Maintenance procedure · Written to ASD-STE100 Issue 9 · Formatted to ANSI Z535

The Battery That Tests Good but Is Flat Every Morning: Find the Parasitic Draw

12 V vehicle batteries — rule out the free causes, then measure the current the vehicle draws while it is parked
Applicability12 V lead-acid and AGM starting or accessory batteries in passenger cars, light trucks, and hybrid or electric vehicles with a 12 V system
Task time35 min to 1 hr 36 min editorial estimate, not a measurement
PersonnelOne
Revisionv1 · 2026-08-04
BEFORE YOU BEGIN

Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.

Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.

Summary

A battery that reads a normal voltage on a multimeter and is still flat the next morning usually has one of two explanations. Either something is drawing power that should not be, in a way you can see and stop for free, or the vehicle draws a small constant current with the ignition off and that current is higher than it should be. This page starts with the free half: a dome light, a door not fully shut, a glove box light, a phone charger left in the socket, and a pattern of short trips that never lets the alternator finish a full charge. Only after those are ruled out does the page move to a current measurement.

The measurement half needs a multimeter, a wait for the vehicle's own computers to go to sleep, and a number to compare against — and that number is the part this page will not guess. A vehicle with two onboard computers and a vehicle with forty do not share one acceptable current, and the wait time before an accurate reading is just as specific to the vehicle. This page shows you how to take the measurement and how to narrow it to one fuse. It sends you to your vehicle's own manufacturer documentation for the two numbers that decide whether what you measured is normal.

Necessary equipment

ItemNote
Digital multimeterThe multimeter must have a DC current function.
Safety glasses or gogglesUse the eye protection near a battery at all times.
GlovesGloves give protection from the acid in the battery.
Wrench or socket for the terminal nutThe size of the nut is different for each vehicle.
Flashlight optionalThe light in the engine compartment is often low.
Pen and paperRecord the value and the date of each measurement.
Manufacturer documentation for the vehicleThe documentation gives the fuse chart, the acceptable current, and the correct wait time.
Baking soda materialUse a paste of baking soda and water to neutralize a small acid spill.

Do a check of the causes you can see

Estimated 3 min to 5 min

NOTEMost flat batteries have one simple, visible cause.
  1. Look at every light inside the vehicle.
    • A dome light, a map light, or a glove box light can stay on.
    <1m
  2. Close the doors, the trunk lid, and the tailgate.
    • A door that is not fully closed can keep a courtesy light on.
    <1m
  3. Look at the accessory socket and each USB port for a connected device.
    • A dash camera, a phone charger, or a tire inflator can draw current with the ignition off.
    <1m
  4. Remove any device from the accessory socket and each USB port.
    <1m
  5. Write down the length of your most recent trips.
    • A pattern of short trips can stop a full charge of the battery.
    <1m
  6. Look at the age of the battery.
    • An old battery can fail under a small load at a full voltage.
    <1m
  7. If you correct a cause in this section, wait for one night before you continue.
    • Most flat batteries have one simple cause.
    <1m

Prepare for the current measurement

Estimated 6 min to 11 min

Position of the battery and the fuse box
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THIS VEHICLE.

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Stop the vehicle on a level surface away from traffic.
    <1m
  2. Set the parking brake.
    <1m
  3. Stop the engine.
    <1m
WARNING

PUT ON THE EYE PROTECTION AND THE GLOVES BEFORE YOU GO NEAR THE BATTERY.
Hydrogen gas is a product of a lead-acid battery. An explosion at the battery can blind you.

  1. Put on the eye protection.
    <1m
  2. Put on the gloves.
    <1m
WARNING

KEEP FLAMES, SPARKS, AND CIGARETTES AWAY FROM THE BATTERY BEFORE YOU OPEN THE HOOD.
Hydrogen gas can explode. Do this work in a space with good airflow.

  1. Open the hood.
    • The vehicle documentation gives the position of the release for the hood.
    <1m
WARNING

DO NOT TOUCH THE METAL HOUSING OF A HIGH-VOLTAGE BATTERY.
On some vehicles, this housing is next to the 12 V battery. Tesla warns owners not to touch this housing. Contact with a high-voltage system can give you an electric shock.

