An increasingly common question, now that many solar installations add batteries later: the car charger, with dynamic load control and its Shelly clamps, has been working with the panels for some time. Batteries go in, the client wants to charge the car from them, and the car does not drain them. It looks as if the charger cannot see the battery. Does something need changing on the clamp?
No. And the reason also explains how to make it happen.
What the clamp does, and what it does not
The toroidal clamp is a current sensor: it goes round a cable and measures how much current flows through it. The charger uses that reading for two things — not exceeding the contracted power, and using the solar surplus — which is why it sits, as always, on the grid side, after the inverter.
What it does not do matters just as much: it does not see the battery, does not know its charge and gives it no orders. That it “does not detect the battery” is to be expected, because that was never its job. And so there is no clamp setting to change when batteries are added.
What the battery does is decided by the hybrid inverter: it shares energy between house, battery and grid. The charger, with its clamp, only sees the result at the point it measures.
Why, by default, the car does not draw from the battery
By day, with sun, the panels cover the house and charge the battery, and any surplus charges the car. All fine.
At night, the battery discharges, but only enough to cover the house: it produces no surplus towards the grid. And a charger in surplus mode is looking for exactly that. With none there, it does not charge. It is not a fault; it is how surplus charging works.
There is a second layer: the charger itself protects the battery. On the V2C Trydan, for instance, the documentation says so plainly: by default it barely draws on it, to give the sun priority, and if charging the car would mean discharging it further, it stops charging rather than empty it. That “never quite drains the batteries” is the designed behaviour.
Making the battery feed the car
It can be done, but not at the clamp: it is integration and configuration. Three things are needed.
- A compatible inverter and a direct connection. The charger has to read the battery’s state from the inverter, not from the clamp. V2C, for example, requires a direct connection and publishes a list of compatible inverters; if yours is not on it, the feature does not exist however much you move the clamp.
- Forced discharge. Since by default the charger barely touches the battery, forced discharge has to be enabled and adjusted in its app.
- A suitable charging mode. “Surplus only” does nothing at night. To draw on the battery in the evening, use scheduled charging at a power the battery can deliver.
Before promising it to the client, put the limits on the table:
- The battery has a maximum discharge power — often 3 to 5 kW — that falls short of a car’s 7.4 kW.
- It has a capacity and a minimum reserve, so feeding a car empties it quickly.
- Emptying it into the car uses up cycles.
Often it works out better to charge at night on the off-peak rate and leave the battery for the house.
The opposite risk: the battery draining itself into the car
This is the most common problem when charger and battery fight, and it is the reverse. You plug the car in and the house’s consumption rises; the battery discharges to cover it; the charger sees grid consumption fall and mistakes it for solar surplus; it speeds up, and the battery ends up emptying into the car at full rate.
Three things prevent it:
- Care over where each sensor measures, so the battery system does not “see” the car’s load.
- Setting the battery to “charge only” while the car charges, or fixing a minimum reserve.
- A consistent charging mode: real surplus by day, grid at off-peak by night.
In one line: whether the battery feeds the car is decided at the inverter, not at the clamp.
Frequently asked questions
Does moving the clamp help?
Moving the clamp changes what the charger “sees”, and is useful mainly to prevent the opposite problem. But it does not make the charger read the battery: only integration with the inverter does that.
Is it the same with any inverter and any battery?
The principle is: the inverter decides. The details — whether it allows discharging towards the charger, the power limits, the settings — depend on each manufacturer. The first step is to check that inverter’s compatibility with that charger.