My car battery has gone completely flat after leaving my car parked up for about a month. Tried turning over the car today, and none of the lights on the dash light up.

Ran a multimeter across the terminals, and it shows 2 volts. This battery is under a year old in age, and has never been jumped before. It is a value brand battery, so cheap and not great. I can jump the car, but I imagine it will put undue stress on the alternator.

Is it even worth my time bringing the battery to the local car parts place for a trickle charge, or should I bite the bullet and get a new battery for roughly 100EUR ?

  • I would recommend you just have it jumped by someone just so you could get the engine started and that's everything you need to do, then just either run the engine for a hour or drive around for 1 hour that would fully charge your battery the Alternator is made to charge batteries so the extra stress will have no affect on it, this is not only the cheaper solution it's the best solution. I did this over 5 times on tons of batteries, if after you charge it up, the battery is capable of starting the engine for atleast a week, then it's a good battery otherwise you have to replace it. – SSpoke Nov 9 '15 at 23:05
  • Have you done anything? What did you decide? – philcolbourn Nov 28 '15 at 10:41
  • I tried balancing the battery with my jump start pack, just enough so my trickle charger would kick in... No joy, even after three rounds of "jumping". I've ordered a new battery, and plan on bringing the car to an automotive electrician, to figure out why the batteries keep draining. My gut is on the A/C compressor - after it was replaced all these issues cropped up. – Nick Nov 30 '15 at 20:29

Since you have a battery which isn't that old (most batteries have about a five year life span), I'd suggest you put it on a charger and try to recharge it. This will allow the battery to come back to full charge without putting an undue stress on your alternator. You have to decide if the time spent in recharging the battery is worth your time. To me, 100EUR would be well worth my time as long as I have a spare vehicle to utilize in the mean time.

While this episode of having to recharge the battery will have taken a toll on the battery (shortened its lifespan), it will most likely be able to take the charge and will work fine for you for the foreseeable future.

Another thing to think about, though, is if the battery is only a year old, it may still have the manufacturer's warranty in place. If this is available, you may just take it in and say, "I'll take my replacement, please." ... especially if there is a priority need for the vehicle.

  • WRT the warranty - it's out. WRT charging - I have a sealed lead acid jump/boost pack. Should I try removing the battery from the car and directly boosting it with the pack (red to red, black to black) before I invest in a charger? – Nick Nov 9 '15 at 17:52
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    Addendum: Since it's showing 2 volts as opposed to 12 volts, I have a feeling no modern charger will go near it - hence my idea of attaching my jump start pack directly to the battery to charge it up a bit. Since there's gonna be sulfation, the batteries lifespan is shot either way - might as well try and get another year out of it. – Nick Nov 9 '15 at 20:09
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    @Nick - While you are right about no modern charger touching it, that would under normal conditions. You can easily fool the newer chargers into charging by parallel linking a good 12v power source into it. I keep a smaller 12v wet cell battery around which I've pulled out of a garage door opener ... as long as the charger senses a 12v lead, it will charge. As soon as it starts charging, there's no issues with it finishing. This also works for chargers which don't do gas-mat batteries like an Optima Red Top. – Pᴀᴜʟsᴛᴇʀ2 Nov 9 '15 at 22:08
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    @Nick: Most batteries I've purchased have pro-rated warranties even after the full warranty expires. You might be able to get a replacement for only 30% or so of the cost if it's just a few months out of warranty. – R.. GitHub STOP HELPING ICE Nov 9 '15 at 22:42
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    @SSpoke - I hate to say this, but you are wrong. It is neither the best nor the right way to recharge the battery. While the alternator can do this, it isn't made to recharge a dead battery. It's made to replenish it after it is slightly drained due to starting. Using a charger is the best and right way to recharge it in this situation. Besides that, when recharging from this state, the best way to do it is on a trickle charge (2A) setting. It will provide the best charge with the least amount of residual damage. – Pᴀᴜʟsᴛᴇʀ2 Nov 9 '15 at 23:09

Lead sulfate crystals form on the battery plates as the battery discharges. As the battery becomes more discharged the crystals go from being soft and fluffy to much harder. Recharged in time, the lead sulfate is converted back into sulfuric acid and lead. A month is entirely too long for a battery to remain discharged.

Reversing a serious sulfation condition, even for high quality deep cycle batteries, requires special battery chargers which are able to charge at higher voltages (about 15 volts) than what most consumer-grade chargers produce (about 14.4 volts) and well above what trickle chargers produce (about 13.8 volts).

Car batteries aren't deep cycle batteries, they are "Starter, Lights, Ignition" (SLI) batteries with thinner and more numerous plates, designed to provide very high current for very short periods of time. Attempting to perform the controlled over-charging needed to equalize an SLI battery can cause the plates to warp from the heat, or shed plate material and develop shorts.

