AC Coupling with Grid Tie System Retrofit

Hi my name is Tobias,
iam pretty new to this topic, but i read a alot information on the forums. iam playing with the idea of retrofitting a DYI LiFePo4 Battery Bank to my present Grid Tied solar system. I know its mainly about DYI Batteries in this forum here, but i dont see a better place to discuss anywhere else.

current situation:
- 14.4kWp Grid Tied Solar system with 3phases
- Huawei SUN2000 12kW
- Solarlog 300 (Data Collecting and S0 House Power Draw reading)
- Energy Meter with S0 Interface reporting overall House powerdraw to Solarlog
- Feed In allowed but no good rewards. 2 way energy counter is installed (mandatory by electric company).
- UPS functionality i dont need

central power management control device:
- Homematic (control all possible 1phase / 3Phase devices) upon remaining solar power. Data is collected from Solarlog via Webserver
- EV Charging station is set to possible remaining solar power via Homematic script

Retrofit Idea:
- Retrofit AC Coupled Battery Bank 1phase for driving power devices which draws power over night (Heat Pump in Winter / EV charging (low speed <2kW in rare cases). Batteries should be recharged automatically when sun is shining and remaining solar is left.
- Startup Battery Build size 5kWh system with add in 5kwH Steps up to 20-30kWh

planned components:
- LiFePo4 Prismatic Cells BLS 3.2V 100AH 16x
- 48 V Setup should be the best (due to system size target of 20kWh)
- Inverter / Charger - Victron Energy MultiGrid oder Multiplus 48V / 3kW
- Victron Venus GX ?
- BMS DeliGreen 16S 100A (using max. 60A)
- Battery Shunts

Questions:
- Does these components make sense ?
- i dont know how to control the charge and recharge of the of the inverter charger upon solar and house power draw.
i have the actual house power draw / actual solar power via the present energy meter in my homematic control device available.
can i set these parameters on the venus gx to control the charging of the battery ? this is not explained anywhere.
- Is this planned setup in the configuration decribed easy to extend with +5kwH batteries just by adding additional battery + BMS + Cabling ?
- Please have a look of the grid plan i attached of the current system and leave our comments.

Thank you for taking time

regards Tobias

Pretty much all your questions are answered in the threads already posted. A quick search will reveal quite a few bits of information. Also, read the FAQ located on the main page.





I could have sworn I've seen this same exact post about a week ago somewhere else

Hi Tobias
Id recommend using a Batrium BMS, mainly as it has been proven to work with the Venus GS already.
Id also set up your battery pack so its easy to parallel packs as you increase the capacity. To do this is pretty simple, connect your 0v and 48V terminals to bus bars then to your inverter charger and physically lay out your pack in a single line with your BMS attached.
When it comes time to add more capacity connect 0V & 48V to bus bars and then add small jumpers between the parallel packs for balancing and reconfigure your pack capacity in your bms.
The Victron year is well documented, Id be tempted to gets 5kW for the price increment to allow you to do more later if you want. With a CT connected to the Venus itll control the discharge & charge to meet zero import/export as much as it and the battery can, if it is connectonnected to a capable bms with a shunt.

Jon said:

Hi Tobias
Id recommend using a Batrium BMS, mainly as it has been proven to work with the Venus GS already.
Id also set up your battery pack so its easy to parallel packs as you increase the capacity. To do this is pretty simple, connect your 0v and 48V terminals to bus bars then to your inverter charger and physically lay out your pack in a single line with your BMS attached.
When it comes time to add more capacity connect 0V & 48V to bus bars and then add small jumpers between the parallel packs for balancing and reconfigure your pack capacity in your bms.
The Victron year is well documented, Id be tempted to gets 5kW for the price increment to allow you to do more later if you want. With a CT connected to the Venus itll control the discharge & charge to meet zero import/export as much as it and the battery can, if it is connectonnected to a capable bms with a shunt.

Click to expand...

Hi Jon,

thx very much for your helpfull comment. Is far is i get the information i thought for LiFePo4 systems i dont need an expensive BMS for battery management.
a cheaper BMS for Low Voltage CutOFF / Temp Cut OFF / Overvoltage Protection / Undervoltage Protection is fair enough to use.
but maybe you can explain the necessary benefits, i dont have the experience.

