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I read that you should disconnect a car batteries negative cable if not using a car for extended periods as this could lead to electrical devices draining the battery especially in new cars with a lot of electronics.

However apparently you do not need to do this with older cars, perhaps because there are less electronics. Does that mean if I have an old car such as my 1999 civic vti, I can leave the battery connected and not running for 6-12 months then expect that the car will still start up on crank without the battery being drained and a need for a jump start?

I had the impression that most cars including old needed a jump start when not used for a long time.

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With older cars, the only likely key-off drain would be the clock, and maybe feed to the radio for maintaining presets, which won't add up to anything significant, but the battery itself has a self-discharge rate, and for a period of several months or more, I'd be either removing the battery and storing where it could be periodically checked and recharged, or connecting a battery tender (monitor + low current charger) if it's left in place, or you'll find the battery fully discharged, and even though it will start and begin to recover with a jump, the battery capacity tends to suffer with deep discharges.

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  • Just to see I've understood you. You are saying that after 6-12 months the clock and radio would unlikely cause it to drain, however after 6-12 months the car may not start on crank because the battery may have self discharged and hence there will be a need for a jump start? thanks. – James Wilson Oct 9 '19 at 16:12
  • Yes, Clock and radio drain wouldn't fully discharge a battery over a few months. I wouldn't leave a battery unmonitored for more than that, even if disconnected. If it's a new battery it may survive, but older batteries tend to worsen their self-discharge characteristics, probably because of contamination of the electrolyte. My lawn tractor, motorcycles and vehicles not used during winter all have their batteries in the basement during winter. I can then check them every month or so, and charge them if needed. – Phil G Oct 9 '19 at 16:22

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