The long and short of it is that your battery is pretty much ruined. You might be able to get it to appear to hold a charge, but sulfated batteries have significantly reduced capacity due to reduced active plate surface and material.

This is to go-to web site for most battery questions.


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    +1 This should be take note of. Just because the battery starts the car again after a recharge, doesn't mean it is OK. A couple of very short journeys and you find out that the battery has no capacity. – HandyHowie Nov 10 '15 at 8:27
  • The "diagnostic criteria" for sulfation is precisely that -- the battery behaves much like a battery with a much smaller capacity. For example, I (used to ... long story -- in the middle of changing the entire bank) have 430 amp-hours at 48 volts which provide backup power to my house. When sulfated (they are mostly worn out at the moment) they might act like a 100 amp-hour bank and fully charge with little effort, then wind up dead before the should be dead. I run across people say "the battery took a charge!" and don't know how much total energy was stored. – Julie in Austin Nov 10 '15 at 23:50
  • Yes, it's like someone saying there fuel tank that has a hole in it a 1/4 of the way up is fully functional because they are able to drive a short distance. When they come to drive a long distance, they realize its weakness. – HandyHowie Nov 11 '15 at 8:15
  • @HandyHowie - Mostly correct, though it's more like your gas tank is full of junk. Gauge reads full, but you run out of gas too soon and refilling takes less than you expect. When that happens you need a commercial grade 4-stage charger and patience, and probably a new battery anyway. I'll take the time on a $5,000 bank of batteries, but not on something I can pick up at AutoZone and just be done with. – Julie in Austin Nov 12 '15 at 17:02
  • During discharge, does Lead Sulphate deposit on plates in a crystalline form? I don't think this is true as it takes time (weeks? months?) for amorphous Lead Sulphate to form into Lead Sulphate crystals. – philcolbourn Nov 22 '15 at 14:50

I don't think you are likely to "put undue stress on the alternator." It is a myth in my opinion that alternators can't handle charging a battery, Alternators are literally motors run in reverse and are designed to handle a lot of current. The only issue with charging a batter is it will get a bit warmer than usual and that shouldn't effect it as long as it dosn't get so hot you melt the insulation.

  • An overcharging alternator will boil a battery, causing all of the electrolyte (or a lot of it) to evaporate, leaving you with a dead battery. Anything in excess of 14.5vdc is overcharging. 16vdc will boil a battery in short order. – Pᴀᴜʟsᴛᴇʀ2 Nov 9 '15 at 19:50
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    @Paulster2. Has there been a comment deleted here, because I don't think Sam is suggesting anything about an overcharging alternator. He is just saying that a flat battery can't overload an alternator. An alternator is more than capable of supplying all the current a flat battery will ask for, whether that does the battery any good is another thing. – HandyHowie Nov 9 '15 at 20:21
  • I would recommend you just have it jumped by someone just so you could get the engine started and that's everything you need to do, then just either run the engine for a hour or drive around for 1 hour that would fully charge your battery the Alternator is made to charge batteries so the extra stress will have no affect on it, this is not only the cheaper solution it's the best solution. I did this over 5 times on tons of batteries, if after you charge it up, the battery is capable of starting the engine for atleast a week, then it's a good battery otherwise you have to replace it. – SSpoke Nov 9 '15 at 23:06
  • @SSpoke. The alternator is not clever, just powerful. It will push out >70 amps is there is a request for it. Just because the battery starts the car again after a recharge, doesn't mean it is OK. A couple of very short journeys and you find out that the battery has no real capacity anymore. See Julie in Austin's answer. – HandyHowie Nov 10 '15 at 8:31
  • TLDR: It is harder work to charge a very flat battery. Take care. My old Jaguar XJ6 (x300) would eat a 120AH battery in 4 to 6 weeks if left parked up. If I was going to spend more than a fortnight away from it I needed to either disconnect the battery or remember to charge it when I came back. If I could get the car to turn over after leaving the battery connected for a few weeks I would often break the auxiliary drive belt because of the strain generated charging a very flat battery. This might have been a deficiency in the model or a minor fault in my specific alternator. – TafT Nov 10 '15 at 10:10

If the battery puts out just 2V across the terminals then it is severely discharged; in all probability beyond the point of recharging.

There's no point in trying to charge it. I've had a battery that put out only 9V which couldn't be revived.

