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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

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    \$\begingroup\$ 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 \$\endgroup\$ – Brian Drummond Sep 28 '16 at 23:30
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    \$\begingroup\$ Good point Brian, I will link to a PCB from a reputable vendor \$\endgroup\$ – Jens Roland Sep 29 '16 at 8:47
  • \$\begingroup\$ 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... \$\endgroup\$ – Brian Drummond Sep 29 '16 at 9:49
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1) pretty standard. Get a 4S LiFePO balancing-protection board with over/under charge protection. About $8 on eBay. This will prevent the 4S to exceed the limit voltage, and will cut off the current if undervoltage is detected. This board simply goes in parallel with the CV power supply and the laptop. The supply CV voltage must be between 14.2 and 14.4 V. You can get a 5S or 7S board and rewire it to act as a 4S board.

2) I think the best solution is a 5S 18650 LCO Li-ion (i.e. non-LiFePO). These have more charge for the same weight and same cost. This will work well with your 19V laptop charger. You would need a 5S balancer-protection. 5S is 21 V, so the charger will never reach that. If the charger goes bad, the balancer-protector (BP) board shall protect your cells. You will not need a CV charger.

3) Yes, do not use self-protecting cells.

4) The theory is that you need a precise CCCV charger AND a balancer-protection board. I think with a 5S BP board, you can do away with the CCCV charger and use a standard 19 V laptop charger.

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  • \$\begingroup\$ +1 for this answer and for the question. I am interested in this also, to put a tiny up-board fanless quad core atom up-board inside a toshiba nb100, replacing the existing logic board, so that I end up with a more powerful netbook. Would be nice to make use of the existing laptop batteries via the use of a software-defined charger controller logic board which also complies with ACPI so that it appears as the usual power status under Windows 10 running on the up-board core up-board.org/upcore. \$\endgroup\$ – therobyouknow Mar 20 '18 at 13:46

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