How to check, charge, and where to source the 12v battery

My Prius needed jump start, twice, during last couple cold winter weeks. When it failed to start, even the dashboard completely went dark. These symptoms all pointed to a weak 12v battery. Here comes my journey, and I hope it will be useful for this community.

How to charge your 12v battery?

• If you can jump start your Prius and drive it around for i-dont-know-how-long, it should self-charge. But it is a guesswork. Short trips will not be sufficient, because each starting would consume more than the subsequent short trip can charge back.
• You can use a lead-acid battery charger (some inexpensive hobby chargers will do) to decisively charge your 12v battery to all-it-can-absorb. The way I do it, I did not bother taking the battery out. I figure I could wire the charger to the jump start terminals under the hood (as that is the safe way to do jump start anyway), yet I chose to wire my charger directly to the 12v battery in the back trunk, while disconnecting its ground cable from the chassis, this way I am still confident that the charging will not fry my Prius electronics.

How to check whether the 12v battery is approaching end of life?

• Some auto parts store will provide free battery load test. Not sure how long they are supposed to "load test" it, but a technician in Toyota dealership used a handheld device and it gave a "needs replacement" result within minutes.
• You could use a multimeter to check its voltage. But, as it turns out, Prius has a built-in voltage check. Just shut down your Prius, then press the start button once to turn on the dashboard. Press and hold the DISPLAY button, and quickly flip the headlight on and off 3 times. Now your Prius monitor will show a diagnose menu. Go into "Menu", "Display Check", "Vehicle Check". And then you will see the voltage of your 12v battery. It is expected to remain around 12v (duh) when your engine is not running. For a depleted battery, it would likely be around 11v, and even dip down to 10v (which is likely insufficient to start your engine) or even sub-10v (which will not even be enough to light up your dashboard).
• If you went with the hobby charger route to charge it, some hobby charger will give you a stat on how many Ah were charged into the 12v battery. My failing 6-year-old 12v battery can store only 2Ah, which was merely enough to crank up my engine several times. A healthy 12v battery will possibly hold 12~24Ah.

Where to source a replacement 12v battery?

Prius (at least its 2nd gen) does not use regular 12v battery. You need to find battery with "size S46B24R". You should shop around to find the best deal. As of this writing,

• My local O'Reilly Auto Parts lists several options, all with 3 year warranty, ranging from $239 to $289.
• My local Toyota dealership offers Toyota OEM TrueStart battery, with much better 7 year (prorated) warranty, for only $216. The price is (perhaps surprisingly?) cheaper than 3rd-party shops above, but unless you can install it yourself, Toyota Service will also (perhaps understandably?) try to sell you a $199 installation, plus another $1500+ for your approaching scheduled maintenance.
• And then while I'm writing this post, I stumbled upon this info, that the battery is listed at only $158 per Toyota official website, even potentially "Includes Towing and Installation Labor (Ask your dealer for specific details.)" Data source: Go search "TrueStart Battery Part Number: 0054427060675" in official parts @ toyota @ com /p/TrueStart-Battery/69278353/0054427060675.html (sorry, I am not yet allowed to paste a real link in this forum).

So, now you have it, this issue can be solved by $158 plus some up to 30-to-60 minutes work.

Long live our Prius!

Regards,
Ray

 

Now I am able to leave a real link here:
0054427060675 - TrueStart Battery - Genuine Toyota Part


Also, after you procured your new 12v battery, you can follow this official repair manual to replace the 12v battery: https://attachments.priuschat.com/attachment-files/2017/09/131800_19_-_Starting___Charging.pdf

 

Some starter battery will provide a Reserve Capacity (RC) rating in their specs. Here comes one more piece of info for the geeks on how to convert the starter battery's RC (Reserve Capacity) rating to Ah (Amp Hours) so that we can compare it with the hobby charger's Ah counter.

SAE (Society of Automotive Engineers) specifies the capacity of a starter battery by Reserve Capacity (RC). RC reflects the runtime in minutes at a steady discharge of 25A. So, I'll calculate Ah = 25*RC/60. For example, a starter battery with RC=30 will roughly have 12.5Ah capacity.

Don't worry too much if your years-old starter battery's Ah is lower than its original capacity. Some said it would still be sufficient when it is down to 30% of its original capacity. Perhaps you can consider turning off the keyless entry feature of Prius so that it would presumably consume less electricity while parking in your parking lot overnight.

 

Thank you TMR, you made a good point! I was misled by the Toyota website, which does NOT display a red warning message of "this battery will not fit your car" even after I specify my vehicle as Prius 2007. Now I know that they only display a green "this fits your Prius 2007" when browsing the S46B24R or S34B20R. (P.S.: I bought my S46B24R battery over the counter from my local dealership for the same $215 price.)

So, yeah, the proper 12v battery for a gen 2 Prius with SKS is a 46ah battery.

Assuming both 46ah and 34ah would fit a gen 2 Prius regardless whether it is equipped with SKS, I would recommend to always pay the extra 10% ($20) to go with the 35% larger capacity 46ah battery so that you will have much larger room for error in case you accidentally leave your headlights on.

P.S.: Why does this forum not allow me to edit my own old post to change that wrong link error inline?

 

I did not know CTEK 3300 or 4.3 before. It seems they are priced at sub $100, so, technically still at the same magnitude of the "inexpensive hobby chargers". The hobby charger I used can work with many different kind of batteries: Lithium Ion, Ni-MH (i.e. the one used by Prius traction battery), Lead Acid (typically 12v vehicle batteries), etc., thus more versatile. The CTEK seems to specialize on 12v vehicle batteries, thus presumably easier to operate.

Why do you need to keep an on-grid charger in your car for emergency? If your 12v battery fails and you strand on the road where typically has no 110v grid power, what you would need is a starter battery, so that you can ignite the car immediately, and be able to make it to your home.

Very impressive. Does your wife also get used to such a routine of "remember to unplug it before each drive (otherwise the charger on the wall - or the wall itself - would be torn down)", and you two never made any mistake? That is even more impressive.

It is eye-opening to see how you meticulously maintain your 12v battery which worth only around $200. Now I wonder whether/how you maintain your traction battery which is typically listed at around $2000.