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I'm making a battery out of two 3.75V Li-Ion cells to go into a speaker system that powers an amplifier. I was wondering on the PCM, how many volts would be max to charge these cells? The cells I have are from an old laptop battery and the batteries still work fine. I need a way to charge the batteries without removing them from their casing and still power the amp and everything always in closed. I'm sorry if I'm unclear at all.

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  • \$\begingroup\$ Be aware that bare Li-ion cells have no protection circuitry; you will need to design it into your circuit. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 13 '14 at 3:34
  • \$\begingroup\$ Recycling laptop batteries if you don't know what you are doing can be very dangerous. Liion batteries have a tendancy to "vent with flame" if mistreated, including overcharging, draining too far, shorting or overheating. Read up on liion safety before proceeding. That reading will include appropriate charging methods. \$\endgroup\$ – Grant Apr 13 '14 at 3:36
  • \$\begingroup\$ Get a system power + 2 cell Lipo balance charge module, with undervoltage lockout and fuse built in. If you design your own PCB there are numberous ICs from Texas instruments in the BQ24*** series of Lipo charger ICs which include system powering. Do not do that unless you are experienced in circuit design and electronics in general though. \$\endgroup\$ – KyranF Apr 13 '14 at 3:59
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The answer to your question is: 8.4 volts. This will give you an idea of the voltages you will be working with.

However, first read and please understand the comments about safety of these li-ion cells. They are an excellent source of power, but also a good way to hurt yourself and start a fire. The li-ion cells from the laptop are VERY sensitive to overcharge, over-discharge, over heating, and physical damage. As a result you must built or incorporate a battery management system into your charger that will deal with these issues.

To get you started: Li-ion cells are charged via a constant current/constant voltage system. You start by charging them at a constant, controlled current (less than 1C) until the voltage of the cell becomes 4.2 (or 8.4 with 2 cells in series), then you hold the voltage constant at 4.2 and allow the current to drop as the li-ion battery fills up. Charging stops when the current drops to a pre-determined rate (for example 1/10 of the starting current).

Li-ion batteries do not need a trickle charge, and in fact a trickle charge is bad for the battery.

Also, when charging more than one Li-ion in series, after many charges, the batteries will start to become unbalanced. This means that that each individual battery in the series will not be the exact same voltage. This could result in over-discharge in one or more cells during use, or over charge in one or more cells when being charged. The end result is a high likely hood of venting, fire and bad things. So you need to add a Balancing charger if you intend to charge the two batteries in series.

So..... a little more than just set a voltage! Hopefully this will give you an idea of where to get started, and give you a healthy respect for the potential danger of this technology.

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