I recently purchased a x735 because I was under the impression that it could detect power loss and initiate a safe shutdown. After installing it I have noticed this is not the case. Now I am trying to find a solution to my needs and found the x728 does this. I was about to buy this and then found out that it is not compatible with the x735 on the same Raspberry Pi.
I purchased the x735 as a means to convert power from my 12v car battery to 5v so that I can use inside the car. Since it is a car battery I can give it constant power as well as switched power. Ultimately I am looking for a way to be able to trigger the safe shutdown script upon detection of the car being switched off.
It appears I may have been able to do all of this with the x729, so perhaps I purchased the wrong board.
Is there any options available if I wanted to keep my existing x735 and somehow integrate an additional hat that would achieve the same result?
12v Power Loss Solution?
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Harry Huang Staff member
Hello
X735 definitely does not support the power failure detection and safe shutdown you mentioned, because X735 is not a UPS shield. It will stop working immediately after the power failure. You cannot expect it to continue working for a while after the power failure. This is the job of the UPS shield.
I don't know if you are using Raspberry Pi 4 or Raspberry Pi 5. If you use it on RPI 4, x728 should be the best choice. If you use it on RPI 5, x1205 x1202 x1201 x1200 are all good choices. They can meet your needs.
It should be noted that for UPS shield, a good power adapter is very important. After all, the power adapter needs to charge and power the device at the same time.
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Harry Huang Staff member
I saw that you bought X729, so I guess you are using it on RPi 4. Since X728 only supports DC 5V, you bought X729 which can support 12V input, which is correct.
Thank you for your support.
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Thank you for your quick reply. I am using a Pi 4 and found the x735 while looking to replace Mausberry Circuit's 4amp Car Supply / Switch. It failed on me and I believe many others and is not available for purchase any more. It worked perfect while it did. The x735 is great as it provides a clean and efficent way of powering my Raspberry Pi within the car and I can even avoid draining the car battery using the script. Currently I am feeding the x735 Power through the DC 5521 volts with the constant power wire that goes to my car stereo.
It is very nice to know that I made the right decision to opt for the x729 as I would have likely of fried my entire board setup if I had went with an x728 and x728a and plugged it in with the wiring harness I made.
I am however concerned about the increased risk of charging batteries in my car and I am wondering, is it possible to bypass the charging functionality?
I have found out that I am lucky because my vehicle's manufacturer placed a capacitor behind the switched power of the radio harness. Because of this I have access to car battery for a few seconds after switching off the ignition.
If I were to use some other wire to provide switched power to that is not ahead of this capacitor then I would have way of telling the pi to shut down. I understand I would have to convert this 12v signal down to 0-5v and I could do that with a buck converter or a voltage divider. But this wire would have power until it suddenly loses power.
I could actually skip using the x735off script entirely if I were to do this as I could just as well connect this wire to the GPIO that would signal the pi to safe shutdown when power loss is detected on this wire. The car would cut power to the x735 when it's completely off, by using the switched power cable with a capacitor ahead of it.
I think this might be a safer approach than allowing a battery to be charged. I already have a RTC header so losing power really isn't a concern other than ensuring that the safe shutdown works as intended.
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