question

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tris-stock asked

How to plan a two stage build?

Stage One:

The basic premise is, that I start with this single 200w PV panel on the roof of my 2015 mkVIII Transit L4H3 with a standard Ford alternator. These are facts. They are not negotiable. I am aware of the possible problems with flexi panels, and am prepared to roll with the consequences. Thank you.

What do I need to do, then, - with Stage Two in mind - to build a suitable 12v only system powered by both the 200w PV panel, and whatever the starter battery/alternator throws at it?

Also, do I really need a discrete DC-DC converter for the starter battery, while the PV panels are hooked up directly to the MPPT charge controller? If so, I might have to shift starter battery/alternator power into Stage Two.

My initial consumption projections for Stage One are low (pretty much just running the roof fan occasionally), but with Stage Two in mind, I am looking at a single lithium battery, of as yet unknown properties (probably one of Fogstar Drift's non-Pro 12v 230-300ah jobbies), that will later be doubled up to maximise the Stage Two goal.

Stage Two:

Another of the same PV panels on the roof (now 400w total, but in series or parallel?)

Another battery (I'm guessing the same make and capacity of Stage One's Fogstar selection.

Installation of a 240v inverter (Pure wave, fo sure, but what wattage, before Star Trek's Scotty would have palpitations?)

Precise consumption figures are inherently unknown, as I will ultimately design my consumption around the power system I am building. Yes, maybe that's an unusual way of doing things, but that's what I'm doing. In Stage Two, for example, I'd like to 'go in hard', so to speak, with an induction hob, before any other consideration. Is this even possible, and to what end?

Optimal performance after Stage Two is the ultimate goal, but I don't want to have to buy replacement components for it, if I can possibly get away with it. So, Stage One - where conceivably possible - needs to posses 'scalability'.

Other considerations:

1. I am not overly mindful to include a shore connection at any time in the future, so it really needn't be much of a consideration. But if one can be shoe-horned into a post-Stage Two build, that'd be great.

2. I definitely want to operate it via an app (or two). Other control systems are a secondary consideration.

3. Considering the series/parallel output of the PV panels, would the two Fogstar batteries be wired in series or parallel themselves? I feel sure this makes some sort of difference.

Well, I guess that's enough to be getting on with for now, and I have, no doubt, overlooked something of great importance. Reading back over this, I'm reminded I come from a background of building PCs, rather than RVs. And that It seems to me, that camper power systems are a little bit like therapists: each one appears to take a different approach to resolving effectively same issue. Hence my confusion.

Thank you in advance for your thoughts. I very much appreciate it.

MPPT Controllers
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2 Answers
kevgermany avatar image
kevgermany answered ·

Stage one. A dedicated DC:DC charger suitable for lithium is mandatory to charge lithium from an alternator.

Exception is if you use so called drop in lithiums which are designed to replace lead acid. But using these will not isolate house and starter battery, so the DC loads on the house bank will draw from house and starter batteries.


Stage 2. Start from the desired result. You don't build without this. Part of the design must be expandability. If you don't want to be replacing components, then they have to be large enough to start with

Induction hob, fine. Batteries/charging/inverter/wiring must be sized correctly from the start


This community isn't for large design questions. You might find Greg Virgo's YouTube channel helpful in resolving things.

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tris-stock avatar image
tris-stock answered ·

I think I've found my solution to the PV panel/Starter battery problem I was trying to resolve.

https://www.youtube.com/watch?v=JtAUrS4zNSA

*Edit - Just realised that is a competitor product. Does Victron have a similar product?

This unit appears to act as both a PV panel MPPT, and a DC-DC charger for the starter battery, which is exactly what I was looking for, and I think it sufficient to cover both stages of the build. Maybe?

With that problem solved, I shall heed your advice against generalised questions, and move on to the remaining specificity of battery capacity/wiring configuration. I don't think I want more than two batteries, so what capacity battery should I go with for Stage One (with a single 200w PV panel, so that I can double it up for Stage Two (with 400w PV panels) to best overall effect? By that, I mean not only battery capacity, but which series/parallel wiring for both PV Panels and house bank would be most effective?

Thanks for your thoughts thus far. Very helpful.

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