DC-DC charging voltage question.

Hi all,

I've just temporarily installed a volt meter in my Prius to monitor the 12v battery voltages more conveniently than using the service menu on the MDF.

I suspect my 12v is getting to the end of its life. About three weeks ago I was getting the Triangle warning with all the other dash lights coming up, so I checked the ACC-mode voltage and it was 10.5. So when I got home that night I put the battery on charge overnight and it seems to have improved as I haven't had a repeat. Since then I am typically seeing these voltages now:

Accessory Mode: 12.4v
Ig-on Mode: 11.8v - 12.1v
Ready: 14.0v

I have been checking the ACC-mode voltage immediately after a 40-45 min run and it shows 12.7 - 12.9 and after it has rested for about 4 hours is 12.4. So that would seem to indicate that the battery is not too bad, normal even.

I'm just concerned at what might have caused the sudden drop to 10.5 a few weeks ago. The car is regularly driven and no lights or accessories had been left on, as far as I know. The battery was fully discharged about 4 months ago and sat for one or two weeks before being charged, which was done by my Toyota service dept to get the car to start. At the time I did not have a battery charger to do it myself.

Anyway, all that is a bit of background and my actual question is this: I have noticed that as I drive the voltage always stays at 14.0 v. This is not what was expecting, as I would have thought it would drop down to 13.6 v from time to time once the battery reached a state of full charge. Can anyone who has a scangauge or the like verify whether what I am observing is normal behaviour?

A second question in a slightly similar vein for those familiar with the CTEK MXS 5.0 charger: is it normal to stay on stage 7 (float) or should a healthy battery be on stage 8 (pulse) by the morning? I've been able to leave the battery on charge continuously for 3 full days and it hasn't gone past stage 7. I also put the battery on charge for three nights in a row (driving the car in-between during the day) and again it only got to stage 7, but not stage 8 at any time.

I hope I've been clear, but ask any questions if I have left out any important info. (Likely lol.)

Thanks for any help and insights offered.

 

I am having difficulty finding an AGM battery to drop in. Here Toyota seem to have the battery market sewn up. As soon as I mention that I want a Prius 12v battery, they say, "oh, we don't do those."

Batteries that are the correct size, posts and aH rating are all flooded acid types.

I have also read about AGM and am not sure now I want one as they seem to be a bit finicky and more prone to easily be internally damaged, from what I've read. What is the reasoning for AGM, exactly, as I used a Bosch S4 rated at 45 aH and found it to be excellent?

 

After reading here that the OEM battery from Toyota was an excellent choice I thought I'd just check on the price from my local Toyota parts dept - ~NZ$600 (US$506 or £303) - Yikes! Someone is making a killing. Had an interesting discussion with the parts guy around needing to supply a heavier duty battery in Taxis. It sounded like they just used a heavier duty flooded acid battery from a 3rd party supplier. He also mentioned that one of the reputable battery companies here were going to offer a specific battery for the Prius and that should be available in about 6 - 8 weeks time. No pricing info at this stage, sadly. I'll try and ring them directly for more info.

 

I wasn't aware of the different lead/acid battery technologies back then. The Bosch S4 I bought was a sealed and vented unit and recommended as an alternative to the OEM from Toyota. I thought (maybe assumed) it was a flooded battery, but being sealed and vented, does that mean it might have been an AGM?

 

No, the Bosch S4 was in my previous Prius that I had in the UK. I owned that for two years and had to replace the 12V battery about 4 months in. Never had a problem with the 12V for the rest of the time. Unfortunately, I never felt inclined to take any voltage measurements after installing the new battery.

I know what you mean about prices. Compared to the prices charged in UK for servicing, pricing is similar here, a tad more expensive. It just seems worse as our numbers are double UK £'s - NZ$400 just seems so much more expensive that £200. I just wish wages were on par with the UK.

Back to the here and now, the Toyota OEM battery is ~$600 (and I didn't ask if that incl GST (15%), but assume it does) vs a new Bosch S3 55B24R (45 aH) for $86. It is the same length as the OEM (238 mm) but has a higher rating - 45 aH; 80 RC; 430 CCA - than the OEM. The Bosch is flooded though as I haven't been able to locate an AGM that will just drop in so far. The S4 range is somewhat limited here in NZ and I can't get the same battery I had in the UK. I can also get a Bosch S5 46B19R for $106 which has the same dimensions as yours and is also rated at 45 aH but has a lower RC (63) and CCA (370). I'm not sure how that works, as the Bosch blurb says:

Bosch Battery S3
Maintenance free and provides economical and reliable power for standard vehicle applications.

Bosch Battery S5
Premium maintenance free battery for high performance. Includes Silver Alloy technology to ensure the highest level of starting power and longer service life.

Out of these two technologies, I want to choose the one that will tolerate deep cycling better and supply current for longer. It doesn't have to have a high CCA ability. Anyone with a knowledgeable opinion of which one of these two batteries will deep cycle better and supply current longer is most welcome to have their 2c worth.

As far as AGM vs flooded, I'd obviously prefer an AGM for safety, but I could live with the risk of flooded. After all, the battery is not exactly "in the cabin with you." It is isolated in the wheel well and I feel comfortable, that in the event of a crash that any passengers will not be splashed with acid and any spillage will be mostly contained within the wheel well.