Q: battery keeps going flat when left overnight

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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.

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