Lithium users

[tx2sturgis]

John this is all theory on my part, I dont know 100% that it will work....

Sternwake to the bat-phone!

 

[John61CT]

As I said no problem. As well as above, also long as the charge sources are set for LFP of course, bit low for lead but no harm.

Same strategy for "drop-ins" with BMS built-in, otherwise the cutout can fry alt diodes.

 

[VanKitten]

Ok...so every mppt I can get will all have this fatal flaw?????

That is the most important question I have right now.

As I said....I have no idea where I could find the room for a battery.

Also....what do you mean "Load terminal of the mppt". What load terminals? Huh?

What does it mean? The only "load" is thru power bar controlled by the other contactor ... right?

 

[jimindenver]

Lots of charge controllers call for the battery to be connected before the panels, not just MPPT.

 

[tx2sturgis]

Also....what do you mean "Load terminal of the mppt".  What load terminals?   Huh?

What does it mean?   The only "load" is thru power bar controlled by the other contactor ... right?

Since I (or we) dont know what model of controller you have, I (or we) make some generalizations.

Most solar controllers have some way of disconnecting some (or all) loads when the battery voltage drops to a certain level, usually below 12v.

If yours does not, then just disregard that part.

 

[John61CT]

The better SC vendors have responsive tech support, so if you really want to continue this route, contact them and ask:

> I want an external OVD device to terminate charging by disconnecting my bank from your controller. Will this **really** damage the controller?

However, my advice is to disconnect the panel from the SC, not under BMS control but an adjustable OVD or SoC monitor.

Again BMS should not be used for routine operations, but as failsafe protection ideally never needed.

And a little $20 lead batt will also work.

 

[jonyjoe303]

one thing that might work is to connect a smaller battery (any lead acid should work, agm preferably) to the battery "out" of the controller and then connect your lithium to the aux out of the controller. The aux out should charge your battery, my ecoworthy mppt which doesnt show in the directions it can do it, will charge another battery at maximum amps when connected that way. 

I was charging my 11.1 volt lithium that way because when the bms on the battery tripped, I was encountering overvoltage when the controller was trying to forcibly charge the battery, the aux out on the otherhand will turn off completely at first sign of overvoltage. 

Then the other method is to get a small overvoltage relay (about 7 dollars) have that trip a larger 30/40 amp standard automotive relay (about 6 dollars) and connect this bigger relay between the solar panel and the controller. And set the relay to disconnect the panel when it reaches the max voltage programmed. I actually use these relays as a deadman switch, I would only charge my lithium with a balance charger if I didnt have an overvoltage relay. Disconnecting the panel from the controller won't damage anything, the controller will still be connected to the battery, the controller will think its night time.

Balance charger is the only safe way to charge lithium's, but its too easy and a hassle to connect. I only do it once in a long while.

overvoltage relay
       
30/40 amp automotive relay

 

[tripper]

So, I have encountered an issue.   This is something every lithium user must have figured out.....  so,

    The BMS has a few functions, one of which is to disconnect the battery positive side when the discharge falls below a certain level, or when the charge reaches the top defined level.   This is to protect the battery.   This is an automatic function.

So..now, the solar charge controller enters the picture.  

The installation guide is quite specific that cutting the battery side positive before FIRST disconnecting the solar array will damage and possible destroy the charge controller.

Ok..how do I solve this?   The BMS has no connection nor access to the array side of the mppt.    Turning the battery positive off in the event of reaching max charge is a totally normal thing.  Will happen routinely.

My system is 400ah   60amp max from the solar array.    

It is obvious that the Morningstar mppt I have will not be a workable one for this system.

Other lithium users...how did you solve this?

I am just using my MPPT as is, seems to work.  I am not worried about overvoltage as the controller is set to 14v max, basically the controller is pumping in max amperage at 14V until full. For the undervoltage, I can't see that happening during the daytime as my solar system is capable of 110 amps max, and the only heavy loads I have are items like recharging my ebike battery which I only do in the daytime. Night time is the only time I could potentially drain the battery, but then there would be no solar coming in anyways so the controller wont get fried if the battery disconnects. In the 6 months I've had the lithium battery I haven't had a problem using the standard MPPT controller ( a Victron btw).

I don't see how you could overvoltage the battery unless the controller was set wrong. Undervoltage should only happen at night, in which case the solar wont blow up the controller as the solar wont be outputting any energy. I think most inverters will cut voltage at a higher level than the BMS will. There are cheap low voltage alarms you can get that will sound an alarm if your battery drops below a certain voltage, might be good for night use, set it higher than the bms cutout.

I know this really isn't an answer to the problem, but I don't see OV happening and UV shouldn't happen while the panels are putting out power.

 

[VanKitten]

The use of the contactor (relay) is needed because the panels are not the only charging happeneing. There is the inverter/charger and wind power in the not too distant future too.
Someday I may have to break down and run a generator.

So... one place to test charge....shunt with battery monitor ... which controls the relay (contactor). See picture of the wiring done.

So, setting the charge from the solar to cut off at the mppt....how does it know the true charge when only part of the charge is going thru the mppt. If I set the contactor to shut down only at really high charge and count on the mppt to shut down at a normal "full" .. how will it know?
remember...all charge goes thru the power busbar then through the contactor to the battery.
The contactor is controlled by the battery monitor on the shunt.

