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One of my cars was parked for a few months and it couldn't start after that. I could jump start the car and gave it some good drives (2hrs in total) to recharge the battery. However, that didn't help as once the car engine is shut down the car won't start as normal.

Checked the battery it is supposed to hold 12v but now only holds 5.2v max right after the drive. Alternator is good giving 14v when engine is on. Tried some car charger to charge the battery directly but it only recognise the battery as a 6V battery (as oppose to 12v).

Two questions: 1. is there any other way to restore the battery life? 2. what caused the battery to hold only 5v (and being recognised as a 6v battery) after parked for a few months? (I am interested in the internal mechanism)

Thanks mate

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The battery has been damaged by letting it go too far below 12v for too long.

It needs replacing. When you fit, or before you fit the new one, make sure it is fully charged.

If the vehicle will only see occasional use, then use a battery tender or charger to keep the battery topped up.

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  • Thanks. Is the "damage" irreversible? What exactly is this "damage"? Why it only happens when the voltage is below 12v but not when it is fully charged (otherwise batteries can all be damaged for sitting on the shelf for too long)? Mar 2 at 8:33
  • Batteries can, and do, loose capacity if held fully charged permanently - ups systems - good ones - have an algorithm to run charge / discharge cycles to prolong the battery life. Lots of info exists about the chemistry of batteries so I am not going to search and repeat it here.
    – Solar Mike
    Mar 2 at 8:47
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    @user1589188 Batteries work by chemistry. In "normal" use, the chemical reactions during discharging are reversed during charging. However if the discharging is continued too long different irreversible reactions take place. (Irreversible reactions also occur because the battery chemicals are never "100% pure," which is why batteries eventually fail even if they are not mistreated.)
    – alephzero
    Mar 2 at 14:05
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    @user1589188 All types of batteries self-discharge when "sitting on the shelf" even if they were originally fully charged. Car batteries typically lose about 4% of their charge per week, depending on temperature and other conditions.
    – alephzero
    Mar 2 at 14:08

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