There is a demonstrated need for an alternative replacement battery for the Classic Prius. This results from several facts: First, the accessory battery is subject to failure from repetitive discharges and second, the OEM battery is priced inordinately higher than its market value. The project detailed here uses a replacement battery which meets or exceeds all electrical and safety requirements of the OEM accessory battery, costs around $100 for all parts; and the project is reversible, should you wish to put Humpty back together again.
WARNING AND DISCLAIMER: Work with storage batteries can be dangerous. These small batteries contain a tremendous amount of energy. Please do not undertake this project unless you are familiar with the appropriate precautions for working around storage batteries. Should you decide to undertake this project it is up to you to verify independently that the project is safe and correct in every detail. If anything bad happens it is your responsibility, not mine.
Physical differences: The replacement battery fits the OEM battery holder, but is a bit shorter and wider. It uses larger battery posts than the OEM battery.
Electrical characteristics: 12 volt, 475 CCA, 31 Ah. The CCA and Ah ratings exceed the OEM battery.
Safety characteristics: It uses dry, AGM technology, so there is no danger of acid spilling in the trunk. It is vented so that potentially flammable hydrogen and oxygen gases do not accumulate in the trunk.
WHY THIS BATTERY: This battery was designed for the Mazda Miata. The Miata has safety requirements similar to the Prius; the battery is mounted in the trunk so that hydrogen gas produced during charging must be vented to the outside, and the battery contains no liquid sulfuric acid, which if it leaked could destroy the trunk contents. Fortunately this battery exceeds electrical specifications for the Prius OEM battery and fits in the existing Prius battery holder.
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UPDATE added 1/22/2006
A kit is available, based on this website. It includes
all parts needed, a fresh Wesco battery and free shipping. I have
no financial interest in this kit, but it appears to be a great bargain.
Part Kit for upgrading Prius 12 volt (12v) battery on Prius 2001, 2002, 2003
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Battery availability: Check with your local battery store. Several Internet sites sell this battery for its factory list price with free shipping and state tax free and no core charge.
There may be other Internet sites. Search Google for Miata Battery.
These batteries are available occasionally on eBay (search MIATA WESTCO BATTERY).
Above is a photo of the new, modified battery in the OEM Prius battery
holder:
Note new negative cable at the far left, and wood block extension for
battery
hold-down center. At the far right, beneath the red hood covering
the terminal/block assembly
is a larger post terminal to fit the standard auto battery post size.
BATTERY SIZE COMPARISON
Above is a photo of the OEM battery (left) next to the new battery.
The "front" of the new battery is reversed so that the post positions
correspond to the OEM battery. Note the new battery is 1 1/2
inches
shorter than the OEM battery. Note vent tube nipple on the far
left
upper side of the OEM battery . The new battery contains 2 nipples
on
the upper back side in this photo, not seen here.
PROJECT DESCRIPTION STARTS HERE.
Positive Terminal:
The OEM battery uses small style posts. The replacement battery
uses standard sized posts common to most cars in the US. The positive
terminal must be replaced. The steps for this project are detailed
in a Toyota Service Bulletin. The document contains drawings which
make the process of changing the battery clearer, as well as providing
inspiration to undertake this project. Adobe
Reader is required. (download Adobe Reader here).
Toyota Technical Service Bulletin .
Fashion a Positive Terminal:
Consider the replacement terminal you purchased from the auto parts
store. Note its shaft is too wide to fit into the Prius Terminal/Block
Assembly. It must be hack sawed to fit. Next a hole must be drilled
as shown in the photo to the left below. Insert the hack-sawed terminal
into the Block/Assembly and mark where a hole must be drilled. Drill
a 5/16 inch hole through the modified replacement as shown in the
photo below. Sawing and drilling are easily accomplished as the terminal
is made of soft lead. Attach the modified terminal with a 5/16
inch bolt 3/4 inch long, lock washer and nut similar to the instructions
in the TSB. Save the original terminal and
hardware in case you wish to reverse this modification. The photo
at the right shows the finished block/assembly with standard sized battery
post terminal.
Before and After Positive Terminal. On the right note how terminal is cut down.
Cut down post attached to terminal block assembly.
