Door lock solenoid battery life: the inrush current mistake
Battery life is almost never limited by the coil's steady-state current, because the coil does not run at steady state. It runs as a millisecond pulse fed from a bulk capacitor, and the battery pays for recharging that capacitor as a high-rate burst, where its usable capacity is well below the mAh figure on the label. Measure the current on the battery side, not the coil side.
Why this happens
Ask how long the battery will last and you will usually be given an arithmetic answer: capacity divided by current times on-time. Every term in that calculation is wrong for a solenoid lock, and they are wrong in the direction that flatters the design.
The coil never reaches the current you divided by. At energisation the current rises as the inductance allows, following a time constant set by inductance over resistance. The plunger begins to move when the current reaches the pull-in value and the stroke is over in tens of milliseconds, so the coil spends most of its pulse in the rising part of the curve and is de-energised before it ever reaches the value on the datasheet. The datasheet figure is a ceiling, not a duty point.
In most designs the battery does not feed the coil at all — it recharges the capacitor that feeds the coil. A small alkaline cell cannot source the amp-level pulse a lock solenoid needs, so the circuit charges a bulk capacitor between operations and dumps it into the coil. That is a good design, and it moves the problem: the battery’s cost per operation is the charge needed to refill the capacitor, delivered as a burst. Peak current during that burst is set by the capacitor’s empty voltage divided by everything in the path, and it is several times the average current over the cycle.
A battery delivers less charge at high rate than its label says. Capacity in mAh is quoted at a specified continuous load, typically tens of milliamps. Discharge it in bursts at amp level and the usable capacity falls, partly because of internal resistance and heating, partly because the endpoint voltage is reached earlier. Add cold and it falls further still. So the correct denominator is not the label capacity but the capacity at the actual regime and temperature — and that number is not on the datasheet, which is why it has to be measured or at least derated deliberately.
Standby is a budget that never rests, and it is frequently the larger one. An actuation is tens of milliamps of charge. A receiver or a wake-on-radio that draws tens of microamp-hours every hour of every day of the design life is a steady drip that does not care how rarely the lock is used. For a lock opened ten times a day, the standby term is very often the dominant one, and no amount of optimisation of the stroke will touch it.
Degradation is what the customer actually complains about. As the cell discharges, its internal resistance rises, so the capacitor recharges to a lower voltage and the pulse is weaker. The lock does not stop; it starts being slow, or it needs two attempts. That is also where the estimate goes badly wrong, because a retry doubles the cost of the operation, and a marginal design retries more often as the battery ages. The failure feeds itself.
Check these in order
1. Measure the battery-side current over one full cycle. Shunt between the pack and the driver, then integrate the charge across the actuation plus the capacitor recharge plus the settling. This is the only number that answers the question, and it belongs on the datasheet of any battery-powered lock.
2. Measure standby current with a microamp meter, over long enough to see the duty cycle of any radio. A radio that wakes every second has a very different average from its sleep figure. Average it properly.
3. Scope the battery terminal voltage through the stroke. A sag below the level the driver needs turns one operation into two or three, and a retry is invisible in a bench test that has a fresh battery and a good supply.
4. Measure the bulk capacitor’s value and equivalent series resistance. Electrolytics lose capacitance as they age and in the cold, and a degraded capacitor produces exactly the symptom of a weak solenoid with a healthy coil. A stroke that got slow over a year is often a capacitor, not a battery.
5. Measure the actual on-time of the coil. If the firmware holds the coil energised well past the point where the plunger has seated, the excess is pure heat. Anything past the end of the stroke is wasted energy and free temperature rise.
6. Check whether the design de-energises after the stroke or has to hold. A mechanism that latches the stroke mechanically spends the pulse and stops. One that must be held continuously spends the pulse and then holds current while the customer’s finger is on the button — a different order of cost.
7. Count the operations honestly, including the ones nobody counts. Auto-relock, app polling, status queries, and diagnostic retries all appear in the real budget.
