Why Li-Ion cells have huge difference between their charge/discharge rates?

Batteries, Chargers, and Battery Management Systems.
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thunderstorm80   1 kW

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Why Li-Ion cells have huge difference between their charge/discharge rates?

Post by thunderstorm80 » Jan 29 2018 12:13pm

Hi,
Can anyone explain why the huge differences?
For example, MG1 cells can be charged at ~0.5C but discharged at ~3C with no problem.
Further more, many LiPo's (or as I call them - gunpowder :mrgreen: ) can be discharged at a whopping rate of 90C but only charged at 0.5C~1C or so.
Chemistry is not my strong side, but from what I was able to gather pieces of information from the internet - it's about the ions that move around during charge/discharge, and that during the final stages of a charge, the ions start to "compete" over the remaining sites making the cell less responsive/reliable if you try to force a fast charge rate at this point which would result in heat/outgassing/etc.
Another source also suggests that during an initial charge when the %SOC is around 0%, you are allowed to inject a much larger (than the spec's) C-rate up to the 70% SOC mark, more or less, due to the same reason.
I would assume that the same would be valid for discharge? For the initial discharge at 90-100% SOC you can implement the fully allowed discharge current, but from like 30% or so you should be limiting your discharge C rate more and more as you approach the empty cell?

Would be nice if someone can turn this mesh of information/different sources to a more clear picture.

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dogman dan   100 GW

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Re: Why Li-Ion cells have huge difference between their charge/discharge rates?

Post by dogman dan » Jan 30 2018 8:37am

With no science at all to back me up, I definitely agree that the last 10% of your discharge should be lower c rate than the rest. I'm not so sure about the last 30%. I think as long as you are still above the cliff in the discharge curve, things are not so bad. I just judge how things are going by the sag under load. Any increase in sag, I back off speed till its not so bad again. Generally this is at or close to 10% left. ( on hobby king lipo anyway)

Lowering the rate as you limp home for me, results in a cooler pack when you get home, and less sag under load means you can still wring out that last bit without tripping an lvc. Any avoiding getting the pack really hot has to be good.

It does also seem to help lessen the tendency to drive the pack out of balance, but this effect may be from multiple reasons. Not discharging at all that last 10% most cycles, for example.

thunderstorm80   1 kW

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Re: Why Li-Ion cells have huge difference between their charge/discharge rates?

Post by thunderstorm80 » Jan 30 2018 12:32pm

First, before going on with the subject of the topic, you realize that if you are afraid of limping home due to approaching the lvc, you just need to slow down... I remind you that wind resistance energy loss works with the square of speed. With modern CA firmware, you can see your instant Wh/Km ratio and it really helps to "back off" the throttle a bit. Before long (and flat) journeys, I calculate my not-to-exceed "Wh/Km" value, and monitor it while I ride. Very simple to do.

Now, with the subject of the topic.
I tend to believe (with no science to back me up too, just some logic), that the max healthy C-rate (discharge OR charge) is a function of the remaining %SOC (to reach 0% or 100%, respectively), declining linearly (or maybe exponentially?).
If we assume that when a cell approach one %SOC extreme the Ions start to compete and slow down the reaction, then this makes a lot of sense. It's possible to measure this by temperature rise, but I think that in terms of electricity this would be reflected as a rise in IR as you approach either edge of the %SOC. So, if you suddenly changed from discharge to charge at 20% SOC, for example, you would measure a sharp drop in the IR.
I can verify the increased IR at the bottom end of the discharge from my small A123 26650 packs. I noticed they get significantly WARMER when being discharged at the lower %SOC spectrum. I was actually surprised at start, because the discharge currents were the same all over the spectrum, but now it makes a lot of sense!
I would hope, therefore, that the IR of each cell should be rated at %50 SOC, which is exactly in the middle, but more likely it's being given for the beginning of discharge at 100% SOC.
Keeping on with the logic, we can assume that at 0% SOC the initial charge current can be the same as the initial discharge current at 100% SOC. I still wouldn't be brave enough to test this on gunpowder-cells (LiPo's) which are rated for 90C discharge.... :twisted: :lol:

I can assume that cells are rated so purely with their charge rating for the following reasons, which take into account the "Average Joe":
1. Charging is usually being done while being stationary, meaning no heat convection from air flow. Pack will get hotter than the same discharge rate while having cooling air flow. (assuming for the sake of the example that we test this at 50% SOC)
2. Overcharge is more dangerous than under-discharge due to the more stored electrical energy, especially if you use LiPo.
3. Continuing with 2, the probability for meltdowns (over time) goes lower with lower charge rates. A manufacturer would be more lawyer-proof if he rates his cells for lower charge rates than discharges.
4. The average-joe has enough hard time with understanding the difference between "voltage" and "power", let alone to understand that the max C-rate is a function of %SOC, as noted earlier. It's easier to put just one value for each direction, and you would get less complains and less lawyers.

And so I would like to finalize with what I think determines the overall kWh you can draw from a Li-Ion cell:
1. Heat - The above theory about variable C-rates vs your remaining %SOC (in either direction) can maintain a cool battery.
2. Heat - Sustained high than the recommended C-rates on 1#. Pulse rates, for example during regen, should pose no problem.
3. Reaching the 4.2V mark. (even for brief periods - the cells just don't like it and it can be proofed by the official specs which shows what happens when you fully charge/discharge cells with relation to the life cycle. They don't spend time on the 4.2V. They just reach it and start discharging again )
4. Spending time on the 4.2V mark. (the aging effect)
5. Limiting your max %SOC to 4.05V (80%) and discharging down to 20% SOC seem to maximize the total kWh you can draw over the life of the cell, while still making sense economically and not carrying too much unused battery weight). Of course, extra battery storage is available at any time by topping up to 4.1V or 4.2V and fully discharging.
6. With lower temperature, the variable C-rates vs %SOC which were discussed earlier should be lowered linearly according to the battery spec discharge graphs vs temperature.

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