Question battery questions

Discussion in 'General Discussion' started by timestopMachinist, Aug 12, 2019.

  1. timestopMachinist .

    Aug 10, 2019
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    I'm making a portable emulation machine with the pi 4, and im confused about power solutions.
    im using this screen with this controller board.

    I'm new to this, if you couldnt tell. I realize that it would be difficult to power both with one battery, so i dont mind using more than one. I was going to use a lipo and the adafruit powerboost 1000 to power the pi, but am rethinking this due to lipo safety concerns. it is also worth noting that i intend to use a shim safe shutdown switch to power the pi through gpio. It seems like li ion might be the best option to power the screen, and if that seems to be cost effective enough, i see no reason not to also use this for the pi. I dont really know how i would go about setting up li lion batteries, or where to get them for the best price, or even everything i would need really, so advice is appreciated. space is not really a concern, as while i do intend my build to be portable, the case i have is very large.

    if any additional details are needed, i have basically every other detail ready, so this can be planned precisely.

    i considered posting this in the raspberry pi topic, but the actual area of question has more to do with batteries and power than the pi itself, i think.

  2. hazardousracerx .

    May 4, 2019
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    Hell (Texas)
    Take a look at this resource on the forum:
    Portables and Batteries: A Guide and Explanation

    I have previously built a RasPi portable and there is quite a bit to consider when planning layout and component placement, but you are on the right track with working out the power source first. That was the biggest PITA of the entire device because of the myriad ways to provide power and the numerous size/form factors of the various batteries. The link above will give you more info than I can provide, but my analysis of the info you provided is below. Feel free to ignore everything I said in favor of what is in the battery guide above if anything contradicts.

    TL;DR: You're gonna need a bigger battery

    That is a massive screen/driver setup with matching massive power requirements. The adafruit powerboost you mentioned will barely manage to provide enough current to power screen by itself (powerboost 1000 will only provide 1A). The screen is using the older fluorescent backlight technology rather than the newer LED backlight tech. (Datasheet for screen) The screen says it will only use 878 mA under normal conditions, but has an inrush current of 3A (this does not necessarily factor into power calculations because the driver board is providing this power).

    The input power requirement for the control board is 12V at 4A according to the Ebay listing. If you plan to use a Lipo battery meant for a drone or RC car then you would be ok (still need to donvert voltage for the Pi), but your battery life will be relatively short for a portable.

    The Pi 4 uses even more power than the earlier Pi boards coming in at 3A or as low as 2.5A if no USB peripherals are used (Source). Since you are building a portable, it is safe to assume you will be using at least one of the USB ports on the Pi so 3A.

    The driver board and Pi together will draw approx. 7A of power. Using this figure with a 6000mAh battery you are looking at about a 36-45 minute runtime depending on how you convert the 12V for the screen down to the 5V for the Pi.

    This site seems to have the best prices I have found for cylindrical lithium cells if you are located in the US. They say the verify the cells they sell, and I can confirm that the ones I have purchased from them have accurate information on the site.
    Stitches likes this.
  3. Stitches 2 and a Half Dollarydoos Staff Member . .

    Feb 5, 2017
    Likes Received:
    Banana Bender Land, Australia
    You're mostly correct, but the screen won't use 4A. That number is an ass covering strategy to stop people using just any 12v power source and complaining that the 500mAh supply for their baby monitor doesn't run the screen properly. He will definitely need a more powerful setup for those parts than a Powerboost, but 7A is a bit much.
    hazardousracerx likes this.

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