Q: battery keeps going flat when left overnight

asked by on

to begin with I had a bad alternator as the battery wouldn't hold charge. I changed that and that worked. I've also changed what I thought were bad coil pack and spark plugs. Car was working fine for a while. Now it won't start again will start from jump leads then the alternator charges the battery and car will stop/start all day. Then I leave it for 10+ hours and it won't start again.

My car has 100000 miles.
My car has a manual transmission.

You didn’t indicate the age of the battery. If the battery is "new" (less than 2.5 years old) and passes a load test, you should check for parasitic loads. With all car accessories off and key out of the ignition, use an ammeter to measure the current flow from battery terminal to heavy battery cable.

What you measure should be in the low milliamps, like 20 to 40 milliamps (varies by car model and whether you have for example a security system), but parasitic load should be very low. If you are measuring a high current draw, that is a problem and will discharge your battery. If your battery is old, it could be self discharging overnight and/or have a bad cell(s). If you’d like help with this diagnosis, a certified technician from YourMechanic can come to your car’s location to inspect the battery and perform necessary repairs.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1,000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!
  1. Home
  2. Questions
  3. battery keeps going flat when left overnight

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car would not respond when I turned the key...Tried 2 or 3 times same...came back 1/2 hour it is fine.

Hello! An intermittent failure of a start response can be many things - a bad ignition switch, starter solenoid, or in your case - the keys use a "raised-chip" design for security. With years of wear, the key/ignition switch become...

Q: Replaced alternator and now there's a low sounding, high pitched noise, like a jet engine whistling.

Hi and thanks for contacting YourMechanic. This can be an easy fix. The alternator may have a problem with the fan blades inside it if you got a brand new design. If you got a design with the fan located...

Q: Can this van's alternator be put in a 2004 chevy impala?

The alternators on the two vehicles are not compatible with each other. The computers on these vehicles control the alternator charge rates and you could have a computer problem and should have the system diagnosed by a qualified mechanic like...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.