LiFePO4 planning

[guyman]

Solar panels 4-24V 250W in parallel
Victron SmartSolar MPPT 100/50
LiFePO4 battery (8x120A)
As per Will's DIY LiFePO4 instructions
I have read Will's book (thank you Will) and watched the videos several times.
1) I have read/seen somewhere "LiFePO4 batteries do not require a float when fully charged" or have I made this up?
2) In Australia it can get to 50C 122F degrees plus in a vehicle in full sun. How do I keep these batteries cool fora longer life. One idea I had was mini 12V fridge with small hole in one side, with duct to closed in battery box with smallcomputer fan sucking cold air through.
3) Should I fit a breaker from the panels to avoid frying the MPPT when I'm setting up and doing maintenance.
I'm still at the spreadsheet stage working out cost, parts, calcs etc. Any thoughts please

 

[svetz]

2) In Australia it can get to 50C 122F degrees plus in a vehicle in full sun.

I'd say look for a low Pmax on your panels. Check out Comparing Solar Panels if you're not sure what that is.
Don't know about the rest, interested in what people have to say, but dubious on the fridge idea.

 

[guyman]

Thank you svetv. I'll will look for low Pmax panels. I agree the fridge idea is not my finest hour, back to the drawing board.
As regards the breaker from panels to the MPPT this may be the solution Breaker

 

[gnubie]

The lifepo4 is 8 cells in series, ie 24v? If so you should run the panels in series, at least 2s2p, but honestly 4s is the way to go unless you are trying to work around shading. The Victron will be happy with that. Check the VoC of the panels to ensure that 4 in series do not exceed the 100v rating of the Victron as what passes as a 24v panel is not standardised. If they do exceed the Victron's ratings you'll have to go with 2s2p.

I can speak from personal experience because I was running that exact configuration for a while, 4 x 24v panels in series, right down to the specs of the Victron. The Victron is now running 10 panels arranged as 5p x 2s, with a VoC per set of 88v.

 

[chi]

As @gnubie mentioned, use as much as serial as possible (better efficiency, thinner cabling). Preferably 4s, but I think this probbaly too much voltage for your Victron MPPT, since Voc of 24V panels is ~35V (=> 140V). So the next best solution is to go for 2s2p what fits well with the specs of your Victron MPPT. Shading should be considered, but with a 2s2p you are generally better off than with a 4s concerning this topic.

 

[guyman]

Thank you gnubie and chi. I was off to a bad start, back to the spreadsheet to get the correct panel configuration. I've used the Vmp and Imp, when I should use Voc and Isc

 

[guyman]

Following the above advice. My main concern is, are my maths correct?
Using 4-24v 250W solar panels
Voc 37.9V
Isc 8.65A
These are my results
1) Series connect: 151.6V 8.65A Voltage too high for Victron MPPT
2) Parallel connect: 37.9V 34.9A Too large a wire size required
3) 2S2P connect: 75.8V 17.3A Within Victron MPPT parameters
Is my wiring diagram correct using the 2S2P setup?

 

[gnubie]

Yes, maths and wiring is good.

 

[guyman]

Thank you gnubie
Is there some where I can calculate the wire size from the panel to the MPPT, or just base the wire size on using 24V sizes

 

[gnubie]

Yes, there is. We have a FAQ that is slowly growing.

 

[guyman]

Thank you.
I used the Renogy site