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I am currently trying to implement an active balancer based on the LTC3300-1 (I have proposed the diagram based on the directions in the datasheet) but, about the connection for the secondaries of the transformers, I do not know what is the right potential to connect to.

I saw the circuit diagram in the LTSPICE model given by Analog Devices (Manufacturer) which seems to be a 6-Cell balancer but, they connect the secondaries to a POWER SOURCE CALLED V7 which is 43.2 V (I think it corresponds to 12 cells: 3.6 V * 12 = 43.2 V). Which lets me exactly in the doubt, the datasheet in figure 3. 6-cell active balancer module just says a label (UP TO CELL 12).

But which one is the right point if 6-cell balancer is only for 6-cell, are the connection telling us that we must have 12 cells (6 not balanced)?
How could I translate that to a 3-cell active balancer?

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greybeard
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imt_blake
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    Are you trying to balance 3 cells? Is that your question? – Andy aka Jul 21 '23 at 19:38
  • Yes I am, I trying to do it! I have everything connected in consistency with the datasheet but my doubt is with the secondary of the transformers which is marked with red! I think I should connect it to maximum voltage level of my circuit which in my case is 11.1 V, does that have sense? – imt_blake Jul 21 '23 at 19:54
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    Yes, that has sense. – Andy aka Jul 21 '23 at 19:55
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    Does make sense that we have a transformer with primary and secondary with to the same voltage level? I think yes, given I performed simulations and currents are opposite, so I will perform the suggested connection and let you know, thank you a lot – imt_blake Jul 21 '23 at 19:57

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I don't think you quite understand how this balancer works. The value of V7 is not particularly important and it does not correspond to cell voltage or number of cells. The voltage of 43.2V seems to have been chosen to yield the highest efficiency for that specific circuit so that anyone playing with it would see the best performance results (in regard to efficiency).

However, that same circuit will just as readily run off of 8V or 58V. The current draw will be more or less the higher or lower the voltage and thus the overall efficiency will change some, but it will still balance the cells without any issue.

It is a little irritating as the datasheet mislabels those coupled inductors as transformers, as they are not being used as transformers, they're being used as coupled inductors. Transformers transfer energy magnetically, while an inductor stores energy magnetically to be released at later point in time. They're two very different things.

if you take an inductor and add a second winding (or simply split the existing one) such that you have two windings on the same core, you still can use it just like an inductor. The only difference is the stored energy will be released through whichever winding(s) have a conductive path to receive the stored magnetic energy.

This split inductor arrangement is what a flyback "transformer" is. It's just an inductor with extra tricks, and works by storing energy in a magnetic field and then releasing it through a different winding. This provides isolation and, in the case of this chip, has been cleverly used to make what is effectively a 2 switch buck-boost converter for each cell.

By varying the duty cycle ratio of a given flyback transformer's two windings, it can buck or boost the input voltage to whatever is needed. The main limiting factor of the input voltage is the MOSFETs you select, but the chip itself will only see ~22V or so (up to 25V depending on the state of the cells) from C6 to V-. The max rating across C6 and V- pins is 36V, so it's good it can buck down a voltage like 43.2V which would destroy the chip if connected directly across those two pins!

This is a really neat chip and the nice thing about it is you have a huge range of voltages that you can potentially use. The main constraint is efficiency and staying below current limits.

As for the schematic that mentions 'up to cell 12', they mean you can add a second chip and continue the daisy chain to as many as 12 cells. And there are examples of even more. But the LTC3300-1 is definitely a 6 cell balancer and they want you to buy several of them for more than that.

For 3 cells, just omit the the unused sections from the circuit.

metacollin
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  • thank you for the nice and detailed explanation! I will give you the award in 1 hour (it displays a message about waiting an hour until being able to give the award) One more question please, I performed the simulation about a 3-Cell (Array) and I deleted the V7 source and I shorted circuit the Primary and secondary windings (coupled inductors as you said) of the V6 cell and the results seems to be reasonable, does that have sense? – imt_blake Jul 25 '23 at 14:41