Alternators Are Not Battery Chargers

I participate in 4wd forums with recurring discussions from people believing they need to bypass their smart alternator to have a constant charge alternator so their battery fully charges, or their second battery charges, or just so there is more available charging voltage.

This is misinformation perpetuated across the Internet by backyard tinkerers.

What Is An Alternator?

Very simple explanation. Your car alternator is an electrical generation device, the same as a power station for your home, just smaller. Your engine runs, the belt spins the rotor past the fixed windings to generate an AC output. That 3-phase AC output is to meet the high currents needed for vehicles, sent through a set of diodes (rectifier) converting it to DC for use in the car.

Raw electrical generation is basic and crude compared to intelligent regulated power supplies and inverters we have today. Windings are calculated to achieve a voltage and current for use. Crude is acceptable for vehicle power with some basic voltage regulation.

What An Alternator Is Not

An alternator is not a 3 stage battery charger nor intelligent battery charger. A smart alternator has basic voltage regulation and control via a Powertrain Control Module (PCM), varying the voltage to keep the start battery optimally charged for its next start and supply loads as needed, then disengage to reduce battery damage from over-charge, reduce alternator wear and save fuel as a result.

To be blunt as possible, an alternator is not designed to 100% charge or maintain 100% charge of a battery located in a vehicle. Your start battery does not require 100% state of charge to do its job.

Alternators and Additional Batteries

A standard basic alternator was never designed to charge a second battery. Companies market voltage solenoids that encourage the alternator to do this, however, the device was never engineered to accomplish anything beyond a start battery and vehicle electrics with some reasonable after-market accessories. Modern vehicles are engineered to maintain the start battery around 90% capacity to perform its next start task. Sometimes more, sometimes less, it just depends on use.

Modern Vehicle Engineering

From what I’ve read online, people need to get out of this head-space surrounding old technology and methods and applying them to newly engineered vehicles. New vehicles make more power, use less fuel, and generally do things better than older vehicles. Engine capacities are smaller, oil viscosity variance allow easier engine cranking, they use stop start features and much more. Modern cars are complex. When you mess with certain engineering, you risk creating issues elsewhere.

Stop with the backyard old school mentality when modifying new vehicles.

Circumventing Smart Alternators

You’re doing potential damage circumventing a smart alternator. A smart alternator is designed to vary its voltage and output based on feedback from your start battery. Put that in the back of your mind. It monitors the start battery. Not other batteries, just the start battery. Circumventing a smart alternator may cause engine vibration and rough idling, because the PCM talks with the ECU or other module, where the ECU may now make incorrect calculations based on misleading data because you circumvented battery feedback.

Start Battery Maintenance

The missing sauce from all backyard mentality about batteries. All battery manufacturers recommend you maintain your battery by periodically connecting it to an appropriate battery charger to fully charge the battery. Go find yourself a battery manufacturer website, look at their battery maintenance page, and there you will read it.

A start battery should last approximately 3 – 4 years. Everything after that point is a risk. The moment you have a minor issue from power beyond this point, just replace the battery. You can’t fix it. If you follow simple advice, such as periodically connecting your start battery to an intelligent charger overnight once every couple of months or so, your battery will easily last the recommended time, most likely beyond it.

Start Battery Charger

A start battery is not designed to be rapid charged. The general battery manufacturer recommendation for charging is to charge at 10% of the batteries output. Most start batteries in modern vehicles are 70ah – 100ah. That means a charger should be no bigger than 10A. Bigger is not better for a start battery.

Buy a 10amp intelligent battery charger and connect periodically overnight. Do that, and your car battery will last longer. You can pick one up around $100 – $200.

Charging Additional Batteries

Every additional battery you add within a vehicle requires a DC-DC battery charger. These systems are intelligent chargers that monitor the specific make-up of the battery and charge it according to that batteries charge profile for 100% capacity. DC-DC chargers also use an ignition override specifically for smart alternators, so the charger turns on when the ignition does, shows a load to the system, negating reliance upon alternator voltage output.