  1. Find the battery.
    • The vehicle documentation gives the position of the battery.
    1m
WARNING

DO NOT TOUCH A BATTERY THAT HAS A CRACK OR A LEAK.
Sulfuric acid is inside a lead-acid battery. The acid can burn your skin.

  1. Look at the case and the cover of the battery for a crack.
    <1m
  2. Look at the terminals for corrosion or damage.
    <1m
  3. If you find damage or a leak, stop this procedure.
    • Speak to a vehicle repair shop about a damaged battery.
    <1m
  4. Find the fuse box for the vehicle.
    • Some vehicles have more than one fuse box.
    • The vehicle documentation gives the position of each fuse box.
    1m
  5. Find the fuse chart for the vehicle.
    • The fuse chart names the circuit for each fuse.
    • The vehicle documentation gives the fuse chart.
    1m

Measure the current with the ignition off

Estimated 20 min to 48 min

Maximum acceptable current draw with the ignition off (quiescent or key-off current)
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THIS VEHICLE. THE ACCEPTABLE CURRENT DEPENDS ON THE NUMBER OF ELECTRONIC MODULES ON THE VEHICLE.

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Wait time for the modules of the vehicle to enter sleep mode
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THIS VEHICLE.

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Torque for the negative battery terminal nut
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THIS VEHICLE. DO NOT GUESS THIS VALUE.

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Multimeter function and range for this test
Unverified — do not rely on this value
DC CURRENT. CONSULT THE DOCUMENTATION FOR YOUR OWN MULTIMETER FOR THE CORRECT RANGE.

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

  1. Set the multimeter to the DC current function.
    • Select the highest current range on the meter.
    • Consult the documentation for your multimeter.
    <1m
WARNING

DO NOT LET A TOOL TOUCH BOTH TERMINALS AT THE SAME TIME.
A short circuit at the battery makes sparks and heat. Sparks and heat can burn you.

NOTICE

WRITE DOWN THE RADIO PRESETS AND THE CLOCK SETTING BEFORE YOU REMOVE THE NUT.
Disconnection of the battery can clear these settings.

  1. Remove the nut from the negative terminal clamp.
    • The negative terminal has the symbol -.
    <1m
  2. Move the negative cable away from the terminal.
    • Do not let the cable touch the terminal or any metal part of the vehicle.
    <1m
  3. Touch one probe of the meter to the end of the negative cable.
    <1m
  4. Touch the other probe of the meter to the negative terminal.
    • The meter is now in the path of the current to the battery.
    <1m
WARNING

STOP THE TEST IF THE METER SHOWS A HIGH CURRENT.
A high current at this point can mean a short circuit. A short circuit can cause a fire.

  1. Look at the value on the display.
    <1m
  2. Close each door of the vehicle.
    • An open door can keep a module of the vehicle awake.
    <1m
  3. Wait for the modules of the vehicle to enter sleep mode.
    • The wait time is different for each vehicle.
    • Consult the manufacturer documentation for the correct wait time for this vehicle.
    15m
  4. Read the value on the display.
    <1m
  5. Write down the value and the date.
    <1m
  6. Compare the value with the maximum from the manufacturer documentation.
    • The acceptable value is different for each vehicle.
    • The number of modules on the vehicle changes the acceptable value.
    <1m
  7. Remove the probes from the terminals.
    <1m
NOTICE

DO NOT GUESS THE TORQUE FOR THE TERMINAL NUT.
CONSULT THE MANUFACTURER DOCUMENTATION FOR THE CORRECT VALUE. A LOOSE NUT CAN CAUSE A POOR CONNECTION.

  1. Install the cable on the negative terminal.
    • Install the nut on the terminal clamp.
    <1m

Find the circuit with a fuse

Estimated 2 min to 22 min

NOTEThis section needs the fuse chart from the vehicle documentation.
  1. If the value is higher than the manufacturer value, find the circuit with a fuse.
    • This step needs the fuse chart from the vehicle documentation.
    <1m
NOTICE

CONSULT THE FUSE CHART BEFORE YOU REMOVE A FUSE.
DO NOT DRIVE THE VEHICLE WITH A FUSE REMOVED. A REMOVED FUSE CAN STOP A SYSTEM OF THE VEHICLE FROM ITS NORMAL FUNCTION.

