Answered

Charge controller voltages?

Hi there. I bought a Rover MPPT 20 Li, with the intention of setting up a kit in the back yard for a couple different purposes.


The problem I'm running into is that I don't understand what some of the voltages mean, in the context of 18650 Lithium Ion batteries, so I don't know how to program the Rover for my setup.


Can someone please let me know what I should set these voltages to? My setup will be a 6S16P, with minimum 18V, nominal 22.2V, and maximum 25.2V, with full BMS protection, but realistically, if I can even just be told what the voltages actually represent, I should be able to extrapolate to my setup.


High Voltage Disconnect    Definitely 25.2V, no questions asked
Equalization Voltage    Appears to be N/A
Boost Voltage     No idea?
Float Voltage        Appears to be N/A
Boost Return Voltage    No idea?
Low Voltage Reconnect    Looks like 18.6V
Under Voltage Warning    Probably 18.3V
Low Voltage Disconnect    Definitely 18V, no questions asked
Discharging Limit Voltage I guess 15V?
Equalization Duration    Appears to be N/A
Boost Duration        Appears to be N/A


Thanks,

Chris


  •  So from what I understand this is not a 24V system, this is a 22.2V system? Ok, interesting, I'll try to help out best I can.


    From what I understand with lithium batteries, you aren't supposed to have them go into boost(also called absorb) or equalize mode.


    The Rover supports a 3 stage charging cycle, but since you aren't going to do the boost charging stage, this is more like a 2 stage charging cycle.


    To make this a 2 stage charging cycle, do this first: Set boost and equalization duration to 0. Set the boost, equalize and float voltage to the same thing (if you change the float voltage later, make sure to change boost and equalize as well)


    All under voltage warning is just a warning. That's it. It's not super important.


    Now, you're gonna want the float voltage to be something. The rover usually goes into float after the boost(absorb) cycle. Since we're not doing the boost cycle, it will go to float after that. While in float, the rover maintains the voltage at the float setpoint. So you will set float to a voltage that you basically want to stop at. You also have to keep in mind that if you have a lot of sun, the battery voltage will stay at this voltage until the end of the day, so you don't want it too high.

    I believe the boost return voltage is used if you're going to do the boost mode, but since you aren't, I'm not sure if it affects anything. Usually this is the voltage used to restart the boost cycle if the battery voltage gets lower than this.

    It would be nice if you could get an official response from Renogy. I'm not sure exactly how the charge controller reacts without the boost stage, so most of the above are just guesses based on my experience with other chargers.

    Here's a site that might help you understand the multi stage charging (just replace absorb with boost when you read this): https://www.chargetek.com/chargingbasics.html

    I'm not sure how well the rover will work with your unique battery, but I hope the above helps.
  •  Thanks for the reply! Correct, it's not 24V, but it's pretty close to it.


    Lithium Ion 18650's have a nominal voltage of 3.6 to 3.7V, and a charging voltage of 4.2V. Once the voltage reaches 3V during discharge, if you don't stop, you'll start doing damage to the cells. So in my case, six in series would be a cutoff at 18V, a charge voltage of 25.2V, and then an operating voltage of like 21.6V to 22.2V. It's my understanding that SLAs have similar characteristics -- you charge at a higher voltage than the cell is rated at, and then wait for the current to stop, or some such. (I'm a techie, but my expertise is not in battery technologies.)


    Thanks for the link, I have been to that site before, while looking for information, but I didn't know that "absorb" and "boost" are interchangeable.


    So, thanks again for the info. In the absence of an actual suggestion from Renogy, what I could do is set up a 6S1P (so just six of these 18650 batteries in series, as opposed to the eventual 6S16P, which is six in series, but 16 groups of them), and try out some parameters, and go through a whole bunch of charge/discharge cycles and see how it behaves.


    The batteries are all pulls from old laptop battery packs, so it's not like I would be out a lot of money if the batteries can't handle it, but I'd prefer not to unwittingly damage them. 


    In the meantime, just an FYI to Renogy support, please do offer some suggestions if you have any.


    Thanks,

    Chris


  • Without a BMS you may over charge! BMS prevents overcharge and discharge ...

