Alternator as Battery Maintainer

Delco Remy has prepared this information and we wanted to share it with you.

alternator and batteryThe vehicle charging system is comprised of three components:

  • The alternator
  • The battery
  • The cabling connecting the alternator and battery

These three components make up the charging system on any vehicle, large or small. Each component depends on the other. If the battery isn’t large enough, the alternator has to work overtime (and will need to be replaced sooner than it should). If the alternator isn’t working efficiently, the battery will be overworked.

By the end of this blog, you’ll be able to lay out how each interacts with the other. You’ll also be able to perform tests to isolate a problem component and schedule it for replacement.

The battery

Alternator and battery in a carThe battery is there to regulate voltage throughout the system. It provides resistance to the alternator, so the alternator doesn’t flood the system with juice. How well the battery does this depends greatly on their state of charge.

Batteries act as variable resisters. When the voltage is low, they accept a lot of current. When the voltage is high, the battery actually pushes back against the alternator’s output current, keeping everything in balance.

Without resistance from the battery, the alternator would be running itself ragged all the time, and batteries would degrade rapidly from deep cycling, sulfation and overheated plates.

Yes, the alternator can charge the battery, but it is primarily designed to maintain the battery.

Think about it this way.

You jumpstart a truck with 4 dead batteries and hit the road. The truck now houses 4 low voltage batteries, each accepting 40 amps or more from the alternator. That’s 160 amps dedicated to just charging the batteries.

Then there are the electrical systems just turned on by the driver, drawing even more.

If the truck is traveling across the province, state or country, the driver shouldn’t worry. The alternator will have plenty of time to refill the depleted batteries.

But what if the stop is only 45 minutes across the city?

The whole charging system is now destined to fail.

Alternator performance is much better when used for its designed purpose:

  • Maintaining a charge on the batteries.

The design of your charging system should include:

  • An alternator running only at 35-50% of its maximum capacity (the area where an alternator is most efficient). The alternator should include Remote Sense, so proper voltage is maintained at the battery.
  • Charging cables that meet TMC RP 129A specs, to keep voltage loss at a minimum.
  • The proper amount and type of batteries. Different batteries are meant for different jobs. Pay attention to the reserve capacity and ability to cycle.

When your voltage gets low

To find the source of the low voltage, you can work through the following scenario:

  1. Load test the battery bank and ensure it’s at 12.4 volts minimum. If not, isolate the battery bank and test each battery individually.
  2. If the bank passes the test, check the alternator. Test the output of the alternator with the vehicle running to ensure the alternator is between 13.8 and 14.2 volts. If you’re using Remote Sense, the alternator could exceed 14.5 volts.
  3. Perform a voltage drop test between the alternator and battery bank on the alternator charging cables.
  4. If the alternator is found to be the root cause, replace the alternator and recheck the battery bank to ensure the batteries are at 12.4 volts, minimum. If not, charge the batteries and bring them up to between:
    – 12.4 and 12.6 for lead acid.
    – 12.4 to 12.8 volts for AGMs.

This will give you and your drivers (or just you, if you’re the jack of all trades) confidence in the status of your charging system. You can safely hit the road.

Make sure you have the right alternators by placing your order from us today.

Our thanks to Delco Remy for helping us with this content.