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I believe if a car battery is left uncharged, it will eventually ruin the battery's ability to hold a charge due to irreversible chemical reactions in the car battery.

How long would it take for the average car battery to be ruined by a lack of charge (on average)?

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There's no specific time that will determine how long a battery will last without damage. There are a number of factors, not limited to temperature, age of the battery, what the "open circuit voltage" was when it was last charged, and most importantly: the load drains that are created by various "keep alive" computers, memories, and other modern electronics that consume the battery capacity even when the vehicle is completely turned "off". Because it isn't.

I can say this with confidence: any sealed lead acid battery no rated for deep discharge will incur some level of permanent damage if it spends time below 10 volts open circuit voltage.

Classic cars or hotrods or rarely driven vehicles benefit from either an AGM "absorbed glass mat" type battery, or use of a "battery tender" trickle charger to maintain the battery and offset parasitic and memory "keep alive" consumption while the vehicle is not being used.

Another solution (which is cheap and cheerful) is to use one of those battery disconnect terminal devices, which completely disconnects the battery while not in use. The downside is you lose radio presets and ECM codes and training. This may or may not be an issue to you, depending on the vehicle.

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