My 12v battery is 5 years old=Still working fine=How do I test the 'health' of the 12v battery?

swing by an auto parts store that does free load testing

 

You can simply measure the voltage for a quick information: directly at the battery (right side in the trunk), at the main fuse (under the hood) or at the cigarette lighter. The car must not be in ready state!
The value shouldn't be too low.
(Mine is often very low around 11.3...11.7 V, but I have no problems.)

If you have some lower level voltage, charge fully, let your car rest over night and measure the next day. Then it shouldn't have discharged too much. I'm not sure, maybe 11.8 V or higher.

However the 12V battery don't have to start the engine of the Prius. It only boots the car computer. Then the HV battery with inverter and converter can charge the 12v battery. This is a very big advantage compared to common cars, whom batteries have to do a hard job, especially in winter.

For a better feeling (against German Angst) I've bought a compact LiPo power pack with the usual connectors and additional jump start clamps for the glove compartment. I've ordered directly from China for less than 40€ ≈ $40. Used it two times with success, after long cleaning sessions (seats, windows, washing, waxing, polishing) with interior lights and radio on.

 

But they interpret the result like for common starter batteries.
For starting an engine a current of 100 A or more may be needed. A too old battery can't do that. The voltage would collapse while trying to do. Then they like to sell a new one.

But for booting the Prius much less current is enough.

 

Here's a good instruction page:
12V-Batterie – Prius Wiki

I hope you understand some German. If not, paste the link to Google translator.

They say:
Up to 10 V: charge and replace as fast as possible
Up to 11 V: replace until next winter
11.4 V: common value, normal in winter

If you don't have measuring instruments, you can use the diagnostic menu of the MFD.

 

Turn Ig OFF, turn on high beams for 30 seconds to remove surface charge, at the battery terminals measure voltage:

12.5 V and above: good
11.0 V - 12.5 V: Recharge
Below 11.0 V: replace

 

You will. And probably soon.
Why not replace it BEFORE you have to call a tow truck ??
Under no load, a reading below 12.5 is reason to be a bit nervous.

Any test that you do will be only a rough idea of the condition.
It can be good today and bad tomorrow.
And the older it is, the higher the odds are of a failure.

True.
But failing the "cold cranking test" for that size battery is a good indication that it is on the way out.......even in a Prius.

 

Solar BA5

 

You can test the battery using the onboard computer test AFTER A FULL NIGHT'S REST. With a 5 year old battery, you are starting to play Russian roulette. Many have gotten 6 years before failure. Some people have gotten 8 years (via regular overnight trickle charging). Someday it will fail. Why not just replace it before you get stranded? Question is: do you feel lucky PUNK?!?

 

I have a 2010 Prius with the original Batt. I have it load tested yearly at Walmart. Still good.

 

I’m kind of surprised nobody has mentioned there is a procedure on how to check it in the owners manual. You could try that method.

 

Post number 6 is from TechStream...

 

I don't believe you can selftest 2010 and up.

 

I doubt it. I'm having a <$3 plug with temperature/voltage display in the cigarette lighter. For the last years it never showed 12 v or more before driving (besides the car is in ready state).
So why should I spend this much money for replacing a battery, which is operating well?!
And if it won't anytime, I have such a <$30-power-bank in my glove compartment: https://s.click.aliexpress.com/e/b5AXZo2z
There won't be no need for a tow truck!

I replaced the original one after 6.5 years, because I had your thoughts at first, too. But the new one showed the same readings. I won't replace it until the first problems will appear.

 

Is there a casino nearby ?
If not, maybe you should be playing the LOTTO.

At 8 years, you are skating on thin ice.

 

EXACTLY when you take the measurement is critical.
So is the accuracy of the measuring instrument.

If you open the door and get into the car before looking at the meter, then it is NOT in a "no load" condition.......so your readings might be perfectly good and normal.

My comment about getting nervous at 12.5 and less was in a NO LOAD CONDITION.......which you can't test for except by getting a meter directly on a power lead without opening any doors or the truck lid.

 

I'm not seeing anything. In the battery maintenance section, which starts at page 436?

My take, pony up for a Solar BA5 (or similar electronic load tester), done.

 

Okay, you're right.

 

There is exactly one way to answer that... LOAD TEST. Just be SURE the test is appropriate for a small AGM battery like this (closer to a motorcycle than typical car CCA).