Cars today use lead-acid batteries for the starting/lighting/ignition system. Hybrid vehicles that start the engine by a high-voltage battery do the same except for starting. Similarly, many electric vehicles have a 12-volt lead-acid battery that's recharged from the high-voltage battery when the vehicle is turned on.

All lead-acid batteries have a finite lifetime. For example, my computer UPS where lead-acid AGM batteries are kept on continuous 13.65 volt float charge has a typical battery lifetime of 5 years after which the batteries fail. I'm not completely sure what the failure mode is, but it could be shorted cells, since the last time my UPS batteries failed, it caused a failed self-test that probably failed due to suddenly providing too low voltage, so low that it was incapable of powering the inverter for the time it needed to switch back to mains electricity. But it also could be permanent hard sulfation reducing the capacity so much that at the failure event, the batteries simply couldn't hold any charge.

On the other hand, on my Toyota RAV4 hybrid, the lead-acid AGM battery is 7.5 years old but I'm not sure what its health is. It so far hasn't caused any problems for me.

In my past vehicle, a Toyota Yaris that had an EFB battery, there was one incident of forgetting the lights on, after which the car was jump started after a day of two. The battery subsequently probably showed momentary voltage depression then when starting the engine (so the headlights dimmed for a split second), after which I requested the dealership to do a battery check since they had a special offer where it was free. The report was: battery is fine (probably measured by cranking amperage), but should be recharged (probably measured by open-circuit voltage which was too low). But I know I drove to the dealership for 20-30 minutes and hadn't been driving any short trips, so the battery should have been full. I don't think they had enough time to do a proper discharge test to determine the amp-hour capacity of the battery.

I know that old batteries don't die by slowly reducing their cranking amperage; instead, they die by having constant cranking amperage but slowly reducing capacity, after which the capacity becomes so low they can't crank the engine even once when the idle drains and few days of not driving the car had resulted the battery becoming totally flat due to having only few percent of the capacity left.

Could the open circuit resting voltage of a lead-acid battery be used to estimate its health?

I know that old batteries usually fail due to permanent hard sulfation resulting in reduced capacity. I also know that as lead-acid batteries are discharged, they discharge by soft reversible sulfation, which reduces the voltage as more charge is drawn.

So if I have a full battery that's 80% permanent hard sulfated and only has 20% of the capacity left, doesn't that mean that the battery should show the same open circuit voltage than an identical new battery that's 0% permanent hard sulfated but has been 80% discharged, and only has 20% of the charge left?

If this is the case, I think resting open circuit voltage of a lead-acid battery should quite easily tell its health, which could be used to replace it in time before failing, instead of having an event where the car can't be started anymore.

  • Mostly by its age. Anything over 5 years is lucky, as soon as it starts misbehaving, your luck has run out. Voice of experience, feel free to disregard and learn it through your own experience, but your time also has value, and lead-acid is cheap. Commented May 14 at 0:03
  • I just started an experiment, I bought one cheap 6V 4Ah AGM battery. I'm going to destroy it by "leaving the lights on" (a power resistor) for first 1 days, then 2 days, then 4 days, then 8 days, etc. until it's completely destroyed. Every time, I charge it and measure its capacity in Ah, its open circuit voltage and how the voltage sags when providing different current amounts. I'll eventually have enough data to post an answer when the battery has been completely destroyed by being left discharged to zero volts for a large enough number of days.
    – juhist
    Commented May 14 at 14:59
  • that will be interesting! Commented May 14 at 18:51

1 Answer 1


There are some misconceptions here.

First, in a word, no, you can't judge the health of a lead acid battery by its resting open circuit voltage at full charge. The exception would be if one or more cells are physically shorted or if the electrolyte has become contaminated.

Resting voltage at no load can reveal state of charge though. For example a flooded lead acid 12V battery at 78F would measure 12.6 volts at 100% charge but 12.33 volts at 50%.

A recently charged, fully charged starting battery can have a perfectly lovely standing voltage, and then drop to 5 volts or less as soon as you engage the starter. It's toast, but you would never know it by the standing voltage.

A 12V lead acid battery contains six electrochemical cells. A fundamental property of an electrochemical cell is its constant standing voltage at a given temperature as long as it is fully charged and no current is flowing.

I've actually used an electrochemical cell in a lab to calibrate voltage measuring equipment to 4 decimal places. The cell was at least 25 years old, and it had to be hooked up to a wheatstone bridge for a voltage measurement without current flow. With an accurate temperature measurement, the cell's voltage can be used as THE standard because it doesn't change with age, only with temperature.

I know that old batteries don't die by slowly reducing their cranking amperage; instead, they die by having constant cranking amperage but slowly reducing capacity . . .

Only half right. Old batteries certainly do die (age) by reducing their available cranking amperage, and also by a reduction in capacity. That's why the only automotive industry accepted test of the health of a starting battery is to put it under a measured heavy load and observe the voltage drop. One common test is to set the heavy load at one-half of the battery's rated CCA (cold cranking amps), and hold the current there for 15 seconds. So an 800 CCA battery is tested at a 400 amp load. Depending on ambient temperature, the battery must have a voltage under load at the end of that time above a certain number -- typically in the 10 volt range -- or it fails the test. If the battery is approaching end of life, you find that it is unable to put out half of its CCA, and it fails the test without even considering voltage.

Your UPS is programmed to test its battery by periodically putting the battery under load for a specific time, then measuring how much the voltage has dropped at the end of the test. When the voltage drop under load becomes too great, the UPS signals that it's time to replace the battery. That doesn't mean the battery is dead, but that the battery won't be capable of producing power for the rated time at the rated current. Next time that happens to you, you can see how long the UPS will run and judge for yourself if it has enough capacity.

Re the Yaris, driving 20-30 minutes is not nearly enough to fully charge a deeply discharged battery. It's certainly enough to be able to start reliably, but that's not a full charge. It actually takes a few hours on a smart charger to bring a battery to 100% charge, and only then can the battery be load tested accurately.

A carbon pile battery load tester designed for occasional home use costs way less than a new battery. I use mine twice a year on all the family cars, once before winter and once before summer. I think it's an invaluable tool for any DIY mechanic because it allows you to know the health of a starting battery for sure rather than replacing a battery on a hunch, or worse, getting stuck somewhere with a battery that seemed fine . . . until it wasn't!

  • Wouldn't the voltage of an electrochemical cell depend of the specific gravity of the electrolyte? Also, if I have 90% discharged, 10% charged lead-acid battery, why doesn't it have the same open circuit voltage if the open circuit voltage of an electrochemical cell is constant?
    – juhist
    Commented May 12 at 8:23
  • @juhist The voltage of a fully charged electrochemical cell is a physical constant. And the state of charge of a lead-acid cell is reflected in the specific gravity of the electrolyte, yes. Both true, and not mutually exclusive. But neither is related to the remaining useful life, or "health" as used in your question, of a starting battery. NB: Lead-acid as implemented in car batteries would not make a good reference cell for precision work because you'd never know state of charge. My reference cell was silver/saturated calomel with excess calomel floating around so always "fully charged."
    – MTA
    Commented May 12 at 12:27
  • So true. I have a 9 year old battery in one car. The car has run badly with several smog/driveability issues. As soon as I swapped it for a 4 year old battery, all that stuff went away. Commented May 14 at 0:06

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