    Set battery type to Li which turns off eq and temp comp, set to 24v and adjust voltage to 1/2 of 25.2 (i believe the controller doubles voltage for 24v)

  • Bupkis, I noted in my OP that I will be using a BMS. But I don't want the Rover and the BMS fighting with each other, I'd prefer that they cooperate, so that's why I was looking for information on what all those parameters should be.


    Sorry, I'm not sure how to specify "set to 24v and adjust voltage to 1/2 of 25.2 (i believe the controller doubles voltage for 24v)" in the Rover. I do have the BT interface, but I haven't started to play with it yet, because I still don't know what all the parameters should be.


    Thanks,

    Chris

  • https://www.renogy.com/content/RNG-CTRL-RVR40/RVR203040-Manual.pdf


    see bottom of page 7 for setting to 24v, you'll need batt type user to set parameters.

    LVD is for the load terminals, anything connected to the battery will rely on the BMS, I'd set (page 28), as follows (assuming the controller X2 for 24v). eq and boost to 1/2 of 25.2v, float to anything much lower since a much lower voltage will not charge, boost return to 1/2 of ~10v, the rest to 1/2 your suggest values noting that 9v is the lower limit. eq and boost to zero minutes. Parameter setting is shown in the manual.


  • well that's a place to start as see what is going on!

  •  @Bupkis are you recommending that he basically makes it go into Bulk mode until it reaches the Boost setpoint, then goes into float with a really low setpoint to where it doesn't even charge it?


    So the float is supposed to be really low for lithium batteries?


    Also, if he were to use Renogy's "lithium" battery setpoints, do you know if it would even go into float mode? On their battery setpoint chart it doesn't show anything for the float voltage.


    I'm just trying to learn a bit here. I'm pretty familiar with recommended setpoints for other battery types, not as much with lithium battery types.

  • charge (bulk) until his 25.2v set pt is reached then quit (the BMS takes of the rest and will shut off charging @ 25.2 based on what the OP has suggested), return to charging at whatever the OP wants, I'd base that on how the battery operates with his loads. I'd certainly want it in bulk each morning!

  •  So you don't think that he should charge his batteries with boost or float at all? Shouldn't the batteries be charged with at least one constant voltage stage?

  • Thanks for the replies. I have that PDF, which is where the table came from in my OP.


    But, sorry, I'm very confused as to how 1/2 of 25.2V, 1/2 of 10V, and 9V come into play... If the batteries go below 18V, then the Lithium Ion batteries start to take on a little bit of damage. Significantly below 18V, and the batteries are ruined. 


    Thanks,

    Chris


  •  I think when he was talking about 1/2 of 25.5, I think he meant to make sure that when you set the values, you set them at half of what you actually want them to be. The Rover internally stores the values between 7V and 17V, so you might have to divide the values you want by 2 in order to tell the rover what you really want.


    Or, we could both be wrong. I've never used the Rover's app with a 24V battery, it might do this automatically. However, in some places, it doesn't do this automatically. So if the app allows you to set it to a 25.2 voltage, don't worry about what we're saying. If the app doesn't allow you to enter a number over 17V, then just divide what you really want by 2, and the rover will multiply it by 2 when it detects a 24V battery.

  • When charged very fast @ high current yes one would let the current taper to some cut off value.


    slow solar chagrining, it is my understanding that once the batteries get to the voltage suggested by the manufacture that they are 100%, ie a charge profile is bulk only with the BMS doing the balancing. That is my general take on Li charge profile with solar, no need to 100% charge!

  •  Aaah, I see, ok. Interesting that that's how Renogy decided to store the parameters (and being an embedded software engineer myself, I can see where they're coming from). And then just double the values in a 24V battery environment.


    So the only thing that Li mode does that's different is it doesn't go through the equalization phase and also disables temperature sensing? (Temperature sensing would be helpful, but I'm planning on having separate temperature sensing devices for cutting relays in and out, and will have the batteries themselves inside a 12V "refrigerator" with a Peltier element in it, so I'm hoping I'll have both hot and cold temperatures covered.)


    The ~250W worth of solar panels will be distributed across 96 Li-Ion batteries, so what I'm hoping is that the battery array is so oversized that the batteries themselves will be charged and discharged slowly,


    Well, I'll take a closer look at the chargetek site, and will keep the 12V/24V parameters in mind, and figure out if I can get this figured out.


    Thanks,

    Chris


  • It is my understanding that one does not adjust charge voltage for battery temp with Li. and BMS will not charge below 32F.

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