The Battery LED Build Kit

The battery read fresh, on the label and spent, the LED resistor sized at the worst corner and rated for twice its heat, the light checked at the spent voltage, and the battery life budgeted so it never flatters the build

VoltLab's free tools each answer one question, and a battery-powered LED build asks them all at once. The LED Series-Resistor Calculator sizes a resistor from one supply voltage, the one on the label, though a battery reads above it when fresh and far below it when spent, and the LED in your hand may sit a little under its typical forward voltage. The Battery Life Calculator then asks for a current that nothing on the site worked out. This kit puts the tools in order for one build. It reads the battery at three voltages, sizes the resistor at the worst corner (fresh cells, the lowest forward voltage, a 5% resistor 5% low) and rounds up to a real part, rates it for twice its heat, and checks the LED is still lit when the cells are spent. In the worked example, a lantern of three white LEDs on four AA cells, the label sizing's 150 ohm part can pass 23.86 mA; the worst-corner part, 180 ohm, never passes more than 19.88 mA, and the battery budget, derated and rounded down, is 28.49 hours from 2,000 mAh. Every figure is computed by the site's own calculators; the lantern, capacities and meter reading are examples, and the per-cell voltages, spread, margin and derating are named rules of thumb. Low-voltage battery circuits only, with the safety lines for lithium and coin cells, soldering and clipping leads. Included: 23 printable pages with 7 steps, a build sheet, a battery budget sheet, a bench log, a resistor reference with color bands, and a personalised sheet worked from the numbers you enter in the calculators.

Look inside

Two full pages from the kit, exactly as printed. Click either to read it at full size before you decide.

What is in it

• The Three Voltages Rule: the battery read fresh, on the label and spent, for alkaline and NiMH cells
• The Worst-Corner Rule: the resistor sized for fresh cells, the lowest forward voltage and a resistor at its low tolerance, rounded up to E12
• The Twice-the-Heat Rule: the power at the worst corner, doubled, and the standard rating that clears it
• The Headroom Rule: whether the LED is still lit when the battery is spent, and the voltage it goes dark
• The Own-Resistor Rule: how many LEDs a string can hold on your battery, and one resistor per string
• The Floor Rule: the battery budgeted with the worst-case current, derated and rounded down
• The Meter Rule: the current checked with one reading across a resistor, and the safety lines for batteries and soldering
• Three working pages: a build sheet, a battery budget sheet and a bench log with a pre-power checklist
• A resistor reference: the worst-corner part, its color bands, rating and spent-voltage check across common batteries and LED colors
• A personalised sheet worked from the figures you enter in the LED and battery calculators

The kit picks up where the free LED Series-Resistor Calculator leaves off. The calculator sizes a resistor from one supply voltage; the kit reads the battery fresh, on the label and spent, sizes the resistor at the worst corner and rates it for twice its heat, checks the LED is still lit when the cells are spent, and hands the worst-case current to the Battery Life Calculator, derated and rounded down. Low-voltage battery circuits only: anything on mains power is work for a licensed electrician. The worked figures are examples.