Do You Have To Replace A Battery When Replacing An Alternator? Help & Advice


You’ve got to find the money for a new alternator but could do without the cost of buying a new battery too.

In this article, we’ll explain how a bad alternator can affect a battery and when you may need to replace both.

Briefly though,

Generally, you don’t have to replace your battery when replacing the alternator. However, should your alternator be drawing charge away from the battery – parasitic draw- it may have left the battery damaged.

The question you ask is a good one. The only way to truly know the answer is to run some battery checks to make sure.

The thing is, a bad alternator can kill a battery. 

Can A Bad Alternator Damage A Car Battery?

Car batteries don’t like to be drained. They are designed to lose a small charge while starting the car and then be topped up by the alternator as the car runs.

This leaves the battery with enough charge to start the car the next time you turn the car key. 

When the alternator is bad, it may not charge the battery effectively, leaving it undercharging permanently.

A lead car battery is classed as being fully discharged when it loses just 80% of its charge.

At 80%, it can only produce around 10.5 volts which aren’t enough to turn a car starter motor.

The more times the battery is taken to 80% before being slightly recharged by an alternator, the more likely it can’t be salvaged.

If you have a multimeter, it’s very easy to check if your battery is permanently damaged.

  • The test is to start the car and let it run for 10 minutes. 
  • Turn off the engine and connect the multimeter.
  • Get someone to start the car while you check the multimeter readings.
  • If at any time when starting the car or right afterward, the reading drops below 10 volts, the battery is bad and needs replacing.

Here is a good video on How To Test A Car Battery

You’ll probably also have sulfation on the plates within the battery itself and even some on the battery connectors. The chemistry in the six cells has been disrupted, and the electron flow can no longer create the voltage needed.

Extreme Battery Sulfation

Other visitors also read: Car Battery Gauge Reads Low – Here’s What It Means

Can A Bad Battery Harm An Alternator?

An alternator is not designed to charge a battery that is near a state of full discharge. It is designed to top up the charge lost during starting the car on that duty cycle and provide the power needed by the car’s electrical systems when running.

Any excess demand from the battery will cause the alternator to work harder, get hotter and even provide less power than the other systems need. 

All this can shorten the life of an alternator.

What Are Good Replacement Alternators?

It’s really difficult to answer these questions truthfully unless someone has had the time to fit all the different makes of cars and then wait five years before giving an honest review.

I’m not in a position to do that, and neither are most of the reviews you see online.

I would always check out Amazon and read their reviews. Even if you’re not buying the alternator yourself, you’ll have a good idea of which brand you want the car shop to fit for you. 

Once again, even with Amazon, the reviews you read will be from people that have recently bought and fitted their alternator and not from car owners giving an update after five years of having it fitted. 

I see good things about these brands:

  • DB Electrical 
  • AC Delco 
  • Remy 
  • Napa

Do I Need To Disconnect The Battery To Change the Alternator?

Yes, you must disconnect. Always take off the negative cable first, too, as this reduces the chances of sparks. Consider wearing eye protection when doing this to protect against sparks or corrosion flying off the connectors into your face.

What Happens If You Change An Alternator Without Disconnecting The Battery?

You risk getting a shock from the battery if you don’t disconnect it. This is the same for any part that draws power from the battery.

It’s unlikely that you’ll cause damage to other circuits, but you may blow fuses related to them.

What Else To Replace When Changing An Alternator?

Always check the serpentine belt when changing the alternator. Your alternator may have failed due to the serpentine belt not running smoothly. Check to see if it’s brittle.

Check and replace the tensioner and idler roller too. These parts are very cheap, but going bad will impact the performance of the belt and, consequently, the alternators too.

If you don’t know the history of your car and have doubts about the water pump, you may want to change this out too. It also runs from the serpentine belt and is easy to change when the alternator has been removed.

Water pumps are cheap, so this can make financial sense.

Do You Need To Charge A Battery After Replacing An Alternator?

Was your battery flat before you changed the alternator? If so, the alternator won’t charge the battery when your car is turned off. You’ll need to charge it enough to get it to start the car. 

Once the car is started, the alternator will then take over the role of charging the battery.

If the battery was fully charged, then you don’t need to charge it. Just turn the ignition, start the car, and the alternator will maintain the battery charge.

Do You Need To Jump A Car After Charging The Alternator?

If you’ve got no charge in the battery, you can jumpstart it.

Don’t do this, though, if you already have enough charge in the battery, as it serves no purpose.

What To Do After Changing An Alternator?

  • First, ensure that the serpentine belt has been reconnected to the correct tension.
  • Make sure the grounding strap is firmly attached to the chassis. 
  • Make sure the cables are connected back to the battery.

Does A New Alternator Need to Be Broken In?

Not many people even consider this. Yes, a new alternator does need to be broken in. 

Here’s what to do:

  1. Turn the key and let the engine run with no other electrical systems on for 10 – 15 minutes
  2. Turn it off and let the alternator cool for another 15 minutes
  3. Start the engine again but this time, turn on your heater, car sound system, heated seats, and heated screens.
  4. Turn the car off again, let it cool for 10 minutes, and check the battery voltage above 12.4 v. This indicates the alternator has been changing the battery effectively.
  5. Repeat steps  3 and 4 three times, remembering to check the voltage is being maintained each time.
  6. If all looks good, all is well with the alternator and the battery.

In Conclusion

The battery does not have to be changed when the alternator is replaced. Batteries typically last three to four years, whereas alternators typically last seven to ten years. The only time you need to replace the battery at the same time is if the alternator has drained it beyond repair.

Further reading:

How Long Can You Drive A Car With A Bad Alternator?

Car Battery Smells Like Rotten Eggs – Yes, It’s Dangerous

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