4xAA Ni-MH =2800mA won't give more than 320mA to a 1W LED????

Hi,

I was testing a 1W led (3,4V 400mA) for using it in my bike and as a normal flashlight.

I was planing to test it with a 4xAA Ni-MH (2800mA 1,2V each).

but , when I conected it to my "constant current driver" which works fine with a 12V car battery, the led works fine on 400mA (as calculated for the current driver)

Later when testing it with the 4xAA pack I couldn't get it to work with more than 300-310mA (and 3,2V) , always using the same current drive and same 1W led.


Could it be that the 4xAA pack couldn't give enough current at ONE time , and this is causing it to just tops 300mA only????

Probably something to do with the driver, unless the batteries are really old and worn out they should easily supply the required current. Is the driver definetly constant current?

there coudl be a number of things, i'll start witht he newb things in the hopes it is somehting simple:

1) where are you measuting, if you are measuring tail cap current then 300mA seems fine (300mA*4.2v = 1.26watts for 80% efficiency)

2) what driver? is it amc7135 based?

3) does it look less bright than it did? maybe hte driver is suppose to have a relativly high voltage input and if it is loswer it gives a lower output.

and incase youre super new to batteries, when a battery says it is 2800mAh (mili - Amp - hours) it means it can put out 1.2 volts at: 2800mA for 1 hour or 1400mA for 2 hours, or 700mA for 4 hours, etc. it is a measure of how much energy the battery can store and not how much power it puts out or how much current it can supply

1)I measure the current just between the battery and the Driver... the driver had work fine by giving me the calculated value of current when working on 12v car batterys or deep cycle batterys.. so I was a little confused that it could be something with the 4xAA pack.

2) I'm using this driver: http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/ which works just as calculated with other 3W leds ... and it even works fine with the 1W led I was testing with a 12V car battery (at 400mA, just as calculated)

3) I looks a little dimmer that testing it with the car battery... as with the 4xAA pack it only runs at 300mA and with the car battery it runs at 400mA (as suppoused to)

I know that 2800mA AA battey should work fine with it... but I ask here as I got completly confused by my results.

any ideas of what to test?

PS: the 4xAA pack gives 5.2V total (when the led is connected it drops to 5.05V)

True 2800mAh AA's don't exist as far as I know. The highest "true" capacity are Sanyo 2700mAh's AFAIK. Where did you buy these cells? eBay by chance? I know there are tons of rubbish batteries on ebay, some people have purchased 2800-3000mAh AA Ni-mh's that deliver 100-200mAh in actual capacity and have very high resistance. Could it be that you are using garbage batteries?

richon said:

but , when I conected it to my "constant current driver" which works fine with a 12V car battery, the led works fine on 400mA (as calculated for the current driver)

Later when testing it with the 4xAA pack I couldn't get it to work with more than 300-310mA (and 3,2V) , always using the same current drive and same 1W led.

Click to expand...

your driver needs more volts to supply 400ma than 4xaa can provide, either use different pack (either more nimh cells, or li ions), or get different driver, i had similar situation before, even thou on paper it should work , in reality it didn't.

You said it works fine with a 12v car battery. This suggest to me that your driver is a buck. Most buck drivers need about 1 volt over the led vf to operate correctly. Its possible that the 4.8v input is not enough for the driver to operate properly especially under load. Try connecting one more battery to see if this extra 1.2v will give you your current you desire. If it does then you may need a different driver if you are stuck with the battery configuration.

excellent ideas to test....

will test all of them and post some results

First test: GOAL!!!

I added a single AA in series to the 4xAA pack... and bingo... LEDs working on 400mA , as calculated...


So... I just need a 5xAA pack :duh2:

o well: a 6xAA pack ... but it's good to know this will work fine!!!

Now.. gona test if a 6xAA pack would work on my single 3W led also.

With the 5 AA pack you may want to run a complete test cycle to ensure it will work correctly through the packs discharge cycle. When the batteries are maybe half drained then the lower voltage may start the driver under driving problem again. You would have to test it. A six pack would probably ensure that the driver should regulate through the batteries full discharge cycle.

So the explanation would be that the 4xAA voltage/current wasn't powerfull enough to let the driver work at a constant current rate. Then the 5xAA would work fine with "100% charged" batterys.... so it would be advisable to work with 6xAA to ensure the correct driver work at the batterys get discharged by use... right?

so , in few words: the driver needs a few more volts than the 0,6V (drop voltage of the MOSFET) with less the 12V battery "pack"

As there discharged there voltage drops. So they could drop to low during the cycle with just 5. The driver needs the extra voltage to work correctly. The batteries can handle that current without problems.

as a keep on thinking about the battery pack... would it be better to just have a 2 lithium pack (2x3,7V and 2,4A) instead of the 6xAA Ni-MH for this???

richon said:

as a keep on thinking about the battery pack... would it be better to just have a 2 lithium pack (2x3,7V and 2,4A) instead of the 6xAA Ni-MH for this???

Click to expand...

The lithium pack should work fine. The runtime will depend on the capacity of the lithium battery used. Have you considered using li-ion's. 2 good quality 18650's in series would give you fresh off the charger a voltage of 8.4v. With a depleted voltage of roughly 6.8v measured at rest. This voltage range should power the driver to stay in full regulation. The 18650's at 8.4v would have a capacity at that voltage of around 3000mah depending on the battery used. All six of the AA's will give you roughly a voltage of 7.5v at there rated capacity which some of the best are 2700mah at that voltage. Not to mention the 2 18650's will probably be lighter on your pedal.

some test with 3W led...

with 6xAA works fine barely... so a 7xAA works perfect... and a 8xAA would be the format to use.

well, you just gave me a "way to cool idea" and I have to find out how to buy (from CHILE) thos 18650 Li.ion batterys