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I am newby in electronics. I planning to design a 4s2p (3.7 V, 3000mAh) li-ion battery pack and a charger. I want to use TI BQ series (BQ24618) ICs for charging. I know we can charge battery pack with 14.8V+ DC.

Can we charge the battery pack also from a USB (5V) input. I would like to know how is it possible to charge a large battery pack like 4s2p from USB.

Multicell BM Unit

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  • \$\begingroup\$ With enough electronics and a long enough charge time, yes. The principal thing you need to add is called a "boost convertor". \$\endgroup\$
    – user16324
    Commented Dec 31, 2015 at 10:53
  • \$\begingroup\$ Li-Ion does not like trickle charge much, and you probably need a balancer anyway. \$\endgroup\$
    – Turbo J
    Commented Dec 31, 2015 at 12:35
  • \$\begingroup\$ When you say 4s2p (3.7 V, 3000mAh), what do you mean? Is each cell 3.7V 3Ah? \$\endgroup\$
    – user57037
    Commented Dec 31, 2015 at 19:24
  • \$\begingroup\$ Hi, yes. 3.7V and thanks to you all for comments. \$\endgroup\$
    – juds
    Commented Dec 31, 2015 at 23:26

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You can certainly charge a battery like this from a USB port -- all you need to do is boost the voltage to the appropriate charging voltage.

With a 4s pack, the trickle charge voltage should be approximately 4.2V * 4 or 16.8V. So, you'd have to boost your 5V supply up to 16.8V, which isn't an extraordinary demand -- a boost controller IC or module can do this for you.

Here is where I think you may need to consider practicality over feasibility (e.g. yes it is technically possible, the best kind of possible :), but is it practical?). If you want to charge from a 'regular' USB port, they are specified for 5V @ 0.5A, or 2.5W. When applying 16.8V, 2.5W means you deliver a current of approximately 148mA. It will take quite some time to fully charge 8 cells at 3000mAh each, considering you can deliver at best (neglecting conversion losses) 2.5W.

So yes, it's possible to do what you ask, but I don't know if it's the most practical or not.

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  • \$\begingroup\$ Thanks. Get it. What about this design. multicell bm unit \$\endgroup\$
    – juds
    Commented Dec 31, 2015 at 23:30
  • \$\begingroup\$ I will admit I don't really have time to deep dive into this problem, but if it helps, think about it in terms of power. A conventional USB port will source 2.5W, no matter what. That's all the power you have available to run your conversion and charging circuitry, and then dump into the battery. \$\endgroup\$
    – Krunal Desai
    Commented Jan 1, 2016 at 0:12
  • \$\begingroup\$ Or up to 7.5W from a "dedicated charging port" or similar. \$\endgroup\$
    – user57037
    Commented Jan 1, 2016 at 4:06
  • \$\begingroup\$ By the way, charging Li ion cells with a single boosted voltage should never be mentioned without explicitly mentioning some mechanism to 'balance' out the cells. \$\endgroup\$
    – paki eng
    Commented Jun 15, 2020 at 10:10
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It's better to charge individual cells with a Li ion charge controller, if you are using a USB port. Because a USB port may not have enough power to be boosted to 16 V. If you are using a battery pack like the common ones used in RC stuff, you definitely have access to individual cells.

You can use a mini charge controller like this or this. But make sure you connect only a single cell at a time with a single source. If you try to connect more than one cells in series to different ports of the same computer, it creates a short circuit path and you will burn one or more of the charge controller, the USB cable, the USB port or your computer's motherboard.

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  • \$\begingroup\$ Can anyone explain why it was downvoted? \$\endgroup\$
    – paki eng
    Commented Jun 15, 2020 at 10:50

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