Negative Terminal:
Now construct a completely new negative cable, consisting of
a post connector for the battery post and a ground connector for the opposite
end of the cable. Save the old one in case you want to reverse this
modification. Take a 6 inch length of insulated stranded #8 copper
wire with black insulation (if black is not available, then choose any
color but red). Remove 1/2 inch of insulation from one end and 3/4
inch of insulation from the other end. Flatten the 3/4 inch end with
a hammer (This flattened end will become the chassis connection and the
1/2 inch end will become the battery post terminal end of the new cable
described later).
A word about the terminal kit I used. It is made of steel, not lead, and is a ring type terminal. Mine contained the words "lead-free" battery terminals. The issue here is not what kind of metal the terminal is made of but its size and shape. It must fit properly in the space provided. This kit comes with 2 cable connectors as well as a ring-type battery post terminal. (Feedback from one individual who replaced his battery using the information here: A regular lead negative battery terminal can be made to fit in the available space, but just barely.)
Back to the cable. Take the prepared cable and install cable connectors on each end of the cable. To ensure lasting mechanical connections you may want to tin the exposed copper on each end of the cable with solder and a soldering iron to prevent oxidation leading to poor connections. With both ends of the cable inserted into the connectors crimp the connectors against the cable with pliers to insure good contact. Then lightly hammer the connectors against the cables. Attach the ring type, "non lead" battery terminal to the non-flattened end of the cable (the end with the 1/2 inch length of exposed wire). Use provided hardware. The new terminal is complete.
In the photo below: To the left note the kit. The name on
the package contains the words "lead-free". The photo is poor, so
you will have to take my word for this. This kit comes with 2 cable
connectors; one ultimately attaches to the chassis of the car and the other
bolts to the "lead-free", ring-type negative post terminal. That
terminal is located on the far right side of the photo. It might
appear to be separate from the end of the cable but it isn't.
The cable with connectors on both ends is a single unit.
Completed negative battery terminal on right with kit on the left
VENT TUBE MODIFICATIONS:
The OEM battery (in the photo below labelled GS) has a single vent nipple on its left side. Look closely at the picture below comparing the two batteries side by side. The battery on the left is the OEM battery. On its left upper side note the right angle vent tube nipple.
Fashion a new vent tube for the new battery. Cut two 1 inch
sections of 1/4 inch outer diameter tubing. Cut 1 six inch section
of tubing and 1 three inch section. The 1 inch sections connect to
the vent nipples on the back of the replacement battery. Insert an
L conector on the right short tube and a T connector onto the left short
tube. Insert the 6 inch section between the two connectors and the
3 inch section to the end of the T section. Insert the straight connector
into the open end of the 3 inch section. The pictures shows the result:
.
Now attach the new vent tube assembly to the new
battery:
Now place the new battery with new vent tube assembly into the Prius.
Above. Battery with new vent tube hookup (white) attaches to Prius vent tube (black) far left.
BATTERY HOLD DOWN MODIFICATIONS:
The replacement battery is slightly under 2 inches shorter than the OEM battery. The difference must be made up with a spacer between the replacement battery and the OEM hold down bracket. Cut 2 five to six inch lengths of 1X2 lumber. You must make a groove across one of the sections to mate with a ridge on the new battery. (I used a wood rasp). Fasten one wood length to the other with screws or nails. Then stack the wood unit one atop the battery as a spacer. (In the photo below the two lengths of wood have not yet been fastened to each other). See photos:
Above. Two 6 inch lengths of 1X2 lumber. Note bottom piece
has groove (fashioned with a wood rasp) to mate with battery.
Above. Battery hold down in place on new battery.
NEW BATTERY INSTALLATION:
Now insert the modified replacement battery into the Prius battery holder. You will note it fits more snugly than the OEM battery. Attach the Prius vent tube to the battery vent tube assembly. Next reinstall the battery hold down bracket. The back part of the bracket slides down a threaded rod behind the battery. Tighten the bracket down in the back. The front part will not mate flush with the floor of the trunk. Place 3 or 4 washers beneath the bracket so that the bracket fits snugly and bolt the bracket to the trunk floor with the bolt provided.
Install the Battery Cables onto the Battery.:
Follow the instruction sequence below to minimize risk of shorting
the battery. Please reread the WARNING AND DISCLAIMER
at the beginning of this document. If you have any doubt about how
to do this then obtain help from someone familiar with the proper
handling of storage batteries. What follows is the sequence for connecting
up the new battery. Don't attach anything to the battery posts yet.
