Replace factory second battery to DC/DC

[RoadRunnerDU]

My T6.1 came with the leisure battery. I want to take it out and replace with my own dual battery setup with DC/DC charger 200ah lithium and so on.
I read multiple discussion here and had a look under my seat and there it LOTS of stuff. I get this is all connected to the brains of the car ECU and has smart alternator and other trickery built in.
Is there a way to remove/bypass most of this ?
Assume the little thing on negative is a BMS. 200/300A fuses seem a bit excessive for those cables. Also never figured out what actually is connected to that battery.

Anyway. What I'm after is just to disconnect most of this and connect a DC/DC charger that I can feed into my own battery. If someone knows a discussion in here and can point me in the right direction that would be great.

 

[RoadRunnerDU]

Followed a few cables and it looks like the 100A fuse goes to that green 40A fuse and the 60A yellow fuse goes to the fuse block with all the 30A fuses in it.
200A fuse comes from the relay from the main battery that a lot of people mention in here. From what I read I can just disconnect the 200A fuse and use that to feed the DC/DC charger ?
If so what is supplied with power from the 60/100A fuse and how do I power that stuff again after ?

 

[Richard T6.1]

I've been trying to get my head around the factory 2nd battery setup I have in my van too. Although still a little beyond my understanding of car electrics, I did find a lot more info in this huge document I found here, VWs Body Builders Guidelines for the Transporter. Section 2.5.4 Vehicle battery provides the most info > clickable from the Table of Contents.

It's a monitored system as VW calls it. The white/grey-ish thingy on the minus side of the battery appears to be a shunt. The starter battery under de bonnet has one too and they are used to monitor the batteries state of charge, working together with a CFCU (customer function control unit) under the dash and the isolation relay.
I tried to figure out what the charging system actually does with a Volt meter in the 12V socket on top of the dash. The charging voltage from the alternator varies depending on the SoC from the batteries when you start the engine (or that's how it looks). In my van it settles on a voltage - even as high as 14.6V - and stays there during that trip. On a next start-up it can sit at 12.5V and stay there as well appearing not to charge at all. I'm still a bit puzzled by the charging behaviour/pattern tbo.

To bypass or change this setup, or parts of it, requires a good understanding of how it works imo. I'm not there yet.

Hope this provides some pointers.

 

[RoadRunnerDU]

Thank you for linking the Body Builder Guide. Sure that will come in handy one day. Yeah they have a smart alternator so voltage keeps changing and everything is controlled with ECUs. All those shunts talking back and so on is a pain. Surely someone has a workaround or some electronic gimmick to deal with all this ? I have access to a VCDS as well if that helps. For testing I disconnected the first wire from the 100A fuse to see what stops working and so far everything works and no warnings. There must be some wiring diagram to wee what all those fuses are for.

 

[roadtripper]

This is a very common discussion - I'd recommend searching for @Dellmassive "How I dun it" threads around electrics.

With the "full" second battery you have 3 areas you need to think about

1. You want to replace the split charge relay with a DC-DC to get charging to 100% or use a different battery chemistry
2. You will need to find some way of replacing the battery mounted fuse box - this moves several important things in the van to the second battery like most of the 12v sockets, auxiliary heaters and one of the trailer feeds
3. You seem to have the absolute full setup with a shunt on the negative of the second battery - so that will need removing and the coding updating in the van so that your starter battery and start/stop continue to work correctly.

The first point is quite easy to do and will improve the fitted system, just use the signal wire to the relay as the DC-DC charger ignition enable.

The others take more work.

As an aside if you take out VIP Memberhsip the wiring diagrams for all this are available in the download section.

 

[RoadRunnerDU]

Excellent. I had a feeling my van has the lot also it's just an empty van. I have nothing in the back. No 12V sockets, heater or anything. I've seen the towbar module under the driver seat even though I have no towbar someone must have ticked the box.
I have
8AR Optional infotainment system
8GV Alternator 180A
1D7 Preparation for detachable trailer coupling
J0V Battery 420A (70Ah)
So maybe gets powered from the second battery.

