t6 t32 Is it possible (or worth it) to combine leisure battery systems?

[t6t32]

How far do you think I should go with a Lithium replacement?
My 2018 t32 t6 is a pop-top conversion which has a factory fit diesel heater and 75Ah leisure battery under front seat and an independently wired 105Ah leisure battery system (VSR, EHU mains charger and EHU) in the rear supporting camper electrics [fridge,lights,hob].
It has a dodgy 5 year old solar panel and MPPT to the 105Ah with no isolation other than connectors that I can see.
Downside is that the front leisure battery is dead weight for most of the year when we are not using the diesel heater.
I want to upgrade to Lithium and fit an inverter.
I think my options are as follows:
a) replace dodgy Solar panel (it only works some of the time) - currently 100W - is it worth changing it to 200W using existing wiring but insert isolator and replace the MPPT at the same time?
b) just do the camper side of the set up: so DC-DC to replace VSR, change mains charger for an Li one and new Li battery.
c) as b) and add inverter to this set up (only need 1kw).
d) as b) above but add inverter to 75Ah set up but put it under the drivers seat with its fuses/relay/sockets etc. What is max inverter you can get from 75Ah? (900W?)

e) be brave and rewire as per VW set up so
twin 105Ah Lithium,
replace VW relay with DC DC, [will the feed to this from the alternator support charging both batteries?]
run battery leads (+ and -) between both batteries (unless can rely on chassis earth?). Fuse in both ends of long run +ve (may be able to reroute current lead that runs Alternator to rear).
Ditch current VSR as both batteries will now be connected to the DC-DC.
Replace current mains charger with charger/inverter so that it has enough grunt to charge both leisure batteries [or will this melt the wiring so separate charger and inverter instead?]
Break the existing EHU to consumer unit connection and insert new relay between EHU and charger/inverter
route o/p of charger/inverter to current consumer unit to feed power sockets

So b) is a relative breeze as like for like swap out, but still leaves the 75Ah as a passenger
e) looks ideal but a PITA as lots of cable routing and parts replacement

Anyone got any recommendations/suggestions?
Thanks

 

[roadtripper]

Remember that like for like capacity LiFePo will be smaller and lighter - plus you can easily use 80-90% of the capacity without hitting battery health unlike 50% for AGM

So I'd really favour simplicity - you've got a lot of kit in a small van otherwise.

Pick one location and stick a 200ah seat base type battery in it.

Anytime you mess around with forming batteries into a bank it brings issues unless they are identical in every way including age. It's more complex with LiFePo as they have to have BMS that understands that too. Avoid the complexity if you can.

 

[t6t32]

Remember that like for like capacity LiFePo will be smaller and lighter - plus you can easily use 80-90% of the capacity without hitting battery health unlike 50% for AGM

So I'd really favour simplicity - you've got a lot of kit in a small van otherwise.

Pick one location and stick a 200ah seat base type battery in it.

Anytime you mess around with forming batteries into a bank it brings issues unless they are identical in every way including age. It's more complex with LiFePo as they have to have BMS that understands that too. Avoid the complexity if you can.

Thanks that's what I suspected as must admit that is what I thought I would do until I looked into it and checking on here for diagrams and kit suggestions ie replace the existing 105 AGM in the rear of the van - however had never looked into the diesel heater set up so when I found another leisure battery under the front seat seemed like a waste.

 

[roadtripper]

I suspect what's happened is that the base van had an aux battery specced to protect the starter from the heater running it down - a sensible choice.

Then on conversion the 75aH is a bit small but the heater useful so you add a whole leisure setup - again a sensible choice.

But you probably wouldn't do what you have in one step. There is little point now in the heater having it's own battery, if you add an inverter the power draw of that will dwarf everything else.

When you plan you want your inverter physically very close to the battery otherwise you are going to need route very heavy duty cables between them.

 

[True Romance]

Why did you feel the need to add a second LB to start with? My factory LB can run my van quite happily (fridge, USB points, lights).

I have upgraded recently so I can run 240v off grid and in the end just brought a power station to keep things simple and can take it with us if/when we swap vans.

 

[t6t32]

We got the van last year and it came with both batteries.

 

[roadtripper]

I have upgraded recently so I can run 240v off grid and in the end just brought a power station to keep things simple and can take it with us if/when we swap vans.

The larger power packs are an option for some situations but the OP has a diesel heater and they tend to need permanent power which those types of devices generally can't provide.

It's an alternative to a fixed inverter though, the challenge is recharging then effectively. The ones that can take higher current via the solar input can help there.

 

[True Romance]

The larger power packs are an option for some situations but the OP has a diesel heater and they tend to need permanent power which those types of devices generally can't provide.

It's an alternative to a fixed inverter though, the challenge is recharging then effectively. The ones that can take higher current via the solar input can help there.

The factory fit heater will run off the factory fit LB so has its own power source.

I've tested my power station by just plugging into the vans EHU point as if on a site hook up and carried on as normal, using a mixture of 12v and 240v appliances. Charging via solar atm but do need to sort power from alternator to charge while on the move.

 

[roadtripper]

Break the existing EHU to consumer unit connection and insert new relay between EHU and charger/inverter
route o/p of charger/inverter to current consumer unit to feed power sockets

One thing to avoid is this, there are highly specific ways an RCD needs to be wired in a plugged in EHU that differs from an unplugged in inverter fed system.

It's possible to do that but it's much much easier just to keep invert fed and EHU fed sockets separate.

 

[roadtripper]

I've tested my power station by just plugging into the vans EHU point as if on a site hook up and carried on as normal, using a mixture of 12v and 240v appliances.

Depending on the wiring of your van and how your power pack is configured that's a potentially unsafe thing to do.

Your power pack almost certainly relies on the inherently safe small generator approach of having neither output referenced to ground, the only way to get a shock is to touch both outputs at once which is unlikely.

Your EHU however is required to be grounded to the van body for safety, and will usually have an RCD. This will only operate correctly on plugged in EHU as your van is largely insulated from ground but the wheels so requires the ground conductor in that cable.

Plugging a power pack into EHU connects one of the outputs to your van ground, but does not connect any of the van to an actual ground. In the event of a fault the RCD will not operate but the van body is potentially live.

Properly configured change over systems in things like a Clayton or an inverter/charger use relays to change the grounding according to the power source.