Charge and discharge patterns for an RV with Lithium Batteries

[MountainBiker]

I have 2 Power Queen lithiums in parallel and 3 power sources

1. solar (Renogy 30a charge controller)
2. dc-dc (Rengogy dc-dc 40a charger)
3. ac-dc (power queen 20a charger)

These charger are always hooked up. If i drive, dc-dc charges, when plug in to shore power, the ac-dc charges and solar charges when there's sun hitting the panels. The devices hooked up to my battery bank are always consuming power when they need it. Mostly its the dometic fridge but lights sometimes and the diesel heater at other times and various other intermittent power uses. I asked Power Queen about DOD and charge cycles and how to extend the life of the batteries. Here's their response.

"Hi Tim (that's me). If you want to expand the lifespan(cycle time) of the batteries, we recommend charging the battery to a certain level first and then discharging it to a certain level. This operation is more useful than charging and discharging at the same time all the time. Because it you keep the battery charging or discharging all the time, if the batteries are plugged into shore power or driving all the time, it is possible to reach a deep DOD(100% charge or discharge). In this case, it is not easier to control the DOD."

Obviously this isn't easy to do unless all my chargers can be configured to

1. turn on at say 50% DOD which in my case is 13.15 - 13.2v
2. turn off before 100% which should be around 13.3v

I think maybe my solar charge controller has that kind of setting. My renogy dc-dc and power queen ac-dc do not. If I want to do something like this with my current setup, I'll need to set alerts with my Victron shunt and manually turn off/on my devices by hand. Is this something that you folks worry about? My hunch is that I just need to manage this by turning off these devices when the charge voltage gets near 100% and turning them back on when at 50%. I guess that's not a big deal when I am in or near the RV but not super practical.

 

[sunshine_eggo]

IMHO, you're picking fly turds out of pepper with little potential for benefit and high potential for hassle/inconvenience. Just keep using it like you do except lower charge voltages to 13.8V bulk/boost/absorption and 13.5V float.

Charging is the most stressful thing to the cells. The lower peak charge voltage will slow the charge process, so they are topped off at a lower current and a lower voltage.

 

[MountainBiker]

Perfect. Ty for your advice.

 

[time2roll]

Disconnect the 20a charger until the battery gets low. Depending on conditions this might be very rare.
Reduce the float settings of the Renogy and solar closer to 13.0 - 13.2

 

[marionw]

The Renogy charge controller like so many other like it have a load output that is somewhat programmable. This output can operate in a "streetlight mode" to turn on loads after dark and to also not turn on the loads based on battery state of charge. It may be possible to operate a relay (not any loads) that would control your dc-dc and ac-dc chargers.

In addition if you have programming capability and your charge controller has an RS232 port you could create some software for an Arduino or Raspberry Pi that could control the chargers via relays. Just search the internet for "ROVER MODBUS.pdf". There are projects on Github for this purpose.

 

[MountainBiker]

I am "techcnical" but my area of expertise is AI / ML and data. I could probably get it done tho. It just it might take more time that I have currently allocated for this bit ! I am going to look into what each charge device can to to help given all the above advice and then my legs and arms to handle the rest that I am willing to do !

 

[marionw]

Relay operated by the "Load" output would be the easiest. Load always on except when battery SOC/Voltage falls below your set point at which time the relay is de-energized activating the dc-dc or ac-dc charger. The relay is re-energized when the SOC/Voltage set point for turning the load back on is reached. Just make sure you use a 12vdc relay whose contacts can handle the current. A heavy duty automobile relay from an auto parts store might work. Just keep in mind the relay will be a "load" on the inverter/battery so you want one that does not draw a lot of power to operate the relay coil

 

[MarcoWasRight]

I have 2 Power Queen lithiums in parallel and 3 power sources

1. solar (Renogy 30a charge controller)
2. dc-dc (Rengogy dc-dc 40a charger)
3. ac-dc (power queen 20a charger)

These charger are always hooked up. If i drive, dc-dc charges, when plug in to shore power, the ac-dc charges and solar charges when there's sun hitting the panels. The devices hooked up to my battery bank are always consuming power when they need it. Mostly its the dometic fridge but lights sometimes and the diesel heater at other times and various other intermittent power uses. I asked Power Queen about DOD and charge cycles and how to extend the life of the batteries. Here's their response.

"Hi Tim (that's me). If you want to expand the lifespan(cycle time) of the batteries, we recommend charging the battery to a certain level first and then discharging it to a certain level. This operation is more useful than charging and discharging at the same time all the time. Because it you keep the battery charging or discharging all the time, if the batteries are plugged into shore power or driving all the time, it is possible to reach a deep DOD(100% charge or discharge). In this case, it is not easier to control the DOD."

Obviously this isn't easy to do unless all my chargers can be configured to

1. turn on at say 50% DOD which in my case is 13.15 - 13.2v
2. turn off before 100% which should be around 13.3v

I think maybe my solar charge controller has that kind of setting. My renogy dc-dc and power queen ac-dc do not. If I want to do something like this with my current setup, I'll need to set alerts with my Victron shunt and manually turn off/on my devices by hand. Is this something that you folks worry about? My hunch is that I just need to manage this by turning off these devices when the charge voltage gets near 100% and turning them back on when at 50%. I guess that's not a big deal when I am in or near the RV but not super practical.

There are simple relay modules that you can set up to turn a device on and off when a voltage point is hit.

You'd have voltage sensing wires from the battery going to the input of the module and then you'd set the module to turn on 12.6V and then turn off at 14.0V

The problem is the size of your loads are going to impact your battery voltage at the terminal...as if voltage already wasn't a good representation of LFP SOC this makes it ten times harder. If your loads are fairly consistent then it is possible to set this up. Don't fall into the whole 80-20 or 100-50 BS...just pick some numbers that are above empty and below full.

With three forms of charging...two of which I guess could be on at any given time....or perhaps only one? your voltage rise from charging could vary...so I guess if you were driving and getting some decent solar charging you could hit 14.0V on your battery bank fairly easily despite your no load voltage being ~13.30V and your SOC being somewhere around 40% to 60%.

So I think it would be easier for you to try to implement such a system using the Smart Shunt's SOC measurement and controlling relays that way.