What State of Health Actually Tells You
State of Health (SOH) is a single number that summarizes how much useful life is left in a battery. It is not directly measurable the way voltage or current is, so it is estimated from a combination of inputs: open circuit voltage with the engine off, measured cold cranking amps under load, the battery’s age in months, and the manufacturer’s rated CCA when new. A fresh battery scores 100 percent. A battery scoring under 60 percent will leave you stranded on the next cold morning.
The most important input is the measured CCA. A digital battery tester pulses a known load through the battery and reads the voltage sag. The ratio between rated CCA and measured CCA tells you how much of the original plate capacity is still active. A battery rated 650 CCA that now tests at 400 CCA has lost roughly 40 percent of its cranking ability. The other inputs — voltage, age, surface charge — refine that estimate and help separate a healthy battery that just needs charging from a battery that is genuinely worn out.
If you do not have a CCA reading on hand, this calculator will give you a useful estimate from voltage and age alone, but the answer is only as accurate as your inputs. For a real diagnosis you want a conductance reading from a battery tester. The YOUCANIC Battery and Charging System Tester measures CCA directly, checks alternator output under load, and prints a result card you can keep for warranty claims. Pair it with this calculator and you have a complete picture in under five minutes.
How to Read the Score
Above 80 percent: healthy. The battery will start the vehicle reliably in any weather your climate produces, and it has at least another year of normal service ahead. No action needed beyond keeping the terminals clean and the connections tight.
60 to 80 percent: declining but still usable. The battery still starts the vehicle, but margin against cold weather is shrinking. Plan for replacement before next winter if you live in a cold climate, or within 6 to 12 months in moderate climates. Keep jumper cables in the vehicle just in case.
Below 60 percent: end of life. The battery may still start a warm engine, but a cold morning, a parasitic draw overnight, or a few weeks of short trips will leave you with a no-start. Replace soon. Below 50 percent the battery is essentially scrap and a no-start is a question of when, not if.
Why Batteries Wear Out
Two failure modes dominate. The first is plate sulfation, where lead sulfate crystals grow on the plates over time and reduce the active surface area that holds charge. Sulfation happens fastest when a battery sits in a partial state of charge — exactly what happens with short trip driving, infrequent use, or a parasitic draw. The second is grid corrosion, where heat slowly eats the positive plate grid. Grid corrosion is why batteries in hot climates die at half the age of batteries in cold climates.
Modern AGM batteries resist sulfation better than flooded batteries because the electrolyte is held in a glass mat that prevents stratification. But AGM batteries still fail from grid corrosion in hot climates, from chronic undercharging in vehicles where the alternator profile was never updated after a battery swap, and from deep discharge cycles in vehicles with start-stop systems that exceed the battery’s design cycle count.
Diagnosing a Weak Battery Before You Replace It
Before spending money on a new battery, confirm the old one is actually the problem. A surprising percentage of “bad battery” complaints turn out to be loose terminals, a parasitic draw, or an undercharging alternator. The diagnostic sequence is simple: clean and tighten the terminals, measure resting voltage after the vehicle has sat at least one hour, test CCA under load, and finally check alternator output at idle and at 2000 rpm with the headlights and rear defroster on.
This is the workflow the YOUCANIC Battery and Charging System Tester walks you through automatically. It tests the battery first, then the cranking circuit, then the charging system, then prints the verdict. Skipping any one of those steps is how people end up installing a new battery in a vehicle whose alternator is the real culprit, watching the new battery fail in three months, and blaming the parts store.
Frequently Asked Questions
How do I measure CCA without a professional tester?
You cannot, accurately. A multimeter measures voltage, not cranking amps. To measure CCA you need a load tester or a conductance tester. Inexpensive conductance testers run $40 to $80 and give a reasonable estimate within 5 to 10 percent of professional units. The YOUCANIC Battery and Charging System Tester reads CCA and alternator output directly and is what we use in shop. Most auto parts stores will also test a battery for free if you bring the vehicle in, though they have an obvious incentive to find a reason to sell you a new one.