Ok i read due to the flat discharge voltage curve of LiFePo4 the shunt maybe very important to safe the battery from undervoltage.

What do you mean by "CT" connected to the venus ? you said it must be connected to a capable BMS with a shunt ? the cheap BMS is not capable ?
furthermore, i want to use my present energy meter to set the parameters of actual power on the venus. i dont want to buy and connect an additional energy meter of victron the get the data to the venus / Inverter.
and that information i havent found in the FAQ or anywhere else, so far.

greetz Tobias

CT = Current Transformer

The shunt is used to measure coulombs and amps in/out. It doesn't measure voltage of the battery packs. So the shunt alone can't keep from undervoltage discharge.
However, the bms can still auto-shutoff when the voltage gets too low as there is a drop off when that happens.
Same thing when charging as there's a sudden rise in voltage once full.

Hi Tobias
For the Multiplus (or whichever inverter charger you use) to know when to change the battery from surplus power or discharge so that you dont import power. The easiest way this is achieved (in the case of using Victron gear) is to use a CT clamp on the incoming power of the circuit your inverter charger is on to measure import/export. If you are able do that with your current meter communicating with the Venus easier than the $70 for the price of a CT that is an option I guess.

Thats the AC side, for the DC side the inverter charger needs to know how all your chosen parameters of the battery and where your pack is within those parameters, whichever BMS you use it needs to be able to communicate with the inverter charger, I suggested the Batrium brand as I know they can communicate with the Venus.

For the amount youre going to spend on the system including inverter/charger, controller, shunts, breakers, cells etc I personally wouldnt try to save money on a cheaper BMS. my opinion only.

Thank you all for your explanation.

Then i need 2 energy meter in total ? (1. for the overall house power draw (all phases) which i already have and 2. on the input inverter charger AC side) see attached drawing ?

I have read now a bit about the batrium, looks very solid with the advantage of communication to venus and a shunt with a meter included.
i understand your thoughts and i think it makes sense to spend this extra bucks on this equipment as it results in extra safety also.

so if i would add another bank in parallel i would need an additional watchman5 + shunt.

i think the watchmon5 would be the ideal component for my setup and for beginners (less wiring). netherless its only available up to 15s.
iam quite not sure if i can go 15s with LiFePo4 3.2 volt (the victron inverter allows 38V at lowest so it will end up with 2.53V lowest cell voltage which should be ok, as i would like to use only 30-80% of the cell capacity.

If you add a second bank use heavy Busbars to connect 0v and 48V of both packs and run from these bus bars to your inverter charger, single shunt on the 0V side.

Then connect each cell to its parallel twin with smaller cable, this will let your BMS treat each parallel cell as a single cell, you only need relatively light cabling here as all your doing is balancing your cells.

All your BMS settings other than battery capacity will stay the same.

---

Victron make a CT that you will need to put on the incoming mains for the phase you put the inverter charger on.

You can add more shunts if you want to know the performance of each string. In other words, is string A discharging/charging with more Amps than string B. This can be useful info if you happen to have supposedly identical strings.
Another it can tell you is if power is moving from one string to the other. This would indicate a weak pack in the string and is being charged by the other one.

thx again,

great information.
So its better to spend every bank with a monitored Shunt.
the best setup in terms of BMS would be the WatchMon Plus i suppose. This only have 15s
Do you see a problem with the voltage for 15s (too less) ?
the other "problem" is that WatchMon Plus cannot handle additional banks (2x WatchMon Plus in Series) so far, do you know when it is supported ?

Meanwhile i found a good resource about the VIctron API.
i know found out that Victron has a very good API for local (dbus-mqtt) on Venus to access over the local network.
in that case i can set the charging / discharging without any additional meter as i have a present meter house power draw (complete draw all 3 phases) and Solar production available on my homematic.
In this way the Victron Multi can be controlled in ESS Mode 3 via just a few parameters and is emitting power on AC IN (discharge battery) or is drawing power on AC IN (charging battery) be setting these parameters frequenctly. Thats the way i would like to achive it.


bye tobias

I think the Batrium kits are 15s. But you can you add more longmons to increase the series count. You can even have 1 Watchmon with 28 longsmons monitoring 2 strings of 14s ( I think this is how Daromer monitors two of his strings, not sure tho)

Also, going with LiFePO4's, your top voltage at 16s would be 57.6V (3.6V per cell), and Batrium can go higher than that as I've seen readouts over 60V.
16s would also give you a bottom voltage of 44.8V (2.8V per cell). Should fit the voltage range of your inverter just fine.

