My primary setup consists of a DC coupled solar, Quattro and batteries. I cannot extend the battery capacity, and I need to install a secondary battery bank around around 20m away. (both systems 48V)
My thinking is to run DC cabling between the locations and have a DC to DC charger providing energy to the DC based on the primary battery percentage and current consumption. The secondary battery bank will be charged using DC coupled solar and/or AC charger (overnight from grid).
My question is, what DC to DC charger is there what would allow me to control its output say between zero and 100A? I can even get away with 50A if such device does not exist. My intention is not to charge the primary battery but to provide energy from the secondary one roughly equating to what is currently consumed by the inverter.
If the battery voltages are compatible, I would look at just installing thick cables between the 2 banks, and protect the cable with breakers at each end.
Considering they will be different brand BMS, wouldn’t that be an issue? I always thought the batteries needed to be all the same but I am not very experienced!
The primary bank is BYD 15.4kWh and I am looking to either buy or build around 60kWh secondary battery. Both LiFePO4.
I have just had a quick look and it seems that 35mm2 cable is not cheap but doable for the distance.
However - how can one make sure that both batteries are discharged relatively simultaneously? I have seen dual battery setups on a smaller scale where one connects the load + to battery 1 and load - to battery 2 while the terminals are also connected, but that I think assumes similar battery capacities.
With 35mm2 cable and 20m distance one way, the voltage drop works out 0.18V at 250A, which is the max continuous output I can expect.
My understanding is that you are suggesting this layout, correct? This means that I would need to double-up one of the cable legs to achieve some relative balance, right? (2x negative in the diagram)
Most LiFePO4 batteries are compatible in voltage, so the SOC will equalise over time. With the BMS: as long as the voltage set points for max charge are identical, then there is no problem. If not, then ALL of the connected chargers need to be set to just less than the lower voltage. I normally charge my banks to about 3.55V, 0.1V below the max of 3.65V. Discharge I terminate at 2.8V - well above the 2.5V limit.
As for you schematic, the negative of the inverter can be connected to the primary bank, there is no need to run this to the secondary bank. The 95% charge efficiency of the Li chemistry renders this un-necesary.
35mm cable should be protected with a 120A fuse at each end. You should also have an isolation switch in the circuit too. Charging can be applied to either bank.
BMS Data: you should be able to connect both to a Cerbo / GX device, though I’ve not tried this.
The primary bank will handle any sudden load, and then recharge from the secondary bank.
Charge distribution will be controlled by the sum of cable and internal resistances, driven by the slope of the battery cell charge/discharge voltage characteristic.
Hm, that is very interesting, thank you for the information! Is the battery wiring info effectively not that relevant for LiFePO4?
So in reality, I will still be under-charging and under-discharging the secondary battery, but the difference will not be worth it, right?
re. 120A - isn’t that a bit too little? My inverter is the Quattro 15kVA, wouldn’t there be a concern that the fuse would be blown if the max current is drawn? Or is the proximity of the primary battery effectively limiting the max current that may be drawn from the secondary one? If so, how can I calculate the max current possible for the secondary battery, give the wire parameters?
The 120A is the limit for 35mm^2.
csa Max amps
25 107
35 128
50 157
70 194
95 242
120 276
150 330
185 385
So with the large inverters, you want to be looking at a much larger cable.Calculating the current split is down to the internal resistance of the batteries, plus the respective cabling. Add a few mOhm for each connection too.
