Using BMS to control SoC level

[Boondockorbust]

I apologise in advance if the title of the thread might not be worded appropriately for the question I have.

I have recently ordered a set of 130AH AL cased cells, and will pair them with the Overkill BMS which has already arrived. I just finished installing a Progressive Dynamics PD9145ALV 45 amp charger/converter. These will be in use on my teardrop camper which already has a renogy Rover and 100w panel.

My question is regarding the ability to maintain the battery at a lower state of charge to increase longevity. I know that I can set custom voltage parameters for the Rover and would be able to keep the battery at something like a 30-70% charge while on solar.

The issue it seems, is that although the converter is meant for lithium, it's goal is to fully charge the battery with no adjustment possible to the values it abides by. I know that the BMS should be able to cut charge and discharge current at levels of my choosing, but I wondered if this might affect the charge adversely if the level ends up not accurate due to charge voltage vs actual etc. Also would the charger possibly be stopped before it entered an important phase of the charge by simply disconnecting via the BMS?

If it ended up that I was best to just set the solar controller as I wanted and whenever I was plugged in I ended up fully charged then I could easily live with that for the sake of simplifying things. I will primarily use the trailer off grid as I move cross country later this year.

 

[Ampster]

If your converter cannot be adjusted then there are several ways to control it. One is with a voltage activated relay that could turn off the AC voltage to the converter . Alternately the relay could disconnect the DC but if that is powering the inverter it won't work as well. You could also wire the relay to your BMS if there is a voltage setting there that can be set to your lower voltage. Each BMS is different but those are some workarounds.

 

[Boondockorbust]

Ideally I would prefer to not cut power to the charger as it is also providing 12v power to the trailer to prevent use of the battery while plugged in. The charger will supply power to everything without the battery connected so perhaps the relay removing the battery from the circuit while leaving the loads connected to the supply might be useful. The issue then is having to disable the relay setup just to be able to use the battery when it's time to.

 

[Craig]

I think a Daly smart BMS might work for you. It is a common port config so it will stop charging but allow discharging. I just got one in the mail and am in the process of testing now. But from what I have read it should work.

 

[Boondockorbust]

My understanding is that the BMS that I already have will do this as well.

 

[Craig]

My understanding is that the BMS that I already have will do this as well.

Sorry i didnt see that you already had one. Well then hopefully it does this would make things work very slick for you.

 

[Boondockorbust]

I am just looking for any sort of assurance that if I cut the charge on the BMS at a certain voltage that it is actually at that charge level rather than ever the higher input voltage might have been. For instance, will the battery rise to say 13.4v, cut off, then drop to a lower voltage revealing that it was actually not charged to the point I want it. Or, is the BMS capable of sensing better than that?

Then again, as I type this I am beginning to realize that the BMS would reconnect when the voltage dropped lower and begin to charge again. I'm not sure if I am fond of the on-and-off cycling that would be necessary to reach the final voltage that I want though

 

[Craig]

Well i can speak as to what and how Chargery works. t sets both a per cell and total voltage cut uff and will cut charging when those limits are reached. Then you program a resume voltage. So for instance if you stop charging at 3.5 volts you can set the resume to 3.2 volts so its not sitting there fluctuating all day long.

 

[Boondockorbust]

That's good information thank you. Now, having not dealt with anything other than lead acid, does the cell voltage rise steadily that the BMS senses, or does it jump considerably when a charge is applied? It seems charging voltage on the battery will be much higher than the resting voltage I want and so controlling with the BMS limit doesn't seem like a great idea. the more I look into it, the more it seems I will just have to accept a full charge whenever I'm plugged in I want to go through the effort of adding extra components.

 

[Craig]

With my lithium cells there is not the rise like on lead acid. They do sag after charging but not instantly. I think the problem you are worried about will not hapoen