Fast charge confusion. Crank up the V? 1

[Flox]

Does anyone have experience with charging voltages for high C/charge batteries? I'm looking at LTO chemistry specifically, which is generally rated to take a 10C charge. Fully charged voltage is 2.8v/cell. All the literature I can find regarding the fast charging of cells, i.e. high C does not discuss the voltage element. Standard charge rates, 1C and below, do show the voltage/current charge profile, which is never to exceed the fully charged cell voltage (but for some marginal amount---referring to lithium type chem.).

In my case I am working with LTO 66160 cells rated to handle a 350A (10C) charge. But if I apply the max rated 2.8V the cells won't even remotely approach that. At 3.8V they take 25A. What am I missing here? I don't see a way around blasting voltages that are way above the cell's max volts, and yet no literature about fast charging LTO cells touches on this.

 

[moltenmetal]

3.8V measured at the terminals of the cell itself? If so, I'd be very concerned about local electrolyte damage at those voltages. Voltages that high begin to exceed the electrochemical window of the electrolyte itself, leading to reactions which result in electrolyte being degraded and turned into "gunk" which results in loss of capacity.

A cell might be able to "handle" 10C, but if its internal resistance is too high, you can't get there within the maximum voltage permitted for the cell.

I'd take a look at the spec sheets for LTO batteries from other vendors, if your own vendor doesn't have a spec sheet listing maximum voltage as one of the criteria.

 

[Flox]

@moltenmetal

Ha! You know what, the voltage at the cell was only 2.5V. I turned the supply up to 4V and got 2.6V at the cell @45A. It didn't occur to me that the supply voltage (bench) was not indicative.

Thanks, I can stop scratching my head about this !!!

 

[moltenmetal]

All high current voltage sources need voltage sense wires which do NOT carry the current...nobody could afford enough copper to make it work otherwise!

 

[EdStainless]

Each cell or bank of cells should have its own dedicated sense wire back to the charge controller.
The controller should be regulating the V and A for each cell (or bank).
The controller should also have temp sense built in (another set of wires) unless your arrangement implements this externally.
If the cells are banked (as yours are) then the charge rate and capacity will be limited by the weakest cell in the bank.
After all 85% is considered full practical repeated charge level.
And the cells don't charge worth anything unless they are significantly drained (down under 40% or so depending).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Flox]

@EdStainless

Yes, however I am approaching this slightly differently from the usual charge strategies. To avoid the issues mentioned I am going to charge each cell individually and do away with the need to account for asymmetric cell performance. Each cell will have its own charge voltage rather than a series/balance/monitor.

The target charge current is 100A-200A and there is nothing off the shelf that will even come close to doing this at a reasonable cost. Planning to charge ~1kw in 10-20min.

@moltenmetal

Good info. It got me thinking about current shunts but I see that is not quite the same issue.

 

[waross]

Can I save you some money.
Monitor the cells and charge at the highest current that you want the lowest cell to see.
Then use relatively smaller bias voltages to trim the charge rate on the other cells.
If the greatest cell difference is 3% the capacity of your individual cell chargers will be reduced from 100% to 3%.
You may also use the bias to even the discharge between cells. That may further reduce the needed capacity of the bias supplies.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Flox]

@waross

The strategy you describe sounds similar to the various charge/balance circuits that I have been exposed to for series charging. From the non EE perspective, I understand how these circuits work and have implemented some balanced chargers such as an 8s iteration-- but developing my own high current brew is a bit more involved than I'm equipped for.

With microcontroller + relays, I don't have to worry about the balance/low-cell/high-cell issues. Each cell is fed via buck converter and independently brought to full charge. I like this because I don't have to worry about the balance. And for discharge I can monitor each cell and shut down the pack as soon as the first cell reaches min. voltage. So far I'm still bare bones with some relays and a microcontroller.

I've found prepackaged buck converters that will get me to the low end of a "fast charge" ~70A for about 70$/cell. A more ambitious/powerful/affordable option I'm interested in is the LTC3729 polyphase regulator that can be daisy-chained for 200A and trimmed to the required 2.8v (or whatever the highest charge voltage needs to be) via resistor. Having just mentioned that the goal is simplicity, tackling the LTC3729 is a bit contrary, I'll admit.

Given a 12S pack @2.8V/cell and a charge current of 100-200A/cell (these LTOs will handle 350-400A), what would you do if you were technically limited, as in non EE? If you are still inclined to charge the pack in series, what components would you go with?