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    I brought a battery back from death like this with a "smart charger". I had to charge the battery a little with a jump start just so the smart charger would even turn on (it has to sense battery voltage to work), but it charged over the next 24 hours or so, and the battery worked great later. Batteries that dead are usually gonners, but charging might be worth a shot. – JPhi1618 Nov 9 '15 at 17:15
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    I agree with Zaid. Lead acid batteries (I am presuming it is lead acid) don't like a deep discharge. That is why you pay extra for leisure batteries that will put up with the occasional deep discharge. You may find you can add charge and use it, but it's capacity will be severely reduced. For the cost of a new battery, why take the risk of it letting you down. – HandyHowie Nov 9 '15 at 17:23
  • @JPhi1618 : Any idea what the voltage was when you brought it back from the brink? – Zaid Nov 9 '15 at 17:57
  • It was less than 2V. I had already looked up the model number for the battery and called around for prices, but decided to try charging before I bought it the next day. I probably got lucky. "Results not typical"... I used a Noco Genius. – JPhi1618 Nov 9 '15 at 18:30
  • @JPhi1618 - I'm doing that with a 215Ah, 48VDC nominal bank at the moment. Replacement cost is right at $1,500 and I think I'm going to see about desulfating them before I just give up. They should probably be scrapped (7 years old ...) but they have nice heavy plates and might be salvageable. – Julie in Austin Nov 12 '15 at 17:07

A 12v lead acid battery is flat dead at no less than 10v. If it really were 2v, that would indicate that several cells had shorted out, and you wouldn't even be able to jump start it, as the battery itself would be burning off most of the power the jumper cables can transfer as heat. The fact that you managed to jump it indicates that you read it incorrectly. Double check your multimeter settings ( make sure it reads 12-15v on the running car you are using as the power source to jump ). If you somehow did measure it right, and still managed to jump it, it is a miracle and you should drive straight to the auto parts store and get a new battery.

  • I have read well under 10V on a battery that was able to be recharged and subsequently start a vehicle. I think modern batteries will show a very low level if they have been discharged for a very long time, it is not a sign that the battery is dead (although I agree it is probably not good for the lifespan of the battery). – TafT Nov 10 '15 at 12:15
  • @TafT, by dead I meant fully discharged, and going any lower than that damages the battery. 7 or 8v and it can probably come back enough to be alright for a while, but all the way down at 2v pretty much only happens when there are some shorted cells and there's no coming back from that. This is of course, measured while not under any load. – psusi Nov 10 '15 at 23:11

From personal experience 2V is way under the safe discharge zone (considering yours is the typical lead-acid). Your battery is now damaged. Even if you charge it with an industrial charger and miraculously get it back to 13.2 Volts (of full charge) you will still get a lousy capacity and current discharge. In the best case scenario you will get a few seconds to start the car but not enough charge for a second or third try (in case it won't start the first time).

In your case i would attempt a revival if it was below 20$ (if you urgently need it and don't want to hasty buy the first battery you find). You could also buy a charger and try to revive it yourself (basic ones go for 20-30$ on amazon). But even if i revived it i would still pay the 100$ for a new one as soon as possible (I wouldn't want to pray every time i start) and maybe keep the old one in my trunk just in case i need a little extra charge.

Please remember to disconnect the battery if you intend to leave your car idle for more than two weeks.


I think only you can decide if it is worth your time and money to take it out and get it charged compared to purchasing a new battery.

To assist, if we assume a car battery will last 5 years, and you have had it for, say, 1 year, then you would expect this battery to provide another 4 years of service which I'll say is a value of 80EUR.

But if, as others believe, that this battery has lost life then you may not get another 4 years of service - without any justification, say you only get 2 years of future service, compare your time and costs against a battery with a value of 40EUR.

Re 2V: it would be odd that with load removed that you are getting 2V. This suggests that 3 of battery cells have gone reverse polarity or that 5 cells are shorted or some combination (I will not consider shorted cells beyond this point since, without justification, I will deem to be terminal).

Given that this took place over 1 month, then there are 2 possibilities:

1) your car has some fault that is discharging battery; or 2) your car's quiescent current is high and flattened battery; or (again) 3) some combination.

if 1), then you might want to recover this battery to allow you to determine if your car has a fault (eg. boot light stuck on) before you invest in a new battery and have same issue.

if 2), then since battery current has been low, then 3 weaker cells have been reverse charged by stronger 3 cells. If this current is small, then recharging your battery will re-reverse these cells and battery may be saved. Some have thought that sulfation may have occurred where lead sulphate crystals have reduced surface area of your battery plates. Perhaps: a flat battery has 2 plates of PbSO4 but it takes time to make lead sulphate crystals that are hard to dissolve. Therefore charging these cells will be fine.

Rational thing to do is to replace battery, but if you have 2 cars and are interested in spending some time you might like to jump start car and go for a drive - perhaps to a shop to buy another battery just-in-case?

You alternator and regulator will have no problem recharging your battery - they run your car headlights at night with are typically 5A each, and a alternator can produce at least 70A (I think mine is 100A).

