J.L. Classics

Battery goes Flat when left.

MGB Battery goes flat when parked. Suggested method to trace such a problem.
This could be the condition of the battery, the connections of the battery or an electrical leak upon your system.
You could have the battery condition checked for you or if the battery is fully charged, leave the car with the battery disconnected when parked. If it loses charge despite not being connected, the problem is your battery.
Next check the connections of your battery. This could be the connections onto the battery, any link leads if two batteries exist, the connection of the earth lead to the body work and the live lead to the main loom. This is usually at your starter motor. If the connection onto the batteries are covered in white dust/fuzzy material, this corrosive material can be dissolved by pouring boiling water onto it. If there are any poor connections, your battery cannot perform as it should, and you are just guessing as to what is really happening. I am not suggesting that such poor connections are the reason a battery will go flat. If your vehicle is fitted with twin 6 v batteries both have to be in good condition.
If it is not the condition of the battery, or your battery leads/connections, you have an electrical leak upon the system.

How to tell what`s going on ? A volt meter is of no use. This will just show you the voltage of the leak and we already know that will be 12volts and could be something like a clock, which would not flatten your battery. The simples test equipment a 21w blub test light rig. That could be a single filament indicator bulb with an earth wire and a live wire. Best method is to have it in a bulb holder with the two wires coming from it. A lower wattage test light could be used but again does not indicate the scale of your problem because it lights under very low amperage.
Start with a charged 12v car battery connected to the car by both its leads and the ignition, lights and radio turned off. All doors shut to turn off curtesy lights etc. Take your test light and connect it with one lead onto the either battery terminal. At this point it will light up bright. If it does not, your battery is flat, or your test rig/bulb is faulty. Now remove the battery earth lead from the battery. Now joint your test rig with one wire of the test rig to the battery earth terminal (the one from which you have just removed the battery earth lead) and the other end of the rig to the lose battery earth lead. If the bulb does not light – you have no electrical leak upon the system. If it glows dimly you have a small leak, or a clock fitted. If it lights up your 21w bulb brightly you have a major drain upon the battery which requires investigation.
Leaving the bulb connected and lit brightly, keep your doors shut and look into the engine bay. Remove one fuse at a time and watch the light. If the light goes out, note which circuit the fault is on. You may have to replace some fuses to isolate which has the problem. Having removed all the fuses and you still have your test light on bright – unplug the plug on the back of the alternator. There is usually a wire clip holding this in place. Prise this to one side, wiggle the plug and pull it out. If this turns out your test light – its time to get another alternator or have it repaired. Alternators do cause problems which will fatten your battery despite ever thing is turned off and it works as it should when the engine is running. It could be that you have not played with the wiring but been drilling holes to fit seatbelts or similar, pinched a wire under the seat, fitted a new extra that is not fused and has an electrical leak. This is not comforting news but if it was a major electrical short - there would be smoke coming from a burning wiring loom, perhaps followed by a fire. – it is probably your alternator

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