12 Volt Battery Replacement

The single most important thing when doing anything with the battery is to connect the polarity correctly first time, as there is no second chance if you get it wrong. The gen1 battery has the positive post on the right hand side with the posts closest to you. The gen2 and three have them the other way round, so check carefully the dealer supplies the correct battery.

John (Britprius)

 

The original battery has a relatively smaller terminal post. If you find the replacement battery terminals are larger, you can be a terminal post that fits and bolt it in to replace the Japanese original. You can also replace the ground cable too.

I would recommend making sure you can return the parts and buy them. If a problem occurs, you fix it and move on. If not needed, return them and get the money back.

Also,

• Disconnect the ground wire first so the tools won't 'spark'
• The red-cover has plastic tabs
• Disconnect the B+
• Disconnect the vent tube and pull out the battery
• Compare the terminals to determine if they are the same size
  • Replace terminal clamp and ground wire as needed
• Compare the size to see if you'll need some shims (wooden blocks to hold in place)
• After the ground wire and terminal clamps are installed, install the battery
• Connect the B+ first, NOT THE GROUND, and make sure everything is secure
• Connect the ground cable (don't worry about little flash as you just lit the trunk lamp)
• Start the car to make sure everything is OK
  • Test battery voltage with car running, ~13.95V, if less than 13V, there is a problem.
• Clean everything up
• Properly dispose of old battery

Bob Wilson

 

The original original battery, if it was never previously replaced in the life of the car, said GS on the side and had the smaller posts. The upgraded original battery (the one the dealer just sold you, and the one in the car if it was ever previously replaced at a dealer after October 2003) has the larger posts and should say Panasonic.

The cable/hardware changeover kit is only needed if the battery coming out is the old GS and was never replaced before. The dealer can get that too, part number 04003-23147; it's not expensive. That will change over the cables/tray/holddown hardware to fit the upgraded battery perfectly. Again, if the battery coming out was already a Panasonic then the hardware was already changed when that was put in.

-Chap

 

Can I get in here? Looks like I also need a 12v battery replacement ('02 bought new,with original battery dealer-replaced immediately after purchase due to sitting on the lot too long unattended). Using the test recomendations here (Optima battery and installation kit for Prius 2001 to 2003) I got 11.2-11.3 with no load and 10.2 - 10.4 with low-beam lights, heater fan on high and rear window heater active. Testing across the battery terminals directly with the car off showed about 12.6v. Mileage this winter HAS been abysmal, barely hitting 40 most tanks (Lucious Garage replaced the HV battery with a new one about a year back). If it's a given (as it appears) that I need a replacement, should I go with a Panasonic or an Optima? I've heard mixed recommendations.

 

I'm seeing similar performance with my Odyssey PC925 so I would not 'set your hair on fire' just yet. My old Odyssey would drop to 11.3V as soon as the car was turned off and yours is still in the 'good range', 11.9-12.8.

How are you measuring your 12V battery voltage? I'm beginning to question some of the values the Graham miniscanner is reporting. But more about this later.

Bob Wilson

 

Using the Prius Gen1 test screen set to Vehicle Signal Check, and following the recommendations given at the link in my original post. Mine is well under 11.9 without load and under 10.2 under load, at least sometimes. You'd say that's still not too bad? Admittedly, the following is from someone selling Optima batteries & install kit (for $180 shipped)...

"You will see the 12 volt voltage displayed. With no electrical accessories on including lights the voltage should be fluctuate between 12.2 and 11.9. The lower the voltage, the less of a charge your battery currently has. If the voltage is low do not proceed as the load test might totally discharge your battery. (If you see a voltage around 13.5-13.8 you are seeing voltage from the high voltage battery being converted to around 13.5-13.8 in an attempt to recharge your battery. This normally does not occur till after you start the engine but might occur earlier if the battery is very drained.)

• You can also test the battery by turning on the headlights, rear window heater and the heater fan. For a new battery the voltage would be around 11.3. If the voltage drops below 10.2 it should definitely be replaced. For voltages in between the lower the voltage, the lower current charge of your battery."

 

The 'good range' you quote, 11.9 - 12.8 is better than my 'no-load' figure of 11.2-11.3 from the test screen. My 12.6 was measuring across the battery terminals directly with a digital vom. Which figure is the one to use to measure no-load goodness? The battery is over 10 years old, by the way. Couldn't find a date on it (have not taken it out) but it says 'nonspillable battery' on the front and S34B' on the top (disappears under top clamp), along with yellow stickers w/ Japanese characters.

 

This is what I'm seeing with a new PC925:

• 12.3-12.4V - right after the car is turned off and turned back on to IGN
• 11.3-11.4V - after sitting overnight

The PC925 that was failing:

• 11.3V - right after the car is turned off and turned back on to IGN
• 10.3-6V - after sitting overnight
• 9.8V - after sitting for two days (yet it still started)

Your battery sound 'weak' so an early replacement may make sense. But what bothered me yesterday was doing some early solar cell, tickle charger integration. I saw a higher voltage on the 12V battery than I was seeing on the Graham miniscanner. It has me a little confused so I'll be investigating it but it suggests:

• unexpected voltage drop between actual 12V battery and the front of the car 12V buss
• inaccurate 12V battery measurements on the OBD buss

Fuses are funny in they can often have non-linear, resistance effects, needed so they can blow at a rated current. It is entirely possible that we may see an artificially distorted voltage up front due to:

1. lower battery voltage - requires more current for the same electrical load
2. non-linear fuse resistance - that drops the voltage more as the current begins to approach blow-threshold

PLEASE understand these are hypothesis of potentially subtle effects. I want to do more measurements before making a hard call. As for changing the 12V battery:

• Does the car lock and unlock doors from the keyfob? If not, the battery is weak enough to merit a change. Once I put the new PC925 in, the keyfob door lock/unlock function fully returned.

Bob Wilson

 

I've never had an issue starting the car (but did note a 9.9-10.1 no-load voltage on the test screen last night, after the car had been garaged for a few hours). The keyfob has issues with the lock if the keyfob is cold. I usually wear it on a belt clip, and if it's a cold day, I might have to use the key. If the fob's in my pocket, it has always worked. I replaced the battery in one of the fobs last year, but not the other. DK which I've been using lately, but it could have a 10-yr old battery in the fob!

 

Slightly off topic, but certainly related I have checked a number of gen2 Prius voltage readings with the MFD against readings taken with a calibrated DVM at the front jumper point. All of the cars under read the voltage at the MFD by up to 0.5 volts making the readings very unreliable and only useful as comparisons against readings taken by the same method when the battery was new and in a known good state of charge.

John (Britprius)

 

The Elm 327 OBD reader through "Torque App" or "Priisdash" has the ability for the vehicle 12 volt buss reading to be adjusted and corrected via a programmed input to a known accurate value.

John (Britprius)