AMAZINGLY I got my 48 5000 70 from someone who was doing exactly that upgrading to 48 1000 140 inverters BUT he is a farmer with a Three Phase Supply
I think the first sensible step is to spend my money on a smart EV charger and change my tariff to Octopus Intelligent GO
… is probably to wait and see how you get on with your new system.
You’ll need to know what your typical daily battery requirement actually is - and it’s likely to be greater in the middle of winter.
There’s no point in over or under provisioning the system for a particular moment.
The IOG tariff is in a state of flux just now. I’d wait until the new interpretation of its rules is clear before jumping onto it.
There’s also a school of thought to size the battery pack so that it does get fully cycled every day. This is not for any battery chemistry reason, but because it represents the best use of the considerable investment. If the US5000 battery purchase price will be £100 less in 12 months time (which wouldn’t be unprecedented), you’d need to save £100 in grid costs in the next 12 months to make buying additional capacity now a worthwhile investment.
You don’t need to. You can see graphs of past system metrics in the advanced tab of VRM. Enable the battery monitor widgets.
My own situation is that I’m finding the 48/5000’s 4kW output sufficient for my domestic loads, but I’d feel more comfortable with a larger battery in winter when I do almost all charging from Octopus Go.
You are quite right that the MP2’s 3.5kW charging current gives an effective upper limit of 17.5kWh that can be fed to the batteries in a 5-hour slot. If I want full autonomy till the next off-peak period that 17.5kWh has to run my heatpump and domestic loads for 19 hours. This is a bit tight (especially if the weather gets colder) and like you I feel the need for a bit more overhead in the winter size of the battery.
This thread has encouraged me to look again at adding a DC charger into the mix - like this one ELTEK FLATPACK2 48V/2000 HE and set its output voltage at around 52V.
I should also say that from about March/April I take half of the house batteries away to another MP2 system in an off-grid cabin and do all my charging in both places with PV until about October.
a charger is always a cc - constant current unit
It will deliver its maximum or set current, the voltage will be - as long as the batteries are not full - lower then the set voltage
the set voltage is more like the maximum voltage
That is NOT going to change really in any significant way (probably some additional lighting needed but I cannot think of anything else that would change in the winter) Apart from I am normally sailing the Caribbean during the horrible Scottish winter months {this year being an exception for reasons that I wont go into here}
I have done my homework before planning this BSS so ALL this information I do know, as the Octopus App is really good at showing energy used, and my Solar Edge app is great for tracking solar performance.
So typically the house loads are 22/24 kWh per day. The only difference season wise is the lack of solar power in the winter BUT I only have ATM a small 3.7 FIT system. This is A/C coupled and over this year 2025 it has produced around 3MWh up until today.
This month, a whole THREE DAYS, managed to generate just over 3kWh each day, but overall this month typically solar energy generation is anywhere between 300kWh to 1.2kWh a day SUN dependent of course. And with the low winter sun and my neighbours TWO VERY BIG WIDE and TALL Fir Trees shading my panels for about three hours mid day (Best time for solar generation at my location) unless I become a sneaky in the middle of the night lumberjack I will have to live with that situation.
In the Spring , Summer and Autumn months this year, the daily figure, taken by averaging out the monthly figure {The Solar Edge App is great} to get a daily solar energy generation figure, was from about 9kWh to 15kWh a day.
So for the system to work for me in the winter months (minimising using the more expensive energy) I need to purchase another US5000 battery AND install a Smart EV charger so I can change my tariff to the Octopus Intelligent GO Tariff
I have for the moment reduced my minium SOC setting to 10% and will see how that works for a few days before reducing it to 5% to see what if any benefit I can get from that change.
BUT how about some comments on using a charger and spoofing a VE MPPT Solar Controller to generate additional battery charging energy when the cheap rate energy is available. This MPPT Solar charger can be easily replaced {so it is not money down the drain} by some solar panels on my 99% completed new big garage which was orientated to maximise the solar generation capabilities. I did screw up the design as I should have placed it on the other side of my garden as the planned attached car port could then have been on the sunny side of the garage and of course another location for fitting more solar panels. BUT when locating the garage the layout of the garden and landscaping the garden was more critical. BUT I did orientate the ridge line to maximise solar generation from the sunny side roof panel and set the pitch of the roof to do that as well.
20:00 State of the System UPDATE Batteries are at 30% 
I recommend:
The PSU can be set to your batteries float voltage (55,2V?) and will adjust voltage to limit amperage to the PSU’s maximum. This will boost up to ~90% SoC. The MP2 will deliver the difference between PSU und CCL reported by the BMS.
The PSU can be controlled via Relais or some open-source projects. NodeRed might help.
The MP2 and the whole system is in a fairly cool part of my basement / cellar and in fact my regular house loads are NOT HELPED by a large number of tube heaters placed in various other parts of the cellar to minimise any possible dampness . The cellar has rooms mimicking the rooms above. The BSS is located in the front part of the house directly underneath what was ‘The Telephone Room’ way back then {when the house was built coming up 90 years now} telephone calls were a very private thing so it is quite cool even in the summer months. I have not as yet looked at the charge rate graphs to see IF OR when the rate decreases from the listed 70A. If there is a big drop off in charging and I think it is a thermal issue I can fit four 12V muffin fans in series on the bottom removable part of the MP2 and run them off the 48V DC power. Initially they can run 24/7 (until I sort out a timer circuit not a thermal one) as even a big one will draw only 500ma
Today at 05:28 two minutes before the charging stops my batteries SOC was 99% and it never reached 100%
one option for control is to wire the fans through one of the relays on your Cerbo. The relays can be switched using a simple NodeRed flow running on the Cerbo.