First reconnect the positive battery cable and the sensing wire to
the modified positive battery terminal/block assembly. This is the
reverse of removal which is referenced in the TSB. Now place the
positive battery terminal onto the positive battery post and tighten it.
Next ensure that no exposed metal part of the negative cable makes contact
with the chassis of the car. To be perfectly sure it would be wise
to wrap electrical tape around the chassis terminal such that no
metal is exposed. Now attach the negative battery terminal
to the negative battery post and tighten. Remove the tape from the
free terminal. Now, fasten to the chassis of the car in the threaded
hole provided with the bolt provided. You may notice a spark
when the connector comes in contact with the chassis of the car.
This is normal and expected. Now, reinstall the red plastic hood
over the positive battery terminal/block assembly. It may fit more
snugly than before, as it covers a larger battery post terminal.
Here is a picture of the job so far:
At this point the car should work. Turn the ignition to ON. The display panel should light up as expected. If nothing happens, confirm the integrity of all connections, as your battery is not providing electricity to the Prius.
Now reattach the battery compartment protector to holes in the trunk with plastic bolts provided . You may have to adjust the position of the negative battery cable slightly so that it inserts properly into the groove on the bottom of the battery compartment protector. Finally replace the trunk lining cover. These last steps are the reverse of disassembly per the TSB.
You will need to reset your radio selections and the clock. You will lose your trip meter settings. The odometer will not be affected by battery removal/replacement.
MY OEM BATTERY IS STILL GOOD...WHAT TO DO?
That was my situation. Since I didn't know if I would ever need
to reverse the modification I decided to save the OEM battery along with
all the parts original negative cable and the OEM positive terminal in
case I wished to reverse the modification. So my OEM battery remains
hooked up to a Battery Tender (TM) 24 hours a day. This device holds
the battery at full charge but does not overcharge it. I expect the
battery to remain good for a long time.
PARTS LIST:
1 Westco battery.
1 Battery Terminal. This is a lead replacement positive terminal.
1 "Lead-Free" Battery Terminal kit. This is a 3 part package. It includes a ring-type terminal (steel, not lead) and 2 cable connectors. Needed to make a negative cable.
6 inches #8 stranded copper wire with insulation. Color of insulation preferably black (any color but red will do).
1 foot section 1X2 lumber. Alternatively, one 6 inch length of 2X2 lumber
1 foot section of PVC (vinyl or rubber will work) 1/4 inch outer diameter tubing.
Plastic fittings for 1/4 inch outer diameter tubing. One each L connector, T connector, straight connector.
1 bolt, 5/16 inch diameter, 3/4 inch long with matching nut and lockwasher.
4 washers with 5/16 inch hole.
CLICK HERE TO EMAIL ME QUESTIONS OR COMMENTS
MODIFICATIONS SUGGESTED BY OTHERS
Alternate method of mating OEM Battery cables to the Miata Battery.
Suggested by DAS. Rather than building new cable connectors and a
new negative cable, this method involves shaving down the lead posts on
the Miata battery to fit the existing cables. Here is a description
by the author. I have emphasized safety warning:
Jul 17, 2005
Tools involved are a hacksaw, a metal file, and a
set of hole punches. The last item is optional, but is helpful and
sells for about $5 at Harbor Freight Tools; well-known to all
cheapskate DIY-ers.
Battery posts are conic sections, for the geometrically inclined. I
selected the hole punch matching the top diameters of the OEM battery
posts, and used those to mark a circle on the top of each of the
replacement battery's posts. Then used the hacksaw to cut down the
new posts at an angle visually similar to that of the OEM.
At this point it is appropriate to mention that shorting the battery
posts with the metal hacksaw blade would make big sparks. Don't do
that. Then used the file to smooth off the cut-down posts (same
short-circuit warning applies). This is all easier than it may
sound, because the lead metal cuts real easy. Best to sweep up your
removed lead bits, for environmentally responsible disposal.
At this point it is nearly certain that the OEM clamps will nicely
fit the new battery's posts. If not, remove more lead with the saw
or file, as appropriate. Install new battery and enjoy several more
years of 12 volt Prius functionality.
The replacement battery has been in service for several months now,
and my voltmeter says it is very happy. My removed
OEM battery will go to Municipal
hazardous waster disposal next weekend, in fact.
DAS
Photo of DAS conversion process:
Above. Sawing down battery post.
OEM battery (left). Note Westco battery posts (right) now same size as OEM posts.
Last Revised 2/12/2006