Free tool: quiescent current
Two readings and a stopwatch: the quiescent current to size the battery on
Does the battery current rise and fall while you measure it? On a voice alarm that checks its loudspeaker lines in pulses, the current settles, rises for the length of each pulse, and settles again, and a clamp meter shows whichever part it catches. Enter both readings and the timing, and the tool gives the time-weighted average: the quiescent current, I₁, to size the standby battery on. Everything runs in your browser. No email, just the number.
The tool
Quiescent current, I₁ 1.00 A
0.8 + (2.0 − 0.8) × 5 ÷ 30 = 1.00 A
Use this figure in the Battery Calculator- Take both readings with the mains isolated and the reading settled.
- If the pulse is too short for your clamp meter to resolve, use the manufacturer's figure rather than a reading.
- If the monitoring is continuous, the current does not rise and fall, and one settled reading is the figure.
Anyone with the link sees these inputs and this result. Your name is not included.
How the figure is worked out
For most of each period the current sits at the reading between pulses. For the length of the pulse it sits at the reading during one. Averaged over the period, that is:
I₁ = I_between + (I_during − I_between) × pulse ÷ period
It is the formula in the Battery Calculator's monitoring tip. The period runs from the start of one pulse to the start of the next. The figures the tool opens with are 0.8 A between pulses and 2.0 A during them, 5 s in every 30 s: the timing is the Battery Calculator's own example, and the currents are chosen for the example. They give 0.8 + (2.0 − 0.8) × 5 ÷ 30 = 1.00 A.
Taking the figure to the Battery Calculator
"Use this figure in the Battery Calculator" opens the Battery Calculator at Level 2 with this figure as the quiescent current, I1, and the monitoring set to "Continuous or unknown", which adds nothing to it. The average already includes the monitoring, so entering the monitoring again would count it twice. The link to this calculation carries your four figures, so anyone you send it to sees the same working.