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Harsha Alva
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Mechanical Engineer + Software Developer (Project Leader)
 · MangaluruIndia
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TUTORIAL

12V battery pack from 18650-size Li-ion cells

Warning
Do not try this at home. There is a serious risk of fire and/or explosion.
I’m not responsible if you were injured while doing this.
Unless you know what you are doing, don’t try this, like seriously.

 

See this thread to extract cells from laptop batteries.

With the 6 previously extracted Li-ion cells in hand, let’s start making a battery.

First, a few things to keep in mind:

  • A Li-ion cell has a nominal voltage of 3.7V, a low voltage of 3.2V and fully charged voltage of 4.2V.
  • To get a nominal voltage of 12V, we need at least 3 cells in series. This configuration is represented as 3s.
  • Battery 101: Series connections add the voltage of the batteries but keep the same capacity, whereas, parallel connections add the capacity, but the voltage will stay the same.
  • To maintain all the cells in the battery at the same state of charge, we need balancing

 

Battery Design

As stated earlier, to get 12V output, we need 3s configuration.

To increase capacity, we can stack cells in series. In our case, we choose a 3s2p configuration.

For added protection, a Battery Management System (BMS) circuit is added to the battery. You can find these in any consumer goods that uses a Li-ion battery, like a cell phone.
The BMS adds overcharge, over discharge, over current and short circuit protection.

A typical BMS is shown below.

Unfortunately, low-cost BMS don’t charge the cells in a balanced way. For that, we need an external charger. To reuse the BMS and to support an external charger, we will add 4-pin JST-XH connectors to the battery (male) as well as the BMS (female).

We don’t need the summation output from the battery as we’re taking the 12V output from the BMS.

Finally, we can add a female XT60 connector to the BMS output.

 

Links to components

BMS
JST-XH
XT60

Electrical Engineering
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Harsha Alva

Very useful explanation on Battery packs and their charging considerations

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Ravi Pujar
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Embedded systems engineer, Founder @Valetron Systems, Blogger @www.raviyp.com
 · DharwadIndia
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Harsha Nice post. Very good information

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Harsha Alva
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Mechanical Engineer + Software Developer (Project Leader)
 · MangaluruIndia
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Thanks Ravi

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Steven Reubenstone
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Mechanical Engineer & Tinkerer of Things.
 · New YorkU.S.
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Tutorial of the Week awarded to Harsha Alva!

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Harsha Alva
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Mechanical Engineer + Software Developer (Project Leader)
 · MangaluruIndia
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Thanks Steven :)

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