  1. Remove one fuse from the fuse box.
    <1m
  2. Look at the value on the display.
    • A drop in the value shows the circuit for that fuse.
    <1m
  3. Install the fuse again before you remove the next fuse.
    • Never remove two fuses at the same time.
    <1m
  4. If the value dropped, write down the number of the fuse and the name of its circuit.
    <1m
  5. If the value did not drop, move to the next fuse.
    • Repeat this test for each fuse in the fuse box.
    <1m

Decide what to do

Estimated 2 min to 8 min

  1. If you found no cause, speak to a vehicle repair shop.
    • Give the technician the value, the date, and the results of the fuse test.
    1m
  2. If the value is higher than the manufacturer value and you found no circuit, speak to a vehicle repair shop.
    • A shop has the equipment and the training for a deeper electrical check.
    <1m
  3. Ask the technician for a written estimate before the work starts.
    1m

Record the result

Estimated 2 min

  1. Record the date, the value, and the results of each check in this procedure.
    1m
  2. Keep the record with the documentation for the vehicle.
    <1m

Terminology

This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.

parasitic draw
Also called: parasitic drain, battery drain, current draw, phantom drain, vampire drain, dark current, battery draining overnight
quiescent current
Also called: key-off current, resting current, standby current, sleep current, parked current
module
Also called: computer, control unit, ECU, onboard computer, car computer
fuse box
Also called: fuse panel, fuse block, relay box, fuse compartment
negative terminal
Also called: ground terminal, negative post, the minus post, black terminal

When to use a professional

This procedure does not repair a fault. It finds the circuit and the value for a technician. If the fuse test does not find a circuit, speak to a vehicle repair shop. A vehicle with many modules needs a wiring diagram and a scan tool for a full check. Do not remove a fuse for a system you do not want to disable. Consult the fuse chart before you remove any fuse.

On a hybrid or an electric vehicle, do not touch a high-voltage system. Speak to a qualified technician for any work near a high-voltage battery. Give the technician the value, the date, and the fuse that dropped the value. A written record helps the technician find the fault faster. This procedure is for a 12 V system only. It does not cover the high-voltage propulsion battery of a hybrid or an electric vehicle.

Common questions

Why is my car battery dead every morning even though it tested fine?

A normal voltage reading only shows the battery can hold a charge, not that nothing is drawing it down overnight. Two things explain most cases: a light, a door, or an accessory that stays on and drains the battery for free, or a small constant current the vehicle itself draws with the ignition off. Every modern vehicle draws some current at rest to run its clock, its alarm, and its computers, and that draw is only a problem when it is higher than the vehicle maker allows. This page checks the free causes first and only then moves to measuring that current.

What is a parasitic draw on a car battery?

It is the current a vehicle's electrical system pulls from the battery after the ignition is off and every door is closed. A small draw is normal and keeps the clock, the alarm, and some modules ready. A parasitic draw becomes a problem when a module fails to go to sleep, or a short sends current somewhere it should not go, and the draw is high enough to empty the battery overnight.

What is draining my car battery overnight?

Start with what you can see for free: an interior light, a trunk or glove box light, a door that is not fully shut, or a phone charger, dash camera, or tire inflator left in a socket. A pattern of short trips can also stop the alternator from ever fully charging the battery, so the battery starts every night a little lower than the last. If none of those explain it, the vehicle may be drawing more current at rest than it should, which this page shows you how to measure.

My battery drains even when the car is parked and untouched. What is normal?

Some draw is normal on every modern vehicle: modules do not switch off the instant you lock the door, and some take minutes to finish their own checks and enter a low-power state. What counts as a normal amount of current after that settling time is different for every vehicle, because it depends on how many modules the vehicle carries. This page will not print one number for every car; it sends you to your own vehicle's manufacturer documentation for the number that applies to it.

I just bought a new battery and it keeps dying overnight. Why?

A new battery rules out age as the cause, but it does not rule out a parasitic draw, a pattern of short trips, or a light or accessory left on. A new battery can also read a normal voltage while its terminals or its ground connection are loose enough to add resistance, so the visual inspection in this page's early steps is still worth doing. If the free checks find nothing, the current measurement further down the page applies just as much to a new battery as to an old one.

