Why Does My Alternator Test Good but My Battery Keeps Dying?

The Frustrating Paradox
There is a special kind of frustration reserved for a car that leaves you stranded in a San Angelo parking lot, even after you just had the battery and alternator checked. You turn the key, and you get that dreaded click, click, click. You head to the parts store, they test your alternator, and the machine gives it a passing grade. You put in a new battery, and three days later, you are stuck again. If this sounds familiar, you are not alone. This is one of the most common diagnostic puzzles we solve here at Ric Henry's Auto Service, and the answer usually lies in the details that a quick parts-store test simply cannot see.

The Ripple Effect: When AC Invades DC
The short answer to this mystery often comes down to ripple voltage. A standard alternator test at a parts store measures voltage and amperage at idle, but it does not measure ripple. A failing diode inside the alternator can allow AC voltage to leak into your vehicle's DC system. This "ripple" confuses the onboard computer modules and slowly drains the battery without ever tripping a warning light on your dash. We measure this with a scope or a quality multimeter set to AC volts; ripple should be under 0.5V AC. If it is higher, that alternator is failing, even if it passed the basic test.

The Parasitic Draw: The Silent Battery Killer
If your ripple voltage is fine, the next culprit is a parasitic draw. Modern cars take 20 to 60 minutes to "go to sleep" after you turn them off. During this time, modules stop communicating, and the current draw should drop to 20 to 50 milliamps. However, a module that refuses to sleep, a stuck relay, or an aftermarket accessory wired to constant power can draw 500mA or more. We perform a timed draw test with an inductive ammeter, monitoring the current as the car sleeps, then pull fuses or use a power probe to isolate the exact circuit causing the drain.

Why Cheap Parts Cost More in the Long Run
This is also where part quality plays a massive role. OEM alternators have quality diodes and regulators designed to last. Rebuilt units from big-box stores often fail the ripple test because the internal components are not up to the same standard. Throwing batteries and alternators at a draw problem gets expensive fast. Measuring ripple and sleep current is the only way to find the real cause. We explain the repair before we sell the repair, ensuring you understand exactly why a specific component needs replacement.

The Legacy of Trust in San Angelo
Just because we have been doing this since the 1960s does not mean we have not kept up with the times. We are a fully modernized shop with all the latest tools and tech to take care of San Angelo. Our family auto repair roots date back to 1966, proudly serving San Angelo drivers for generations. We are equipped to handle a wide range of repairs, from minor fixes to major overhauls, and we use the same tools and equipment that the dealership does. We offer comprehensive auto repair services and carry a 3-year/36,000-mile warranty for all services we provide.

Dealer-Level Care Without Rearranging Your Life
You need a shop you can trust, one that has evolved with the industry. We offer concierge pickup, early-bird drop-off, online booking, and phone status updates. We are San Angelo’s trusted dealership alternative with honest diagnostics. Cars are more than our industry, it is our passion, and we take big Texas pride in being who San Angelo can trust for generations.

Trust your car in the hands of Ric Henry's Auto Service. We will find the real problem, so you can get back on the road.