My car battery recently died and I was wondering if it is more cost effective to re-charge it or replace it with a completely brand new battery.

I know car batteries are charged with the alternator when the car is on, but I was not aware that car batteries could be charged back to full using a portable battery pack or a battery charger that plugs into a standard AC outlet. I always thought people just threw out car batteries once they died.

If I charge a completely dead battery at 12.0V to 12.6V for example, how much money does it cost in terms of electricity to charge it back to full? Also how long would it take to charge it back to full? I'm not exactly sure what units of measurement, conversion, exact mathematics, etc. to use to calculate such a problem.

Edit #1

I should have corrected my question to remove the cost of the charger. Both charger and new battery will cost money but I meant to ask, over a longer period of time, which method would save the most money.

Edit #2

Almost everywhere that took the time to answer brought value to the question, thus it is my opinion that awarding an answer to one person would be unfair. Originally, I should have cleared up the confusion about a discharged better versus a battery that is dead (i.e., cannot hold a charge anymore). The battery in question is discharged and since I was not aware that batteries could be recharged, it makes sense that charging it is the most cost-effective method.

  • It very much depends on if you have a friend from whom you can borrow a battery charger; that makes it a no-brainer. Feb 6, 2017 at 11:46
  • 3
    I think there is a bit of confusion on how car batteries work here. They have 1.) a Discharge life, and a total lifetime. Batteries can only handle being charged/discharged so many times. A battery can either be 1.) Discharged, from say leaving your lights on or not driving the car in a long period of time, or 2.) Dead (will not take a charge) which is typically due to battery age. There are several ways to accidentally prematurely kill a battery (it will not recharge). To answer the actual question though, if your battery is discharged, it would be cheapest to get a jump from a friend!
    – Sidney
    Feb 6, 2017 at 15:18
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    I've voted to reopen this question. It's not about shopping advice, so not off-topic IMO.
    – Hobbes
    Feb 6, 2017 at 20:50
  • In another question, OP clearly believes a car battery is a primary battery, non-rechargeable, like a Duracell AA. That starting the engine irrevocably draws energy out of it, and the draw stops when the engine is running because the alternator takes over. That there's some "gypsy" method of recharging the car battery like the way some people try to recharge Duracell AA's (which works a little). This premise is incorrect, so any conclusions fail as well. Feb 7, 2017 at 2:07

6 Answers 6


The cost has been addressed, but there's an underlying question: is the battery dead (i.e. it cannot hold enough charge anymore to start the car), or has it just been discharged?

If it's just been discharged (e.g. by leaving the headlights on while the car's parked), charging the battery is the best option. If the battery is dead, you'll have to replace it. A repair shop can test the battery to see if it's still usable.

  • 1
    The question might be off-topic but this is certainly the most appropriate and on-topic answer, +1.
    – DizzyFool
    Feb 6, 2017 at 13:03
  • This is pretty much the answer to the question, though it could use an edit to add that when a battery does fail to start a car it's far more likely damaged than just discharged, assuming a somewhat competent owner (doesn't regularly leave the lights on)
    – Leliel
    Feb 6, 2017 at 19:09
  • This answer in addition to the answer by Agent_L, mentioned confusing an issue with two separate problems helped me understand my problem more.
    – Narcotixs
    Feb 6, 2017 at 20:43

It is nice that some people put complicated math here (which is mostly correct also) but IMHO there was no reason to complicate it so much. As a rule of thumb:

  • IF battery can be recharged then it is always cheaper and more environmentally friendly to simply try to recharge it. (it costs almost nothing to try)

  • However if the battery damaged due to discharge (for example sulfation damage if it was empty for very long time) then you may not be able to recharge it. So in that case the only solution would be buying a new battery.

About the cost. Here is an example:

1- Most car batteries are under 200Ah (this means battery stores 200Ah worth of energy in perfect conditions). Let's assume our hyphothetical battery is 200Ah

2- Let's assume the battery is charged to full with 50% efficiency. Meaning half of the energy is lost due to chemical process, and transformer inefficiencies etc. In reality efficiency would be so much higher, usually the sites I looked mention that lead-acid battery can do average 85% efficiency when charging.

