question

@martinwinlow

I am always a fan of a mix if battery cycling is happening.

AC for loads DC for charging the battery.

It really depends on its entire purpose.

@Alexandra

Hi, and thank you (belatedly) for your comment.

Since my original post, I have completed the tracking array which, on a good day, generates about 33 kWh. I am quite pleased with this given my 56° latitude.

However the question of re-jigging things to better suit my situation has come up again and I would welcome any advice.

The main issue, here, is that I do not own the property and will be leaving it in the next 3 years or so as I am building a house of my own, locally. The owner of the house I am in currently and I are good friends and it is a bit of a test-bed for technology that I am interested in and the landlord is as well, hence his tolerance of me ‘messing around with it’. This includes not only alterations and a small extension I have made to the house but also the renewable energy side of things; renewable microgeneration, battery storage and heat pump technology being the main subjects trialled, thus far.

When I move, I will be leaving much of this behind, and whilst that may seem strange, the landlord will compensate me adequately in due course (directly or indirectly) as is our long-standing arrangement. When I leave this property, I do not want to leave the landlord or occupier with a large complex DIY-based ‘entity’ for them to deal with. To that end it is my intention to hand the property back with functional PV array with microinverters (which are currently installed and operational) but remove all the batteries and MultiplusIIs and rewire the consumer unit to a straight-forward and conventional setup.

As things stand, the garage is connected to the mains supply via its own consumer unit (and meter) effectively bypassing the house’s 2 x 5kW MultiplusIIs and 15 kWh battery. So, the PV array currently provides power for the garage only and any excess is dumped onto the grid (which I do not get compensated for).

What I would much prefer, Obviously, is to use the PV generation to charge the house's battery and only use mains power if the battery state of charge dropped below a predefined level.

On top of that I would like to install a bigger battery in the garage where there is more space and safer, and add a third separate MultiplusII to supply the garage (including light EV charging use). As I see it, to do this using AC would involve replacing the existing house to garage cable with a considerably heavier cable which is something I do not want to get into.

Therefore the only ‘easy’ way I can see to avoid the need for a heavier interconnecting cable, is to use the existing 4 mm² AC cable as a 48 V DC connection between the house and garage and then charge both house battery and garage battery via a Single MPPT located in the garage. Thus the garage AC loads would only be supplied via the third 5 kW MultiplusII and the house would continue to be powered (mainly) by its MultiplusIIs and existing battery.

The only downside is to this idea as far as I can see, is that the tracker would require a small inverter to power it (as it is designed for an AC connection - tho this maybe getaroundable by rejigging the controller, conceivably).

I guess the main question is: is the 4mm2 interconnecting cable gong to be large enough to deal with any possible charging/discharging *of either or both battery* scenario?

It's also worth mentioning that a future project will be a small (0.5-1kW) Wind Turbine addition which will charge the two batteries. Up here, this will generate considerably more energy in winter than the PV array will in winter (on average).

Question: Is this idea a sensible/feasible arrangement?

Regards, MW.

4mm2 wont work, its just to small, cable will have a high voltage drop and could get overheated really fast… dangerous

Well you wrote about a dc interconnection between house and garage, so i assume its not low on amperage…

5m (back and forth) on 48v 30A is already a 3V drop/heat

I think having two batteries separated by a long length of cable is asking for trouble. Which end will the BMS be and how will the other battery be managed?

Can you not rewire the system so the garage CU runs off the main house CU with a meter on the grid tails, then you can export from garage to house without the power going outside the Victron ecosystem?

So, schoolboy error: I completely forgot that whereas the output from the 10 module PV array may well be only 20 A, after it has gone through the MPPT (for a 48 V battery) the current will be considerably more. So that answers that question and the previously discussed scheme will not work.

My next alternative would be to have two use 2 x (250V/70A) MPPTs, one on each battery, linked in the same way to the PV array as before i.e. at 250VDC via the existing cables. This would mean that when the garage battery was flat that I would have no way of charging it from the mains but as it is going to be a comparatively large battery and the garage consumption amounts to less than 3 kWh per day I don't think this will be an issue.

I have attached the sketch below and wonder if this is feasible?

The next alternative is to just stick with AC coupling. The question then would be (and this is the reason I went down the rabbit hole of DC coupling in the first place - aside from efficiency issues) how would I do that easily, leaving the cabling between house and garage as it is?

I am beginning to think that just putting in 2 heavier cables (bi-directional AC - capable of running the house loads from the garage as well as supplying at least 30A to the garage for EV charging from the mans at night and charging the garage battery simultaneously) between house and garage and then move everything into the garage would be the better plan… but that is going to be very disruptive (digging up patios, etc) and the 10mm2 x 4 armoured cable won’t be cheap, either (at £8/m - but cheaper than a second MPPT) …

Oooft!… My head is beginning to hurt - and I may be over-thinking this!

OK - I have realised that the link from house to garage is actually up to handling the max likely current flow in either direction (being 6mm2 SWA - continuous 12kW).

So, I am going to move the battery and inverters into the garage and DC couple the PV array until I move out (taking all the batteries and inverters with me) whereupon I'll reconnect the microinverters and just have a very basic system little changed from the original setup.

This also gets around the problem of getting the EICR done as - as far as the house goes - it will be easy for an EICR electrician (without any renewables experience) to get their head around.

The question is, is there a device that will connect and disconnect the mains supply (that goes into the house) according to the SOC of the garage battery? Obvs, if we are in the midst of a sunny spell of weather, all of the house's power will be derived from the PV array which will have filled up the battery each day (let alone when I add a wind turbine's output to the battery as well)...

But will this mean having to run the mains to the garage first?