[halestorm]

The Elithion Lithiumate BMS manual describes the charging phases of CCCV charging here, and with the attached figure. I understand the CC phase; I'm interested in understanding the CV part of the charging phase, where the voltage is held constant and the current is tapered off exponentially. Is anyone allowing their Elithion to control their charger?

I'm puzzled by their statement at the bottom of their web page,

If the charger is capable of current control, you may also set up a way for the BMS to tell the charger the maximum allowed current, either through the CCL line or through the CCL data in the standard CAN messages.
That is not absolutely necessary, though.

How can it not be necessary? How else is the current going to exponentially decay unless the charger is told explicitly to do so? All throughout their page on charging they advocate a dumb charger and letting the BMS do the thinking. And in order for the BMS to get it right then it must know what the charger is capable of.

Presumably the BMS must know the max value of each charger make/model that it is communicating with. I guess I'll have to conduct an experiment, and tell the BMS that I have a different charger, and watch the CAN messages to see if it changes the output current message.

 

[halestorm]

Okay, thanks guys, that's kind of what I wondered about but wasn't sure. 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.

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.

How do I ensure I get this current limit correct?

I can enter two current-related parameters into the Elithion BMS: the max continuous and max peak charging currents (where I think "peak" is defined as < 10 sec). Beyond that, the BMS calculates a charge current limit. Exactly how it calculates this number will forever be a mystery.

For my nominally 90Ah pack, I have the max set to 90A, and the peak set to 135A. I picked max as 1C and peak as 1.5C (which it will only ever see for some seconds on regen). If I ask the BMS right now, sitting in the garage on a fully charged pack, what is the charge limit, it tells me 53A. That's nowhere near the tail of any exponential curve! If I plugged the charger in right now however, the BMS wouldn't let it charge because the cell voltage is too high.

 

[halestorm]

What is the cell voltage to which you are charging?

I have LG Chem Chevy Volt batteries (LiMn2O4). I have programmed the Elithion for Vcell-max=4.00 and Vcell-high=3.95.

According to Elithion, Vcell-max is the maximum cell voltage reached at the end of charge. If the voltage of any cell goes above the value the BMS disabled charging. Vcell-high is the cell voltage at the bottom of the steep rise that occurs when the cell is fully charged. If the voltage of any cell drops below this value (typically because of balancing) the BMS may re-enable charging.

I've read somewhere that the max for these cells is 4.15v. I'm not 100% sure about the sources I've read this, plus I don't want to charge to the max anyway, so I somewhat arbitrarily set the high to 3.95v and the max to 4.0v.

With 90AHr cells, someone has set your charger cutoff limit to a conservative 53A, probably Davide did this since he knows the dangers of overcharging. He just cut the long tail off the exponential to save wear and tear on your pack. So this leads back to the question, what's your CV setting?

First I have to tell you more about the pack. I have two "batteries" in parallel, each consisting of 3p54s cells. 54 cells in series at the max 4.15v/cell is 224v.

Elithion says somewhere (I can't find the reference right now) that the CV should be set higher than the pack voltage. Somewhere along the line this got set to 226v, though now with my new charger I have complete flexibility to adjust this (more on that in a moment). I gave my pack info to Elcon when they supplied my previous charger (a PFC2500) which they programmed for 226v output.

That PFC2500 just died and I have now purchased Elcon's new UHF charger. This charger requires a CAN message at 1Hz to stay on, a message which contains the voltage and current. So I have to tell it the max voltage and current. The max voltage I figured I would just keep at 226v. 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.

Actually what kind of charger are you using and what is the max current that it can do?

I have the new Elcon UHF IP67 Sealed CAN Bus Charger, 6.6kW, which is capable of 20A output. I will probably never use the full output. I can only get 15.5A from my home EVSE, and the EVSEs at work are throttled based on demand, and I think give only 8A. Other than the EVSEs at work I have never had to use a public station, say on the street, so I don't know how much current they supply (but by the fact I've never used one, I'm not too concerned -- all of my charging has been at home or work).

That brings up another point. It seems that no one must be using J1772 because even though the Elithion BMS is capable of talking to the charger directly, you certainly couldn't have your BMS telling your charger to put out 53A (it can't anyway) nor even the 20A that it is capable of, because the EVSE may put out substantially less. And that is all part of the communications in J1772; the EVSE gets to tell the car how much current it can supply. I ran into this problem at work. I used to have the AVC2 from Modular EV Power, which was fine at home, but it completely ignores the message from the EVSE. Once they added charging stations at work the AVC2 didn't work for me. The EVSE would announce it was supplying 8A but since the AVC2 ignores that, my Elcon tried to run flat out at 12A, which the EVSE didn't like so it shut down. In order to charge at work I ended up having to build my own AVC2-equivalent, which listens to the EVSE's message and then drives the (optional) 2-5v analog input signal to throttle back the PFC2500's output.

With my new CAN bus Elcon, I've replaced the analog diver with a CAN driver, so it can tell the charger directly what the EVSE can supply. But I suppose I should also be listening to the BMS, too.

Now you see the origin of this post! I've got to tell my new charger SOMETHING, and I have to listen to the EVSE and I have to listen to the BMS, and I want to make sure I'm telling the charger the right thing!

 

[halestorm]

These chargers do limit the current as the battery fills. The software limits the current to 0.2C in stage three cv stage and then in stage four it drops to 0.02C until the end point voltage is reached.

What are stages? Maybe you're thinking of a different charger? Because the charger doesn't do anything unless I tell it; it requires a CAN message once per second or it will turn off.

 

[halestorm]

My bad then, I thought your speaking about an Elcon Charger.

It is an Elcon. I have the new Elcon UHF IP67 Sealed CAN Bus Charger, 6.6kW but like I said, it requires a CAN message containing voltage and current @ 1Hz to stay on.