Do The amp hour ratings on all car batteries really matter as much if your car alternator is working well? Ah is the capacity but if you have reliable charging do it still matter as much? Cause its going to keep charging the car battery so as long as your not drawling over 50% or 60% you should be fine.


The main thing your battery has to do it start the car, and act as a 'buffer' for the alternator.

Starting the car is by far the highest load on the battery. Most car batteries are rated in Cold Cranking Amps (CCA) to determine how well they will turn the starter when it is cold out.

The battery stores power, accessories draw power, the alternator feeds power to the battery. The battery keeps a steady supply of power. The alternator's power isn't always 'clean', but the output from the battery is. When you use something like the horn, there is a spike in power draw, and the alternator is not able to handle spikes well, so the battery picks up the slack. At idle the alternator is not putting out full power, so the battery may need to pick up the slack. This will drain the battery slightly, but the alternator will recharge it once the vehicle is at higher RPM again.

Any time you use accessories with the engine off, you are drawing from the battery. Common items are electric locks, alarms, automatic lights, etc.

Overall, the amp hour (ah) rating does not matter much as long as you do not use accessories with the engine off. The main thing you need to worry about is your alternator. You can watch the voltage to determine if the alternator is able to keep up.

engine off - ~12.5-12.8 engine on - ~13.8-14.2

If you turn on your headlights, fog light, radio, AC, window defroster, then check the voltage and it is lower than 13.8, it means your alternator is having a hard time keeping up. You can try reving the engine to about 2000 RPM to see if it will keep up better while you are moving.

Another note on voltage dropping at idle. I've had several cars with underdrive pulleys. They spin the accessories slower to reduce load on the engine. I would always see my headlights dim at idle indicating that the voltage was dropping. Everything was fine once I started moving. Overall, I never had issues.

| improve this answer | |
  • Thanks for the detail response. I want to install new headlights that use about 100-111 watts of power on a 12v 45ah battery which gives about 540watt hours and u just wanted to know could the battery/ alternator handle that much power usage plus with all the other electrical components – LostPecti Jul 22 '16 at 14:46
  • 1
    So the main thing our worried about here is how many amps the headlights draw and how many amps your alternator is putting out. 111 watts / 12 volts = 9.25 amps. I would need to know the year/make/model to determine your current alternator, but it SHOULD be able to keep up just fine. – rpmerf Jul 22 '16 at 15:17
  • Its a 2013 Dodge Challenger – LostPecti Jul 22 '16 at 15:47
  • 1
    According to rockauto, it looks like your alternator is 160 - 220 amps. looks like your stock bulbs are around 64 watts, so around 5.4 amps. This means your going to have a 3.85 amp increase. This will be a 1.7% - 2.4% load increase on your alternator. – rpmerf Jul 22 '16 at 15:59
  • Thanks for your help man. Awesome and thanks for looking it up – LostPecti Jul 22 '16 at 16:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.