BMS question

Nojus33

Member
2019-10-12 9:18 am
Hello,

I'm currently working on a DIY bluetooth speaker. The amplifier that I've chosen needs about 12V for 2x20W speakers that I'm planning to use. The part I don't understand is the 18650 battery management. Most of them have a nominal voltage of 3,7V and maximum voltage of 4,2V. If I understand correctly, using 3S2P configuration would make an 11,1V - 12,6V battery pack, which is good for the amplifier. However, most of the BMS circuits I'm able to find have an over-discharge cutoff at 2,5V. At 3S2P configuration 7,5V would be way too low for the amplfier. Would those BMS's keep the voltage between 11,1V- 12,6V or not? Or is there any way too keep the voltage somewhere around 12V?

For example:
(Title says 11,1V-12,6V, however in the description over-discharge protection for each cell is 2,5V)
3S 40A BMS 11.1V 12.6V 18650 lithium battery protection Board with balanced Version for drill 40A current-in Battery Accessories from Consumer Electronics on Aliexpress.com | Alibaba Group
 
Here you can see the specifications for the nowadays Sanyo Panasonic 18650.

https://www.orbtronic.com/content/Datasheet-specs-Sanyo-Panasonic-NCR18650GA-3500mah.pdf

Chemistry starts to be weak below 3V per cell. The 2.5V setting is just an emergency mechanism, not a convenience mechanism.

You may consider making a monitor circuit with a low power or micropower comparator IC and some resistors and capacitors. But the main question is: Does the amplifier misbehave or just shut down below a certain voltage? An adjustable power supply may be required to test that.