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Thanks for looking. I would like some advice on a charging/power circuit for a raspberry pi laptop I would like to make. It needs to power both the Rpi 3 and an LCD display. The LCD is a 10" 1280x800 with its own controller board. I am not sure it will run off of 5.2V like the pi so I have a couple of designs.

These are based on the TP4056 board to charge the 18650 Li-ion batteries, and an XL6009 boost converter to get 5v+ from the 3.7V batteries.

Question: could you please look at my three designs below and provide advice on the "best" one? Option 1 (not shown) would run both Rpi and LCD from the same XL6009. Option 2 has two XL6009s from the same battery pack. Option 3 is similar to the Adafruit design I believe. BTW, there will be a toggle switch between the batteries and the XL6009s.

Also, can any of these be charging and running the laptop at the same time?

circuit idea #2 circuit idea #3

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  • \$\begingroup\$ Advice is "don't". The Raspberry pi series is based on set top box chips that are meant to have AC power, hence both the chips and the boards lack the kind of power management you'd want in a battery powered device. With an appropriate chip, you wouldn't be boost converting to 5v only to convert back down to run logic (you'd only need that for USB peripherals), and you would have good sleep modes. \$\endgroup\$ – Chris Stratton Feb 13 '17 at 17:55
  • \$\begingroup\$ Before starting a power management design, it is advisable to determine power (voltage/current) requirements for all functional blocks. Please do so first, for RPI (including possible USB devices), and for the particular LCD. \$\endgroup\$ – Ale..chenski Feb 13 '17 at 20:13
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All three approaches are deficient. Weak points:

  1. Manual toggle switch will fail to manage battery discharge phase, so the batteries will likely be accidentally overdischarged and die soon. The switch should be designed to work automatically.

  2. Connecting two cells in parallel requires careful cell match, otherwise the charge/discharge can be unbalanced, and lead to excessive self-discharge and therefore to reduced battery capacity.

What you need is a complete PMIC - power management IC, which contains both Li-Ion charger AND "power-path management" (automatic toggle). Here is an example, from SparkFun.

If a single 18650 cell does not have enough capacity to run your device, use two or three cells, but in-series, not in parallel. In this case you need to use PMICs designed for balanced charge of multiple Li-Ion cells. Look up Digi-key or Mouser for 2-3 cell PMIC/chargers, pick the price point/brand you like, then go to manufacturer website and get application notes.

And yes, if you use a right PMIC and if the delta between supply current capability and battery charge current (which is set by resistors) is enough to run your device, the PMIC should be able to charge the battery and feed your device. But you should check with IC specifications.

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  • \$\begingroup\$ Ok. What you said makes sense. I will be careful with matching batteries in parallel or only use one. Question: Is the TP4056 suitable if I ONLY use it when the load is disconnected (physical toggle switch to disconnect batteries from XL6009)? \$\endgroup\$ – dr_fallout Feb 14 '17 at 15:06
  • \$\begingroup\$ Actually, I could just charge single cells on the bench with one TP4056 per cell, then pop them into my enclosure to power the XL6009s? \$\endgroup\$ – dr_fallout Feb 14 '17 at 15:08
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Ok. I have the circuit working. The key is this: newer TP4056 boards come with battery protection circuitry and separate pads for battery and load. They support pass-through charging and battery low-voltage protection (as far as I can tell). I have not rigorously tested the circuits yet, but they appear to be working fine.

I went with option 3, but will explore option two in the future to give the LCD more amp-hours.

BTW, the LCD driver seems to work fine at 6V. It will not turn-on at 5V unfortunately. Some smaller screens do work at 5V, but I like the larger one. It pulls about an amp at 5V.

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