Prioritising the battery

In combining both battery storage and water heating from surplus PV output I was keen to ensure that the two solutions worked together well. I consider that there are potentially two issues with such an arrangement:

  1. Firstly, the system may go unstable. With two independent systems each trying to absorb the same PV output, they may overreact both claiming the excess and putting the house into import, then both back-out in response to the input restoring the system to export, and then repeat the cycle endlessly etc. In control system terms you might consider that it had too much gain.
  2. Secondly, even if stable, the resulting split of diversion into two systems (battery and immersion heater) may not be optimal. In my case a kWh of stored electricity is worth more than four times a kWh of gas saved – so I’d clearly want to prioritise battery charging.

I considered various relatively complex solutions and then thought of one of great simplicity which required no additional parts.

My solution is simply to install the current clamp for the high priority device around both the incoming power cable and the cable to the lower priority device. The nature of a current clamp is that the current it reports is the sum of all the cables that it encircles. Thus, by placing the battery clamp around both the incoming supply and immersion heater feed, it doesn’t matter how much of the potential export has been diverted to the immerSUN, the battery clamp still sees what would have been exported were the immerSUN not active and gradually takes that power. The immerSUN in turn sees the reducing export / increasing import on the meter tail and backs off to restore the balance.

My installer had previously combined immerSUN and battery without such an arrangement and advised that the slow response of the battery compared to the immerSUN remained stable, but as the immerSUN responded most quickly it was prioritised.

However the above graph from the immerSUN shows my battery successfully tracking the PV output, so my arrangement is working successfully.

The only downside, which for me isn’t an issue, is that the immersion heater becomes invisible to the battery. Even if the immersion heater is operated via an immerSUN boost when no PV output is available, the incoming mains current is entirely matched by current in the immersion heater feeder in the opposite direction, and the battery’s current clamp sees nothing. Thus the battery cannot support the immersion heater because it doesn’t know that the immersion heater is drawing any current. I however don’t see this an issue since we use gas to heat the water when the ImmerSUN’s diversion is insufficient. Even if we did use electricity the battery is not able to deliver sufficient power to run the immersion heater at 3kW.

In my installation the battery current clamp is actually inside the consumer unit to minimise the additional wiring to allow the clamp to encircle both the incoming mains and the immersion heater feed.

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4 thoughts on “Prioritising the battery

  1. codm80

    I have just installed a battery and have a mark 1 immersun.
    The battery and immersun at first seemed to work but when lights drops the battery started to discharge to the immersun for heating the water.
    I have put the CT clamp of the battery around the main live coming into the house as well as the wire to the Immersun (as you indicated) but I am now thinking that the immersun wire has liev and neutral so may cancel each other out. WHen you connected yours did you separate the live and neutral, so essentially the CT clamp was on 2 live wires? Any help pictures will help as it looks like I will need to bring back the electrician!

    1. Greening Me

      If the clamp is around both the live and the neutral then they do indeed cancel each other out. It’s conventional to put the clamps around just the live, but just the neutral works equally well. Some of my own clamps are inside the consumer unit or inside the rotary isolator switch in order to get easy access to just the live. However you should be technically competent to open either.

      1. codm80

        Thats what I thought. If I connect though to just the live, if battery is charging up it may work. But when battery is charged up and there is a grid load as well as the immersun being activated, won’t the discharging of the battery keep the diverter operating even if other grid loads are switched off. I think this will still have risk of discharging the battery.
        I had another idea which may not be ideal but would at least prevent battery discharge. I was thinking to put the CT clamp of the immersun about the live feed to the battery inverter, this way the immersun will only operate when battery is being charged. It may not be exactly prioritising teh battery but at least both may work in parallel. I do realise when battery is full, excess power won’t be diverted but I have solar thermal as well so probably enough. In winter months when I dont generate much I probably will just switch the immersun off.

  2. codm80

    just wired up the ct as you recommend and so far seems to work today —hurray!
    . I will monitor over the coming days, but to be honest I still thought The battery might flip to discharge when the immersun switched on the load.

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