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Beginners Questions about my 2P4S setup

  • M

    Hello,

     

    I have built my first setup which I want to use on my livaboard boat. 

    2P4S from EVE 280Ah A Grade

    REC ABMS Victron compatible

    REC Wifi Module

    2x Kilovac EV200AAANA contactors

    Victron Cerbo GX

    Victron 100/30 MPPT

    Victron Orion Tr Smart 12/12 30 DCDC Charger

    Configured the MAIN relay to be the charge bus, and the Relay 1 to be the discharge relay.

     

    1. The REC manual shows how to connect the cells for a 2P4S setup. But it can only measure 4 cells. How does it affect the accuracy? Is there anything special that I must be aware of when using this setup?

    2. I have top balanced the cells to 3.6V, checked with my DVU. However, the voltage when assembling the battery was only 14.16 Volts. Now I have connected the BMS and it reads 13.81V. The only power consumption is BMS and contactors, which draw 150mA and seem to be in the specs. Is this quick drop of voltage normal?

    3. The REC Wifi Module homepage shows 48.8% state of charge at 13.81V, which I do not understand. Since I had top balanced the cells, I expected them to be nearly fully charged. How can it be explained?

    4. I have no inverter, but the configuration does not allow me to set an inverter number to 0. Why is this? I will be charging from alternator or shore power, going through the DCDC charger, and the MPPT which the BMS can control over the Cerbo.

  • R

    1)  In a 2P4S configuration, the 2P means that you have 2 cells wired + to + and - to -, which in effect creates one large 560Ah cell for the BMS to monitor and balance.  This means you only need the 4 cell tap wires plus the ground/cell 1-.

    This is standard operating procedure for many systems.

    2)  At 3.6 volts you are at the maximum range of the cell and it is not unusual to see a large voltage change for a very small change in capacity.  This is why top balancing works so well on LFP cells, because in the mid range of 3.1 to 3.4 voltages, there can be a large change in capacity with almost no change in cell voltage

    3)  SoC is a calculated value and you need to fully cycle your cells a few times to get it to zero in on the correct value.  It defaults to 50% at initial startup. Dont forget to set your CAPA cell capacity either, in your case it will 2 x 280 = 560

    4)  Dont worry about that.  That is for controlling an inverter over CANBus and is a calculated value, so leaving it at one is fine.

  • M

    Thank you Rick, that helped clear things up for me. The system is now running fine.

    0 likes
  • T

    @Rick Hi thanks for your reply, I am currently building a 3P4S setup with EVE 304ah cells. Just checking to make sure the Rec Active BMS is fine with 3 cells in parallel as well? Thanks!

    0 likes
  • R

    The EVE cells are proving to be a good cell and so long as they are well matched you shouldnt have any problems with a 3P4S configuration.

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