Help with this Lithium Battery Balancer idea.

Thread Starter

Khaleef

Joined Mar 22, 2017
105
Good day all.

I would like to know if this would be a viable idea.

I deal a lot with Lithium batteries and as y'all migth know, these battery design are made of several cells in series of which they have to be equal in voltage at all time to ensure the effectiveness and safety. For this, there's a need to have Active Balancers hooked up to these cells.

I know there are already made Balancers in the market but as an electronic enthusiast, I am thinking of designing mine.

In a previous thread, I asked if anyone could help with a Bi-directonal circuit to switch cells(connected in series) of a battery into an individual small toroidal transformer and then the AC output of the transformer can be connected in parallel but many said I couldn't achieve that.

So here's a new idea I got...

I am looking to switch a battery voltage (say 12v, 24v or 48v DC) into a primary side of a single ferrite transformer or toroidal core and the secondary would be wound with several strands of copper (the number of cell in series) with each output of the cell voltage in AC.

So then I can rectify each output back to DC and then connect them to each cell. What this would mean is the total battery voltage is divided to charge up each of the cells it's made of.

And because the output are of exactly same voltage, the almost full cells takes less current while the lower take more.

Sorry for the long epistle, I just wanted to explain clearly. I appreciate the contributions.

cc: @crutschow

***Edit
I found this research work about this exact design I wrote up there.

https://www.semanticscholar.org/pap...sche/f1094f2c1c4b6be96856aff8d59982b953660b93
 
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Alec_t

Joined Sep 17, 2013
14,330
What this would mean is the total battery voltage is divided to charge up each of the cells it's made of.
But, because of circuit inefficiencies and the Law of Conservation of Energy, the amount of charge you could put back into the cells would be less than the amount withdrawn from the switched battery.
 

Thread Starter

Khaleef

Joined Mar 22, 2017
105
But, because of circuit inefficiencies and the Law of Conservation of Energy, the amount of charge you could put back into the cells would be less than the amount withdrawn from the switched battery.
Yes, I am aware of that. The goal is to have same voltages on all cells.
 

BobTPH

Joined Jun 5, 2013
8,998
You seem to say you will
connect a battery to the primary of a transformer and the secondary would produce AC. Is that your understanding?
 

Thread Starter

Khaleef

Joined Mar 22, 2017
105
You seem to say you will
connect a battery to the primary of a transformer and the secondary would produce AC. Is that your understanding?
Not just "connect a battery..."
I already know how to switch voltage into a transformer using one of the several oscillator ICs out there.

What waveform does a transformer output please?

At least, I can build a crude square wave DC-AC transformerless inverter, so I know what I mean when I say "connect a battery..." to the primary side of a transformer.
 
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Thread Starter

Khaleef

Joined Mar 22, 2017
105
You seem to say you will
connect a battery to the primary of a transformer and the secondary would produce AC. Is that your understanding?
Besides, my OP says "... switch a battery voltage..." not "Connect". I may be a Noob but I know what either terms mean.
 

BobTPH

Joined Jun 5, 2013
8,998
Besides, my OP says "... switch a battery voltage..." not "Connect". I may be a Noob but I know what either terms mean.
You need more than simply switching it, you need to invert the current every cycle. This is either done with a center tapped transformer, alternating the two windings, or with an H-bridge. Perhaps you understand that, but it was not obvious in your post.
 

Thread Starter

Khaleef

Joined Mar 22, 2017
105
You need more than simply switching it, you need to invert the current every cycle. This is either done with a center tapped transformer, alternating the two windings, or with an H-bridge. Perhaps you understand that, but it was not obvious in your post.
I knew it would be with a centre tapped transformer. Like I said, I can build a simple DC-AC Square Wave inverter which also make use of the center-tapped tranx design so I already got that part in my head.

The purpose for my thread was if the idea was going to work and maybe some extra points I could pick up.
 

BobTPH

Joined Jun 5, 2013
8,998
Your idea might possibly be made work, but it would be very inefficient, as others have said.