This is the #1 issue amongst this entire discussion. People use cheap chargers or voltage control relays, trying to get an alternator to do something it was never designed to do, smart or dumb.

More volts is not better. In fact, constant unrestrained charging to any battery causes battery degradation. Batteries are expensive, be smart.

Not Enough Amps

If you need more output, get a high output alternator. That simple. Find a good mechanic and ask them. They will source one for your vehicle. If you’re running a vehicle second battery (DC-DC charger) and towed vehicle additional battery (DC-DC charger), and additional vehicle accessories, then chances are all and any issues you have with charging is due to your alternator not being sufficient for the job you’re asking it to do.


Batteries are not cheap. From what I’ve personally experienced and further read online, people spend large sums on buying additional batteries, then skimp on charging those expensive batteries. Stop being a backyard auto electrician. Stop listening to backyard advice. Batteries and their respective means to charge them are expensive. If you can’t afford to do it right, keep saving until you can. This way you keep your investment in tip top shape, working when you rely upon it to work, all done right for long-term use.

Am I Qualified To Write This?

Yes. I’m an electrical fitter by trade. My experience is turbines, transformers, power stations, inverters, industrial electronics, railway and automotive. Everything from low to high voltage and current, both AC and DC. I’m an electrical mechanic as well, but never continued to my A grade license for residential / commercial work, as that was not my interest and I find that work boring.

3 thoughts on “Alternators Are Not Battery Chargers”

  1. Hi, great articles that you write – I have a query. I am installing a 12V 40A DC to DC Battery Charger & a 50AH lithium battery in the cargo area of my Prado. I can adjust the maximum charging current setting on the DC-DC charger between the range 5 to 40 A. The lithium battery manufacturer indicates a maximum charging current of 50A. I am uncertain what value (ie 5 to 40A) to set the maximum charging current on the DC DC charger to avoid damaging the lithium battery. I would appreciate your advice, Thanks, Ian

  2. Hey Ian, thanks for your kind words. Based on what you’ve provided, the options come down to a couple of factors. What size alternator output does your model have? Doing a quick look, Prado use between an 80A to 150A depending on year model. Then you used the word “maximum” which is not the same as “recommended” charge.

    For example, the iTech 54AH has a maximum charge of 55A. It has a recommended standard charge of 25A. Maximum charging can shorten the life of the battery, as charging too fast is just as bad as completely draining a lithium to the point it requires a jump to accept charge again. The manufacturer in this example says the sweet spot for battery longevity is 25A.

    Half the AH rating is a good measure for lithium, so if your alternator can manage it, set it for 25A would be my recommendation, giving your lithium the best chance of a long healthy lifespan.

    You can typically fast charge a lithium at its rating, but its not recommended for everyday charging.

    Many a person bring themselves undone with DCDC chargers, going large, when the vehicle has other heavy draw accessories, like spotlights / lightbars. I’ve got a 100AH lithium charging at 25A, lightbar at 18A, alternator 90A OEM. If I added anything beyond that from my alternator, I would need to increase the alternator, as the car itself can draw 50-60A at times. People start burning out alternators and wonder why.

  3. Oh… the best advice I can add, get hold of a clamp amp meter, turn on everything in your car, heater, high beam, open doors, radio, hazards, everything you can at once. Clamp the meter onto your battery positive and be shocked how much your car draws. Minus that from your alternator output and that’s what you safely have available for accessories that require your alternator, ie. setting a DCDC charger value. If your car draws 60A with everything running, has a 80A alternator, then 25A DCDC is sustainable, but upgrading that to its max 40A may cause you issues, drain your start battery and damage it.


Leave a comment

The maximum upload file size: 5 MB. You can upload: image. Links to YouTube, Facebook, Twitter and other services inserted in the comment text will be automatically embedded. Drop files here