I am building a DIY laptop, and I need a battery pack that will allow me to use and charge my device as you would any modern laptop, i.e. by connecting a regular 12-19V DC power brick to a panel mounted barrel jack. Specifically, I don't want to have to stop using the laptop in order to charge the batteries.
But with Lithium-Ion and LiPo batteries having a habit of venting with flame when you mistreat them, I figure I could use some advice on how to build a safe and solid charging circuitry for a DIY laptop.
The project specifics:
My DIY laptop contains an ODROID XU4 single board computer and an iPad2 replacement LCD panel. From a power perspective, the needs are:
Single board computer: 5V @ 1A continuous, spiking briefly to 3A during startup
LCD + controller: 12V @ 1A (it technically accepts 5-24V, though I've read that it's unstable at 5V)
To power the 5V rail I have a buck converter which accepts 9-35V and outputs a nice stable 5V (up to 5A) for the ODROID. So my power source should be between 9V-24V. I've found that a 12V/4A power brick works perfectly fine.
For the best safety-to-power-density profile, I would like to use LiFePO4 cells, and I figure a 4S configuration is ideal, yielding a total voltage of 11.2V-14.4V.
Assuming around 1200mAh per cell, I would need 8 cells in a 2P4S configuration to achieve 2400mAh, which by my math should give me up to 2 hours of use between charges, which I think is adequate for a 'show pony' device like this.
Now for the question:
How would you go about charging a 4S configuration of LiFePO4 cells while the device was in use?
So far, the best idea I've come up with involves this batteryspace.com listing for a combined charging / protection PCB:
or alternatively, this one for 3S (11.1V) Li-Ion packs:
If I read the listing correctly, that PCB would allow me to do 'pass-through charging' and it would keep the cells balanced and protected from overcharge, overdischarge, etc.
I would hook it up to my 2P4S cells, add a fuse between the PCB and my panel mounted DC jack for a bit of extra safety, and possibly another buck converter to allow me to use a wider variety of power bricks and still feed the correct voltage to the PCB.
But would that work? Would it actually balance the cells? Or am I missing something?
And for a bonus question: With a protection PCB like that, I assume I should be using cells that don't have their own built-in protection circuits - is that correct?