Hybrid coolant and the inverter
Hybrids such as the Prius, Aqua, Insight, Fit Hybrid, Vezel and Accord Hybrid are ordinary cars in Georgia by now. But many owners do not know their car has two independent cooling systems, and have never checked the second one.
The second reservoir nobody notices
Under the bonnet a hybrid has two expansion tanks. One belongs to the ordinary engine system, the other serves the inverter, the power control unit and often the electric motor.
That second circuit runs on its own electric pump and frequently has its own small radiator. It is not connected to the engine system at all.
Honda advises checking the inverter reservoir level every time you refuel. In practice almost nobody does.
Why this circuit is more sensitive
There are three reasons.
The passages are narrow. An inverter heat exchanger uses far finer passages than an engine radiator. A deposit that would pass unnoticed for years in the engine system registers here immediately.
The temperature window is narrow. Power electronics run cooler than an engine and tolerate overheating less well.
The consequence is expensive. An overheating inverter first limits power, then throws a fault. Replacing an inverter or PCU is among the most expensive hybrid repairs there is.
What to look for in the fluid
For this circuit what matters most is that the fluid leaves no deposits. Zerex Asian Blue has three documented properties here:
- Silicon under 10 ppm, where the JIS limit allows 250 ppm. Silicate dropout is the principal risk here.
- A phosphate package that works against deposits and scale.
- Hard-water compatibility, which reduces the risk if you used tap water.
And one practical piece of advice: on a hybrid, use distilled water anyway. It is a five-lari decision that reduces a very expensive risk.
Leave the actual inverter fluid change to a dealer or hybrid specialist, because bleeding that circuit is a procedure of its own.