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My battery was fully discharged (forgot to turn off headlights for a few hours). The car wouldn't even unlock from the remote.

What should I do after getting a jumpstart?

6

It is very important to charge back the battery as soon as possible.

As per how fast does the alternator charge the battery?, it may take as much as 10 hours or more for the battery to be re-charged if it was truly fully discharged (which might not necessarily have been the case in itself, due to the potential Negative surface charge?).

It is much cheaper to do that directly with the electricity than plan an unexpected road trip (and especially if it means extra delay of the battery being left in a partially-charged state, as well as potentially not getting that much alternator capacity left over if you're already using AC, lights, radio, and it's so hot outside that the engine cooling fans have to be running at full-speed, too). As such, the best course of action, bar none, is to purchase something like a 6A or 8A charger, and let it charge the battery overnight for a couple of nights, right away from the first night the battery got discharged. Note that the charger automatically charges the battery at 14.4V (2.4V per cell), but after it determines that it becomes fully charged, then it's supposed to float it at 13.5V or so (2.25V per cell), so, these charges are often designed to be left unattended and for an extended periods of time without ill effects.

Once fully recharged, if it's a maintenance-free flooded lead-acid battery, it's also very important to check the fluid level in the battery.

If the battery has been in service for a couple of years already, this step is highly recommended, especially after a deep discharge. If necessary, peel off the label from the top, revealing the cells. As per how much water can be safely added to a battery?, use protective gloves and eye-protection, unscrew the cap, check fluid level with a flashlight -- it should definitely touch at least one side of the filler tube, or potentially two independent sides of the filler tube, forming an eye-like meniscus. (If it doesn't touch either side, make sure to fill in enough distilled water (use at most 10ml per try) to make sure at least one side of each filler tube has the meniscus present.) But make sure to not overfill, and make sure the battery is fully charged in the first place, prior to adding any water.

  • 1
    I'm not sure all maintenance free batteries have caps under the label. And further, I would NOT charge the battery until the plates are covered, if the electrolyte level is low. In fact, you can't fully charge, as the capacity is dimished due to the reduction of "battery" chemistry surface area. Another point is a a full discharge below 10 volts OCV may cause permanent damage in sulfation, and no amount of charging or adding water can completely correct this. – SteveRacer Aug 18 '16 at 2:52
  • @SteveRacer, good points! My assumption is that since the battery was good to start with, it's supposed to still have adequate water levels after a discharge event; but you are correct, the metal plates must be fully covered prior to charging. – cnst Aug 19 '16 at 3:14
  • Usually overcharging is what boils electrolyte out of the battery. In any case, your advice is fairly sound. I would say get after a deep discharge event as quickly as possible, for the best hope of a maximum recovery. – SteveRacer Aug 19 '16 at 5:07
  • If your going to buy a charger, get one that does desulphating like the BatteryMINDer 1510 Battery Charger. – Robert S. Barnes Apr 27 '17 at 16:27
  • @RobertS.Barnes, no, there is no need to buy any special charges in the situation described. – cnst Apr 29 '17 at 1:17
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I recently had my battery discharge just when the warranty expired; I had been working on the car in the garage for a considerable period of time and was using the 12V outlet to power some tools (and the OBD port for my insurance company's metering device). When I was done my work, the battery would no longer hold a charge, even after boosting.

I took the the battery to the local auto parts store for a free 45-minute rapid-charge (AutoZone is one such company that does this for free). The charger will also run a diagnostic on the battery and determine if the battery is no longer usable, or if it can still operate once charged to capacity.

If the battery is still usable, your alternator will charge the battery after it starts the car, and you have nothing to worry about anymore. If it is no longer usable, check to see if electrolyte and/or water can be added to the battery to boost its capacity. If the battery isn't serviceable, you will likely need a new battery.

I'm sure someone has figured out how to service non-serviceable batteries, but I'll leave that up to you to determine if is worth your time and if it meets your risk tolerance level :)

  • This advice is simply incorrect. (1). One should never add electrolyte into a battery, only add distilled water. (2). A depleted battery is best charged for 10 or more hours, at 1/10th the capacity; rapid charging and/or expecting that an hour each week of charging through the alternator from the normal driving will best restore the battery is simply ensuring that the damage will set in, and that even a good battery will require full replacement within merely a year or two from the event. A home charger can be had for only 20 bucks at Walmart, plus they have a 90-day return policy, too. – cnst Aug 19 '16 at 3:10
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    @cnst Comment about charging rates is valid. I have an expensive professional charger that handles FLA, AGM, and "deep cycle". It has a myraid of digital modes, one of which is designed to recover a deep discharge on a conventional FLA battery. It starts for a few minutes ~50 amps, in theory to reverse any sulfation, and then slowly (2-5 amp) recharges, checking State of Charge against rated capacity. You can discharge/repeat the process for maximum effect. This process can easily take 24-48 hours. I've sucessfully "vampired" batteries with OCV < 5V with this method, back to 85% rated. – SteveRacer Aug 19 '16 at 5:12

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