maanantai 8. huhtikuuta 2013

Why you shouldn't fully charge your cells

In my previous entry I wrote about the Swedish paper on Lithium battery cycle life. I kept thinking about the results and came to an interesting conclusion on why you really shouldn't fully charge your cells.

Consider a scenario like the one in the paper where you would Top Balance your cells and fully recharge them to 100% state of charge every time and then discharge them to empty. You'd be at 80% capacity in about 2000 cycles as expected.

But if you didn't fully charge your cells, but instead chose to undercharge your cells like we do in addition to Bottom Balancing? You might not see the 9000 cycles that they got when they only charged to 50% SOC, but I'd bet you'd get to 6000 easy. Even that would be triple compared to Top Balancing.

In order to get maximum range a lot of people will Top Balance, use a BMS or try to find other ways to maximize their traction pack capacity, for example by trying to even the cell charges on the fly. What they don't realise is that if you try to push as much into the cells as you can you are killing them much faster than you could be.

By 1000 cycles of 100% charging you'll have less capacity in the cells that you would have had if you just undercharged them in the first place. And the loss is permanent. There's no way to get them back. Undercharge and those cells will have that 90% charge much longer than the fully charged, Top Balanced pack takes to reach 80% capacity.

Top Balancing might give you the maximum range right now, but after a couple of thousand cycles they guy who only charged to 90% will probably have longer range and healthier cells.

If you have more money than you can ever use, or happen to sell batteries, then Top Balancing is for you. Otherwise, in the long run, undercharging is the smart thing to do. Don't forget to Bottom Balance.

Update! Here's a link to the paper:

Disclaimer: All of my battery ramblings are based on my own experience and Jack Rickard's findings. They are applicable to CALB SE- and CA-series cells. Other cells and chemistries may at least require different voltages.

3 kommenttia:

  1. Nissan also seems to have realized this. Their new Leaf has the recommended option to charge only to 80% of the capacity or so and they recommend you top the batteries 100% only when leaving for a longer trip.

    1. It's anybody's guess what the OEMs actually charge their cells to. They don't give you the amp hours or the volts so you could tell what's really going on. And don't even get me started on the turtles... It is however known that the Volt/Ampera never uses most of it's pack capacity. They've done it to make it last forever.

      Easy way to make your vehicles "quick charge": Instead of doing a normal charge of Constant Current and then a slow taper with Constant Voltage, just charge to the end of the Constant Current phase and stop immediately when you reach the desired voltage. It's faster and gets your about 80-90% charge, which is just about right for maximum battery cycle life.

  2. The big question for the Nissan Leaf.. What do they mean by 80%, 100% or even the turtle modes. ;) Plus they do have a BMS the engineers have staked their jobs on. BMS failure on the Leaf is not unknown though. I think all the Leafs made have a settable 80 or 100%.
    Nissan engineers have set their own limits ---- and I agree totally with you all; keep the charge "middling" and attempt not to shock or stress them for a good life. We have seen less fires with the Nissan Leaf than the Boeing 787. A scary irony.