MPG going down and battery question

I am assuming you are talking about the 12 volt battery. (I am not awareit has any warranty) There is one way (of many) that a 12 volt battery can fail where one cell dies.

the car's charging circuit keeps trying to bring it up to 14 volts even though it is now a 10 volt battey, so it usue a lot of energy vainly charging, so MPG goes down.

I am not at my computer, bu tthere is a screen you can test the 12 v battery on, without trusting the dealer.

 

Don't forget, you have two batteries: a 12V, and a 201.8V. The 12V has the usual pro-rated warranty which has expired by now, and this is your problem. The car is basically not capable of recharging a dead 12V battery unless you leave it on for a long time. Try putting it on a 12V charger at 4 amps or less, or buy a new one. Also, don't forget that mileage declines in cold weather. If you had 48 last summer, 40 in the dead of winter, and 43 right now, that's normal.

 

Your 12V battery, if original equipment, is 3 years old. You could try charging it overnight with a battery charger.

If you continue to experience the symptoms that you noted in your OP, it is highly likely that you need a new 12V battery. It would be very unusual to have a warranty that covers the 12V battery to 100K miles.

Other posters have reported that their 12V battery passed the test at the Toyota dealer, nevertheless the battery failed shortly thereafter. Hence I would view a battery test "pass" result with skepticism.

 

It's almost certainly your 12V battery. Unfortunately the 12V battery is not warranted. On the other hand, it's not very expensive (comparatively).

It often happens that a battery test will show a failing 12V as still good. Get it replaced and life will be good again.

Tom

 

If your oem battery went dead it's toast.
When you buy and install an optima yellow top(and installation kit) which is a deep cycle battery you will no longer have to worry about draining the battery and killing it, deep cycle batteries can be drained and recharged over and over oem batteries only last a short period and it's much shorter when they get drained, as a matter of fact it's over.

 

The Optima battery Yellow Top (size 51) is a better battery, but not that much better. Try to avoid full battery discharges as after a small number, it will fail too.

An OEM battery at the dealer should cost less than $200 installed (don't let them charge you more). The Elearnaid Optima kit (includes required installation adapters and instructions) is about $160 and and decent mechanic can install it for you. If 12v is lost during the installation process:

1. The ECUs will have to relearn the parameters of your Prius. It happens automatically and restores efficiency.
2. Your radio presets will be lost and need to be re-established.
3. The Automatic driver window function needs to be restored. Just manually fully open the window and then fully close it to reset the settings.

JeffD

 

I'm fascinated by the brand loyalty that Optima enjoys here.

I am wondering how much better of a battery it really is. I guess we'll need to see how long they last in Prius service.

Note that comparing the life of the original equipment battery to a replacement Optima is not a completely fair test since the short life of the OE battery can sometimes be attributed to owner error (like leaving a cabin light switch on or the hatch open.) Then, after the owner had to pay ~$200 for a replacement battery, s/he might exercise more care. Hence the replacement battery should have a longer life due to more careful owner behavior, regardless of any improvement in battery technology that might or might not be present.

 

I have two Prii; an 05 for my wife, mine an 06. Both have needed new 12 volt batteries at about 3 1/2 years. My 05 just one day died and would not boot up the computer. I charged it on a stand alone charger and 1 week later it failed again. I bought the Elearn aid battery kit mentioned here. It solved all my troubles. My battery in the 06 one day about a year later, just started acting strangely. After listening to the radio for about 5 minutes it would not boot up. I walked away and came back about 30 minutes later and all was good. 4 days later again it would not boot up.I pulled it out and charged it and the battery worked fine until winter and it finally died also.I also replaced it with the Elearn aid battery kit. Since replacing both batteries, I have had 0 troubles with either car. By the way ,both cars when the dealer looked at them, dealer said both batteries were good despite showing low voltage readings on the on board diagnostics in both boot up and ignition on mode. We shall see if these yellow top optima batteries perform better or longer then the OEM. so far however they both are good but they both are less then two years old. Even the OEM stuff lasted that long without troubles.
-Paul R. Haller-

 

As an example of how use influences the life of a battery, I have a 2006 Prius with 60,000 miles. My original OEM battery is still going strong. Other than luck, there are to reasons for this: 1) Mild climate, and 2) I baby batteries.

The weather in this part of the world gets cold, but it seldom gets hot. Hot temperatures are hard on batteries.

As for babying, I am an electrical engineer, and as such, I understand charge cycles and what they do to a battery when you discharge it fully. My OEM battery has never been allowed to discharge. It has lived a very easy life. Consequently, it still works fine. Every deep discharge takes a big bite out of a battery's life.

Even though my battery is working fine, at 4+ years I keep a jump box in the back. It's only a matter of time before I need it, and I have no intention of letting anyone else jump start my Prius.

Tom

 

+1. 2005 prius with 80,000 miles and original battery. We're in NC which has a moderate climate (fairly hot in the summer but only a few weeks of sub-freezing tempearatures in winter).