12V Battery Load (Headlights) Question

Okay, I am going to say this. I have a battery that is 14 months old. It is a "Yuasa" battery that is made for the Prius. I bought it from Toyota. It is a 45 amp/hr battery. I put the Prius in "Ignition On Mode" (two pushes of power button without stepping on brake) and observed the MFD voltage read out. I did this for 60 seconds. Went by clock on dash... Test started at 12.1 volts (headlights on - low beam). Voltage fluctuated between 12.1 and 12.0 volts the entire 60 seconds. After that I turned every thing off and came inside to report the findings. FWIW, the fan for the climate control was also running at the lowest speed setting so that also added a little more load to the system.

I would venture to say that if your battery is dropping off to 10 like you said, it has probably seen better days. I would not be surprised if the battery in your Prius is the original battery. Base model Prius battery can sometimes last a long time cause they don't have the SKS system dragging it down like the package #2 and above cars do. But those cars also have a bigger battery.

Just for grins, what kind of numbers are you reading if you run the test that most guys run using the MFD? If you don't mind, run the test with the headlights OFF and the blower fan turned OFF along with the doors closed (minimal loads) for consistency. The test is best run after the Prius has set for at least 12 hours.

Link is below in my signature (test the 12 volt battery) if you are not sure what I am talking about.

Please run the test that is linked below and post back all three of the values you read. Don't forget, when you run the test keep the headlights OFF and the fan blower turned OFF also. Close the doors etc.

FYI, If you change the battery in the future AND use a aftermarket battery, you may find that the battery doesn't seem to fit right. That is because the battery in the base model is "smaller" than most aftermarket batteries for the Prius. The trick is to change the "tray" the battery sits on. You can get the "tray" from Toyota at the parts desk. Just tell them you want a tray for a car that has the "Smart Key System". If you tell them you have a base model, they will sell you the same tray you have now. There are two (2) different trays and two (2) different batteries (12 volt) for the Prius. It all depends on if you have "base" model or one that has at least "Package #2" on it.

Best of luck to you and "Welcome to Prius Chat"!

 

Okay, here's some more data for you.

Ambient temperature here 57 F - time of day 12:20 A.M.

Ran your test A/C ON, Temp set on LO, Fan speed on High with A/C compressor running. Headlights ON for 60 seconds in "Ignition Mode ON" (No Ready Light) MFD showed voltage at 11.6 and fluctuated between that and 11.5 for the 60 second duration. Note: I said "A/C compressor running" above, but I believe the compressor only operates when you are in "Ready Mode" as it is supposed to run off of the "traction battery" and I don't think the traction battery kicks in until you are in "Ready Mode". I hope my terminology is correct and not misleading. What I will say is this, the A/C light was on and the green line on the MFD under A/C was lit.

I think more than likely your battery is probably the original battery. If your Prius is a base model, the amp/hr rating is 35 IIRC. Chances are the battery is getting weak due to usage and sulfation built up on the plates which reduces the capacity of the battery.

Your MFD numbers are not too shabby, but after sitting only a couple of hours they can really be misleading.

If I am not mistaken, you are approaching Winter? I think the seasons are reversed from where I am. If that is the case, you might consider thinking about a battery replacement. However don't get me wrong. If you have a good AGM type battery charger you could probably get by for a while longer maybe even a year or so. But then again, it might die when you need it the most.

If you have doubts, invest in a new one when you can. There are a few guys from Australia that post here on Prius Chat. Hook up with one of them if you need help finding a battery as the sources and brands vary from place to place. It would be a good idea if you were to update your profile here on Prius Chat also to show your location.

Best of luck to you and I hope this helps.

Ron (dorunron)

 

Hi ciaw, your battery is definitely not holding up well to high load levels. You're probably putting your battery under a 30+ amps load there. I'd estimate that ig-on mode is 10 to 15 amps just to power up all the systems, headlights another 10+ amps, and I'm not sure about the fan but I think it's quite a significant load on higher speeds. (Note that the AC compressor runs off the traction battery not the 12V, so it's only actually the fan running in ignition on mode).