So, with (1) and (2) in mind... To calculate total energy we need, we need to sum of capacity of the battery(1) + lost energy(2). Which is 200Ah + 200Ah%50 = 300Ah to fully charge it in worst case.

Where I live electric costs about 0.1 Euro per kW/h. We want to put 300Ah in 12V so 300x12=3.6kW/h per joule's/ohm's law[1][2][3]. Therefore it will cost 0.36Euro! (same in USD if you paid $0.1 per kW/h)

You can easily see that charging 200Ah battery would cost so little that it is IMHO IRRELEVANT. Because, as you put it adequately the car alternator already charges the battery under normal conditions. So, you will need to do this operation very rarely. Your costs will be so small that you can consider them IRRELEVANT compared to cost and installation of new battery.

  • I believe the math is incorrect. If electricity is $1 / kWh, it does not mean the same thing as $1 / 1000Ah = $1 / kAh. kWh and kAh are different! To get kWh from kAh, you need to multiply by the voltage of the battery which is 12V. So, now 1 kAh = 1000 Ah = 12 000 Wh = 12 kWh. Besides, where does electricity cost $1 / kWh? It is usually about tenth of that. The only thing correct in this answer is the conclusion, i.e. costs are irrelevant.
    – juhist
    Feb 6, 2017 at 12:40
  • You are absolutely correct. I should have drunk more coffee today. :) I corrected it now. Feb 6, 2017 at 12:57
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    Actually, you are still using the incorrect voltage. The kWh should be calculated using the voltage of the battery which is 12V. It doesn't matter whether the power grid voltage is 110V or 220V or whatever. So, 300Ah means 12V*300Ah = 3600Wh = 3.6kWh = 0.36 euro. No way will it cost 7 euros!
    – juhist
    Feb 6, 2017 at 13:01
  • @juhist thank you very much, actually it is so much easier to see the cost when converted to w/h. I am sorry for the stupid answer I created. I should have been more careful :) Feb 6, 2017 at 13:08
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    Now it seems to be correct, so changed the downvote into an upvote.
    – juhist
    Feb 6, 2017 at 13:09

As the other answer didn't contain a calculation, let's see how much energy a car battery has. 12 V * 65 Ah = 780 Wh = 0.78 kWh. Taking into account charging losses, perhaps 1 kWh. Where I live, energy costs 0.1 EUR per kWh, and a new large good quality car battery costs 100 EUR, so it is actually not 1% of the cost of a new battery but rather 0.1% of the cost of a new battery.

I know there are places where value-added tax is lower (and thus stuff is cheaper), and also electricity is ridiculously expensive. But even in those places, the cost is below 1% of the cost of a new battery.

Note that this didn't take into account the cost of the charger. Chargers may contain electrolytic capacitors that don't last forever, and thus, in the worst case (Chinese capacitors, not Japanese capacitors) you may have to replace your charger as often as you replace your car. However, a good quality slow microprocessor controlled charger costs somewhat less than a large car battery. Of course, you can spend practically any amount of money you want: there are fast chargers that are actually more expensive than car batteries.

Thus, I would heavily recommend you to purchase a slow microprocessor controlled charger and charge a car battery if you believe it is not fully charged.


You have mixed up 2 issues : battery empty vs full and battery operational vs damaged. It's an easy error to make, as situation from one question (discharging too deep) CAN lead to situation from the other (damage).

  1. Is it cheaper to let your car's alternator charge the battery, or charge it from the grid at home via wall charger?

Car alternator doesn't charge the battery for free. Engine burns more gas to generate the extra HP to be converted into electricity. Electricity from gas is always more expensive, otherwise all powerplants would run on gas. In theory it's always cheaper to charge your battery from the wall at home. But the savings are marginal and it's huge inconvenience to do it every day. Nevertheless, you SHOULD top up your battery from the wall at least once every few months to keep it in best condition.

