D-cell Alkaline VS AA nimh

lightseeker2009

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This may sound unfair but here is my situation and concern.
I've bought a Osram crosser 3-led spotlight a while ago. The light runs off 4 D-cell alkalines. I was under the impression that you can't use D-cell rechargebles in them as they dont have a metal positive that covers the top of the battery like its the case with alkalines. The light has wide springs so to get the positive tab to make contact the springs would require some modding.
So I've decided to cut everything out and use 4 AA nimh batteries in an adapter. Actually I've got 3 packs to run in parallel for increased runtimes.

I then thought that since the light only draws about 0.96 amps and the Nimh is only 1.2V against the 1.5V of the D-cells, that the light might be a little less bright now.
So I went to my father who owns the same light but still in standerd form.

Now here is my concern, at long last.. My light is twice as bright as my fathers? How can this be possible with 1.2V less, at this relatively low ampdraw? Will this mean that the led's might burn out prematurely?

I really thought that big D-cell alkalines would sag a lot less under a small load like this
 

Lynx_Arc

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Nimh AAs are not 1.2v, they are "rated" at that voltage but vary from 1.45v off the charger to about 1.1v essentially discharged. Nimh have a lot lower internal resistance so if you had 4 hot off the charger at ~1.4v and put it against alkalines that had been used for awhile sagging to 1.3v or so you would have 0.4v difference. I am thinking your father's light the batteries are nearly depleted and yours are barely used. 0.96A is a decent load for D cell alkalines there would be a little sag on them, more when they get depleted but fresh they probably would closely match nimh off the charger.
 

lightseeker2009

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I got a reading of 5.41V with the pack of 4 Nimh's
The D-cells were fairly new and all were giving between 1.62 and 1.64V for 6.53V collectively

Both figures quoted were unloaded figures.
 

Lynx_Arc

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so we have approx. 1.35v for the nimh and 1.63v for the alkaline. You would have to get voltages when operating as I cannot see the alkalines caving in below 1.4v at that voltage normally. Did you try swapping batteries between the lights?
 

lightseeker2009

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I cannot swop bats between the lights as I had to rip out the insides of mine to put in the 4AA battery holders. So mine can't work with D-cells anymore.
 

Lynx_Arc

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I cannot swop bats between the lights as I had to rip out the insides of mine to put in the 4AA battery holders. So mine can't work with D-cells anymore.

I invested in a bunch of clip leads long ago so I can test things outside of their cases.
 

lightseeker2009

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No I can hereby confirm that AA Nimh batteries lights up a led better than D-cell alkalines.
I have done the following test. I've used a 300 lumen Solarforce drop in (0.8-4.2V) I connected it directly to 2 d-cell batteries, and directly after to 2 AA nimh batteries. The difference is much more than expected. With the AA's its so much brighter that you just wont believe. The resting voltages of the two D-cells are quite a bit more than the voltage recorded for the AA's.

Simply amazing. Another reason to go green I supose.
 

45/70

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Interesting results, lightseeker. I can't say I'm too surprised though. It is a well known fact that NiMh cells deliver higher voltage under load than alkaline cells, at higher current rates.

Alkaline cells, are primarily designed to deliver current loads in the <500mA range. Even the "D" size cells are designed in this way, and their larger size is more to extend runtime in devices, rather than to increase the actual power output. That's not to say that a D alkaline isn't able to provide more power (ie. Watts, not Watt/hours) than an AA alkaline, it's bigger so probably does. As I said though, it really doesn't surprise me that an AA NiMh cell can provide more power than a D alkaline. Now, if you were to compare the two cell sizes at a rate such as 50mA, the D size (or maybe even an AA) would most likely do better than the NiMh cell.

Dave
 

lightseeker2009

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Interesting results, lightseeker. I can't say I'm too surprised though. It is a well known fact that NiMh cells deliver higher voltage under load than alkaline cells, at higher current rates.

