[john61ct]

Is anyone allowing their Elithion to control their charger?

I have not yet seen a BMS I would use for charge regulation, so I'll answer from a "generic" POV.


> How can it not be necessary?

The point at which charging transitions from CC (Bulk) to CV (Absorb) is not controlled by any outside regulation, but

by battery chemistry, SoC vs internal resistance and the rate of charge.

At some point these factors also cause current **accepted** by the bank to drop, regardless how many amps are "on offer" by the source.

All the charge regulation is doing is keeping voltage from climbing above the Absorb setpoint.

Now, the other critical requirement of charge regulation is "knowing" when to stop.

Ideally this is based on trailing amps accepted by the bank hitting a a 100% Full current setpoint, adjustable by the user to match the batt mfg spec, or their preference based on desire for longevity, as opposed to squeezing in the last possible Ah / range.

Whether or not it is the BMS controlling this end charge point or not (often dropping voltage from Absorb to Float), no "knowledge" of any values is required wrt the source of current.

It's just volts and amps, and in fact the decision tree is exactly the same for any power input.



Now of course, there are more complex algorithms, a current-limiting controller **could** reduce current in line with the Bulk stage's "striving to hit Absorb" rising voltage, in effect never quite hitting a true CV / Absorb stage.

But this would be a slower process, and I am not yet convinced of the advantage of doing so, other than perhaps such "gentler" treatment may allow for pretty-fast charging while reducing internal heat-production and perhaps extending longevity.

 

[john61ct]

To extend life some folks use a more conservative approach with lower limits than the advertised procedure--this is the current control to which he says can be done, but is not absolutely necessary.

You, or your BMS, must be diligent to ensure that the limits are never exceeded. So if you hold a CV point then the current should be monitored to shut off the charger before or at the current limit.

There is little to no benefit in trying to absolutely fill the cells to the max. Leave yourself some margin at the top and the bottom, enjoy peace of mind and longer lasting cell life.

Well put, and agree completely.

In fact, where longevity and robust simplicity are priority goals and fast charging not,

a lower current charge into a high-CAR chemistry like LI, will get to a **very** high SoC in Bulk / CC stage before hitting Absorb V, so it becomes possible to simply terminate charging at that point, no Absorb / CV required at all.

In fact, at low enough current rates, it is possible to **overcharge** before ever hitting the mfg spec'd charge voltage, so dropping the setpoint by as much as .1V may be indicated.

For example with LFP at a .2C charge rate, my preference in normal cycling is to stop at 3.45Vpc.

When charging at say .1C, I'd stop at 3.40V or even 3.35V.

If determining actual SoC with CC load testing, not Ah counting of charger output, it becomes apparent that there is not more than 1-2% between this stop point and holding 3.6V until .02C endAmps.

 

[john61ct]

So no one is "programming" that exponential curve, but the BMS does program a current limit. If that current limit is "wrong" (let's say it's very high and the charger is programmed for 20A, at a time when it might be at the tail of that curve) it's not necessarily going to output 20A.

OK, so charging from depleted, when the voltage is rising (CC / Bulk stage) then hits the CV setpoint, the charger's regulator is now in Absorb stage keeping V from rising.

At a .5C charge rate say the bank is now at SoC 93%.

At a lower rate the SoC would be higher, at a "low enough" rate there would be no need for any further CV charging at all, better for longevity to just stop at that transitiin point.

At a higher rate SoC would be lower, perhaps unacceptably so wrt your need for range.

What is your desired stop-charging SoC point?

Maybe just 10 more minutes will be enough.

Personally for longevity with LI chemistries, I would not pass trailing amps going below .02C. Without range (max capacity) being critical, .05C would be better.

And that with the CV setpoint being lower than mfg spec.

Some people just blindly keep pushing current until the batt's accepting .005C, or even zero, charging stops completely.

And at the too-high mfg spec voltage!

The difference in actual stored energy available might be 2%, but lots of lifetime cycles are being sacrificed.

 

[john61ct]

The max current I suppose I should get from the BMS, but I think this should have to be scaled proportionally to what the EVSE can provide.

No, the max charge rate is a judgment call, your balancing longevity vs speed of charging.

That .5C rate you mentioned above is oriented toward longevity, possibly hanging around for 1.8 hours to refill the cost.

If the charge source can only put out less, then that will be longer, nothing can be done.

