Hybrid vehicle immobilizer / Theft deterrent system indicator stays on and drains battery

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How far (miles) do you drive the vehicle per day/week?

 

The 12v battery charges very slowly while driving or when in 'Ready' mode, 70 miles a week isn't all that much and you could be slowly draining the 12v over time.

When was the last time the 12v battery was replaced?

 

what do you do when the car is dead?

 

does it actually say 'doing fine'?

 

Welcome!

The immobilizer is supposed to be on when you shut down the car and it will not run down a healthy battery unless you let it sit for weeks and weeks. So, either you're taking too many short trips and it's not getting recharged or it's not holding a charge.

The car doesn't really do a diagnostic of the 12V. The MFD will display the battery voltage but, iirc, the car must be in ready mode for that and that would mean the inverter is charging the 12V battery, so the display would show around 14 V give or take a couple tenths. I tried that once a couple years ago just out of curiosity. But it's not helpful unless you think the charging system is malfunctioning.

You said in your original post that you tested it at 12.3 volts. Was that immediately after shutting down or had it set overnight? Either way, that's a little low but especially if you just shut it down. If you put a load on it like headlights and got that reading, that's not so bad.

One more thought. You didn't say how long you've had the car. If it's new-to-you, there may be something that the previous owner installed that's draining the battery.

Also, please be hyper careful when jumping a Prius. Getting the cables reversed can cost thousands of dollars in damage to the inverter system.

 

1) Your method of getting the 12.3V battery reading is suspect. A load test is needed. Follow post #5 below.

Is my inverter coolant pump slowly failing? | PriusChat
Post 5-quick easy way to load test the 12V battery
Post 8-SOC (State of Charge) chart, some smart, AGM compatible 12V battery chargers.​

2) How many times have you had to get a jump, and over what time period? You give the impression that this is multiple times. A discharged battery will live a shortened life, which is probably your battery's case.

3) 70miles/wk ~ 14miles/work day, may not be enough. 14 miles/work day in Los Angeles would be sufficient, as that short distance could easily be 1hour+ in traffic = 1 hour+ for the 12V battery to get charged. A better gauge is how many minutes/hours is the car in READY mode per week?

4) Do you listen to the radio in ACC (Accessory) mode? Esp for long periods? This will obviously contribute to drain the 12V battery faster, and leave it in a discharged SOC, which will shorten its lifespan.

5) When was the 12V battery last replaced?

6) Should you need to replace the 12V battery , do NOT get an Optima. Why? It is a 38Amp Hour battery; you are loosing 7Amp Hour. Your SKS equipped Prius came with a 45Amp Hour battery. Also, recharge the 12V battery prior to installation; sitting on the parts shelf collecting dust can lead to the battery being partially to severely discharged.

Get a Prius battery from the dealer; price shop this, ask for discounts (AAA, Costco, etc), dealer coupons (online, mailers, other dealers
Camelback Toyota has it for $183.65, 7 year pro-rated warranty; no dealer can ship it, as it is it is considered Hazardous Material, and would require cost prohibitive Hazardous Material Shipping. Maybe your local dealer will price match Camelback?
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A Bosch AGM Prius battery is a great buy from PepBoys. $142.49, 4year straight warranty; not pro-rated, same 45Amp Hour capacity, fits the Gen2 Prius, has the vent tube hole, and JIS battery terminal posts (IIRC). Hopefully there is a PepBoys near you.
Car Batteries | Auto Batteries For Trucks & SUV's | Pep Boys

 

12.2V in ACC mode is a small load and this battery is weak. (50% SOC at best).
Best to take a reading with a DVM (Digital Volt Meter); borrow/buy one if you can. I think you should be able to still do the test w/ the built in menu function with IG-ON, vent fan on HIGH, NO A/C, radio on low. This will be a better load, and provide a better indication of the current battery's SOC. That 12.2 will most likely drop into 11V range or lower.

Get the battery recharged with an AGM compatible charger. Why an AGM compatible charger? AGM chargers will recharge to a higher voltage that an AGM needs. Non AGM battery chargers will cut off before an AGM battery is fully charged, b/c regular battery chargers charge to a lower threshold, that is sufficient for wet cell and maintenance free batteries.

1) Maybe the weakened 12V battery is the cause of the light staying on? People report the Prius doing weird things when the 12V gets weak. Recharge the battery first, then continue below.
2) Try resetting the car by disconnecting the 12V battery ground for a minute or two, then reconnect.
3) Maybe there is a lost FOB in the car, the previous owner(s) could not find?
4) Just deactivate SKS?
5) You'll need to see the dealer. The solution will be costly.

Just a guess, but seems like the SKS ECU remains engaged all the time, and does not "sleep," like it is supposed to after a period of time when the FOB is not in proximity. Even in sleep mode, it will periodically "call/listen" for your FOB.

 

Ahhh! Now I get it. I'm not sure if a low fob battery will do that. Here's what I found in my wife's '07 owner's manual. If it's not the fob battery, then I think I'd take it to the shop.

 

While this is generally interesting information, it does nothing to address your issues. It really is a red-herring as the voltages you reported are not going to cause any faults related to a bad or irregular 12 V supply. For that the battery would have to drop well south of 10 V.

The thought that the immobiliser or SKS ECUs not going to sleep may be relevant.

jerrymildred has the best advise so far. I wonder if reading pages on the Theft Deterrent System starting at page 46 would shed any light or give you some more clues.

If you don't have a hard copy of the OM (owner's manual), you can download a PDF copy from the Toyota owner's section of their website.

Let us know if any further information come to light.