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I'm confused with the various websites and advice on this. I want to use 3×18650 cells (3.7V, 2200mAh each) connected in series to supply my device with +-11V. To charge them, can I just connect my pack to a 12V DC power adapter (2A), or do I need some additional components?

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    \$\begingroup\$ (not quite) instant death! THere is a large amount on this site on charging LiIon batteries, how to do it well , how to do it badly, and why doing it badly is a bad idea. | 3S LiIon needs 12.6V for a full charge. 12.0V is actually potentially good for them. BUT there is more to it than that. Search this site and look at www.batteryuniversity.com and then ask re what you still need to know. \$\endgroup\$
    – Russell McMahon
    Mar 11, 2017 at 12:29
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    \$\begingroup\$ Look through these and these \$\endgroup\$
    – Russell McMahon
    Mar 11, 2017 at 12:31
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    \$\begingroup\$ Indeed, NO. When your batteries are quite empty too much current might flow and that will damage your batteries. A proper charging circuit is needed. See the list of related questions on the right --> and start reading. In my opinion no one should even be allowed to touch unprotected lithium based cells before understanding the basics of how to use them. \$\endgroup\$ Mar 11, 2017 at 12:31
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    \$\begingroup\$ Perhaps this is actually a very useful question that needs a good and "authoritative" answer. That answer being a very long version of NO. I'm sure a lot of people are asking this. \$\endgroup\$
    – pipe
    Mar 11, 2017 at 12:34
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    \$\begingroup\$ Learn all about batteries: batteryuniversity.com/learn Learn why you likely do not have 3.7 volt, 2200mAh Lithium Manganese Oxide batteries. A why it is more likely to be a 3.6V Lithium Cobalt Oxide. \$\endgroup\$ Mar 11, 2017 at 13:14

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You can not.

First off batteries need to be charged with current limiting. If you just give it unregulated access to current the batteries will get hot and potentially catch fire.

In addition to that a series arrangement of batteries must be balanced charged so that no one cell is much higher than the other. A danger is that even if the average voltage is within the acceptable range one of the cells might be higher and get overcharged which can make it, you guessed it, catch fire.

Such arrangements of batteries need a proper balance charging device that has access to not just the + and - at the end, but also taps between each battery in the form of balance leads.

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