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Discussion Starter #1
We have made good progress with the LTO packs and can now obtain the cell voltages. :)
However we can't control the balancing on the BMS boards, so we need another solution.

We could periodically manually balance or install an Orion2, but that's a pita/expensive and not a fit and forget solution. :(

Proposal.

I have used very simple dumb balance boards on previous supercapacitor and lithium packs I built.
They worked very well. Remember these? I use them on my supercaps.

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They had a nice little led light as well so you could see which boards were active.

Now as supercaps have gained popularity similar 2.7V versions have appeared which start balancing at ~2.65V.
That's about right to prevent significant LTO long term over charging.

This example burns off 50ma at any voltage above ~2.65V


It doesn't have any LED and you would need two per 12 cell block, but fit and forget inside the battery case.
The power dissipation is very low heating would not be an issue.

Kindly one of the suppliers on aliexpress etc posted the circuit diagram.

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The actual BW6101 / C1AP control chip costs ~50 cents per cell, so we could roll our own 12S pcb. :)
One for each block to fit in the space we need/have available.

This is a more powerful version that could burn off a lot more power, but then you have to get rid of the heat.

84076


We could add the leds in a central group on the pcb and have a little window in the top of each block so the leds are visible.


Anyway food for thought, it seems like a simple solution to me.

When you want to balance the pack you simply charge it at <50ma for 24hrs or so.
Full cells burn off the excess, lower cells slowly catch up.
It's basically the same as NIMH overcharge self heating, but we are burning off the excess charge in an external device.

While all this is going on we have our nice cell voltages to monitor the process!
 

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I've never heard of such a 'balancing board'. I looked at the Ebay products you sent links for and watched your YouTube but I still don't understand how many would be needed for a 72-cell LTO pack or how they might be connected. Can you elaborate more please.
 

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Discussion Starter #4
John

If you use the 6 cell supercap ones I linked.
Then you need two of those per 12 cell LTO block.

You would need 12 of the boards 6 cap boards for your 72 cells.
The wiring should be fairly straightforward but i will make a video in due course to illustrate it.
Google the protection boards and have a read about them in general.

Our cells are just like the supercaps as far as the board is concerned.
 

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Discussion Starter #6 (Edited)
Well this is another option.

Active balancing, transfers power between the cells in a block rather than burning it off as heat.

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Looks like it comes with wires and plug in connector as well.


Only issue I see with those is that blocks could end up at different voltages
but cells within blocks would be balanced to within 30mv.

The low power passive ones I suggested earlier would bring all cells down to our balance voltage. (Eventually)
They are probably easiest and cheapest if we roll our own i will speak with Bulldog about them

Unless anyone else wants to knock up a 12S pcb based on that schematic in my first post.
 

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Discussion Starter #7
Well after a lot of Googling I have found the BW6101 balance chip data sheet.
It was only available in Chinese.
But I shoved it through Google Translate and we get the attached.

It clearly shows it has two set levels of balancing voltage.
So we can choose to balance at 2.65V or 2.45V by pulling the SEL pin either high or low via a 1k resistor.

It can balance at upto 700ma which is impressive.
We can choose load resistors to give around 50-250ma balance power to keep heat down.

It has a minimal component count and we could fab our own cheap 12S LTO balance boards with led indicators.
Add a suitable connector with pigtails and easy to install. Fit and forget.

I think is likely the best option to keep costs down and voltage consistent across the LTO pack.
We might even be able to get a Chinese supplier to build them.
 

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Discussion Starter #8

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Discussion Starter #9
Discussion of proposed add on balancing section for LTO BMS boards.

Note the LTO boards have all the cell connection pins at one end of the pcb.

We could use a small adapter pcb here to bring these pins to a single in line row of sil 0.1" pins or solder pads for connection to a simple balance board pcb.

For testing I intend to replace the 3.6V balance chip on the cheap chinese boards with the supercap 2.65V ones and make a test block.

Our pcb can be affixed to the LTO pcb using super double sided foam tape.

 

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I talked to Ian about these at InsightFest 2019. A problem he mentioned was that if you do this then it can mask problem cells by having them constantly balanced. He suggested a "bus" type balancing system where balancing is controlled by a computer chip and recorded into a spreadsheet. So it stays balanced but if there's an outlier cell you have a way of detecting it. My understanding is that if a cell internally shorts then voltage is just going to be dumped into it by these "dumb" capacitor balancers which could make things worse.
 

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Discussion Starter #11
There are two types in this thread.

1) The charge shift transfer type which you allude to.

2) The resistor burn off the excess type which would be safer.
 

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The only issue i can see with capacitor style balance boards is that it will not protect from under voltage. For a capacitor it doesn't matter as they wont be damaged, but LTO's will be.
It is quite foreseeable that a cell will slide unto chronic undercharge, specially if there is enough spare capacity in the bank to mask a single under-performing / out of balance cell.
 

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Discussion Starter #13
The only issue i can see with capacitor style balance boards is that it will not protect from under voltage. For a capacitor it doesn't matter as they wont be damaged, but LTO's will be.
It is quite foreseeable that a cell will slide unto chronic undercharge, specially if there is enough spare capacity in the bank to mask a single under-performing / out of balance cell.
However we are voltage monitoring every cell using the original LTO boards so will see this occurring and can intervene.
 

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Discussion Starter #14
These 2.65/2.45V chips finally arrived after 3 months!

I might try and mod one cheap board or channel for testing.

 

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Discussion Starter #15
I modded a 4 channel board, managed to swap the chips but it doesn't do anything :(

Maybe some circuit differences so i will build one up seperately and see what happens.
 

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Discussion Starter #16
I built one of these up using an SMD adapter board and it does work as per the circuit diagram from earlier in the thread :)

However the cheap balancer boards have a different chip pinout.
So we can't just unsolder the 3.6V chips and replace them with our 2.55V versions. :(

We would have to spin our own boards. The component count is minimal.
If you don't want the led then it's one control resistor, one ceramic cap, one load resistor and the special chip.
 

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Discussion Starter #17 (Edited)
I have ordered enough of the 12S active equalizer boards as per post 6 to retro fit one of my 72 Cell LTO packs.


The total cost was about $135

I also have one 12s board here already for testing which I will attach tomorrow to an LTO block.

I will force it out of balance and monitor it's balancing progress.
 

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Only issue I see with those is that blocks could end up at different voltages
but cells within blocks would be balanced to within 30mv.
Looking at another ebay link for similar boards, this example is given of a way to balance multiple packs with each other. I think it would just take one or two extra wires going between packs (or, perhaps the stock harness already has these wires?)
86689
 

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I agree that correct per-cell balancing for these packs is essential. These looked promising. However, you might want to look at the comments in this video before investing too much time into these particular balancers.
 

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Discussion Starter #20
His comment are interesting but I think it's worth testing.

They shunt power up and down a string of cells.
The worst case scenario is if Cell 1 is low and Cell 12 is high.

It has to shift the power from cell 12 to 11 to 10 to 9 etc etc until it finally gets to 1.

He may not have allowed it enough time. We shall see.
 
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