What voltage means a battery is bad?
Open circuit voltage (engine off, no load, after sitting at least one hour) tells you the state of charge: 12.6 V or higher is fully charged, 12.4 V is roughly 75 percent, 12.2 V is 50 percent, below 12.0 V means the battery is deeply discharged. A battery that reads 12.6 V at rest but drops below 9.6 V during cranking has lost capacity even though the voltage looks fine. Voltage alone cannot diagnose a battery — you need a load test or CCA measurement.
Can a bad battery damage my alternator?
Yes. A weak battery forces the alternator to work at maximum output continuously, especially during the first 10 to 15 minutes after a cold start when the battery is trying to recharge. Sustained high output overheats the alternator’s diodes and rotor windings. Many alternator failures that look unrelated to the battery actually start with a worn out battery that was not replaced soon enough. Always test the battery first when diagnosing a charging system problem.
How does cold weather affect battery health?
Cold reduces the chemical reaction rate inside the battery, which lowers the available cranking amps. At 0 degrees F a battery delivers roughly 65 percent of its rated CCA. The engine also gets harder to crank because oil is thicker. A battery that is marginal in October becomes a no-start in January. This is why cold mornings expose weak batteries that seemed fine through the summer.
Why does my battery die after sitting for a few days?
Parasitic draw. Modern vehicles draw 20 to 50 milliamps continuously to keep memory powered. Anything above 80 milliamps after the vehicle goes to sleep (typically 30 to 60 minutes after locking the doors) is excessive and will drain a healthy battery in a week. Common causes are aftermarket accessories wired incorrectly, a faulty body control module, a stuck relay, or a glove box light that does not turn off. Measure parasitic draw with a multimeter in series with the disconnected negative cable.
Does jump starting damage a healthy battery?
No, jump starting is safe for the donor vehicle when done correctly. But if you have to jump start the same battery repeatedly, you are running it down to deep discharge over and over, which sulfates the plates and shortens its life. Two or three deep discharges can take 20 to 30 percent off a battery’s remaining capacity permanently. If you find yourself jumping the same battery twice in a week, replace it before the next cold front.
How long can a car sit before the battery dies?
A healthy battery in a vehicle with normal parasitic draw (30 milliamps or so) will last about 3 to 6 weeks before discharging to the point where it cannot start the engine. A weak battery or a vehicle with excessive parasitic draw can drain in days. A battery tender or trickle charger eliminates the issue entirely and is a worthwhile investment for vehicles that sit for weeks at a time.
What is the difference between a battery tender and a charger?
A traditional charger pushes a constant current until the battery is full and then needs to be unplugged. A battery tender (also called a maintainer or float charger) monitors voltage and pulses small currents only when needed to keep the battery at full charge. Tenders can be left connected indefinitely without overcharging. For a vehicle that sits more than a few weeks at a time, a tender is the cheapest battery insurance available.
Should I replace both batteries in a dual-battery setup at the same time?
Yes, always. In a parallel-connected dual battery setup, one weak battery pulls down the other and forces it to work harder than designed. Mixing a new battery with an old one will degrade the new battery to match the old one within months. Replace both at the same time and with batteries from the same manufacturer and date code if possible. Diesel trucks, large RVs, and emergency vehicles benefit most from this rule.
Why We Built This
Parts stores will sell you a new battery without testing whether the old one actually needs replacing. Dealers will quote $400 for a battery and registration when the real fix might be cleaning the terminals or finding a parasitic draw. This tool gives you a quick honest estimate of where your battery stands before you spend money on a replacement, and tells you what to do about it. You can be the mechanic.
Help Us Make This Tool Better
Spotted an SOH threshold that does not match your fleet’s experience? Want lithium chemistry profiles added? Send us a note and we will look at every message. Tools improve when the people using them tell us what is missing.