Battery charging for DIY laptop

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.

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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.

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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:

http://www.batteryspace.com/cmb-for-12-8v-lifepo4-battery-pack-4a-limited-with-dc-charging-fuel-gauge-and-full-protection.aspx

or alternatively, this one for 3S (11.1V) Li-Ion packs:

http://www.batteryspace.com/CMB-for-11.1V-Li-Ion-Battery-Pack-10A-limit-RoHS-Compliant-with-DC-char.aspx

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?

Sincerely, Jens

• I have a suspicion that device is not a battery charger, but provides additional protection against a battery charger (as well as balancing and short circuit protection). But I can't tell, the data is so sketchy. Which means ... no datasheet, no sale
– user16324
Commented Sep 28, 2016 at 23:30
• Good point Brian, I will link to a PCB from a reputable vendor Commented Sep 29, 2016 at 8:47
• The replacement is explicitly a charger. And the magnetic component on it (necessary for efficient voltage conversion) reinforces my doubts about the cheap eBay one. But I have a suspicious nature ... it doesn't say anything about balancing cells (though over/voltage is detected per cell) ... do you need a separate charge balancer? This company may answer questions...
– user16324
Commented Sep 29, 2016 at 9:49