Car Battery Draining Overnight in Chennai?
A battery that goes flat overnight has a specific cause, parasitic current draw, a failing battery, or an alternator that isn't recharging properly. GM diagnoses the actual fault before recommending a fix. Ambattur workshop, Mon-Sat 10AM-8PM.
Quick answer: A car battery draining overnight is caused by parasitic current draw (a component staying on after ignition-off), a battery that can no longer hold charge, or an alternator not charging properly. Each needs a different diagnosis. Call +91 84280 00085 to book at GM's Ambattur workshop.
Reviewed by G.M. Manigandan, founder and lead technician · Last reviewed
Three Reasons a Battery Drains Overnight
1. Parasitic Current Draw
A component stays on after the ignition is off, a relay, blower motor, interior light, alarm module or after-market accessory. Even 200mA of draw empties a battery in 8-12 hours. Most common cause.
2. Weak or Failed Battery
Chennai heat degrades car batteries faster, typical life is 2-3 years. An old or heat-damaged battery loses capacity and can't retain charge overnight even without a parasitic fault. Confirmed by load test.
3. Alternator Not Charging
If the alternator output is below 13.5V, the battery slowly discharges while driving and goes flat after parking. Checked with a voltmeter under load.
How we diagnose it
GM's Battery Drain Diagnosis Process
We don't replace parts until we know what's wrong.
1Battery Load Test
The battery is tested at its rated CCA. A battery that fails this test needs replacement regardless of other findings. If it passes, the drain is coming from somewhere else.
2Alternator Output Check
With the engine running and electrical loads on (AC blower, lights), the alternator should read 13.5-14.5V. Below this means the battery is not being recharged properly.
3Parasitic Draw Test
A current clamp is placed on the negative battery terminal with ignition off. After 10-20 minutes (for all modules to sleep), draw is measured. If above 50-80mA, fuses are pulled circuit by circuit to locate the faulty system.
Confirm & Fix
Once the fault circuit is isolated, the specific component is identified and the repair is recommended, relay replacement, wiring repair, or battery/alternator replacement. Estimate confirmed before work begins.
Battery Drain: Questions Answered
Why does my car battery drain overnight?
A battery draining overnight is caused by parasitic current draw (a component staying on after ignition-off), a battery that can't hold charge, or an alternator that isn't recharging it properly. Each needs a different fix. GM diagnoses the actual cause first.My battery is new but still drains. What's wrong?
A new battery draining means the battery is not the root problem. The fault is parasitic current draw or an alternator not charging properly. Just replacing the battery again will not fix it.Can an AC relay cause battery drain?
Yes. A sticking AC compressor relay or faulty AC control module can cause the compressor or blower circuit to draw current after the ignition is off. This is checked during parasitic draw diagnosis at GM.How long does battery drain diagnosis take?
Battery load test and alternator check take about 30 minutes. Parasitic draw testing requires 10-20 minutes for modules to sleep, then circuit-by-circuit fuse pulling, typically 1-2 hours total. Call +91 84280 00085 to confirm availability.
Battery Draining Overnight?
Call or WhatsApp GM Car A/C Service, Ambattur. We diagnose the actual cause before recommending a fix, no guesswork replacements.
Call +91 84280 00085 WhatsApp for DiagnosisGet Your Free 30-Point Checkup
We reply in workshop hours · No spam
Prefer to talk? Call +91 84280 00085 or WhatsApp us.
Four steps, in this order, every time
Diagnosis-first is the whole method: measure, write it down, then price it. It is also why every job, gas filling included, is quoted in writing after the checkup.
We test before we quote
The free 30-point A/C and electrical checkup runs first, about 15 minutes, with a written report of what was measured.
You get the estimate in writing
The report says what was found. Gas filling, like everything else, is quoted in writing after the checkup, and nothing starts until you approve it.
No work starts until you approve
Nothing is opened up or replaced before you say yes to the estimate. If the diagnosis changes the scope, you hear about it first.
Repaired, then checked again
The approved work is done and the car is checked again before handover.
Frequently asked questions
Why is my car battery draining overnight in Chennai?
A car battery draining overnight is caused by parasitic current draw (a component staying on after ignition-off), a battery that can no longer hold charge, or an alternator not charging properly. Each cause needs a different fix. GM Car A/C Service diagnoses the actual cause before recommending a solution. Call +91 84280 00085.
What is a parasitic battery drain?
Parasitic drain means a component is drawing current from the battery even when the car is switched off. Every car has a small normal standby draw (typically under 50mA) for the ECU, clock and alarm. A fault causes abnormal draw: 200mA or more, which empties a typical car battery overnight or in a few days. The technician identifies the fault by measuring current with a clamp meter and pulling fuses circuit by circuit to locate the culprit.
My battery is new but still drains. What is wrong?
A new battery draining means the battery itself is not the root problem, a new battery simply has more capacity to absorb the drain before it goes flat. The fault is most likely parasitic current draw or an alternator that is not charging adequately. Both need diagnosis and correction; just replacing the battery again will not fix it.
Can an AC compressor relay cause battery drain?
Yes. A sticking AC compressor relay or a faulty AC control module can cause the compressor or blower circuit to draw current after the ignition is switched off. This is one of the less obvious causes of overnight battery drain and is checked during GM's parasitic draw diagnosis.
How long does a battery drain diagnosis take?
The basic battery load test and alternator check take about 30 minutes. A parasitic drain diagnosis requires the car to sit for 10-20 minutes after switch-off before accurate measurement, then circuit-by-circuit fuse pulling, typically 1-2 hours. Call +91 84280 00085 to confirm availability.
Related pages and areas
Tell us what the car is doing
We will call you back to confirm a slot. Nothing is charged and nothing is booked until we have spoken.
Customer Service · Individual car owners. Mon–Sat 10:00 AM – 8:00 PM, Sunday closed. Prefer not to call? The form takes a minute.
Illustrative image. Not a photograph of GM premises, staff or a customer vehicle.