you double the battery size by paralleling up you double the energy and double the run time. The physics of energy balances ensures that you can't really do better than this.

That's not exactly true. It really depends on what current is being drawn from the cells. Something like the Ultrafire C3 which runs about 50 minutes to 50% on a single alkaline AA cell would likely run much longer than 100 minutes on 2 alkaline AA cells in parallel. This is because at the ~1.4 amps the light draws at a nominal 1.5V, the voltage of an alkaline cell rapidly decreases even though it comes no where near being fully depleted in a chemical sense. Paralleling 2 AA cells would allow that current to be split, allowing both a greater depth of discharge and a reduced voltage depression.

I'll try to dig up some numbers for alkaline cells relating discharge rate and depth of discharge tomorrow.

 

[Mr Happy]

That's not exactly true. It really depends on what current is being drawn from the cells.

Yes, acknowledged. In this thread though I have been visualizing a caving light which I presume is designed to run for several hours, so that would not be putting such a heavy load on the batteries as the kind of light you mention. I wonder what the typical run time would be -- is it 8 hours, 12 hours?

 

[Cavelightchris]

Well I had an epiphany, by parallel wiring up 2 4 AA's in series packs it would make the pack dual redundancy.

The light only needs 4 AA's in series, so you could run it off of only one pack if you wanted to. Say for instance one of the battery packs is brakes, thats ok you only need one of the two packs to run it.

I really plan on making it a dual LED dual redundancy system. Two lights, two cords, two switches, 4 4 AA battery packs total. So I guess you could say the light is going to have quadruple redundancy battery packs if all your trying to do is get some light out of it while your stuck in the dark. I mainly want to use AA's because they're cheap, and cavers are supposed to bring tons of spares anyhow. It would cost ya $50 a spare battery if you bought the Stenlight which is something I really don't like about it.

One of the lights is going to be a 450ma Seoul P4 for the flood, I heard Seoul's are the best for flood because of their emission pattern.

The other light which will be a spot, is going to either be a P4 running at 900ma, or this Q5 but I have no idea what that thing runs at, so I am probably not going to use it unless I can find out.

I have heard that an alkaline battery at 1 amp will last an hour, so I would image that my spotlight will work for over 8.5 hours, and the flood should work for about 17 hours. Can anyone help me confirm, would 2 sets of 4 AA's wired in series, then the packs wired together in parallel(8 batteries total), would they last for 8 hours at 1 amp?

Mr Happy you have been quite helpful and so I am going to award you with these tokens of my appreciation:

 

[Mr Happy]

Mr Happy you have been quite helpful and so I am going to award you with these tokens of my appreciation

Heh heh, thanks!

 

[Mr Happy]

I have heard that an alkaline battery at 1 amp will last an hour, so I would image that my spotlight will work for over 8.5 hours, and the flood should work for about 17 hours. Can anyone help me confirm, would 2 sets of 4 AA's wired in series, then the packs wired together in parallel(8 batteries total), would they last for 8 hours at 1 amp?

Unfortunately no, it depends exactly where the 1 amp is being measured.

If you have an efficient regulator, it may happen that 1 amp at the LED will draw less than 1 amp from the batteries, depending on the relative voltages. You have to look carefully at the specs for the LED driver module. In a perfect world, a 6 V battery driving a 3 V LED through a buck regulator would require half the current from the battery compared to what the LED is using. However, there are always losses in conversion and you never get such perfection.

However, if you do happen to be drawing 1 amp from the battery pack, then 4 AA's in series will last just the same time as a single AA will last (but will deliver four times the total power). For a given current you only increase the run time if you put batteries in parallel, not in series.

To look at something concrete I think a typical 1 AA flashlight can run for about 2 hours at high brightness for an average (not extreme) LED. So with an optimum configuration you could get 16 hours out of 8 AA's, and with 2 LEDs running maybe 8 hours. But this all depends on having a good setup with nicely designed and matched drivers, batteries and lights.

Maybe you should look at some typical examples of real lights and try to find run time graphs made by people who have reviewed them? Then select similar items for your needs.