How do I test for a parasitic draw myself?

With a multimeter set to its DC current function, connected in the path between the negative battery cable and the negative terminal, after every door is closed and enough time has passed for the vehicle's modules to settle. The wait can run to tens of minutes on some vehicles, which is the main reason most readers do not finish this half of the diagnostic. If the reading is high, pulling one fuse at a time and watching for a drop can point to the circuit, but a fault that stays hidden after that is shop work.

Sources

Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.

  1. Tier 1Basic Rules for Battery Care & Maintenance (form 0004)East Penn Manufacturing Co., Inc. (Deka) · accessed 2026-07-29 · re-verification due 2027-01-25
  2. Tier 129 CFR 1926.441 - Batteries and battery chargingOccupational Safety and Health Administration · accessed 2026-07-29 · re-verification due 2027-01-25
  3. Tier 1Replacing the Low Voltage Lead-Acid Battery (Model 3 Do It Yourself Guide)Tesla, Inc. · accessed 2026-08-04 · re-verification due 2027-01-31
  4. Tier 1Jump Starting (Model 3 Do It Yourself Guide)Tesla, Inc. · accessed 2026-08-04 · re-verification due 2027-01-31

Revision history

Current version 1 · updated 2026-08-04 · next review due 2027-08-04

  1. v1 · 2026-08-04First draft, scaffolded as doc_type diagnostic per the backlog note. The backlog's own sourcing assessment is followed exactly: the observational half (a light left on, a door not fully shut, an accessory left in a socket, a short-trip driving pattern, battery age) needs no source and none is cited for it. The measurement half needs a multimeter-based quiescent/key-off current test, and its two safety-adjacent numbers — the acceptable current and the vehicle's own module sleep/settle time — are published as unverified, routed to the manufacturer's own service documentation, because they differ by the number of electronic modules on the vehicle and no general figure exists (rule 1). No specification on this page carries a digit. Four sources are named: East Penn's battery care and maintenance document and OSHA 1926.441, both already registered in data/sources.json from an earlier record, supply the lead-acid hazards (hydrogen gas, sulfuric acid, short-circuit risk, PPE). Two new sources are staged in data/sources-additions-diagnose-a-battery-that-goes-flat-overnight.json: Tesla's own public Model 3 service documentation for 'Replacing the Low Voltage Lead-Acid Battery' and for 'Jump Starting', both fetched and read directly (not through search, not through a reseller). The first supplies the WARNING that on some vehicles the 12 V battery sits next to a high-voltage battery housing, quoting Tesla's own 'DO NOT TOUCH OR PLACE ITEMS ON THE METAL HOUSING!'. The second supplies the basis for the general point that a vehicle's own modules take a real, vehicle-specific settling time after the 12 V circuit is disturbed, quoting 'It may take several minutes for the touchscreen to wake up.' — used only to establish that a settle period exists, never to supply a number. Neither Tesla source's own numeric values (the 45 Ah replacement battery spec, the 6 Nm terminal torque) are published on this page: this is a diagnostic for batteries in general, not a Tesla replacement procedure, and publishing a single manufacturer's number here would misrepresent it as general. The record's own torque and multimeter-range specifications are deliberately left as consult-the-manufacturer for the same reason. Two published pages already in the corpus, test-a-12v-battery-with-a-multimeter and choose-a-replacement-vehicle-battery, are linked as related along with replace-a-blown-fuse-in-a-vehicle (the fuse-isolation half of this diagnostic overlaps directly with that record) and read-a-diagnostic-trouble-code. NOTE FOR THE PUBLISHER: this record cannot pass scripts/validate.py until data/sources-additions-diagnose-a-battery-that-goes-flat-overnight.json is admitted via scripts/sources_admit.py --apply — the two Tesla ids are staged, not yet in data/sources.json, by design: this session was instructed not to touch the registry. A merged in-memory gate run (sources.json plus the staged file) passed with zero errors and only the two expected 'not yet cached' warnings before this note was written.

Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.

Comments and corrections

Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.

Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.

The comment system is not connected yet. Comments are stored in GitHub Discussions through Giscus, outside this site, so that a redeploy cannot destroy them.