Hi,
Removing power to HU is a way to reduce drain on battery so that maximum power can be directed towards starting of the engine. The ignition switch of the car turns off the accessory line that wakes up the HU when you take it to crank position. This causes the HU to turn off.
When the battery is in good condition, leaving HU on will still not cause a serious issue. But as battery becomes weak, there could be a situation that the car could have been started if the HU was not on, but the drain of the HU makes it difficult.
But in my own experience, once battery is in such a state, it is only a matter of a very short amount of time before you won't be able to start the car even with HU off. Moreover, the current consumption is high by the power amplifier. Other than that, the HU consumes very small current (under 1A mostly). That is why, in some cars, the HU itself does not power off, but the power amplifier's power is cut during cranking.
All said and done, HU rebooting due to cranking is quite annoying. Especially with today's scenarios where sometimes you are using navigation, or playing songs from phone using bluetooth etc, the process of getting back to where you were can take a long time.
So, I would want my HU not to reboot. But if I make such modifications, what I would do to reduce the drain on the battery when cranking is to make sure that the volume is turned down, so that the power amplifier is not pulling a lot of current. Especially if you have a sub-woofer fitted. Sub-woofer consumes a very high amount of current when producing powerful base sounds.
Following solutions are possible (and many of you have suggested most of these) with their side-effects:
1. Separate out the wake-up line of the HU and fix a switch. Manually turn on and turn off the unit as you desire. The downside is that you have to do it manually. Also, if you forget to turn it off, the HU will drain battery.
2. A direct capacitor with a diode. This keeps the HU awake for a few seconds (duration of the cranking). This was the first solution I had tried. But I had to use a large capacitor (470uF or larger). The size of the capacitor will depend on how much current your HU consumes from that wake-up line. After some time, the diode got spoilt due to repeated in-rush current of charging the large capacitor.
3. Then I devised a simple delay circuit. I am attaching the diagram. In this, the capacitor is small (100uF). So, the diode did not get spoilt till I sold the car about three years after implementing this. To enhance it further, a small value resistor (~4.7 ohm) could also be added between the diode charging the capacitor and the capacitor.
Hope this helps.
26th July 2021, 13:52
