I have fitted a much larger 12v battery to a car. I modified the clamp and battery tray to make it fit securely, it's almost twice the size of the original and has a much higher CCA rating. It works well, I left the headlights and radio on (at a low volume) overnight in the garage and it still started the next day!

Will this bigger battery cause any problems or issues for the car?

I am specifically concerned with damaging or reducing the life of the alternator, but I would like to know of any other potential problems before fitting the same type of battery on my other cars.

  • No the alternator is used to charge the battery, and to charge 12V batteries the voltage applied is about 14V. It doesn't matter how big the battery is, that is just the duration of energy storage. They will all be 12V. The alternator would just be providing the same charge current for longer time, always spinning (except in BMWs under load), and won't wear it down any more. Ah (amp-hours) is the rating for for the battery to provide 1A for however many hours until it's about 90% discharged. Bigger batteries just take longer to charge, still 12V.
    – Chloe
    Commented Mar 27, 2017 at 17:00
  • 1
    No- The alternator regulator sends a max of 5AH to a charging battery. The battery capacity won't impact it at all except taking longer to charge. The vast majority of the alternators output is directed to running the vehicle and any accessories. The charge rate on any battery is well below what the battery is rated for. This is prevent overheating of the battery, The charging current drops as the battery approaches full charge.
    – Old_Fossil
    Commented Mar 28, 2017 at 4:37
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    @resident_heretic "The alternator regulator sends a max of 5AH to a charging battery." I don't know what "5 AH" (a measure of charge, not of current) means in this context. If you mean 5 A, that's absurdly low - where are you getting that figure? Back when I had a car with an ammeter it was clear that charging current could be several tens of amps, depending on how low its charge was and how fast the engine was turning. Commented Mar 28, 2017 at 7:49
  • Does the old trick with the battery cable off show if alternator is working or not true?
    – Denso
    Commented Dec 30, 2020 at 2:46

8 Answers 8


Great question! Taking a few minutes to think through this logically should come up with something. First of all, realise that a larger battery with a higher amperage (CCA) will not cause any problems on the drawing end. In other words, you're not sending too much electricity to some component or something. The higher amperage simply means that more energy is available, not that more energy is flowing through the components. I'm sure you already knew that; just making sure everyone is on the same page.

Secondly, you've got to realise that it doesn't take any more energy to charge up a bigger battery than a smaller battery. That is, provided they have had equal draws. The difference, of course, is that the bigger battery can get uncharged further, resulting in a larger draw on the alternator.

Basically, this means that your alternator will have to work harder to recharge the battery, but simply because the larger battery was able to output more juice. Since the alternator wear is usually due to age, the answer is yes, the bigger battery will wear down your alternator more.

That being said, it probably won't be significant, unless you regularly leave your radio running overnight, because your alternator is always running when the car is running; the only difference is that it will be demanding more current from the alternator to recharge the battery. Therefore, as far as I and my research are concerned, the wear is

Probably insignificant.

  • 2
    Higher voltage would damage electrical components, of course; but the current is determined by the drawing side, as you said.
    – jpaugh
    Commented Mar 27, 2017 at 14:52
  • I don't understand, I thought alternators worked just as hard under high load as under no load. You could disconnect the alternator completely and it would last just as long and provide the same amount of drag on the engine, right?
    – atraudes
    Commented Mar 27, 2017 at 17:05
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    The battery doesn't ask the alternator for more current. The alternator is rated to provide a fixed amount of current and the battery (or voltage/charge regulator) just takes it. You can charge a battery with 1A, 0.123A, or 10A. The less A the longer it takes. The charge voltage will always be a bit over 12V, around 13-15V. P=IxV, and V is fixed.
    – Chloe
    Commented Mar 27, 2017 at 17:09
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    @atraudes : Completely wrong I'm afraid. The more electrical load on the alternator, the more difficult it is for the engine to turn, due to "back EMF". (This is the same reason that dynamic braking in an electric car works.) This increases wear on the alternator, both due to more heat in the windings and greater mechanical wear on the bearings. However the increased wear on the alt is going to be pretty small. Commented Mar 28, 2017 at 1:37
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    @Chloe : "The alternator is rated to provide a fixed amount of current and the battery (or voltage/charge regulator) just takes it." No. The alternator's rating says it is able to provide up to the stated current, but not that it is a constant-current supply, as your "fixed amount" claim states. If the alternator output voltage is higher than the battery's voltage, the battery is charged; the charging current depends on the difference in their voltages and on the battery's effective series resistance. There is no "charging regulator" as in e.g. a laptop, only a voltage regulator. Commented Mar 28, 2017 at 5:54

For most of the time, the battery doesn't provide any electrical power to the car. All the power is coming from the alternator. Think about it this way: the alternator probably has a maximum current rating of say 90 amps. For a typical "standard size" 40AH battery, that amount of current would flatten the battery completely in less than half an hour, so it's obvious that the battery is not powering the car electrics most of the time!

