http://www.nooutage.com/vdrop.htm
and this:
Table of energy available for various loads
Load Total Internal Available Available Power
Resistance Resistance Current Voltage Drop Voltage For The Load
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0 5 1.000 5.000 0.000 0.000
1 6 0.833 4.167 0.833 0.694
2 7 0.714 3.571 1.429 1.020
3 8 0.625 3.125 1.875 1.172
4 9 0.556 2.778 2.222 1.235
5 10 0.500 2.500 2.500 1.250 <-- **
6 11 0.455 2.273 2.727 1.240
7 12 0.417 2.083 2.917 1.215
8 13 0.385 1.923 3.077 1.183
9 14 0.357 1.786 3.214 1.148
10 15 0.333 1.667 3.333 1.111
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** = Maximum power transfer when load and internal resistance are equal
Voltage = Current x Resistance
Power = Current x Voltage