1. Yes want to take the second battery out and have installed a Lithium battery in the back of the van
2. yeah these fuses. So many of them just seems strange given there is nothing in the back of the van and that would only be powered from that battery as well.
3. the fun bit. Car has start/stop but seems once you push the button that stays off. Since I bought the car it was never on. Removing those shunts and the programming will be fun and scary.

Just going away for a couple of weeks end of April and wanted to get my Lithium setup working as I have an inverter and fridge and so on. Hope I get this sorted before.
Will get the VIP membership as hopefully the diagrams show what those fuses are for. I had a look at @Dellmassive but it seems most in here don't have as much "stuff" around the battery as I have.

 

[roadtripper]

This post from @mmi should put you on the right track with VCDS to make the van forget about actively managing the energy in the second battery (point 3).

Post in thread '6.1 Immobiliser/Battery Short Carista Codes'

26 Oct 2023

134410 - activation of the second battery: Open circuit / short circuit to B+

I have a factory fitted leisure battery and installed a Redarc DC-DC

I guess it's because the energy management doesn't see "proper" behaviour of the second battery.

I would try changing status of second battery to "not installed" as below (in central electrics adaptations):

MAS01490-ENG142037-Energy management-Zweitbatterie

not installed

However, Carista probably can't do it. VCDS, OBDeleven or VW ODIS will do.

• mmi

• 

For point 2 I would seriously consider if you need such a large LiFePo battery. Currently you will have the standard AGM charged to around 80% (as it's split charge relay so will be in a bank with the starter - and start stop/smart alternator leaves headroom for regenerative braking) - so with the nominal 50% discharge rule you only have 30% (~30Ah) usable. Replace that with physically the same format LiFePo and a DC-DC charger and you will get 100% charge and can run them down to at least 20% - giving you 80Ah usable - so nearly tripling your usable capacity.

If you can still use the battery mounted fuse box it's a very compact and neat solution. Even if you have bolt type terminals on the new battery you should be able toi get post converters for this very reason.

 

[roadtripper]

Yep towbar prep means you have the module in and the van is configured for it so you can simply plug in a towbar later. I did a thread on the 6.1 towing socket (at least the UK version) here. I know some of the lower 7 pins behave differently for you with electric braking but I think the higher numbered power supply pins are universal. Pin 10 moves to the second battery if you have one under factory wiring - that's what your 60A fuse is doing feeding that strip of green 30A fuses.

Generally speaking the second battery often gets fitted when there is some other high electrical load option specified in the build so the starter is protected. For instance if the electric cool box was ordered, or one of the diesel cabin heaters that can run with ignition off. I would sort of expect to see the higher output alternator there (250 rather than 180)

If the start/stop remembers the state with the ignition off then someone has probably been tinkering. You may find if you look behind the switch a small plug in module that's not VW as that's quite a popular approach...

Don't remove the shunt on the starter unless you enjoy mysterious electrical issues - just remove the one on the second battery and tell the can to only worry about the starter.

 

[Jamesie]

I'm part way through doing exactly what you are looking to do. I've already got the Victron Orion XS 50 DC to DC charger in and working in place of the split charge relay. I've just replaced the mains charger I had which wasn't lithium compatible with a Victron IP67 one. My new Fogstar 100Ah lithium battery, smartshunt and various fuses and connectors are in a box ready to be installed as soon as I get a chance. I would say though, my T6.1 had the VW factory second battery but did not have that shunt device on the neg battery terminal, and didn't have anything on the 100A fuse on the battery fuse box. See pic of how I installed the DC to DC just before I cut out the SCR and transferred connections across. I'm planning to use in-line midi holders / fuses in place of what's on the battery mount fuse box. I'll post more photos of the install when I get the battery installed in the next few days.