The main function of the battery is when starting. Arguably, a larger battery might discharge less when starting than a small one, because the higher cranking current available is likely to turn the engine over faster and start it quicker - though with a modern engine and ECU, that shouldn't be a big effect unless there is a fault somewhere in the electrical or fuel systems.

The secondary function of the battery is to provide power when the engine is idling, if the electrical load is high and the alternator isn't running fast enough to deliver all the power required. But in that situation the amount of power needed to recharge the battery only depends on the electrical power being used, not on the battery capacity.

If you have added a lot of electrical equipment to the car, that will make the alternator work harder and potentially reduce its life - not the size of the battery.

  • Would be interesting to know the difference in draw between the AC unit being on, and the longer time to charge the battery...
    – Rycochet
    Commented Mar 27, 2017 at 15:55
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    The AC unit in most cars is powered mechanically. It is not an electric compressor!
    – juhist
    Commented Mar 27, 2017 at 17:20

In fact the larger capacity battery may, under certain circumstances, increase your alternator's life.

Many vehicles with enhanced sound systems install a larger battery to smooth out the load spikes on high-draw portions of the music. A typical electrical system upgrade path would start with a higher-capacity battery, then a higher-capacity alternator or even dual alternators, then dual high-capacity batteries.

  • 2
    Interesting you mention that... the previous owner fitted upgraded speakers to my car, and I upgraded the stereo to one that has Bluetooth and USB, and also a higher RMS rating. I noticed that in particularly bassy songs, the interior lights and LCD light for the radio would go dim momentarily. I'll have to check if that still happens with the larger battery.
    – DizzyFool
    Commented Mar 28, 2017 at 7:53
  • If you're already in this territory, then you should also check your spark plugs for excessive carbon. The low electrical power sometimes causes the plugs to fire poorly, which if done often enough fouls them.
    – dotancohen
    Commented Mar 28, 2017 at 8:46

Simple answer : NO, the alternator max charge will just need more time to complete a full charge, but the lifetime of the bearings is down to the speed of rotation and the working temperatures plus the load caused by the belt driving it. This is a good exercise in tribology to work out the bearing lifetime...

  1. A larger battery rated to the same voltage will have a lower internal (electrical) resistance.
  2. Less electrical resistance -> More current will be generated (flow from the alternator to the battery) by the alternator when it turns but it will be harder to turn it due to increased mechanical resistance or torque (the relationship between electrical and mechanical resistance in this instance is inverse).
  3. Thus, one might expect additional mechanical stresses and potentially heating to occur on the alternator when charging a larger battery. Depending on the design specifications of the alternator that may translate to a shorter life for it.

As several people have stated, no, a larger battery will not harm your alternator (or other electrical components), provided that it is putting out the correct voltage. One thing that I'd disagree with most on is the charge time. First, if you're talking about charging from the battery's discharge limit until it has reached capacity, then sure, it will take more. However, that's about where the charge time difference ends. Under normal conditions, if you change only the battery capacity and make no other system modifications, it will take the same amount of time to charge both batteries (obviously if resistance is not the same, this will affect charging, regardless of the total capacity of the battery).

When you use a larger battery, your car's electrical systems don't automatically start pulling more current simply because it's available. The current draw will remain the same. Because of this, a larger battery will not deplete faster than it's smaller counterpart. So, if your vehicle has an average current draw of 75A, it will deplete the battery the same amount. Therefore, you will have the same amount of current to be replaced with both sizes.

  • Welcome to Motor Vehicle Maintenance & Repair!
    – Cullub
    Commented Aug 22, 2018 at 12:29

No. A healthy and fully charged battery draws virtually no current from the alternator regardless of size. I would argue that a larger battery could actually increase the lifespan of the alternator. The reason is that a bigger battery will help to stabilize the voltage in the system compared to a smaller one by acting sort of like a buffer. A real world example is how the lights on a car dim when the bass hits. With all else being equal, the lights wouldn’t dim as much under sudden, brief loads like bass hitting. What DOES, however, wear out the alternator is having a worn out battery that no longer holds a charge and/or undersized/loose/corroded grounds and battery cables. Make sure your battery cables and grounds are clean, reasonably sized and tight, and maybe look into a “Big 3” upgrade if you want to help your electrical system perform better and last longer. Obviously none of this would matter if the alternator suffers a mechanical failure such as failed bearings before it suffers an electrical failure though.


Car can run without battery hense power drawn from alternater. However car can run without alternater as well is it not. Therefor it a circuit charge circukated through a parallel flow. I think

  • How does this answer the question?
    – CharlieRB
    Commented May 16, 2018 at 12:44
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    Agree with @CharlieRB here. Also, while you can run a car without a battery, it's a GREAT way to fry your alternator. The battery does more than just provide power when needed. It also acts as a power buffer, which absorbs power spikes. Without it, the alternator takes those hits and it can fry. Not only your alternator, but all electrical components of your car come under the same surges/spikes, making them vulnerable to these as well. Not a good practice to run a modern day car without a battery. Commented May 16, 2018 at 18:07

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