  1. Is the battery permanently damaged if discharged below 10–11V?

Yes. If you ever let it discharge that deep, some damage will happen. But not necessarily a critical one: the battery will lose some of its capacity and perhaps peek power will be lower, but usually there still is more than enough to start the car just fine. But the deeper it was discharged and the longer it sat at that level, the more damage it sustained (read: capacity lost).

  1. Is it cheaper to try to revive flattened battery or to buy a new one?

And here we arrive at the question I believe you intended to ask. You CAN bring new life into old battery by cycling it (charging it and discharging) a few times. But is it useful? The answer is 3 fold:

  • If the battery was damaged really bad (e.g. discharged and left for months) it'll never recover enough to start a car again. But if the damage was slight, you'll get a working battery for next to nothing.
  • Even with a new battery, you still should get a good charger just to keep it in best condition.
  • The costs of electricity are negligible. Typical trickle charge is let's say 4 amps at 14 volts, that's 56 watts. Sure, you need to run it for like 20 hours, but it's still comparable to an average lightbulb running overnight. (Theoretical energy cost is written on the battery. If it's 40 Ah battery then it should take 40 Ah*12 V = 480watt-hours, 0.48 kWh, let's round it up to 0.5 kWh. That was in perfect world, assuming that as much as half the energy is lost as heat during charging, that's 1 kWh to charge it. Twice as big battery = twice as much electricity, etc. Check how much 1kWh costs you, usually we're talking dimes here.)

So, my recommendation is as follows: Buy a good wall charger. With option to cycle the battery, at best (otherwise you'll have to rig some 12 V lightbulbs to discharge it nicely). Stay clear from chargers with built in batteries. Those are only useful if you often need to jump-start other people (if you often need to jump start your own car then fix the car). With the charger, try to revive your old battery — but be prepared to buy a new one if your efforts don't yield satisfactory results. Also, get a new battery if you're in rush, because it can take 1–2 days to fully cycle a battery just once.

  • Citation needed for "Nevertheless, you SHOULD top up your battery from the wall at least once every few months to keep it in best condition". This does not sound plausible. Feb 6, 2017 at 19:22
  • I did mix up the two issues as my question wasn't specific enough. I should have mentioned that the battery was discharged and more than likely (since I have not tested it yet) can be recharged. So I'm guessing batteries can be recharged, but how many times can they be recharged before not able to hold a charge anymore?
    – Narcotixs
    Feb 6, 2017 at 20:19
  • @Narcotixs But discharged how deep? The battery is considered empty when discharged down to about 11V. That is the designed lower limit so it can be discharged to this limit as many times as you want. The very purpose of any rechargeable battery it to get charged and discharged all the time. It's only when you discharge it below the minimum that damage happens. And it's usually not a 100% damage, even very worn out batteries are able to hold a bit of charge, just not enough to crank the engine. Just like a phone battery, except that phone is a computer than won't allow discharge below 0.
    – Agent_L
    Feb 7, 2017 at 9:12

Depending where you live, electricity is rather cheap. Here's some math and some definitions, to take some of the mystery out of car batteries.

Amp-hours: Car batteries have a range of capacities, measured in Amp-hours. The Ampère is the unit of electrical flow: 1 Amp is a flow of 1 Coulomb (unit of electrical charge) per second. An hour is 3600 seconds, an Amp-hour is a flow of 1 Amp for 1 hour, so an Amp-hour is 1 Coulomb/second x 3600 seconds, so a total electrical charge of 3600 Coulombs.

Battery rating: Car batteries have a range of ratings, lets pick 45 Amp-hours. Since an Amp-hour is an electrical charge of 3600 Coulombs, this means that a car battery with a rating of 45 Amp-hours has an electrical charge of 45 x 3600 Coulombs, or 162,000 Coulombs.