Ask yourself this though: How is it better than current solutions?
 

Alec_t

Joined Sep 17, 2013
14,330
Your proposed system, if I've understood it correctly, would attempt to feed back charging current to the cells at the same time as those cells are driving an inverter. A cell can't be charged and discharged simultaneously, so I don't see how the system could work in practice.
 

Thread Starter

Khaleef

Joined Mar 22, 2017
105
Your proposed system, if I've understood it correctly, would attempt to feed back charging current to the cells at the same time as those cells are driving an inverter. A cell can't be charged and discharged simultaneously, so I don't see how the system could work in practice.
Yes, the cells would get charged and migth be discharged at the same time. But what charges the cells is the same battery they are made of.

With my understanding, the readily available active Balancers in the market also "charges" lower cells from higher cells and same battery goes discharging at the same time.

The difference between the above and mine is that the "cells are charged" from the total voltage of the battery of which they are made of.
The process would be same as running an inverter off a battery which is hooked up to Solar system. It charges and discharges at the same time.
 
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michael8

Joined Jan 11, 2015
415
[disclaimer: no lithium battery experience]

battery design are made of several cells in series of which they have to be equal in voltage
at all time to ensure the effectiveness and safety.


My understanding was that the requirement was to keep each cell voltage between the maximum and minimum
voltage -- I see no requirement for the cells to have equal voltages. If the cells are similar with the same capacity
and start out with the same amount of charge they should track voltage pretty well though.

There are two cases, too high (during charge) and too low (during discharge). Perhaps I can add something to your
charging scheme:

I am looking to switch a battery voltage (say 12v, 24v or 48v DC) into a primary side of a single ferrite transformer or toroidal core and the secondary would be wound with several strands of copper (the number of cell in series) with each output of the cell voltage in AC.

So then I can rectify each output back to DC and then connect them to each cell. What this would mean is the total battery voltage is divided to charge up each of the cells it's made of.

And because the output are of exactly same voltage, the almost full cells takes less current while the lower take more.


If the DC to AC conversion design is a flyback with indentical output windings then all the outputs will be clamped
to the lowest cell voltage and so it will charge that cell until the voltage rises to some other cell voltage.

If you only charged the cells this way it would take care of the maximum voltage check assuming you had an independent
way of watching each cell for full charge and stop when any are full. The problem with this is that the separate
windings/diodes are likely less efficient than a bulk charging of the whole series string (n diode drops vs 1 doode drop).

I think a lot of the balance systems just shunt current around a cell at the maximum voltage thus allowing a bulk series
charge of the cells to continue until all cells are at maximum voltage.
 

Alec_t

Joined Sep 17, 2013
14,330
Because they all pass the same current. If that current is powering the inverter then it is discharging all the cells, not charging any of them.
 

Thread Starter

Khaleef

Joined Mar 22, 2017
105
Because they all pass the same current. If that current is powering the inverter then it is discharging all the cells, not charging any of them.
Okay Alec.
Let me understand it please with this practical.

Is it possible to design a DC-DC Buck converter with multiple isolated output of exact same voltage?
 

BobTPH

Joined Jun 5, 2013
8,998
Thanks Alec
Can you please explain why?
Because current cannot flow in both directions at the same time.

That does not prohibit your scheme, however, because what would happen is some cells would be discharging abd others would be discharging.

You also asked why it would be inefficient. You have four phases in your scheme:

1. An inverter. A good is 90% efficient. zit could be considerably less.

2. A transformer which is alsobless than 100% efficient.

3. Rectification. Using a bridge, the current goes through two diodes. Using good Schottky diodes, this loses about 1V. That is out of the 3 to 4V to charge the battery, so the efficiency is, at best, 67 to 75% efficient.

4. Regulation of the charging current. This will also lose power. A 1V dropout is pretty good, and that is again 67to 75% efficient.

So, let’s take the upper bounds of efficiency, lets be generous snd say:

95% for 1
99% for 2
75% for 3 and 4

Multiply those together and you get a total efficiency of 53%.
 
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