On board electrical design

[bobakers]

Hi all
have start the build of my 6x6 land rover perentie, have the truck where I want it and now working on the body which will be a composite full height design.
Looking at the electrics, I'm going with 400ah lithium with winston cells and no lpg inside the body (will have a bbq on lpg outside). This is what I have come up with so far:

appreciate any feedback or for anyone to point out any glaring errors.

thanks
Bob

 

[Rando]

Do you plan to use this primarily plugged into shore power, or off the grid?

It is a bit hard to tell the specifics here - but to start with I don't seen any monitoring, do you plan on a BMV-712 or something like that?
Secondly, are you planning on a residential fridge run through an inverter? This seems like a very inefficient approach to this. Any reason you aren't using a DC fridge?
What does the DC-DC charger connect to?
Finally, I doubt it will be practical to run the 1.7kW air con off batteries, but maybe you are planning to be plugged in much of the time. You certainly can't run it off a 10A CB.

 

[luthj]

A few questions. What BMS are you using? How are you handling battery disconnect, balancing etc? The Victron inverter is also a charger correct? How will the system react to the inverter being on the load bus, and providing charging in the case of the disconnect?

What is the standby power of your fridge inverter? Are you going to use the fridges thermostat to switch the inverter on/off? Or will it be on 24/7?

Wire sizes are an important consideration, details on run lengths and gauge will tell us if you fuse selections are safe.

What size of cells are you using? 100AH? 200AH? Note that 200AH is the max I would go for a vehicle application. Any larger and they become more prone to vibration damage. It depends on your needs, but I would consider going to a 24V system, and using 8 200AH cells in series. Since you are using a DC-DC charger, there is no requirement to stay with 12V. You can use a small 24-12v converter for your 12V only loads.

The 24V system would allow for a more efficient inverter, light wiring, etc. There are pros and cons to the 8S battery configuration. The major Pros are simpler balancing due to only 1 string. The major Con is that a single cell failing will take the pack off line.

 

[bobakers]

Thanks Rando & Luthj

The plan is to spend most of our time free camping (boon docking). Monitoring will be with Victron 712 and using the customisation of the various Victron units to set charge voltage to stay out of the knees. I have a small genset for those days where we are not travelling and need a top up due to bad weather.

Samsung 221 litre inverter fridge will run comfortably off a 300w inverter and will maintain -18 degrees c freezer and 3-4 degrees c in +35 degrees c outside temp. The waeco compressor I have in our current caravan is much less efficient and has struggled in high temperatures.

The dcdc charger is connected to the vehicle’s alternator via a redarc SB1 Solenoid.

The aircon is a residential inverter split system. The 1.7 kw is the cooling power not the current draw. The current draw is listed at 0.42 kw when cooling and 0.465 kw when heating.

BMS for failsafe will be an EV Works Zeva 8 cell monitor with low and high voltage cell level monitoring controlling tyco 500a latching contactors. Cells I’m looking at are 200ah so 4x2.

I struggled with where to connect the Victron multiplus 3000W but decided to place it on the Load bus with a HV relay to cut the 240v to the victron multiplus if a cell goes high

Balancing. Without starting a war I’m intending to top balance at the assembly of the pack and wont be running cell balancing as I don’t intend to charge to 100% Nor discharge below 20%. I know that there are a lot of theories but this seems to be safe for house batteries.

Wiring will be the appropriate size for each system/run length and fused to protect said wiring. Batteries are 1.5 metres from the electrical cabinet and solar panels up to 4 metres.

Re 24v, would love to go that way but the land rover is 12v and I already have a number of 12 v appliances so it would not be cost effective.

whilst I have a bit of experience on low voltage and generators, and a working knowledge of 240 & 415 ac, this is my first foray into lithium so appreciate your feedback.
Regards
Bob

 

[luthj]

With regards to your pack. Even if you don't have automatic balancing, you need to have a method to manually balance the assembled pack. Assuming the total internal resistance variance is plus/minus 5% across the cells, you will still need to balance every 100-300 cycles. Obviously charge/discharge rates play a large roll. Avoiding the knees will delay balancing needs, but inevitably a cell will trigger pack drop out due to high/low voltage.

Manual balancing can be accomplished with a properly sized resistor in the 3-10W range. Another option is a 2-5A DC-DC power supply with calibrated output. Usually around 3.5-3.6Vpc