Battery current capacity?

[KingGlamis]

I'm wondering how to figure out how many amps (or mA) different batteries can provide? Many batteries that we use don't give any rating at all, other than voltage. Some give the mAh, but that is not at all an indication of the max draw of amps the battery can provide. D cells are a prime example. A 4-D light will have a great runtime, but when modding, how do I know how much draw the batteries can handle? For example, a 50W bulb at 6V would draw 8.3A. I seriously doubt 4D batteries could handle 8 amps for much time. But how do I find out other than just doing real-life tests?

I have done some searching on sites like Duracell and Energizer and others, and I don't see any specs that tell me what I need to know.

Any help would be much appreciated.

 

[VidPro]

8.4 amps is only "1c" or less for a D ni-mhy of 9000+ma capacity.
anything in alkaline will not work good for that consumption rate.
so a ni-cd or ni-mhy D cells would be good, and they are usually rated, although often overrated.
anything that is going to suck down a battery in an hour or so, is a good candidate for a rechargable anyways.
the alkalines have some 9-11AMps in them, but when you use them that hard , they just fold up and go home.

 

[Curious_character]

I detect some possible confusion between the current (mA) and capacity (mAh).

The maximum current you can get from a cell depends (other than the size and chemistry) on how far discharged the cell is and what the load voltage is. A fresh AA alkaline cell will deliver several amps into a short circuit, but much less if the load voltage is, say, one volt. You can get an idea of a cell's short circuit current by looking at the cell impedance given on the manufacturer's data sheet. The data sheet will also tell you what voltage you'll have at various currents as the battery discharges.

You'll find data sheets for a lot of batteries at the major manufacturers' web sites. For example, Energizer at http://data.energizer.com/.

The capacity is a way of showing how much current can be drained for how long -- the more current you draw, the shorter the time you can draw it before the battery is discharged. The capacity of lithium, NiCd, and NiMH cells is relatively constant over a pretty wide range of currents, although the capacity does drop some as current increases. That is, you can draw twice as much current for almost half as long, over a wide range of currents. The capacity of alkaline cells, however, is highly dependent on current, and decreases dramatically at high current drains. You can get a lot more total charge (current times time) at low currents than you can at high currents. You'll find this data in the form of graphs in the data sheets. If you can't find exactly the cell you're using, you can usually find something comparable to get an idea of what to expect.

c_c

 

[LuxLuthor]

I detect some possible confusion between the current (mA) and capacity (mAh).

The maximum current you can get from a cell depends (other than the size and chemistry) on how far discharged the cell is and what the load voltage is. A fresh AA alkaline cell will deliver several amps into a short circuit, but much less if the load voltage is, say, one volt. You can get an idea of a cell's short circuit current by looking at the cell impedance given on the manufacturer's data sheet. The data sheet will also tell you what voltage you'll have at various currents as the battery discharges.

You'll find data sheets for a lot of batteries at the major manufacturers' web sites. For example, Energizer at http://data.energizer.com/.

The capacity is a way of showing how much current can be drained for how long -- the more current you draw, the shorter the time you can draw it before the battery is discharged. The capacity of lithium, NiCd, and NiMH cells is relatively constant over a pretty wide range of currents, although the capacity does drop some as current increases. That is, you can draw twice as much current for almost half as long, over a wide range of currents. The capacity of alkaline cells, however, is highly dependent on current, and decreases dramatically at high current drains. You can get a lot more total charge (current times time) at low currents than you can at high currents. You'll find this data in the form of graphs in the data sheets. If you can't find exactly the cell you're using, you can usually find something comparable to get an idea of what to expect.

c_c

I had this same type of question, and finally realized that I needed to start looking at the various amp draw tests that SilverFox has done, or ask AW for some that he has on his Li-Ion cells to find the real world results. Then I got a CBA-II a few months back which is a nice little tool.