  • That's not at all the case - the voltage is an indication of the specific gravity of the sulfuric acid within the cells. It is possible that the battery has one or more damaged cells, but this is a case of complete discharge and using a hydrometer to measure the density of the acid. The hydrometers most consumers run into have a simple dial gauge that reads something like "Dead", "Charge" and "Full". If one were to start with a "Full" battery and run it down, the density would decline until the dial read "Dead" and the voltage was much lower. – Julie in Austin Nov 14 '15 at 18:47
  • Julie in Austin, do you disagree with all of my answer? I agree that voltage is an indication of SG, but to have 5 of 6 cells with near zero H2SO4 is unlikely (noting that in an open state, cell terminal voltage no longer indicates SG since current is near zero). Once one cell reaches this point (no H2SO4), current from 'stronger' cells begins to reverse charge this cell causing electrolyte H2SO4 concentration to increase and polarity to reverse. – philcolbourn Nov 22 '15 at 14:23
  • Phil - What you write about the improbability of all the cells having low sulfuric acid density is incorrect. Because the cells are in series the discharge current through each cell is reasonably the same, within a very small margin of error. What what you wrote becomes relevant is with batteries are are cycled between partially full and nearly dead. In the present case, the battery was presumably okay because it was brutally murdered, so the probability of something else -- like what you suggested -- happening is very close to zero. – Julie in Austin Nov 27 '15 at 0:41

Yes, it is worth trying to charge it. However it might be worth you purchasing a charger if this scenario is likely to play out again in the future.

TLDR - Try charging the battery it is probably fine. Disconnect the battery from the car when not driving it for a week or more. Consider buying a good battery charger to recover the battery if you forget to dicsonnect it.

The Tool Long Version - Some models of car will drain a perfect battery to near dead in 4~6 weeks while left parked up. This is a fault in the design of the car, not a problem with the battery. It could also be a repairable fault with the car if it suddenly starts happening when it never did before. I had an old Jaguar XJ6 (x300) that would drain a perfect 120AH battery to stone dead in about 5 weeks. This was normal for that make and age of car. Nothing is wrong with the battery when this happens (although it may age the battery rapidly it it happens often).

When my battery was really flat it would only register a few volts. If it was only a little bit flat (after 2~3 weeks) then I would be able to get the car started. If I was unlucky I would get the car started and then the auxiliary drive belt would break under the strain of running the alternator hard to charge the depleted battery. To avoid this I either disconnected the battery when leaving the car more than a few days or when I forgot about it I made sure to charge the battery from a mains charger before hand.

I spent ~£60 on a Halfords Advanced/Smart Charger that could charge the 120AH battery for my car and the much smaller (40AH?) battery of my partners Ford KA. Even from near dead this unit was able to charge my large battery back to a working state. I am sure it did nothing for the lifespan of the battery and it would be better to avoid completely discharging it but it was better than buying a new battery every couple of months just to make sure it was charged. At worst the unit would fail the battery after a 24 hour charge cycle, then I would leave it an hour to cool off and I would try again and the cycle would pass. This did mean I needed 24~48 hours of charging before I could confidently use the car but it only cost me the power to charge a battery each time.

I did replace the battery at one point to see if that helped. It did seem to give me another week before starting the car became difficult or resulted in a lost drive belt but it did not resolve the issue altogether. I replaced the alternator with a refurbished one and the issue persisted. It was only by either disconnecting the battery when a week or more of standing was expect or charging from the mains after 2+ weeks of idle time that I avoided the issue altogether. Driving around for an hour or two might charge the battery if it is a little low but it would take a long time to charge a battery that was very flat this way.

  • Your jag electrics is from Joseph Lucas the Prince of Darkness .Because your car is a classic and not full of microprocessors and crap then OFF should be OFF .But your car has a yuckky leak that is to be expected of modern stuff .I looked after a mates jag while he was in canada and the same bullshit happened .It also happened to my mates brother while he was looking after it .The battery eating jag . – Autistic Feb 14 '16 at 6:41
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    @Autistic my Jaguar was an early 1990s XJ6 (the x300 model) which was known to have plenty of iffy microprocessors in it. The preceding XJ40 model from the 80s was also known to have power hungry electronics in it. I am not sure how far back you have to look in the XJ6 history to find a non-leaky one but it might be quite a long way. – TafT Feb 15 '16 at 10:06

If you have a car where the battery dies after leaving it, try plugging a solar charger on the dashboard into the cigarette lighter. This has kept me out of trouble for quite a while. Note that the cigarette lighter must be the type that is still connected when the ignition is off. Nick B

  • Are you saying you can charge a car battery through the cigarette lighter? – DucatiKiller Feb 13 '16 at 16:53

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