Features → Two cars, one supply

Two cars, one supply

Two cars that each want 32 A, on a supply that can deliver 38. Something has to decide, every few minutes, and "half each" is the answer that sounds fair and charges least.

Applies whenMore than one car is plugged in and wants charge

DecidesWhich car leads, and how the amps divide

DefaultSmart — ranked by what is actually urgent

The problem, concretely

Your installation has two limits: what one car may draw, and what all of them together may draw. On the system these numbers come from, that is 32 A per car and 38 A combined — a second car cannot simply be given its own 32 A, because 64 A is not available and asking for it trips the main breaker.

So 38 A has to become two numbers. There are three honest ways to do it and Sunplug picks between them every tick.

Why not half each

Because a car is not a resistor. An onboard charger burns a couple of hundred watts simply being awake — running its contactor, its cooling, its electronics — before a single watt reaches the battery. Pay that overhead twice and you get less battery for the same electricity.

The consequence is sharp: 5 A into one car puts more energy in a battery than 5 A + 5 A into two. Same electrons off the roof, twice the overhead, and both cars crawling. So there is a floor below which Sunplug will not seat a second car at all — it gives everything to the one it ranked first and leaves the other waiting, which is not meanness but arithmetic.

The two floors, and why they differ

5 AThe solo floor. One car may run this low — better than stopping it.

6 AThe start floor. An idle car needs a little more before it is worth waking.

8 AThe sharing floor. Below this per car, two cars are not seated.

The gap between 5 and 6 is hysteresis — if a car started at the same current it stops at, it would oscillate around the boundary all afternoon. The gap between 5 and 8 is the overhead argument above: 8 A each is 16 A total, and under that the split is worse than not splitting.

Who leads

With the default smart setting, Sunplug ranks the cars into bands and the band is what the reasoning names. In order:

Below its overnight floor — a car that will not reach the level you said it must reach. Nothing outranks this.
A trip is coming — a departure you have told it about.
Furthest from its solar target — the ordinary sunny-afternoon case, and the band most decisions fall in.
Level unknown — a car that will not say where it is. Ranked last because every other band is a measured claim and this one is not.

Being ranked first means different things in different bands, which is why the split has to know which band it is in. Two cars both drifting toward their solar targets are in the same situation and an equal share is right. A car below the floor it has to reach tonight is not in the same situation as one that is merely a bit low, and giving them equal shares would fail the first to be fair to the second.

The thumb on the scale

A car that is already charging is treated as slightly emptier than it is — five percentage points of state of charge. Without that, two similar cars swap places endlessly: the one charging rises above the other, loses the lead, stops, falls behind, wins it back. Each of those swaps costs a contactor cycle and a restart delay, and the cars end up charging less than either would have alone.

The three shapes a split can take

Solo — one car takes everything. Either there is not enough to seat two at a worthwhile rate, or the leader is in a band the other is not.
Weighted — the leader takes more, the other still charges. The usual answer when one car is genuinely more urgent but there is enough to go round.
Equal — both the same. What happens when the cars are in the same band and there is enough for both to be seated properly.

The decision log says which one it chose and names the reason, so an afternoon where one car got nothing is a sentence you can read rather than a mystery.

If you would rather decide yourself

Two other settings. Custom is your own order, applied strictly all the way down — you said who comes first, not who is equally deserving, and Sunplug takes you at your word. Even is a promise about the split rather than a ranking: everybody is in one band and nobody is ahead.

One quiet interaction worth knowing: smart ranking weighs solar targets, so with solar tracking switched off entirely — charging purely on tariff — there is nothing for it to weigh, and smart behaves as even. Your setting is not changed; turn solar tracking back on and smart is smart again.

Stuck? Email me.