Power question

Well if the Op is capable of designing and implementing a dc dc converter I'm sure he is aware of the well known circuit techniques associated with evading dead cells in a parallel battery array. If he isn't he can ask and anyone here I'm sure will tell him what is involved.

exskimos

Well lets just put this one to bed right now shall we (an expression obviously). Your both right, generally C, D and AAA cells initial impedance is higher than AA cells. In some cases C cell impedance is lower than D. All depends on manufacturer, chemistry, the way you or the manufacturer perform the measurements, etc, etc, etc. Sealed lead acid batteries impedance decreases with battery size, as does NiMh battery impedance (but interestingly for NiMH the Ah capacity remains the same).

OK -- I DID just measure four D cells, two C cells, and some AA cells, all reasonably fresh (except for a couple four year old Ds) alkalines of various brands. In each case the current was about 0.16 amps: 1.6V across 10 ohms. The results were quite consistent: the AAs had the highest resistance at around 0.4 to 0.5 ohms, the Cs were about 0.3 ohms, and the Ds were between 0.15 ohms and 0.3 ohms (the latter for the oldest by far of all the cells tested). Sorry, I haven't found any reason to believe that larger cells have higher resistance. I WAS surprised that the difference wasn't greater than I saw, but it wasn't in inverse order.

Cheers, Tom

Could you explain in a little more detail just what you did to determine the internal resistance (IR)? Did you use the delta V / delta current method?

There's a paper on the topic at:

On this page:

Back in Sept 2004 I downloaded some of their data sheets for consumer grade cells, and at that time they said the typical IR of AA cells was 146 milliohms and the IR of D cells was 173 milliohms, for fresh cells.

But now their data sheets for AAA, AA, C and D alkaline cells ALL say 150 to 300 milliohms for a fresh cell.

I measured the IR of 2 AA cells and 1 C cell with the 4192 impedance analyzer and got the following results (in Ohms) for these fresh cells. The 2 AA's are made by different manufacturers:

Frequency AA #1 AA #2 C

I wonder what the result would be for the pulse test described in the paper on IR from Energizer referenced above. Maybe I'll try that tomorrow.

Its possible your readings were affected by parasitic impedances internally in the cell or in your circuit. In any case internal resistance is a function of many factors like the current flowing through the circuit . As the OP wanted to use AA cells I took a look at the Duracell datasheets and the impedance of their MX1500 Alkaline AA cells is

lemonjuice

agreed

lemonjuice

I notice that the datasheets on this page:

Yes, it's a step-up converter. I haven't done the step-down equivalent.

Theres no real reason to think that a step-down converter will be less efficient. To toggle a MOSFET pass transistor's gate N times per second requires a certain number of microwatts. More for a higher current; but the average current in to a step-down converter will be less for a given output wattage, so it might be expected to waste even less power.

Clifford Heath.

if you could find a DC-DC converter that was 100% efficient at such a low load and a 12V battery with 4 times the shelf-life of the 3V battery.

Then yes.

That will not be an easy task.

it's probably much easier to use 2 D cells instead of the AAs.

Bye. Jasen