The higher the load the more it's gonna weed out a weak battery and that's what is happening here. Remember however that in the Prius we usually go straight into ready mode, where the traction battery and DC-DC converter are supporting the 12V. So we can often get way with having a pretty anemic 12V battery for quite a long time, as long as you don't try sitting in accessory mode for any period of time. It's your call on whether you replace it straight away or not, I'd probably watch it for a while or perhaps try it overnight on a charger first.

 

uart,

If you add the fan speed on high to the load mix, it drops the voltage down a good .5 volt at least on our Prius.

I thought the compressor only ran off of the traction battery. I am glad to hear that we agree on that.

Are the seasons reversed down there from the states where I am? In other words, is it fall down there?

Ron

 

Yep. Southern hemisphere seasons are exactly reversed to that of the Northern. So we're just out of summer here, but it's still fairly warm.

Re the fan speed. Yeah, I was surprised that just having the fan speed to a medium-high was enough to drop the voltage from 12.1V to 11.8V on my ignition-on test. And that's with a fairly new battery!

 

Don't forget about the tray thing. It will help if you end up with a bigger battery than the one you have now.

Several folks here on PC with base models have posted about the replacement batteries don't fit well. Rubbing against back end, not sitting level, hold down clamp not fitting right etc. The larger tray fixes all of that. Easy to swap when you have the battery out.

You may not be aware of this. The Prius takes a special AGM type battery that is vented via a tube. It also has small posts on it (JIS type). Get together with uart or someone in your area who is familiar with the Prius. It is not hard to swap the battery. Instructions are in my signature below. I changed ours in an hours time. I am disabled and was able to do it. Also, the battery is NOT cheap. Here in the USA we pay anywhere from $175 to $200 US for just the battery. The dealer (stealers) can charge anywhere from $50 to $150 more just to put the battery in.

Here's a picture of what the battery looks like. Of course this is the 45 Amp/Hr battery, not the 35 you have.



And this is what it look like when the battery is installed in the Prius.

 

Ok, as it stands it is like I said. The stuff you guys get is different from what we get here. Yeah, I think the D51T1 will work in the Prius. However I cannot say if it will fit without changing the tray. Also, some guys mentioned having trouble getting the vent house hooked up properly.

I do know this much. You mentioned the Yuasa battery with the S46B24R part #. That is the battery that is being sold here in the US for the Gen II Prius with the SKS system on it. (bigger battery).

What I would suggest you do is this. Measure the dimensions on your existing battery. Compare that to the new battery. If the new battery is bigger, then you probably will have either change the tray or make a modification to get the battery to fit properly.

It seems different folks do different stuff. One guy here on PC (britprius) in the UK fitted a high amp/hour rating in his Prius because he has a wheel chair lift on his car. I think he put in a 60 amp/hr battery that was a standard flooded battery. He also modded the cables to make them work with the lug type terminals on the battery.

I know all of this just muddies the water a little more. Wish I could clear it up, but sometimes it ends up that way. That is one reason why I suggested you hook up with someone local down under who is familiar with what you are doing. Been there done that sort of thing.

One thing I will comment on. Pay attention to the amp/hr rating of the new battery you get. The bigger the better. That new amp/stereo you are talking about is going to add more load. So get a battery with enough oomph in it, otherwise it won't last very long. Also pay attention to the post configuration. The positive post should be on the left hand side when the posts are closest to you. The vent hole should be on the left hand side towards the backside of the battery when the positive post is to your left. Don't forget about the small (JIS posts). There are a lot of different battery configurations out there.

I have a feeling when it is all over said and done you will think "It wasn't that bad after all".

Best of luck to you and please post back what you finally did and the prices you spent as that helps the community as a whole.

Ron (dorunron)