BTW, LiFePO4 cells fit "extremely" well with pretty much "any" inverter, as they are basically drop in replacements from 12V to 96V and beyond.

It is possible to double up with shunts and BMS, I personally wouldnt, especially if youre using new cells.
Your first string will be 1P??S (Im not familiar with LiPo).
That being 1 cell in parallel and ?? Cells in series.
When youre first setting it up Id decide how many packs you may want to end up with in parallel and make the bus bars that long and lay all your cells out in 1 line, either horizontal bus bars with a single vertical stack of cells or vertical bus bars with a horizontal line of cells, the shunt would go onto the 0V bus bar on the cable running to the inverter/charger. The light weight BMS balancing cables then just go to each individual cell.

Commission your system, settle it down, tweak it and become comfortable with its peculiarities and how it serves your usage pattern.

When/if you decide to add more capacity its a simple matter of putting another line of cells into your system, tying into the feed bus bars and putting lught jump leads between each parallel cell for the BMS. The only software change is then to change the overall capacity of your battery.

If at some stage you have reason to think that a cell is starting to fail (say at that time you are 3P,??S) mark the 3 cells that are questionable, shuffle them so that those 3 are in different Ps and monitor. The new P that is showing signs of failing has the suspect cell in it, youve already marked which ones were suspect the first time, therell only be one marked cell in that P, its a fair bet thats your suspect cell.

You can run totally independent strings and monitor everything with very high accuracy if thats your thing, Im inherently lazy and only like to capture data that will allow me to monitor and maintain at a glance.

I was hanging out for an active balance system rather than a passive balance, I also initially liked the idea of having everything back on the BMS board. I then ended up buying a WM4 on the logic if anything is going to fail over time its likely the unit that is converting electrons to heat, having this as a discrete unit lets me troubleshoot and replace easily.

For clarity on this forum, pack/string construction is listed as XsYp, always listing the series first, the parallel. Other forums may be different, but the vast majority of ppl here use the XsYp nomenclature.

If a pack isn't performing as expected, then you may need to desolder all cells and find the ones that are not the same voltage as the others. Then remove them completely. Don't add them to another pack to test if they are positive suspect as you'll be stressing those cells in that pack, and doubling or tripling your work. Test the cells in a separate charger/tester, plus it will be faster doing it this way.

If it isn't clear, when Jon says the shunt goes on the 0V rail, he means the Negative line.

thx to all for your detailed answer.

Thanks Korishan

Tobias is using large format cells, that is why I suggested swapping cells to troubleshoot.
I would also recommend doing the same with packs where people have packs paralleled to find out which pack has suspect performance.

As the new Watchmon Plus has only 15s capability i would rather use a battery bank setup of 15s3p
so the 3 parallel cells are acting as 1 cell so i only need one watchmon for the complete setup.

i need to take care to link just cells in parallel which are identical in voltage and capacity and Resistance ?
Is there anything else i need to take care of the grouping of cells ?
Is there any specific problem with these large LiFePo cells to put in parallel ?

Hi
Theres no issue with large format cells going in parallel.

All cells in the system obviously need to be the same chemistry for voltages etc.
From there its a bit more flexible.
Think of your pack as a matrix of Series and Parrallel cells, say like a spreadsheet of columns and rows. If your bus bars are horizontal each Column represents a Series of 16 cells and the main current flow is vertical. When you add another string in parallel youre adding another column of 16 cells, the current flow horizontally is negligible, just balancing really.
The cells in one column can be a different capacity to the other column, but you want all the cells in a column to be the same. Eg column A can be 1.5 times the capacity of the cells in column B with no issues, but you want all the cells in column A to be the same capacity and you want all the cells in Column B to be the same capacity.
IR (IMHO) is important if you lean on you cells hard by charging and discharging at a high rate, the kinder you are to your cells the less important IR is.

If youre buying new large format cells as long as each series group is the same spec I think youll end up with a nice setup.