Alkaline cells, are primarily designed to deliver current loads in the <500mA range. Even the "D" size cells are designed in this way, and their larger size is more to extend runtime in devices, rather than to increase the actual power output. That's not to say that a D alkaline isn't able to provide more power (ie. Watts, not Watt/hours) than an AA alkaline, it's bigger so probably does. As I said though, it really doesn't surprise me that an AA NiMh cell can provide more power than a D alkaline. Now, if you were to compare the two cell sizes at a rate such as 50mA, the D size (or maybe even an AA) would most likely do better than the NiMh cell.

Dave

I'm sure. The D-cell have a much higher mah rating than AA nimh cells.

But now that this was established. How will the use of the AA's affect the life of my spotlights led's since they were originally designed to work with alkalines? The Nimh's makes the light output very close to twice as bright, I kid you not!
 

Lynx_Arc

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I'm sure. The D-cell have a much higher mah rating than AA nimh cells.

But now that this was established. How will the use of the AA's affect the life of my spotlights led's since they were originally designed to work with alkalines? The Nimh's makes the light output very close to twice as bright, I kid you not!
have you tried to figure out the current used by both alkalines and nimh in the light? If it is twice as bright you could be using over twice the current.
 

45/70

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I'm sure. The D-cell have a much higher mah rating than AA nimh cells.

The mAh rating of a cell really has nothing to do with how much power the cell can deliver, although it can help. Don't confuse power with power over time (energy), ie. Watts with Watt hours. They are two different things. So, while what you say is probably true, it really has nothing to do with how brightly a cell can drive an LED. For example, the AA NiMh cell may drive the LED much brighter, but for only 10 Minutes. On the other hand the D cell may not drive the LED nearly as bright, but for two hours. These are just figures "pulled from a hat", but this is an example of power vs. power over time. In this example the AA cell has more available power, but less power over time (more Watts, but less Watt hours).

How will the use of the AA's affect the life of my spotlights led's since they were originally designed to work with alkalines?
Just as in the example above, the AA NiMh cells will no doubt not run the light nearly as long as the alkaline D cells. There is a tradeoff when a cell is able to deliver current at a higher rate. Add to this the fact that an AA cell is only approximately one fourth the size of a D cell, and you have your answer, more or less.

Dave
 

qwertyydude

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The average D-cell alkaline battery has about 300 milli-ohms of internal resistance, shorted out this equates to a 5 amp short circuit discharge rate. I know for a fact my good AA nimhs can pump out close to 20 amps maximum discharge on a dead short, this equates to about 60 milli-ohms internal resistance. So it's no wonder why D-cell alkalines don't perform as well as even a smaller AA nimh. In fact the D-cell battery really has no more current capacity than a AA, just simply more capacity. It's their construction that gives it that limitation, the anode is simply a brass pin so that tiny surface area which is pretty much the same size, maybe a little longer in the D-cell would ultimately be what restricts maximum current. Nimh on the other hand uses a coiled up sheet with much more surface area so even a AA would be able to have more surface area, and hence discharge capability, than a D-cell alkaline.
 

45/70

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How will the use of the AA's affect the life of my spotlights led's since they were originally designed to work with alkalines?

My apologies, lightseeker. After rereading your question, I see that I somehow totally missed it!:candle: My answer related to the difference in predicted runtime between AA NiMh cells and alkaline D cells.

As for how the higher drive current applied to the LEDs will affect their lifespan, it depends entirely on how much current they are actually being driven at. Without a measurement of this value, it is hard to predict how they will be affected. It is true that higher drive currents shorten LED longevity, but if the current is still within the LED's specification, the effect may be minimal.

Dave
 
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lightseeker2009

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My apologies, lightseeker. After rereading your question, I see that I somehow totally missed it!:candle: My answer related to the difference in predicted runtime between AA NiMh cells and alkaline D cells.

