Hybrid HV & Auxillary Battery Types

Toyota Prius Gen 1 and Gen 2 Auxiliary Battery

Gen 1 Auxiliary Battery

Gen 1 Auxiliary Battery

Gen 2 Auxiliary Battery

Gen 2 Auxiliary Battery

The Gen 1 is easier, on the left side of the trunk. On the Gen 2, the hatch area floor cover and plastic insert must be removed to access the battery on the right side. Note that disconnecting the AGM auxiliary has been known to cause a no-start after it has been reconnected or replaced, particularly on high mileage vehicles. The reason is a dirty throttle body. The Engine ECU loses the learned memory of the throttle blade position and the vehicle will crank, appear to run for 10 seconds or so, and then shut off. The simple solution is to clean the throttle body.

Absorbed Glass Mat Battery

The AGM battery is sensitive to high rates of charge and needs to be recharged with an appropriate charger. Technicians claim to have good success with a trickle charger set on 2 amps, but it has been reported that this method can damage the battery. There are special chargers available for AGMs, such as one sold by CTEK which has a specific mode for the AGM battery. Because the battery is inside the trunk it subjects the occupants to potential gassing. Therefore the AGM battery is vented externally. It also generates less gas than a conventional lead acid battery. Gen 1 and Gen 2 Prius batteries have a non-standard post size to discourage installation of a non-AGM battery. It is hard to find an aftermarket Prius battery and the OE battery is expensive. The Gen 2 Prius lists two batteries, one for vehicles with Smart Key and one for those without Smart Key.

Mazda Miata Battery Conversion Tip

Battery Terminals

Comparison of the standard and Prius AGM battery terminals. Notice the difference of the battery terminal size.

A Mazda Miata AGM battery has the same overall dimensions except it is shorter and has conventional post sizes. The Miata battery can be purchased from Interstate Battery and other vendors as well. A spacer can be installed beneath it. A short 2×4 works and the cable ends can be replaced with marine terminals. This battery costs less and gives the customer a higher rated battery than the OE Gen 1 Prius AGM Battery. The Miata battery vent connects to the Prius factory vent tubes perfectly. Toyota has a TSB (EL014-03) for a higher capacity battery but it requires the installation of a special kit that is expensive.


The Honda 12 volt battery

Honda uses a conventional12 volt battery mounted under the hood. But it is also equipped with a 12 volt starter, a back up unit that is rarely used. Honda literature says that it is supposed to crank the engine when ambient temperatures are below approximately 5 degrees. Vehicles owners report they have not heard the 12 volt starter work even at -10 degrees. The starter is a backup for extreme conditions. The Honda will start with the HV switch turned OFF, although the check engine lamp will be ON and the DC to DC converter will not charge the 12 volt battery. The 12 volt starter can be used for compression testing. Pulling the IMA and INJ fuses in the underhood fuse block will force the engine to crank on the 12 volt starter without starting. Exercising the 12 volt starter, or at least proving that it is capable of cranking the engine is a useful service to offer customers that might be going to a skiing trip, or in cold country, when expecting a cold winter. Since the starter is rarely used, it could fail and the driver would be unaware until extreme conditions occur.The back up 12 volt starter allows Honda to avoid the need for a HV battery charger. This fact is in direct contrast to Toyota which must have a greater than 17% SOC (state of charge) in the HV battery pack or the engine will not crank. It is very difficult to run down the HV battery as the computer software is specifically designed to prevent it. However, it can be done, usually by running out of fuel and repeatedly cycling the key (or powering down and powering up, in the case of the Gen 2) to get a few more yards out of the electric propulsion system. Such cases generally require the limited availability of a Toyota HV battery charger.

Ford Escape HV Battery

The Ford Escape avoids the problem by installing a HV battery charger on board. It is powered from the vehicle’s 12 volt battery and started via a button located in the kick panel by the driver’s left foot. While it is limited to a 8 minute charge cycle, it is possible to use a jumper battery and start the 8 minute charge multiple times in order to bring. the HV battery up to the level of starting the engine. From this point the vehicle’s own motor generators can recharge the battery. Like Toyota, the Escape HV battery is very difficult to run down as the computer software is specifically designed to prevent such an occurrence.

This post was written by: Martin Hand


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About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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