New system for RV

[Ben1962]

Hello from Canada. Based on what I learned on this website I designed that system to install in our Fifthwheel. My idea is to use as much of the existing system as possible. I already have 8 headway 38120 LiFePo4 cells which is 24V when you put them in series and just received the MPP PS 3KVA unit. So now I just need to put it all together and install it in the travel trailer which will be our home for the next travel year. What do you think?

 

[Off Grid Circuits]

From first glance, it looks like it's missing a few necessary items.

 

[Ben1962]

From first glance, it looks like it's missing a few necessary items.

I admit it's a quick drawing without any fuses or shunt showing but I do intend to put in all necessary protections.

 

[Bigbillsd]

I would suggest you put in a battery meter like a Victron BMV 712 of equivilent, the detailed information it provides about your usage would help when you go to size your solar panels.

 

[Ben1962]

I would suggest you put in a battery meter like a Victron BMV 712 of equivilent, the detailed information it provides about your usage would help when you go to size your solar panels.

Thank you.

 

[Ben1962]

Updated my drawing with a few details.

 

[Ben1962]

With the MPP system do I need to put BMS on the battery pack it's Headway 38120 8 cells pack?

 

[gnubie]

If you have access to the cells it's always a good idea to put a BMS on them.

The MPP can only see the voltage across the battery and current through the battery. It can't see what is happening to each cell. Good lithium cells don't drift that much but over time they will drift and that will reduce the total capacity of the battery until you rebalance the cells manually. As the cells drift the voltage across the cells during charge will be different potentially taking one or more cells above their maximum voltage and you don't want to do that with lithium cells.

 

[Ben1962]

If you have access to the cells it's always a good idea to put a BMS on them.

The MPP can only see the voltage across the battery and current through the battery. It can't see what is happening to each cell. Good lithium cells don't drift that much but over time they will drift and that will reduce the total capacity of the battery until you rebalance the cells manually. As the cells drift the voltage across the cells during charge will be different potentially taking one or more cells above their maximum voltage and you don't want to do that with lithium cells.

Thank you for the explanation.

 

[adas]

Aren't headway 38120 cells 8Ah at 3.2V nominal? So your total capacity with only 8 cells would be about 200Wh, which is very small. Or is this just a proof of concept before buying a much larger batch of cells?

Also, 10ga wire is far too small between a battery and a 3kVA inverter.

 

[Ben1962]

Aren't headway 38120 cells 8Ah at 3.2V nominal? So your total capacity with only 8 cells would be about 200Wh, which is very small. Or is this just a proof of concept before buying a much larger batch of cells?

Also, 10ga wire is far too small between a battery and a 3kVA inverter.

It just proof concept for now. I intend to put 24 more cells when I do the installation in the RV and I have space for a total of 64 cells. About the 10 gage wire, it's what's in the RV, from factory, between the shore power and the fuse boxe. But I'll verify calculation before final install.

 

[adas]

It just proof concept for now. I intend to put 24 more cells when I do the installation in the RV and I have space for a total of 64 cells. About the 10 gage wire, it's what's in the RV, from factory, between the shore power and the fuse boxe. But I'll verify calculation before final install.

10ga is probably fine for the AC connection, but the DC connection between the battery and the inverter could carry 125A or more. I'd probably go 1/0, or even bigger.

 

[Sojourner1]

Read the inverter instructions, they will tell you what size wire to use.
There are plenty of DC wire selection charts here is one.

 

[Ben1962]

I have at home a 20ft section of 2awg welding wire and I intended to use since the distance between the inverter and the battery pac is less then 10ft. The max load I'll run is propably a tv and a laptop which is under 500 watts. All lights in the RV are LED so very low draw. The other items are heater fan, pump, fridge's pump are all temporaray us for minutes so it should be fine. Any sign of over heating would trigger an upgrade whish would be easy considering distances.

 

[Sojourner1]

Some of the items you're mentioning are 12v and would not be going through the inverter.