![]() ![]() ![]() The DC-DC converter on the Prius has two levels of output: 13.5V and 14V. One the Prius, and all Toyota hybrids, the DC-DC converter charges the battery and powers the 12V systems once the car is Ready. 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. 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. 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. You may also need to close the rear hatch to prevent the BCM from staying awake. 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. Disconnect the negative battery cable from the negative battery terminal so all of the current flows through the meter. Connect one lead to the negative battery cable and the other to the battery terminal. Set the meter to measure amperage, which usually requires moving the positive lead to the “amps” port on the meter. It’s easy to check for key off drain if you have a multi-meter. However, the battery should last at least a week while sitting. All modern cars pull some current when the key is off. If something is pulling current while your car is sitting parked, your battery may go dead if it sits for long enough. Key off drainĪ bad battery isn’t the only reason your Prius battery might keep dying. The first two tests will burn off any surface charge. Either let the battery rest for a few hours or test it 3 times, waiting around 5 minutes between each test. Also, the battery produces hydrogen gas while charging and a bad load tester clamp connection can cause a spark and explosion. 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. The only voltage reading that counts is the one measured after the battery has been loaded for 15 seconds. Running a short test will make a bad battery look like it’s good. Test for the full 15 secondsĭon’t be in a hurry. Prius batteries are usually rated between 300 and 450 CCA. If you test at too high an amperage, a good battery may look bad. If you test at too low an amperage, a bad battery may look good. Loading the battery at 1/2 the cold cranking amp (CCA) rating is the proper way to load test a battery. 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. 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. A battery MUST be at least 3/4 charged to load test. Testing a discharged battery will make it look bad even if it’s good. ![]() If you want accurate results, use a load tester and follow these rules: Only test a charged battery One the other hand most tow drivers, auto parts stores, and dealerships use resistance testers. If you’re a professional shop, misdiagnosing 1 out of 20 cars isn’t great for your reputation, so most shops use load testers. 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. However, the tool itself isn’t all that accurate. It’s so easy to use that anybody can use it properly. ![]() The other type of tester is an internal resistance tester. This is the reason that nearly all manufacturers recommend using a different type of tester. There are rules to using a load tester properly, and frequently people test improperly and come up with an incorrect result. A load tester measures battery voltage while drawing current from the battery. The best type of tester is called a “load tester”. The best plan is to bring the car or the battery to a shop or parts store with a battery tester. 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). Unless you have some specialized test equipment, you won’t be able to test your battery, at least not very accurately. ![]()
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