My car was running fine one day and the next wouldn't start at all, not enough power to even open the doors but the lights went on when I turned the ignition on. The mechanic said I need a new battery and this worked a treat till the next day when I tried to start it and the same thing again, no power except the ignition lights go on . Any help would be appreciated thank you

Hi trebuski and welcome, sounds like a ground, may try pulling fuses one at a time to isolate what circuit. Good luck

Battery Drain

Parasitic battery drain is the cumulative load produced by electrical devices (e.g. ECMs, clocks, security alarms, and radio presets), when the ignition is turned off. These items continue to operate, when the ignition has been switched off, and are becoming an ever increasing problem. Normal parasitic loads are below 75 milliamps (.075 amps). When parasitic load is greater than 75 milliamps, batteries will drain even faster. Glove box, trunk, and under hood lights that do not automatically turn off when the door is closed, or shorted alternator diodes, are common offenders. Cooling fans, power seat belt retractors, radios and dome lights left on, alarm systems, and electric car antennas can also cause batteries to drain overnight. Leaving headlights on will usually discharge a fully charged car battery, with 90 minutes of reserve capacity (36 amp hours), in a couple of hours.

Two methods are used to test for parasitic load (engine off, under hood light disconnected, accessories off, and vehicle doors closed):

1. The basic approach is to connect a 12-volt bulb across the positive and negative battery terminals to test the bulb and battery. If it glows brightly, remove the negative battery cable and connect the bulb in series between the negative battery cable connector and the negative battery terminal. If the bulb continues to glow brightly, start removing fuses or connections to the positive battery, one-at-a-time, until the offending electrical component is identified by the bulb dimming.

2. The best approach is to use a DC ammeter, connected in series between the negative battery cable connector and the negative battery terminal, or a clamp meter (set to DC amps), clamped around the negative battery cable. Starting with the highest scale (or set multi-meter to auto range), determine current load. If load is above 75 milliamps, after the initial surge, start removing fuses or connections to the positive battery post, one-at-a-time, until the offending electrical component is identified by the parasitic load dropping to 75 milliamps.