Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[DerKrawallkeks]

Here's what the Quiq charger does when changing from constant current to constant voltage at 100% SOC. Nice data from the BMS serial port although it took some work to filter out all of the DEBUG lines complaining about no CAN traffic.

Nice! Can we infer a charging profile from this?

Sorry for reviving an old thread, I must clear a common misconception. What you can see there is not something the charger controls. It does not actively follow a charging profile.
This goes for most lithium chargers.

For lithium batteries it is not needed to slow down the charging when they go towards 100%. What is happening (in this diagram as well) is CCCV charging, first doing a constant current and then holding a constant voltage.
The point where this changes from CC to CV is not actively controlled, at some point the battery just reaches full voltage. Since the power supply does not pull it any higher, the current starts dropping while the battery soaks to full voltage.

None of this is actively controlled and the current ramp down is not actively controlled.

(Also someone had wondered why the current is so noisy and the voltage is not. The current is not actually noisy, the behavior is just caused by the fact that the resolution of the current sensor here is 1A, and without any hysteresis it jumps +/-1A, even if it just goes from 0,99 to 1,01A in reality.)

[Keith]

Clearly this is a charging profile although it may be much simpler to create than it looks. But the charger could decide to turn off current immediately when full voltage is reached. It does not. It let's the battery "soak" for a period of time, surely useful for cell balancing near full charge. Integrating the declining current would represent a significant amount of charge, perhaps 100Wh or so in this case. It is probably a very simple decision by the charger, stop when the current drops below ~2 amps (caused by the gradual increase in the battery voltage), and not a regulated current/time curve. But regardless, a profile is created. Would battery health be reduced if the full voltage soak time interval was not done? I think so. Would a more sophisticated profile be even better? Possibly, only a lot of real battery life cycle testing could determine that. And as a practical matter, the person wants to use the battery, not wait indefinitely while a reluctant cell (maybe, given more time or a different current) reaches its "ideal" voltage. 

[DerKrawallkeks]

Well yes, in a sense everything is a charging profile of course. But this really is very basic and requires no intelligence. I just wanted to point that out.
In my 2013 Zero, they actually use LED power supplies (for lamps) to charge. Enough for a lithium battery.

Would battery health be reduced if the full voltage soak time interval was not done? I think so.

No, stopping the charge immediately is healthier for the battery (disregarding the balancing need for now). The higher the charge voltage, the more harm is done, with quite a significant difference happening in the last few tenth of a Volt. That's the reason Zero charges the cells only to 4,15V instead of 4,2V, even though it seems like a small difference.

[Keith]

My impression is that the harm done by higher charge voltage occurs over a long time period, not instantly. Store at 60% SOC, charge full and use immediately. But in any case, the transition point from CC to CV can be at whatever voltage is "good." And of course balancing is complicated and important, and takes time. I have a six year old Zero FX pack with two bad (out of balance but functional) cells. BMS won't let it charge. Ideally, charging for each cell would be separately and independently managed, but that would be quite challenging and costly to design. Anyway, I mostly agree with your points, nice to discuss.

[DerKrawallkeks]

Yes nice to discuss!
Sucks with your FX pack.. but happens..
I have one more thing to add. You're right with how to use a battery, but there's a point around 55% (called the central graphite peak in the differential voltage spectrum), above which cell aging is significantly higher. 5% seems like nitpicking, but it really makes a difference. It's not too big but since it's only a 5% difference to the end user, I just wanted to say that. Don't store at 60, store below 55 or 50, to be sure.

Also I checked and it makes a difference how high you charge, even if you use it immediately. It's a pretty significant difference, so I strongly recommend only fully charging if needed.
On the other hand, we buy the bikes to use them, and Zero charges them to 4,15V only, instead of the usual 4,2V for exactly that reason.
But yea.. It makes a big difference, avoid if you can.

I didn't come up with all that knowledge:) it's taken from a study of NMC batteries (exact same chemistry that Zero has)

[gt13013]

there's a point around 55% (called the central graphite peak in the differential voltage spectrum), above which cell aging is significantly higher. 5% seems like nitpicking, but it really makes a difference. It's not too big but since it's only a 5% difference to the end user, I just wanted to say that. Don't store at 60, store below 55 or 50, to be sure.

Are you referring to Keil's thesis "Aging of Lithium-Ion Batteries in Electric Vehicles", p.52-53, in which I saw this surprising information, or do you have any other references?

[DerKrawallkeks]

Hey, yes that's my go-to source. You can find it in different studies as well I'm sure:)