My low MPG is probably attributed to a combination of short trips and poor tire choice by prior owner. However, I thought a low 12V battery may be adding to the problem. Apparently not the case. I used the non-headlight method of accessing the service menu and the vehicle sensors: (I have to do the up down key presses a minimum of 4 times to get the service menu to come up, but come up it does.) I had read 11.7V after an overnight sit and that concerned me and was leading me to research getting a new battery. It also led me to getting a plug in voltmeter to keep an eye on the battery voltage: Amazon.com: LCD Cigarette Lighter Voltage Digital Panel Meter Volt Voltmeter Monitor Car By Buyincoins: Automotive When I got it, I plugged it in and started reading 12.3V at rest. Maybe the battery was doing better than at that one point for some reason..... Today, not satisfied with that, I started questioning the accuracy of this cheapie voltmeter, maybe it's optimistic and not useful. I took out my power probe and measured voltages at both aux ports, the jump point under the hood and directly off the battery. All 4 read 12.3V right in line with the cheapie plugin. Ok great, battery seems to be fine. Time to check the service menu and see what it's saying today: 11.7V This led to me rechecking all the voltages again and still finding the actual aux battery voltage at 12.3V. I don't know what voltage the service menu is tapping but it's not direct voltage so it is a poor choice to determine aux battery health. I would suggest reading it directly. Guess I'm sticking with the battery that's in there.
Have you checked what your probes say while the Service Menu is on? The computers will draw some power from the 12v, which would lower its voltage.
........Yep, just unpacked a Scangauge and will be seeing what it reads after I go through the instructions and configuration. ..........The readings were all taken at the same time, while the Service Menu was on. All the voltages were stable during that time and pictures were taken within a second of eachother. Service Menu voltage was consistently below what the actual voltage was. Worrisome thing is that this shown voltage is often what battery replacement is predicated upon.
i went to place selling batteries because i suspected low voltage......but he tested and said it was "good " ....but he said a overnight charge would be useful..because of my stop/start motoring i was surprised because i expected him to try to sell me a new battery his speicialist tester showed good result
As you discovered, use the Service Menu for a reference point only and not for use to determine 12V battery health. Always take the readings from the battery terminals or at the jump point in the engine fuse box. When I first checked my Service Menu reading versus the jump point terminals, the Service Menu was reading low by 0.7V. Like yours, the Service Menu indicated 11.7V with the car off and the voltmeter read 12.4V.
Is the 12.3 reading at night after just having run the car or in the morning after sitting several hours?
Checked it again this morning. 12.1V everywhere and 11.5V on the service menu. Seems battery is good and the lower reading is consistent on the service menu.
Ok. Not worried about low MPG any longer. Just did a longer steady state highway drive. On the way out, it was flat for about 55 miles then into the mountains. MPG was pretty good but included a long uphill. Reset on the way down and predictably got unrealistically good MPG at somewhere around 90mpg. Once on the flats I reset the trip odo and made the pretty flat highway run home including a couple miles of local driving to get to the house once off the highway: Conditions: On the highway was at mostly 70mph. Average speed shown at 56 mph 84F outside, AC set at 74F auto recirc. 55.5 mi run 60.7 mpg I'm not as worried about the MPG any longer. The 40mpg I get from day to day is a result of the short trips and is 4x better than my last car.
As you are perhaps aware, 12.1 at the terminals is still at the low end (likely at least half discharged); mine typically reads about 12.6V in the morning. Your battery might benefit from a good 24-hr hookup to a charger once every 3 months or so ... I have a 3.7 mile commute (each way) to work. I use the CTEK MUS 4.3, but it charges at 4.3 Amps which is 0.1 Amp more than that recommended in the manual. I have seen others recommend the CTEK 3300, Optimate, etc. Not sure what the latest and greatest is for handling AGM batteries.
Just curious: how short are your day-to-day commutes (one way)? Often my daily commute can be 0.6 miles one way. Once or twice a week I have a one way commute of about 14 miles. I should have checked the voltage this morning after the long drive home last night. I usually use a BatteryTender 800 on my power stroke when it is in the driveway sitting. It's only 800mA and has some good control circuitry so shouldn't be any worry about overcharging it. Think I have an extra permanent charging jack dongle in the garage I can wire into the Prius and check out a full charge.
I have not checked any gen3 Prius but having checked 7 separate gen2's all of them under read the battery voltage on the mfd. The readings varied from 0.3 to 0.7 volts under against calibrated instruments, making me wonder how many good 12 volt batteries have been discarded by using this reading. The only use for this reading is as a comparison "if you know what a good battery fully charged voltage is displayed to start with". Even at 0.5 volts under reading is a large error on the state of charge of a 12 volt lead acid battery of whatever type. John (Britprius)
Do you disconnect battery mains before hooking the charger, or go straight under hood? I have a 20y-old Toyota Carina where I do that in a blink, but it does not have expensive ECUs...
Your charger will be fine for the Prius battery do not worry. You can connect to the jump point for charging no need to disconnect. John (Britprius)
I have always hooked it up directly to the battery. I figured 4.3 Amps isn't much different from the factory recommended 4.2 Amps for charging. This CTEK starts with 15.8 volts for desulphation but switches to 14.4 volts in car mode. I haven't seen it stay in the desulphation mode for even a few seconds. Have used this charger on both my Prius and the HyCam with no adverse effects (that I am aware of ).