Jonyjoe....same question. How will the relay you talk about know the true charge state of the battery?

 

[Weight]

Sorry. I thought you had a conventional cell management system in the Li battery. I still don't think the Morning star cares if the battery is disconnected with a relay or a switch. However, if you added a relay to the high voltage cut off you could disconnect the panels from the morningstar at the same time you disconnect the Li battery.

 

[VanKitten]

Weight. The "best practices" guide put out by Morningstar says it won't work. The relay delay is still enough to damage the mppt.

I want to look for a different manufacturer. If all it needs is a bit of current to stay on....then why not a second wire between power input to relay connect to mppt? Like I do to maintain power to the battery monitor? Seems like a basic design flaw. I realize I cannot run a 12 gauge wire to the mppt to maintain power..it is not designed for that.

So...I need to find a manufacturer that did not do this...or figure out how to do the little battery on the side..

As for the small battery....if I just add a second cable connect to the mppt...basically two ring connectors to the same terminal connector on the mppt...that will work? ... if I have to..I will find a place for a little 12v...maybe under the bed. I do not have venting there...so..I guess I need something that will not out-gas. Do I need to have 2 batteries to be at 12v? Even more space I cannot find room for. Since the mppt will not be setup to delivery power for a lead acid battery needs...will this be a throw away? Need to be replaced often because it isn't properly charged..ever. ?

Will a little battery on the side cause the main battery bank to not full charge? Too much being drawn off by this little one? Will the mppt shut off because the little one is full? While leaving the main battery still not fully charged? How will the mppt react to a full little battery?

 

[jimindenver]

So what it needed is a controller with its own internal battery back up.

 

[VanKitten]

JIm. Great idea. Is there such a beast?

 

[tx2sturgis]

So what it needed is a controller with its own internal battery back up.

Yes!

Except in this situation, it's gonna be external.

 

[jimindenver]

Not that I know of Kat but after trying to keep up on all the controllers for five years I burnt out on them.

So instead of just looking to keep the controller powered once the bank is full, maybe look to keep the bank from getting full in the first place.

Load terminals were mentioned although not all controllers have them. They can be set to turn on a load once the voltage reaches a certain point. Most load terminals are fairly low in how many amps they can handle so you may not be able to run enough load to out draw the solar.

Some controllers can diversion loads I believe it is called. Once they sense that the battery is full the controller sends excess power to a load. Like regular load terminals you might be able to play with the settings. Again fairly low amps.

Aside from the BMS (battery management system) there is also a PMS (power management system) that someone mentioned on the forum. I think it did something like what is needed but I am not up enough to understand.

Eventually as lithium becomes more popular there will be a profit to be made in creating charge controllers designed to work with them. I love the thought that they can be left undercharged but between the price and the tech needing to advance, I am just hoping my 6 year old bank holds out long enough for things to change. I also need to understand their properties more. What kind of charge rate? High is fine, I have 80+ amps coming down. I see they can be discharged deeper meaning more capacity. On the other hand that only seems true under low loads, high loads drop both capacity and voltage? All very confusing.

 

[Weight]

I have a DPST heavy duty switch on my morningstar controller. It switches the battery connection and the solar panel connection with one turn. I have switched it many times. During charging and when controller is in float. When it is off I switch it on during bright sun shine during the month of July. I have also switched my batteries off while the solar panels were still connected to the controller. Every thing continues to work perfectly well. I wish you luck, and all that. You should talk to folks at Morningstar directly and ask them specifically.

 

[John61CT]

RK

I think you're overthinking a bit.

Since there is no reason to get to full with LFP, just stop charging when bank reaches your voltage setpoint.

With a low current, lower setpoint or less Absorb time is more important for longevity, IMO 13.8.

IOW when charging from any source hits that V, just stop, no float is best.

 

[VanKitten]

Again....how does the mppt "know" when the battery reaches a set point?

As I said before. Mppt is connected to a power bar..that in turn is connected to the battery then the contactor.
Multiple devices are charging the batteries...so a common input is needed.

The only device that "knows" the state of charge is the battery monitor connected to the shunt..it directs the actions of the contactor.

How does the mppt know the state of charge? How could it?

 

[John61CT]

No worries discharging down to say 12V, high or low current.

If high, just means actual SoC will be a little higher, volts will bounce back higher.

Low currents in either direction just means V is less far from resting V.

You can go down a bit further, when you look at the V vs SoC chart on any LFP, you will see where the curve's shoulders are.

That's when to stop, at both ends.

But 12V to 13.8 works well too, nice and conservative, and likely usable AH capacity is within 90% of rated.

For many many years, maybe 15-20 capacity walks down by 20%, if coddled. Calendar decline may even be a greater factor than cycling wear.

 

[John61CT]

>how does the mppt "know" when the battery reaches a set point?

It knows charging voltage of the circuit, just minimize Absorb time, it drops to Float (13.0-13.2V).

Your controller's output voltage should be **very** close to the battery's. Many high-end SC have a dedicated V sense wire for that purpose.