[Tremelune]

I'm at the point in my project where I can charge the batteries successfully, but I'm wondering if I could be using better values for the process...I'm using Leaf "Wolf" modules, a Thunderstruck charge controller, BMS, and TSM2500 charger.

My understanding from this thread is that the Leaf calls 4.1v "100%" and 3.6v as "0%". I believe the "real" numbers are more like 3.5v and 4.2v for these batteries, and that Nissan just fudges the numbers to prolong the life of the batteries without having consumers call and say "MY CAR ISN'T CHARGING TO FULL" all the time. Correct?

What's a good balancing current? What's a good voltage to set for it to kick in?

I currently have it set to 10mV below max voltage. For current, I tried 0.5A and it didn't seem to charge at all...I wound up at 3A and it seemed to do the trick. These numbers are pretty much arbitrary, though. The pack is already well-balanced, and in the end the standard deviation was .001, which seems pretty good. All cells are within 5mV of one another.

I'm wondering how this will fair when the cells are further out of sync from driving, charging, driving, charging...As soon as charging stops, the cells lose about 10mV.

Thunderstruck also has provisions for cutoff thresholds...Any value to them? In other words, if a cell hits the trigger voltage, it will stay triggered until it falls back down below a different voltage that you can set...I'm not sure I understand the value of this...

 

[john61ct]

SoC vs resting OCV mapping varies by chemistry.

Absolute 0% does not exist, that voltage level is arbitrary, staying well away from the vendor spec gains longevity.

Under load of course voltage means nothing, need to rest and get OVC. For the Leaf 3.4V would be my choice for absolute 0%, but for the LVC **under load** would be adjusted by max C-rate to stay well above 10% SoC. 20% would be better, greatly extend longevity, but at the cost of range. Personally 3.6V probably works very well as a normal LVC under load, if long lifespan is important to you.

Are the packs new?

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At the high end, there is hardly any useful range gained once resting OCV gets to 4.1V, trying to push in more will just cause harmful heat and stress, most resting OCV gained will just be "surface charge".

Look at a discharge graph even at a 0.1C rate and observe how quickly the 4.1V line is crossed.

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Balance current and start-balance voltage should be enough to get the cells to your delta-spec balanced state within the normal charge cycle at your desired profile.

Or at least not sitting at the harmful top end for hours at a time.

Starting balancing earlier and at a higher rate has no downside I can think of other than a bit longer finish time, and the price of the balancing device.

If you see imbalance growing, that happens as EoL approaches of course inevitably. However, see if going easier on lead-foot driving and factors like stopping charge a little earlier, reducing avg DoD maybe slows the process down.

Be **very** sure to not charge at a high C-rate unless necessary, but especially not at all once you're cooler than in T-shirt weather.

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> As soon as charging stops, the cells lose about 10mV.

That's normal, a small delta in fact, other LI chemistries can be a volt or more. Depends on the finish V setpoint and charge rate of course.

 

[john61ct]

> Thunderstruck also has provisions for cutoff thresholds...Any value to them?

That should be what controls your normal charging cycle, the BMS is there to save the bank when the charger fails to regulate properly or cells balance gets out of whack at the top.



> if a cell hits the trigger voltage, it will stay triggered until it falls back down below a different voltage that you can set...

Not sure what you mean?

The charger itself is measuring cell/group voltages?

Or getting CAN signals from the BMS?

 

[brian_]

Thunderstruck also has provisions for cutoff thresholds...Any value to them? In other words, if a cell hits the trigger voltage, it will stay triggered until it falls back down below a different voltage that you can set...I'm not sure I understand the value of this...

If I understand this application correctly, this providing hysteresis in control, just like a thermostat for a furnace or air conditioner. You can't have a single triggering value, because if you do then as soon as the value is reached the control action (heating, cooling, or in this case balancing) the value drops even slightly and the action stops, starting a rapid cycle. A spread between two switching values allows the system (furnace, air conditioner, balancer) to actually make a useful difference before shutting off, so it doesn't cycle rapidly.