Car Battery Electrical Energy: Electricity from a typical car battery is delivered at 12 Volts. The Volt is the unit of electrical potential and is equivalent to a potential of 1 Joule (unit of energy) per Coulomb. So, at 12 Volts, our 162,000 Coulombs car battery is storing 12 Joules/Coulomb x 162,000 Coulombs, or 1,944,000 Joules.

kW-hours: Home electricity is usually billed in kW-hours. A kW (kiloWatt) is the unit for the rate of energy consumption, equivalent to 1 kJ (1000 Joules) per second. So, 1 kW-hour is a rate of consumption of 1 kW for 1 hour, or 1 kJ/s x 1 hour. An hour is 3600 seconds, so 1 kJ/s x 3600 s means a kW-hour is a consumption of 3600 kJ.

kW-hours needed to charge your battery: Assuming 100% efficiency (which will not be the case), the car battery will use 1,944,000 Joules to get a full charge, or 1,944 kiloJoules (a simple unit conversion). Since each kW-hour represents 3600 kiloJoules, a full charge for our car battery represents 1,944 kJ x 1 kW-hour/3600 kJ, or 0.54 kW-hours.

Cost of home electricity: This varies greatly with location, so let's pick $0.10/kW-hour. Our cost to fully charge our battery (again assuming 100% efficiency, which won't happen) is 0.54 kW-hours x $0.10/kW-hour, or $0.05.

So, fully charging your car battery will cost you about 5 cents of home electricity. A new battery is in the $100 range. How long will the charging take depends on the power of your charger, with 10-16 hours being typical (slow is good, puts less strain on the battery internals).

  • 3
    I believe this answer is incorrect. It doesn't take into account how a transformer operates. A transformer conserves energy/power, but does not conserve charge. So, 1 kWh at 12V is 1 kWh at 110V even though it's a different amount of Coulombs. Obviously, there are transformer losses as well, but most transformers are 90-99% efficient.
    – juhist
    Feb 6, 2017 at 7:04
  • Not considering factoring in a transformer, would the above answer be for the most part correct?
    – Narcotixs
    Feb 6, 2017 at 7:27
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    Most of it is correct, but I won't retract my downvote until the central part of the calculation, i.e. assuming that you need as many coulombs from the power grid as you need to store to the battery, is modified. The kWh answer is off by approximately 10x, and so is the cost!
    – juhist
    Feb 6, 2017 at 12:35
  • 1
    @juhist Looked it up, you was right, fixed it. Energy is conserved, not electrical charge.
    – tlhIngan
    Feb 7, 2017 at 5:50
  • So why did you multiply by 12? What do you mean by "electrical potential if equivalent to 1 Joule per Coulomb? I thought it's not possible to compare Joules to Coulombs as they are two different types of units. Still trying to figure out the math.
    – Narcotixs
    Feb 7, 2017 at 6:14

The standard lead acid car battery does not store much energy so it does not cost much to charge .Most garden variety car batteries actually store less than one Unit of power . In my country NZ one unit ie one KWH costs about .25 $ . Taking into account charging and charger losses you will expend 1% or less of the cost of a new battery.If you are using a lossey chinese charger and you are oversupplying the Amphours by say 40% your cost could get to .5 NZD . In NZ you wont get a chinese battery for less than $50 . I can not think of any place in the world where it would not be a good idea to recharge the flat battery .If the battery is shot due to sulphation then most of the time it wont accept the charge so you save power anyway .If the battery has a shorted cell then if the charger is worth its salt it will detect the condition and shut down saving power.

  • What does one "Unit of power" mean? Firstly, there are many units so it isn't clear what unit you mean. Secondly, I think batteries store energy, not power. Power is just energy divided by time. Also, I have had UPS batteries that actually after failing started to convert charging power into heat, heating up the whole UPS to a high temperature.
    – juhist
    Feb 6, 2017 at 6:53
  • @juhist: Electricity is usually sold by the "Unit", so the answer is perfectly clear. One "Unit" is 1 kWh as stated in the post.
    – Chenmunka
    Feb 6, 2017 at 10:02
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    @Chenmunka Never heard of that "Unit". Where are you? It most certainly is not part of the International System of Units. It's best to use universally understood terms and units. Feb 6, 2017 at 16:32
  • The answer seemed pretty decent using inference, but the personal use of terminology confused me.
    – Narcotixs
    Feb 6, 2017 at 20:11

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