Prius 1st gen – Auxiliary Battery Upgrade

Update 09/11/07 — I’ve been meaning to update this page for a while. We now keep two Miata batteries, manufactured by Interstate/Johnson Controls, in stock for the first generation Prius (and any Miatas that might come in!). We install a standard marine terminal for the positive side, and fabricate a custom negative battery cable. We use a 2×4 under the battery for a height adapter. The standard Toyota vent tubing connects securely with no problems. The upgrade does not require replacing the battery well or any of the mounting equipment. The Interstate battery is 370 CCA, so it’s not as powerful as the Westco and some others, but it does exceed the rating of Toyota’s upgraded battery by 14 CCA, and it’s much more convenient than mail order.

Update 01/18/05 — A customer has sent us a link to a website that sells the upgrade battery for $84.95 with free shipping and has verified that it will work in the Prius. They also sell a conversion kit and supply installation instructions. The do-it-yourself battery test they describe may not catch all bad batteries, but it’s far better than no testing, since the Prius will not start if the 12 volt battery goes dead. Glass mat batteries should be tested using an internal resistance type tester (as opposed to a load tester). We can properly test your Prius 12 volt battery for you, and install the battery conversion for you if you don’t feel like doing it yourself.

Here’s the link–

Toyota decided the glass mat battery in the 2001-2003 Prius did not have the necessary capacity, and issued a service bulletin and retrofit kit for installing a higher capacity battery. (The glass mat battery is the small 12 volt battery in the trunk, not the 274 volt battery array under the trunk). The new, higher capacity, glass mat battery is physically larger than the original battery, and will not fit in the original battery tray. To get around this problem, Toyota offers a conversion kit, which currently costs $123. The cost of the upgraded battery, the conversion kit, and the labor to install both ends up being $300, which seems like a lot of money to replace a pretty ordinary battery.

I began to do some research to find a cheaper way. My preliminary findings are in the e-mail below, which I sent to one of our customers who was considering replacing his Prius battery. We have not actually done the procedure I’ve outlined below yet, but I will update this page when we do if it is successful (or delete this page if it’s not). If any do-it-yourselfers out there try it and succeed before we do, drop me an e-mail and let me know how it went.

************* begin e mail ***************

I’ve attached the TSB (Technical Service Bulletin) explaining the battery upgrade and the reasons for it. $300 + for a battery replacement just seems excessive to me, so I did a little research for alternate solutions. Your Prius has a glass mat battery that is is sealed with a vent tube that runs outside the car. This is necessary when a battery is installed inside a car rather than in the engine compartment. The Mazda Miata has a similar arraignment and also uses a glass mat battery with a vent tube, so I wondered if the Miata battery might fit into the original sized battery tray.

Here’s what I found: The original battery should have the following dimensions:

197mm (7.75″) long
129mm (5.07″) wide
203mm (7.99″) tall

I got these specs off a bulletin board, as they are not available from Toyota, so they should be verified (by measuring your existing battery) before ordering a battery.

The replacement Miata battery has the following dimensions:

7.5″ long
5.0″ wide
7.25″ tall

This battery should fit neatly into your existing battery tray without buying the $123.35 conversion kit. It will need a 3/4″ height adapter, which should be easy to fabricate.

The original battery’s CCA (Cold Cranking Amp) rating is 272 CCA.
The oversized Toyota retrofit battery has a rating of 356 CCA.
The Westco Miata battery has a rating of 475 CCA.

Since bigger numbers mean a better battery, the Westco should outperform both the original battery and the improved retrofit battery, and fit in the existing tray as well.

There may be some downsides to this idea, so here’s a list of pros and cons:

1) more powerful battery
2.) no need to buy retrofit kit
3) the battery costs less.

1) my original battery size specs may be wrong or the vent configuration could be incompatible.
2) Toyota dealers will almost certainly make a lot of noise about it being the wrong battery if you ever bring your car in.
3) future owners may find the battery situation confusing.

Last Updated on Friday, 24 December 2010 11:58