As for how the higher drive current applied to the LEDs will affect their lifespan, it depends entirely on how much current they are actually being driven at. Without a measurement of this value, it is hard to predict how they will be affected. It is true that higher drive currents shorten LED longevity, but if the current is still within the LED's specification, the effect may be minimal.

Dave

I just wanted to correct you but you caught it before me lol.
Well I've figured each LED is being driven at around 0.32 amps. I've looked at the light on the Osram webite and apparently they are driven at 350 ma at 6V, according to the documentation. So I believe I'm well within safe specs but it does not hurt to make sure. With freshly charged AA's it draws 0.96 amps in total for the 3 led's.

@Lynx. I never tested the power consumption with the Alkalines and now that the light was stripped I wont be able too, unless I can get another double D-cell holder.
 

MorePower

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The average D-cell alkaline battery has about 300 milli-ohms of internal resistance, shorted out this equates to a 5 amp short circuit discharge rate. I know for a fact my good AA nimhs can pump out close to 20 amps maximum discharge on a dead short, this equates to about 60 milli-ohms internal resistance. So it's no wonder why D-cell alkalines don't perform as well as even a smaller AA nimh. In fact the D-cell battery really has no more current capacity than a AA, just simply more capacity. It's their construction that gives it that limitation, the anode is simply a brass pin so that tiny surface area which is pretty much the same size, maybe a little longer in the D-cell would ultimately be what restricts maximum current. Nimh on the other hand uses a coiled up sheet with much more surface area so even a AA would be able to have more surface area, and hence discharge capability, than a D-cell alkaline.

Alkaline D cells can put out 20 amps into a dead short without too much trouble. Current will quickly drop, however. This is about twice as much current in to a dead short as a typical alkaline AA cell. Granted, mostly what larger alkaline cells give you is more capacity, but the current collector (brass nail) isn't what's limiting the current capacity of the cells. Inter-facial area between the anode and cathode is a major contributor to rate capability of alkaline cells. D cells simply have a much larger inter-facial area than AA cells.
 

qwertyydude

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I just finished testing a brand new Energizer D-cell, 1.603 volts. Dead shorted it on my multimeter. I got 7 amps. To compare I also tested a brand new AA Kirkland Alkaline 1.610 volts and it actually did better 8.5 amps.

I still stand by my previous observation. D-cell alkaline pretty much has the same discharge capability as AA alkaline.
 

MorePower

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I just finished testing a brand new Energizer D-cell, 1.603 volts. Dead shorted it on my multimeter. I got 7 amps. To compare I also tested a brand new AA Kirkland Alkaline 1.610 volts and it actually did better 8.5 amps.

I still stand by my previous observation. D-cell alkaline pretty much has the same discharge capability as AA alkaline.

Your multimeter has more resistance than a true dead short. We test cells at 10 milliohm resistance when measuring flash amps. I have stacks of data showing D cells putting out 20 to 25 flash amps, while AA cells typically fall between 8 and 12 amps into the same load.
 

czAtlantis

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I tested GP ReCyko AA with clamp, non contact A-metter and thick copper wire and they easily delivered 45Amps. Alkaline AA can deliver max 10A. Will test D cell alkaline as soon as I get home but i think there will be not so big difference due the alkaline design- negative pole is brass rod dipped in powdered zinc paste-and the contact area between zinc (which acts in chemical reaction) and this rod (which is just current collector) in AA is very similar to D.
 

MorePower

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I tested GP ReCyko AA with clamp, non contact A-metter and thick copper wire and they easily delivered 45Amps. Alkaline AA can deliver max 10A. Will test D cell alkaline as soon as I get home but i think there will be not so big difference due the alkaline design- negative pole is brass rod dipped in powdered zinc paste-and the contact area between zinc (which acts in chemical reaction) and this rod (which is just current collector) in AA is very similar to D.

The contact area between the zinc paste and the brass current collector is not the limiting factor. It's the surface area between the zinc paste and the cathode.
 
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