8. Then, and only then, calculate:
life = usable capacity at the real temperature and regime ÷ (charge per operation × operations per day + standby current × 24)
What actually to change
| Finding | What to change | Why not the other thing |
|---|---|---|
| Standby dominates the budget | Remove or duty-cycle the always-on load before touching the stroke | A larger cell buys time but does not change the physics |
| Coil energised past the end of stroke | Switch off at stroke completion, or reduce to a hold current | The excess is heat and charge, and neither is recoverable |
| Capacitor degraded or undersized | Specify capacitance and ESR for end of life and for cold | A bigger battery cannot fix a pulse the capacitor cannot deliver |
| Retries appearing as the cell ages | Add a low-battery threshold that reports early rather than degrading | Letting it retry doubles the cost per operation at the worst moment |
| High operations per day | Reduce the energy the stroke needs — geometry first, then a latching mechanism | A stronger coil solves the force and worsens the energy |
| Operation in the cold | Change cell chemistry or add a capacitor buffer rated for low temperature | Alkaline pulse capability falls steeply as temperature falls |
| Mechanical stroke poorly defined | Fix the latch geometry so less travel and force is required | Geometry converts force more cheaply than turns of wire |
When it IS the harder problem
The lock is outdoors in a cold climate. Alkaline cells lose pulse capability steeply as temperature drops, so a design that is comfortable at 20 °C can fail to complete a stroke at −10 °C while still showing most of its nominal capacity. The fix is chemistry and buffering, not capacity: a lithium cell or a properly rated supercapacitor holds pulse capability where an alkaline does not, and this is a case where the honest answer is that the cell family is wrong, not the pack size.
The lock is high traffic — a hundred operations a day. Now the actuation term genuinely dominates, and the useful question changes from “how big a battery” to “why does this stroke cost this much energy”. The two levers are the mechanical one and the architectural one: reduce the force and travel the mechanism needs, or change the mechanism to something that latches and stops. A motor-driven latch is more efficient per stroke for long travel, which is why it shows up in exactly this application.
The specification demands years of standby with rare use. Then standby is the entire problem, and capacity is not the answer, because the leakage and self-discharge of the cell set a ceiling you cannot design around by adding cells. The work is in the electronics: switch off what can be switched off, and treat the wake-up interval as the primary design parameter.
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Frequently asked
- Why is the steady-state current the wrong number for battery life?
- Because the coil never reaches steady state. The pulse lasts tens of milliseconds and the current is still rising through most of it, so the rated or holding current is a ceiling the coil does not sit at. Worse, in most designs the coil is fed from a bulk capacitor, so the battery's job is not to supply the coil current at all — it is to recharge the capacitor between operations, at whatever rate the capacitor's empty voltage demands. That recharge is a high-rate burst, and a battery delivers less total charge in bursts than its rating implies.
- Where should I measure current to get an honest energy number?
- On the battery side, not the coil side. Put a shunt between the cell pack and the driver and integrate the charge over a full operating cycle, including the recharge of the bulk capacitor and the recovery afterwards. Measuring the coil current tells you what the coil did; it does not tell you what the battery paid, and the difference is where the estimate goes wrong.
- My standby current is only a few microamps. Is it even worth counting?
- Always count it, because it never rests. Twenty microamps is about 175 mAh over a year, which is a meaningful fraction of an alkaline pack once you account for the capacity lost to pulse discharge and to the cold. Actuation energy is a budget you can control by design; standby current is a budget you can only control by switching things off.
- The lock works but goes weak and slow before the battery is flat. Why?
- Because the cell's internal resistance rises as it discharges, so the capacitor recharges to a lower voltage and the force available during the stroke falls. Force in the pre-saturation region roughly tracks current, and the current you can deliver depends on the voltage the capacitor reached. The unit does not fail; it degrades into a stroke that is too slow or too short to release the latch, and the customer reports a dead battery.