Hi,

Could you please suggest best way to make a lithium battery pack (3S) charge level indicator with 3-4leds? Simply each led indicating percentage of the battery?

 

Welcome.

 

Welcome to AAC!

Voltage comparators. Look at the LM3914 datasheet for ideas.

From Battery University:

EDIT: fixed URL

 

The power at a full charge produces a much higher maximum output current. Then the lower-voltage battery voltage will drop when a high current is needed. Then the battery voltage does not show the remaining capacity.

Instead you must measure how much power (V x I) is needed for a reasonable low charge to a full charge and how much power (V x I) is being used now.

 

Voltage (static measurements) only is a poor proxy for lithium battery SoC. Three leds are sufficient. Full -> Good -> Dead. Those cheap percentage indicators are good for that.

https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=8843918

The state-of-charge (SOC) describes the amount of energy left in BESS [12], [13]. SOC is not a physical quantity that can be measured directly. The SOC can only be estimated by measuring strongly correlated proxy quantities such as voltage, current, and temperature [14] and is usually expressed in a percentage in relation to the rated capacity. In the literature, SOC is defined as the ratio of the available amount of charge to the maximum amount of charge of the battery [15].
Equation (1) shows the mathematical definition of SOC.

SOC = Qavailable/Qrated

Despite the straightforward SOC definition expressed in (1), the accurate estimation of SOC for LIB is extremely non-trivial. The reason behind this is because of the rated capacity, Qrated which does not reflect the true capacity of the battery as suggested by battery manufacturers

You really need to track energy in and energy out.

 

I think voltage is a reasonable guide to capacity. 3.0 = 0, 3.6 = 1/2, 4.2 = full
The 0.6 V from 3 to 3.6 has less watt-hours than from 3.6 to 4.2, but indicators for Full - 3/4 - 1/2 - 1/4 are good enough. Measuring real energy is worthwhile for a renewable energy situation where watt-hours are precious, but for most portable things a rough idea of charge status is enough.

 

We are building small dedicated battery pack with 3s lithium cells, so called 12.6V. All that is sorted. Current draw is relatively low.
I just need approximate, voltage measuring is fine, level indicator. Current draw of scheme is more or less not important I will just add NO momentary button.
LM3914 was the first to show up. But is not available from my supplier.