 

[brian_]

> Thunderstruck also has provisions for cutoff thresholds...Any value to them?
...
> if a cell hits the trigger voltage, it will stay triggered until it falls back down below a different voltage that you can set...

Not sure what you mean?

The charger itself is measuring cell/group voltages?

Or getting CAN signals from the BMS?

I assume that Tremelune's charger, charge controller, and BMS are all Thunderstruck products, and that anything about cell voltages is referring to the operation of the BMS. Right?

 

[Tremelune]

My practical concern in all this is understanding and balancing range vs reported State of Charge. A Tesla seems to show accurate SoC while saying "Try not to charge above 80% much" whereas the Leaf will charge to 100%, but secretly it's not 100% to prolong battery life...I suspect there's some magic with Tesla, too, but they're different.

I keep my EVs as close to 50% charge as I can, while anticipating range needs for a given day. So like, if I don't have anywhere in particular to go, I'd charge to 50-60% and let the car sit. If I'm cutting it close on a trip, I'd like the option to charge to as much range as the batteries can safely support from time to time...

It's similar on the low end...When I'm driving an EV, I tend to ease up when the battery is below 20-30%, but I'm not sure if that percentage represents the real battery SoC or a user-friendly interpretation...

Essentially, I'd like my fuel gauge to represent the real battery min/max, and rely on my brain to keep SoC between 20-80% most of the time...It seems like the "real" min/max is a somewhat malleable concept.

My pack is reasonably new, and seemed pretty balanced when it was in the Leaf. I never charged it fully, so I don't know how many "bars" it had...

Interesting point about the discharge from 4.2 vs 4.1. I cribbed this chart from here:

Looks like 3.6V-4.1V is a reasonable operating range. That would imply a balancing voltage of maybe 4.090, which would perhaps have to be lower, depending on how out of sync the cells become under standard operating conditions. I assume the longer it has at balance current, the closer the final balance should be?

Is there a way to get closer to "resting" voltage without actually resting? Perhaps lowering the charge current causes quite a bit of "rest"? I guess 5A isn't particularly high current to begin with...Like, if voltage drops when charging stops, wouldn't you want to charge to a little higher so it rests where you want it? Or might that result in overcharging?

How do I determine C-rate? I've read a few articles on C-rate, and several describe it as a somewhat misunderstood concept.

The more I learn, the more I'm aware of what I don't understand!

 

[Tremelune]

If I understand this application correctly, this providing hysteresis in control, just like a thermostat for a furnace or air conditioner. You can't have a single triggering value, because if you do then as soon as the value is reached the control action (heating, cooling, or in this case balancing) the value drops even slightly and the action stops, starting a rapid cycle. A spread between two switching values allows the system (furnace, air conditioner, balancer) to actually make a useful difference before shutting off, so it doesn't cycle rapidly.

This is correct—thank you for explaining it better!

I'm not sure what useful values to set in this context.

 

[brian_]

My practical concern in all this is understanding and balancing range vs reported State of Charge. A Tesla seems to show accurate SoC while saying "Try not to charge above 80% much" whereas the Leaf will charge to 100%, but secretly it's not 100% to prolong battery life...I suspect there's some magic with Tesla, too, but they're different.

I think Tesla is playing exactly the same game, but is just willing to push to more extreme limits. There have been versions of the same Tesla model with the same battery but different specified capacities and prices, and in some cases owers can even buy a capacity upgrade for their existing car over-the-air, so clearly the stated capacity (and thus SoC shown) depends on a chosen charging limit and software logic, not just the physical battery characteristics.

 

[Tremelune]

I assume that Tremelune's charger, charge controller, and BMS are all Thunderstruck products, and that anything about cell voltages is referring to the operation of the BMS. Right?

Also correct. The BMS tells the controller "A cell has hit the balance voltage" and the controller tells the charger "Lower the current for a bit." Same for high voltage "complete" signals, or high temperature sensed, etc, all over the CAN bus.

 

[john61ct]

Aha, sounds excellent.

In the event of the controller or BMS failing, does the charger halt itself?

 

[Tremelune]

Aha, sounds excellent.

In the event of the controller or BMS failing, does the charger halt itself?

That is a very good question. I shall test.