Hi,
I have set the 2 wire BMS in the ESS to disable charger when BMS needs to balance cells which takes some time, during this time the Multi goes into Passtrough and then the SmartSolar is disabled. Is there a way to use PV power from the SmartSolar while the BMS balances the cells ? I guess there is no since the cables are connected to the same points and thus would charge the battery innevitable or am i wrong ?
/peter
Depends on your BMS. I've designed my BMS to interface to the MPPT charge controller using VE-direct connection. When one cell approaches the limit voltage, the BMS commands the MPPT to throttle back the charge current. This forms a feedback loop, and the two multi's stay in invert mode during balancing. A 2 wire BMS is not smart enough to do this.
Did you think about workarounds?
@Raphael @MikeD thank you for your reply, i really appreciate it.
I am using diyBMS (that's what it's called) and that has only relays i can control based on some rules for example the cell get's to a certain treshold and the relays are connected to the Multi using the mentioned 2wire bms. And yes this constelation is not that smart but does work but unfortunattely i get the mentioned issue.
Now the problem may go away when/if the cells balances (i didn't think to do top balance before i put all 16 cells in series doh...) but until then i have the problem with the bms turning the charger off hence PV power is lost during balancing.
I do have two more relays i don't use which i could use to turn on additional balance and hence cut down the time where the PV power is lost since i guess that what the 2 and 3 will do in your @Raphael suggestion will do right ?
@MikeD what BMS are you using ? Don't you run into the same issue as me i mean you will loose PV power because you throttle down the charger maybe not as much as i do but it doesn't sound like it solve all problems.
Yes, adding further passive balancing resistors or acive balancing, will considerably reduce the balance time. Once the cells are balanced, you should not have the problem anymore.
2. When you connect a further passive balancing module, you don't have to switch it on or off. It'll kick in when the voltage of a cell rises above a certian treshold (for example 3.55V).
3. When you connect a active balancing module, it makes sense to only use it, until the cells are balanced, or you switch it off, if the voltage of the cell enters the flat stage of the discharge curve...otherwise it might work all the time, due to the resistance difference of the cells and the cell-connections when you have higher currents flowing.
Just in case you want to use a BMS that is able to do CAN-BMS communication with a victron GX device, I could recommend the REC-BMS...it works well for me:
https://rec-bms.com/
You can use it with a relay, but you can also save the costs of a relay, because the BMS communicates with the GX-device and therefore you won't run in high or low voltage situations.
I have designed my own BMS around a Maxim chip and a Raspberry pi. I did this because none of the commercially available 3rd party bms units had: 1) High enough balancing current for a solar installation and 2) would share information about the CAN bus protocols. Therefore I built my own, with sufficiently high balancing current and NO CAN bus. Instead, the Pi has a web server and I've designed info pages that I can see on either my phone or computer. Information sharing with other systems is via a Modbus/IP map. The BMS server also reads my EKM energy meter, and controls the MPT over the VE-Direct connection. So when one cell approaches max permitted voltage, the |Solar charge is reduced - not cut off - to the level where it equals the input current to the Multi-plus inverters (I'm running 2 x 1300W units in parallel) plus the cell balancing current. So far, the system has been running for 18 months at 4.5kWh /day typical. During balancing, the output from the MPPT will fluctuate as it follows the load. This allows the battery to settle with a charge current of about 1 - 2 A as the balancing takes place.