If the bank is trying to pull more from a higher current charger, there should be regulation in place keeping to the limit you set, in this case .5C.

Ideally that is part of the charger, thus the BMS acts as a failsafe fallback in case the primary current regulator fails.

For example if your CAN-enabled Elcon is set to max current of 53A, your BMS could be set to 60A as a fallback.

There is no dynamic current regulation going on (usually, with standard CC-CV gear).

After the CC to CV transition, the falling amps is determined by the **battery**, chemistry's SoC / resistance characteristics.

The charge regulation to determine the charge-stop point is ideally based on measuring that falling amps acceptance rate.

But sometimes it's a dumb egg timer. And sometimes it's the human watching an ammeter.

Which IMO should be regularly tested and calibrated, or a known good independent meter used - lots of BMSs and chargers get out of whack over time.

 

[john61ct]

How can it not be necessary? How else is the current going to exponentially decay unless the charger is told explicitly to do so?

First off, usual CC (Bulk)/CV(Absorb) charging involves no current control at all, other than a max limit made available.

Battery determines any current changes, tapering after the CC-to-CV transition, both are functions of resistance as SoC rises, not coltrolled by the charge source.

With LI chemistries, demand amps trailing off only happens at a very high SoC, depends on current C-rate made available by the charger.

Low enough C rate, can get all the way to 99% SoC with CC stage only. At higher C rates, the CV transition happens at much lower SoC, greater bounce back down to resting V.

____
So, no "stage" intelligence is required so far, dumb power supply is fine, keeping V capped to the max Absorb / CV setpoint.

The trick now is how to terminate charging.

If absolute Full is required, don't mind losing lots of life cycles, hold Absorb V until current drops to zero amps.

Lower SoC termination increases longevity, to a point, but sacrifices range.

To prevent too many lost cycles, stop when the current taper reaches an endAmps setpoint of .01C, likely ~99% SoC

.03 or even .05C is better for longevity, maybe .97% SoC

If current is low enough and Absorb voltage high, no CV at all, just Stop when the V setpoint is reached, gets you to 92-96% SoC.

Trying that at high current, or a V lower by a .1V or so, maybe 88-92%.

All the above is true, regardless of equipment used.

Whether the sensing of V and A and decision to terminate charge (or maybe drop to Float V) is handled by a BMS or charger really doesn't matter.

But chargers with a shunt at the bank to measure trailing current are very rare. At best they may have an intelligent Absorb Hold Time algorithm with user adjustability.

Most just have a dumb eggtimer approach.

All the BMS needs to know is what it can measure, stopping charge input can be a relay cutting off the chargers upstream supply.

But a comms protocol is more elegant.

 

[john61ct]

Yes not talking about the internals of **how** the regulator controls voltage.

Many people think that the controller is "in charge of" the CC-to-CC transition point, whereas in effect the battery chemistry is, charge rate vs resistance.

The point is that the intentional control is Voltage only, stopping it from rising over the Absorb setpoint.

There are chargers that allow for de-rating current to protect the upstream source or for charging small batteries,

but that's usually a once-off "sticky" setpoint,

dynamically dropping the current rate ahead of the batt resistance-acceptance limiting that,

although IMO a good idea for exotic profile ideas like "pulse timer" CC-only targeting a precise **resting** voltage,

is not found in normal CC/CV chargers.

Of course with live constant CAN messaging, and sophisticated custom programmable microcontrollers on the scene, anything becomes possible.

 

[john61ct]

Just a nitpick

In the CV stage, when the cell voltage reaches, or nearly reaches, the set point CV voltage,

Bulk / CC stage is voltage "striving" to hit the setpoint.

Absorb / CV stage is **after** the charger is holding voltage at the setpoint.

With lead chemistries, Absorb needs to be held for many hours before trailing amps taper off to the terminate-charge (batt is 100% Full) endAmps spec, usually then dropping to Float V.

With super high CAR of LI chemistries, at low rates Absorb CV stage may be only a few minutes, and even at high rates maybe 10-20.

Everything else there is spot on, 100% agree about gentle charging for longevity.

The standard spec'd charge profile is IMO way too aggressive, "you won't know how much lifespan you're losing, damage is not obvious", and the **only** upside is maybe 5% higher usable Ah.

And the vendors staying in business selling replacement banks more frequently.