The flow could switch the relays at set times of the day, or in response to values in other registers in the MP2.
CHARGING PERFORMANCE well NOT really as I cannot see the actual MP2 charging output data I can only look at the battery data
BATTERY DATA
GENERALLY 69.5 A From 00:30 to 04:11 well within specifications
FIRST DROP to 66 A From 04:11 to 04:42
SECOND DROP to 63.5 at 04:55
THEN it goes down vertically
IF THE BLUE LINE is MP2 CHARGING OUTPUT
THEN WE HAVE
FIRST READING 70/69A from 00:30 to 04:11
FIRST CHANGE 65/66A from 04:12 to 04:55
NEXT CHANGE 10A and decreasing from 05:04
Would the relays handle a 2A load or does the CERBO GX trip an external (SQUARE STYLE PLUG IN) Car type relay then again that would be a 12V relay ooooh I can see 48V relays very cheap
The MP2 and the whole system is in a fairly cool part of my basement / cellar and in fact my regular house loads are NOT HELPED by a large number of tube heaters placed in various other parts of the cellar to minimise any possible dampness . The cellar has rooms mimicking the rooms above. The BSS is located in the front part of the house directly underneath what was ‘The Telephone Room’ way back then {when the house was built coming up 90 years now} telephone calls were a very private thing so it is quite cool even in the summer months. I have not as yet looked at the charge rate graphs to see IF OR when the rate decreases from the listed 70A If there is a big drop off in charging and i think it is a thermal issue i can fit four 12V muffin fans in series and run them off the 48V DC power
NO matter I have found some rather cheap 80x80 24V muffin fans FOUR of THEM 
TWO in series sitting over the air cooling slots on the black bottom detachable part of the MP2 with a 40degC NO thermal switch on the hottest part of the case (Maybe even mount the sensor inside the case) and connected to the 48V DC in terminals will give me auto temperature cooling. I could even modify the case itself and have (2 or 4) of them mounted externally. Maybe even internally if there is enough room and just cut cooling slots in the case. Tonight I will have a look at the case temperature with my laser temperature gauge.
Someone suggested buying a cheap used MP2 48 3000 35 and just setting it up as a battery charger (easily done with the power on switch) as I do not think that you can parallel couple different sized MP2’s and I do not want to split my A/C output to the consumer unit with two separate Inverters. That would be around £400 to do including the wiring whereas another MP2 48 5000 70 (used) could be £600 and new about £700 SO I will see what temperatures i have on the MP2 case and get some 80x80 fans and a temperature controlled switch a 40 degC one and see if that makes any real difference to the stability of the charger output One problem is taht the detachable part of the case with the cooling slots is NOT a flat surface so i will have to think of a way of sealing the fan to that part of the case to maximise air flow into the case and minimise air spilling out of the side of the fan rather than going into the case, it should be fairly easy and simple to seal the fan perfectly onto the bottom removable part of the case.
The MP2 charger has been running for almost 2 hours now, so before I go to bed I will go measure the case temperature and report it here.tomorrow.
After two hours of charging the case temperature was 44 deg C. Measured the temp now as it has been Inverting for six hours now to see what the ‘normal’ running temperature is=was and it was 24 deg C. ATM I think i will get a 30 or 35 deg C thermal switch.
The SOC never {reportedly} reached 100% toping out at 98.9% at 05:26. I think that is within the possible error rate
I use one of these
What I’ve found is thermal switch’s have a large swing to switch on or off.
With the temp controller you can adjust on/off/ offset ect ect, pwm versions also available which also work great as they adjust fan speed to given temp.
That needs 12V which I do not have unless I use a step down DC / DC converter additional unwanted expense and complication really
If its a new system it can take a few days to get the batterys balanced to 100%
I use the mp2 auxiliary 12v power to run the controller, then the controller relay switches fan circuit on/off.
They are balanced, as a few days ago when the batteries were not as deeply discharged with a minimum SOC of 20% set and with MODE {optimised for battery life} the charging stopped at 100% before the five hours were up. BUT now the minimum SOC is set at 5% as they were yesterday and will be tonight so it is a charger issue not a battery issue. Once my electrician has some space in his busy work schedule he will be installing a Hypervolt Home 3 Pro Charger for my EV. And once that is in, I can change tariff to the Intelligent GO version, which will give me an extra hour of battery charging at the low rate. So that extra hour of charging, along with adding the cooling fans should I hope make things a lot better.
5 of them is already more expensive than an MP2 5000. I have 3 MP2 5000, and hence do not have the problem. But I am off-grid, so my problems are different anyway.
MP2 5000’s are pretty cheap, strong and versatile.
Using them multi-phase is probable the best.
But in my case, my new house has some different requirements, so I might as well go for 3 MP2 15000, and use my current MP2 5000 as either DC chargers, or remote AC chargers (assuming they can be a “slave” in a micro grid system, like an inverter). But the final decision of 2..3 parallel 3 phase MP2 5000 vs 3 phase MP2 15000 has not been made yet. The latest video of victron showed a house as big as my new house only sporting 3x10k. For me it’s either multiple mp2 5000 parallel or the 15k.
BUT: DC charging is kind of: if all your chargers charge within spec and follow the voltage and you do not overfeed the batteries, it kind of doesn’t matter what you hook up together.
The only thing I am really interested in right now is that most of my planned lighting use 230V to ~ 36V PSU’s with CC for led lighting. The manufacturer tells me that I can use meanwell PSU’s connected to their controllers, but meanwell sells a lot of ~56V to 36V dimmable CC led drivers… No need for AC.
Sorry for hijacking….
You could try dehumidifiers rather than tube heaters David.
