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MK1 RS2000 electrical issue

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Evening

im having a bit of a stupid issue with my electrical system in the car. I seems to be destroying batteries on a regular basis. The alternator is charging well (13.8v at 1000rpm) but the batter just can't seem to keep everything going. When driving home last night I had to decide if I wanted headlights or the fuel pump (electric fuel pump), not a decision I really want to have to make at 22:30. 

Has anyone had an issue like this before. I have noted that it's very hot in my engine bay which I don't think helps situations. I have been thinking about mounting a bigger battery in the boot but I don't really want to if I don't have to. 

Any advice of info would be greatly received. 

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You don't drive around at 1,000 rpm! ....... what is the output at 3 - 4,000rpm - you could be overcharging and cooking the battery? Batteries are fairly resilient things and overcharging / undercharging is generally what kills them! The differences you state are maybe 5 - 10A which shouldn't trouble a healthy battery / charging system for hours of use - as headlights are usually included in alt. load / battery capacity calcs they can be discounted so the addition of a elec. pump maybe within the excess allowed for battery charging (unlikely but possible) but even so the battery (say a 45ah type) should still run everything + pump for 9hrs with engine running normally!

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very true, revving the engine makes very little difference to the alternators output so i would assume thats ok. i will double check it tomorrow as its always good to be sure.

i am planning on playing about with the power draw on the batter with the engine off and the engine on to see how much current is drawn at any one time, that way i might have a better idea what is going on. 

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Be a strange alternator that didn't increase output with revs! Volts and amps should increase to replace the Wattage load drawn from the battery.

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An alternator output will not change very much upon rising revs if there is no demand, that is the job of the regulator, if the demand is say 7 amps and that can be supplied by the alternator at 1000 RPM, increasing the revs to 2000 RPM the demand would still be 7 amps, so the alternator would not need to supply more, 13.8Volts is not what I would call "Charging Voltage", 14+ Volts would deliver a better charge rate, as long as it does not exceed 14.6 Volts, 13.8 Volts is what I would expect an alternator to give out at tick over on a light load, switch on the headlamps and/or rear window and then increase the revs to 1000 RPM + and the voltage should be closer to 14 Volts indicating that current is being produced.

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