The hybrid battery of a car generates heat normally and it needs cooling for it to work efficiently and to also last for long. The cooling fan is attached to the battery to suck cold air into the battery compartment.

It is normal for the cooling fan speed and noise to increase for a short time and then return to a quiet state since the fan is not a constant speed fan but the speed increase as more cooling is demanded by increase of duty cycle using pulse width modulation (PWM) technique. This technique has been employed in different systems of automobile.
If the fan speed is constantly high and the fan is clearly audible, this signify that there is a problem which needs to be given attention.

Causes of hybrid battery fan noise and high speed operation

1. Weak hybrid battery

When a battery is weak, it draws a lot of current during charging. Same applies to the hybrid battery. As it is being charged through regenerative braking or the internal combustion engine, there is a lot of current drawn, causing a lot of heat. To cool it down, the fan runs continuously at high speed and which causes it to be audible. This is one way that signifies that a hybrid battery is weak.

2. Dust blocking hybrid battery fan duct

After some period of use, dust accumulates in the fans and air duct of the fan. This prevents efficient cooling and causes the computer to increase the fan speed.

3. Dirty DC to DC converter's air filter

In some new hybrid car models, there is air filter installed for the DC to DC converter. DC stands for direct current. When this filter gets clogged or dirty, fan noise and speed increases.