Tech Stuff

Hybrid vehicle air conditioning service

Craig Van Batenburg of ACDC explains the critical handling safety concerns when testing voltage once the HV battery has been removed from a Prius HV.
<p>Craig Van Batenburg of ACDC explains the critical handling safety concerns when testing voltage once the HV battery has been removed from a Prius HV.</p>

There are over 1.5 million Toyota hybrid vehicles in North America (and well over 2 million hybrids total, including Toyota and other brands). Even though these numbers represent perhaps less than 1% of the total North America vehicle population, it’s evident that hybrids are here to stay, and their population will increase. Based on this, it’s time to become familiar with A/C systems in hybrids.

Before attempting any service to the A/C system, you need to be aware of important battery voltage concerns. Remember, hybrids don’t feature a traditional 12-volt battery alone; a high voltage (HV) battery pack is present that, in the case of the Prius, contains from 201 to 273 volts. It’s vital to be familiar with the power system before attempting any service work. We strongly suggest obtaining HV training, either from Toyota or from an independent hybrid specialist training center.

Hybrid battery basics

While there are several variants of hybrid designs among the OEMs, here we’ll take a specific look at the most popular, Toyota’s Prius hybrid.

One of the notable features relates to the battery approach. The hybrid features two battery “divisions”: an HV 201 or 273-volt trunk-mounted AGM (absorbent glass mat) battery group, and a 12-volt auxiliary battery. If charging is required, the primary battery group (AGM) must be either trickle-charged or charged with a charger that is rated for AGM. Otherwise, irreversible battery damage could result. The 12-volt auxiliary battery (this is assigned to provide power to various 12-volt accessories) maintains its charge via a DC-DC converter (instead of using an alternator).

The 2001-2003 Prius (referred to as Gen 1) features a 273-volt HV battery, while 2004-2009 Prius models (Gen II) feature a 201-volt battery. The later generation features a NiMH (Nickel Metal Hydride) battery pack. A “boost inverter” was added to raise the voltage used by MG2 to up to 650 volts in order to boost vehicle acceleration performance.

High voltage powers the A/C compressor, electric motor, generator and inverter/converter. All other conventional automotive electrical devices such as headlights, radio, gauges, etc., are powered by a separate 12-volt auxiliary battery. Numerous safeguards have been designed into the Prius hybrid to ensure that the high voltage Lithium-ion Hybrid Vehicle (HV) battery pack is kept safe and secure in an accident. The Li-ion HV battery pack contains sealed batteries that are similar to rechargeable batteries used in some battery-operated power tools and other consumer products (but at a much higher voltage). The electrolyte is absorbed in the cell plates and will not normally leak out, even if the battery is cracked.

Prius Gen II (2004-2009) compressor with rotor removed and stator still in case.
<p>Prius Gen II (2004-2009) compressor with rotor removed and stator still in case.</p>

In the unlikely event the electrolyte does leak, it can be easily neutralized with a diluted boric acid solution or vinegar.

The HV battery pack is enclosed in a metal case and is mounted to the cargo floor pan cross member behind the rear seat. The metal case is isolated from high voltage and concealed by carpet in the cabin area. The auxiliary 12-volt battery (in the 2010 model, for example) is mounted in the right rear quarter panel area.

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