How to design a simple circuit for detecting unbalanced Li-ion parallel cells?

Question

I would like to design a "safety" device able to tell me if a 3 parallel Li-ion cells module is voltage balanced or not.

EDIT (after @winny's comment): It’s needed if the user connects cells with different SOC/voltage, for e.g. if the cells are easily removable from the battery pack (in order to be easily replaced if needed).

Expected functionalities

N.B. Here I present the full functionality I would like to perform at the end in order to give you the global overview BUT this is not a DO IT FOR ME question. I just ask help for detecting the anomalies (not balanced cells and reversed cells) and then close a switch (starting from that, I will probably succeed in light some LEDs myself, hopefully...).

Considering 3 cells C1, C2 and C3 with corresponding voltages (potential difference at cell terminals): V1, V2 and V3:

1. I want to detect if one of the 3 voltage differences (i.e. V12=V1-V2, V13=V1-V3 and V23=V2-V3) is higher than a threshold Vmin (e.g. Vmin=0.01V).
2. If yes then light a red LED corresponding to the not balanced cells (at least 2 LEDs).
3. If no, then close a 3 normally open switch contacts in order to connect all the cells in parallel and to allow current flow from the parallel module (global module current around 20 A max).
4. Also it would be interesting to check if one cell voltage (e.g. V1) is negative in order to detect a possible cell reversion by the user. So, if the 3 cells are well balanced but V1 is negative, that means: all the cells are reversed and so don't close the switches and light the 3 LEDs. Else, if one cell is reversed, it should be detected at step 1.

My questions are:

1. Is it possible to design this kind of device with few elementary and cheap components (e.g. op amp, resistor, diodes, transistors, etc.) ? Again, I don't ask you to tell me how to design the full functionality but for example if you can tell me how to close a switches after detecting the anomalies (not balanced cells and reversed cells), it will be perfect for me
2. Do you know about a commercially available devices which do quite the same (or could be easily adapted or twisted)?

First naive pseudo-proposal

As neophyte, I thought use an op amp as differential amplifier in order to detect the voltage difference and then close a switch. I fund this kind of configuration which do closely what I want to achieve for one voltage comparison (see explanations at the end):

The problem for me is the need of a lot of components (ressitors, diodes, transistors, etc.) just for one voltage comparison. So I need 3 times this circuit for the whole detection... Any idea of a much more simpler solution?

Circuit explanations (from here)

Here the circuit above acts as a light-activated switch which turns the output relay either “ON” or “OFF” as the light level detected by the LDR resistor exceeds or falls below some pre-set value. A fixed voltage reference is applied to the non-inverting input terminal of the op-amp via the R1 – R2 voltage divider network. The voltage value at V1 sets the op-amps trip point with a feed back potentiometer, VR2 used to set the switching hysteresis. That is the difference between the light level for “ON” and the light level for “OFF”. The second leg of the differential amplifier consists of a standard light dependant resistor, also known as a LDR, photoresistive sensor that changes its resistive value (hence its name) with the amount of light on its cell as their resistive value is a function of illumination. The LDR can be any standard type of cadmium-sulphide (cdS) photoconductive cell such as the common NORP12 that has a resistive range of between about 500Ω in sunlight to about 20kΩ or more in the dark. The NORP12 photoconductive cell has a spectral response similar to that of the human eye making it ideal for use in lighting control type applications. The photocell resistance is proportional to the light level and falls with increasing light intensity so therefore the voltage level at V2 will also change above or below the switching point which can be determined by the position of VR1. Then by adjusting the light level trip or set position using potentiometer VR1 and the switching hysteresis using potentiometer, VR2 an precision light-sensitive switch can be made. Depending upon the application, the output from the op-amp can switch the load directly, or use a transistor switch to control a relay or the lamps themselves.

Answer 1

I think there is a problem with your approach: If the cells are at a different voltage, then the switch doesn't close and the user cannot use the product.

Instead, let me propose a completely different approach that, instead of just detecting the unbalance, it solves the problem.

Each cell has its own DC-DC converter (such as a step-down converter) with low-voltage cut-off (to protect its cell) and with current limit (to ensure load sharing if the load draws more current than an individual converter can provide). Advantages:

1. It uses all the charge from each cell
2. It accepts cells at different SoC without damage
3. It works with any number of cells, 1, 2, or 3
4. The load is powered by a constant voltage, regardless of SoC levels

Answer 2

The idea

The simplest solution is for the switches themselves to sense the battery voltage value and polarity and react accordingly.

The implementation

(Schottky) diodes can act this clever way.

Discharging. So my suggestion is to connect (as many as you want) batteries via decoupling diodes (diode OR) to the load.

^{simulate this circuit – Schematic created using CircuitLab}

If there is a difference in battery voltages, the battery with the highest voltage will run first until it equals the next and then the last. A reversed battery will be disconnected.

Charging. And v.v., you can charge the batteries in this clever way by connecting the charger via reversed decoupling diodes ("diode expander") to (as many as you want) batteries.

^{simulate this circuit}

If there is a difference in battery voltages, the battery with the lowest voltage will be charged first until it equals the next and then the last. A "little" problem is if there is a reversed battery since first it will be fully recharged:-(

More about diode properties

It is interesting to see what is this "magical" property of diodes that allows making so simple and attractive circuits. It can be explained "functionally", that is, without considering the internal structure of the diode.

"Comparing switch". From this conceptual point of view,

• a diode can be considered as a combination of two devices - a comparator and a switch,
• the comparator controls the switch,
• the two terminals of the comparator are connected in parallel to the two terminals of the switch,
• so the whole combination is 2-terminal as well.

^{simulate this circuit}

Switchable voltage sources. The most interesting diode applications are where diodes are connected between voltage sources and switch them.

The simple rule is that when the source with the higher (positive) voltage is connected to the anode and the source with the lower positive voltage to the cathode, the diode connects them (assume the negative source terminals are grounded) and a current begins flowing from high to low voltage.

When a few voltage sources are connected through diodes to a common point, they begin to influence each other through the diode switches - the high voltage source turns off the other sources.

Answer 3

This will never work properly.. it is generally accepted that you need a fet+load resister between each cell plus selection circuitry. This tends to add up to too many parts for small packs, which is why battery management chips exist. Search these out, the associated app notes are also a good education. Note that most of these are designed for packs up to 11 cells...so 3 cells may involve a little thought to use these.

If these are small cells, IMHO you are wasting your time with this approach. As long as you don't deep discharge a lithium cell or leave it on float so it gets hot, you won't see much degradation with 3 cells. Lithiums don't like: water, low/high temps..too long on float...The quality of the cells you buy also has alot to do with this.