Battery Questions

Thank you so much for the detailed response! I did just check the battery under load and here's what I gathered:

Open Circuit Voltage at the jumpstart location: 12.84V
Voltage under load from High beam headlights: 12.17V (didn't change for 12 minutes)
Voltage under load from High beam headlights and A/C blower at high: 11.59V (Dropped to 11.57V after 5 minutes)

I didn't know if I was to continue this any longer, and was afraid I might end up with a dead battery so I stopped the test. Do these results give any indication of a weak aux battery?

Also, could you tell me where the IG1 and IG2 voltages come from? Are they being supplied by the aux battery even when the vehicle is in READY?​

 

The IG1 and IG2 relay voltages come from the 15A ABS MAIN3 fuse via the brake control power supply which is the black rectangular box mounted next to the 12V battery. That fuse gets power from the DC bus. The DC bus is powered by the 12V battery when the car is not READY, and is powered by the DC/DC converter when the car is READY.

 

I hope your measuring everything at the front jump points.

I can tell you that my 12 volt battery just sitting there measures 12.6 volts and with the car off and the high beams on for 5 mins the voltage doesn't drop one bit.
Your 12 battery is not holding a charge very well under load. I would replace it immediately. You can buy a yellowtop online from autozone for $160 and pick it up locally. Since you have a voltmeter and thank god for that bring it with you to check the battery voltage before leaving the store. If its below 12.6 volts its been on the rack too long. Demand a fresher battery.

If the slightest question about the 12 volt comes up its a good idea to replace it. Seen so many instances where posters have destroyed there prius messing up the jump start polarity mostly because there jumping the battery directly in the deep dark trunk. Completely unnecessary as they gave you convenient jump points under the hood.

 

Yes I was measuring the voltages at the jumpstart points in the fuse box.

The 12V battery was last replaced 3 years ago in April 2015 at Toyota and I've had this issue for over a year now. I've taken it back to the dealer twice in the past year and the claim that the battery is in good condition when they use their tester.

I wasn't able to find the battery for the Prius on the Autozone website, could you please include a link or provide the part number if possible?

 

Advance Auto Parts Batteries: Coupon for a 30% Discount

If you have SKS you need Optima Yellowtop DS46B24R

 

Checking Voltage at store, of course. Good idea Ed. Nobody at the store is concerned about shelf life. I must say, Batteries are us, they were using a pretty sophisticated multimeter.

 

These show your battery is in reasonable condition. If the car is not being driven regularly, then keep it on the charger, and it certainly won't hurt to put it on the charger once every 2 weeks or once a month to keep it fully charged.

I wouldn't be rushing out to buy a new battery, just yet, primarily because the Techstream screenshot shows that the car was running (and travelling 15 mph) when the fault triggered. This points to something in the inverter or DC/DC converter pulling the power so that the car is only running off the 12 V battery. Every time this happens you should be charging the 12 V battery back up, but ultimately you need to pin point why the 12 V bus power is going down while running.

When you ran Techstream, did you do the full health check or just for the ABS/TRAC/VSC ECU?

I have attached the diagnostic work up for DTC C1241 as it has a number of checks to harnesses and ECU outputs to check. Focus on the checks for C1241-83.

Let me know if you have any other DTCs show up.

 

The car is now being driven daily but shorter commutes during the week. I charged the battery about 2 months ago. Will keep charging it as necessary.

That's what I was thinking too. I have ordered a used Brake Control Power Supply Unit off of eBay that should arrive in a couple of days and plan on swapping that out to see it if helps. Perhaps the control unit is bad and is causing a power draw?

The issue seems to resurface every 3-4 months or so and I've never charged the battery until a couple of months ago and it's still happening although the battery voltages are the best they've been.

I ran a full health check and this was the only current DTC. There was another alert for an ECU firmware update that was available.

However, the previous time this threw an error about 4 months ago (screenshot below), there was another DTC C1256 (ACCumulator Low Pressure) recorded along with C1241 - 83 which I'm guessing is due to drop in voltage.



I had the brake actuator replaced under warranty extension about 6 months ago and hoped the issue would be resolved but it seems to persist.

I've read through the document and need clarification on which pin "B18-5 (IG) - Body ground" is being referred to to check the Brake Control Power Supply voltage mentioned in Step 3.

Once again, thank you for all your help!

 

I do not have SKS but wouldn't mind a battery with higher capacity if it will fit in my battery compartment.

 

It is easy to fit an SKS battery, you just need to replace the battery bracket, which costs about $25 from the dealer. Part number is 74440-47020.

 

Had a look at the illustration and compared it to my BCPS, and it seems, on my BCPS, B18 is oriented 90º clockwise (i. e. vertically) when compared to the illustration. Interestingly the disassemble/reassemble procedure I have attached also shows B18 in the horizontal position. Is this what was causing you to question the location of B18-5?

I have attached an illustration using photos of B18 on my BCPS with where I think pin 5 is.



Hope that helps.

[EDIT} Not sure if you need to back probe the connector in step 3. Steps 4 and 5 say to back probe in the hint, but the wording of the main instruction is the same in steps 3, 4 & 5. Hopefully you can work it out.

 

Solar BA5 is a decent tester for the DIY'r. Just googling a little: Ancel BA101 Pro looks very interesting now too...

 

Thank you for the illustration and the picture from your BCPS! Yes, I was unsure which pin was the #5 but now that you've clarified that the illustration in the repair manual makes sense!

The replacement part arrives later today, I will do all this testing then when I get to the BCPS and will keep you updated.

That's awesome, I'll get one when I replace my battery next.

 

I finally got round to replacing the BCPS last night. Yes I did need to back probe the B18-5 to test the voltage:
With car in accessory mode: 12.29V
With car in READY: 14.03V

I replaced the part anyways since I had a newer one in hand. The voltage on the newer BCPS was:

With car in accessory mode: 12.42V
With car in READY: 13.97V

Not much of a difference but will monitor and see if the voltage drop happens again.​

 

Good to hear, but shouldn’t the recharge potential be 14.5 V ?, in Ready You may want to look at your 12 V
connections, all 4 of them. The 12V battery could have some internal resistance also. A professional batttery shop can determine that for free. Your 2006 is now a 12 Yr old car, right?

 

What are the 4 connections that I should be checking?

I've had Toyota test the battery and claim its good. I have charged it since and it seems to hold a charge. Yes, it's a 12 year old car but the battery is only 3 years old, and I've been having this issue pop up now and then for over a year now.

 

Have you checked the connection where the negative battery cable attaches to the body of the car?

 

4 connections = One chassis ground/ other end: to neg terminal on Battery
Three: positive on battery and other end ( I don’t know if you had the other end of the
positive 12 V off. It usually helps to remove these ( the 3) completely and scrub with a wire brush especially if you see corrosion.
My thoughts are that you may be able to improve that charge voltage to 14.5, where it should be...
People have claimed increased Mpg by a new or improved 12 V battery.

A corrosion gathering point because the vibration causes it to loosen forming corrosive liquids underneath
thereby causing more resistance.

 

Yeah, the thing is that I don't see any corrosion. It looks very clean, I did disconnect and clean both battery terminal posts as well as where the negative attaches to the body about 2 months ago when I charged the battery.

 

What are the 3 +ve points? The set of components attached to the battery positive terminal? Where does the other end attach to?

I have not been having issues with MPG, its been pretty consistent after I did my spark plugs and transmission fluid exchange, at around 51-52 MPG average.