Prius battery keeps dying

Things don’t always go as planned. You head out to your Prius and touch the door handle, but it doesn’t unlock. Try pressing the unlock button on the smart key, still nothing. Once you’ve opened the door using the emergency key insert, the car won’t Ready. Aarrgh! A dead Prius battery! Why does it keep dying?

If you’ve landed here looking for instructions to jump start your Prius, click the link. This article is for those whose Prius 12V batteries keep going dead and want to know why it’s happening.

There are 4 possible reasons your Prius battery might keep dying:

  • The battery has very low capacity
  • Something is draining the battery while the car is sitting
  • The battery isn’t being charged while you’re driving
  • The battery terminals are loose
A picture of a "dead" Prius battery with a face drawn on it. X's for eyes, frown, and tongue sticking out.

How to check your Prius battery

Unless you have some specialized test equipment, you won’t be able to test your battery, at least not very accurately. Many DIY’ers check their battery voltage using a multimeter, but unfortunately using a voltage reading won’t tell you much about the condition of the battery, other than its state of charge (maybe).

The best plan is to bring the car or the battery to a shop or parts store with a battery tester. The best type of tester is called a “load tester”. A load tester measures battery voltage while drawing current from the battery. There are rules to using a load tester properly, and frequently people test improperly and come up with an incorrect result. This is the reason that nearly all manufacturers recommend using a different type of tester.

The other type of tester is an internal resistance tester. It’s so easy to use that anybody can use it properly. There’s no danger of user error. However, the tool itself isn’t all that accurate. An internal resistance tester will get the right results most of the time, but I’d guess that it’s completely wrong somewhere around 5% of the time. If you’re a professional shop, misdiagnosing 1 out of 20 cars isn’t great for your reputation, so most shops use load testers. One the other hand most tow drivers, auto parts stores, and dealerships use resistance testers.

If you want accurate results, use a load tester and follow these rules:

Only test a charged battery

Testing a discharged battery will make it look bad even if it’s good. A battery MUST be at least 3/4 charged to load test. How do you measure the state of charge? Ideally you’d use a battery hydrometer, but the Prius 12V battery is sealed and uses gel electrolyte, so that won’t be an option. Check the static voltage. It should be 12.45V – 12.7V; that’s 75% to 100% charged. However, if you’ve just had the battery on a charger, you’ll need to allow time for the surface charge to settle before using voltage as a indicator of state of charge.

Test at the proper current

Loading the battery at 1/2 the cold cranking amp (CCA) rating is the proper way to load test a battery. If you test at too low an amperage, a bad battery may look good. If you test at too high an amperage, a good battery may look bad. Prius batteries are usually rated between 300 and 450 CCA. Check your rating before testing.

Test for the full 15 seconds

Don’t be in a hurry. Running a short test will make a bad battery look like it’s good. A load test is a 15 second test. The only voltage reading that counts is the one measured after the battery has been loaded for 15 seconds.

Don’t Test right after charging

More often than not, you’ll need to charge the battery before you can test it, but if you test it right after charging, a bad battery may seem like it’s OK. Also, the battery produces hydrogen gas while charging and a bad load tester clamp connection can cause a spark and explosion. Either let the battery rest for a few hours or test it 3 times, waiting around 5 minutes between each test. The first two tests will burn off any surface charge.

Key off drain

A bad battery isn’t the only reason your Prius battery might keep dying. If something is pulling current while your car is sitting parked, your battery may go dead if it sits for long enough. All modern cars pull some current when the key is off. That’s normal. However, the battery should last at least a week while sitting.

It’s easy to check for key off drain if you have a multi-meter. Set the meter to measure amperage, which usually requires moving the positive lead to the “amps” port on the meter. Connect one lead to the negative battery cable and the other to the battery terminal. Disconnect the negative battery cable from the negative battery terminal so all of the current flows through the meter. If you disconnected the cable before the meter was in place, it will take a while for the current draw to settle down as body control functions will “wake” and take a while to go back to sleep. You may also need to close the rear hatch to prevent the BCM from staying awake.

How much key off current draw is normal? Well for a Prius around 25mA is normal, but up to 60mA might be acceptable for any vehicle. Most people teach that 80mA is the limit, but I’ve never seen any car pulling that much current without a problem or some sort of aftermarket accessory sucking power.

Charging system problems

You’ve checked your battery condition and key off drain but your Prius battery still keeps dying? Maybe it’s not charging while you’re driving. One the Prius, and all Toyota hybrids, the DC-DC converter charges the battery and powers the 12V systems once the car is Ready. The DC-DC converter on the Prius has two levels of output: 13.5V and 14V.

Testing is easy. Use your multi-meter in voltage mode (don’t forget to switch the positive lead back if you were just testing amperage) and measure the voltage across the battery terminals when the car is Ready. If you have 13.5V or 14V, go ahead and check with a bunch of electrical load on too. Just turn on the high beams, radio, defroster, seat heaters, etc. The voltage should remain at 13.5V or 14V.

Loose battery terminals or B+ connector

Maybe your Prius battery isn’t dying. Maybe it’s just not well connected to the car. Loose battery terminals are a very common problem, and aren’t as immediately obvious on a Toyota hybrid since the 12V battery isn’t used to start the gas engine.

The best way to check is to grab the terminal and twist. It shouldn’t move at all. If you can twist the terminal clamp, the terminal is too loose. Also, many generations of the Prius have a B+ cable connector that snaps into place. Sometimes I find that they’re not “snapped” and gravity is holding the connector in place (most of the time).

Additional reading