What the kW number on a charger means
A stall’s rating is the most that hardware can deliver, to a car that can take all of it. It isn’t a speed you’re promised.
The app words it the same way. Open the Stalls list on a station page and each stall shows up to 150 kW, or whatever that unit is rated for, and opening the connector picker repeats the figure before you start.
The charger offers power, and your car decides how much of it to accept. Aalto can’t raise either limit, and neither can the operator.
Charging slows as the battery fills
This is the single biggest reason a fast charger feels slow, and it’s normal.
A battery accepts power fastest when it’s fairly empty. As it fills, your car steps the rate down to protect the cells. On a DC fast charger that’s dramatic: past roughly 80% most cars are taking a fraction of their peak, and the last stretch to 100% can take about as long as everything before it. On a slower AC charger you barely notice it.
Two things follow from that:
- Arriving with a fairly full battery means you never see the peak. Plugging in at 60% and expecting the rate you would get at 10% is the most common version of this question.
- On a DC fast charger, stopping around 80% usually gets you moving again sooner. Past that you spend a long time for a little range.
There’s no setting for this on our side or the operator’s. It’s your car protecting its battery.
Check the charge limit in your car
Most cars let you cap how full they will charge, often at 80%, and many are set that way from the factory or from a habit at home. As the battery gets close to that cap the car slows hard and then stops, which at a fast charger looks like the stall giving up. If you plugged in at 70% with the limit at 80%, that’s what you’re seeing.
That cap does the same job as stopping around 80% yourself. It only catches you out when you plugged in above it, or forgot it was set.
Two settings alongside it do something similar on a slower charger. Scheduled charging delays the start until an off-peak window opens, or so the car finishes just in time for a departure you’ve set. A charging current limit, which many cars remember per location, holds the rate down. Both live in your car’s charging screen or its own app, not in Aalto.
Cold and hot batteries
Temperature changes what a battery will accept, and cold is the one that surprises people.
In cold weather a battery accepts far less power than it would on a mild day, and the first several minutes of a fast charge can be slow while the pack warms itself. It usually improves as the session runs. On a slower AC charger the cold shows up differently: the rate looks normal, but some of the power goes to warming the battery instead of filling it.
Many cars can pre-warm the battery on the way to a charger, and where they can, it makes more difference to a winter fast charge than anything else you control. How you trigger it varies. On most cars it only starts if you set the charger as the destination in the car’s own built-in navigation, so a route set in Aalto or another phone app generally won’t do it. Some cars have a preconditioning setting you turn on before you set off instead, and some can’t pre-warm at all.
Heat limits things from the other end, and that can come from either side. On a hot day, or after a run of fast charges in a row, your car may hold the rate down to keep the battery in range. The charger can reduce its own power too: cabinets and liquid-cooled cables have temperature limits of their own, and high-powered units step down on very hot days and after heavy back-to-back use.
Your car’s own maximum
Every car has a limit of its own, and on many cars it sits well below what a fast charger can offer. A car that peaks below the stall’s rating charges at its own maximum on the fastest hardware you can find.
Slower AC chargers, and your own cable
On the slower AC chargers, called Level 2 in North America, the power is converted inside your car, and the rate is set by whichever of these is lowest:
- Your car’s onboard charger. Usually the binding limit. If your car accepts 7 kW, a stall rated far higher still delivers 7 kW.
- How many phases each end can use. A three-phase supply only helps if your car can use all three, and a car that can use one phase gets about 7.4 kW however the stall is rated.
- Your own cable, where you bring one. The rating is printed on the plug, as an amp figure and a number of phases, and a 16 A or single-phase cable holds you down whatever both ends could manage.
- Any charging current limit set in the car.
Nothing here is a fault, and on an AC charger the cable and that limit are the two worth checking before you blame the stall.
An older battery charges a little slower
Batteries change as they age. They hold slightly less than when new, and the management system tends to be more cautious with high power, so on an older car you may see a slightly lower peak and the slowdown starting a little earlier than it used to.
It’s a small effect next to how cold and how full the battery is, and a slow session almost always comes down to those first. If you want to see it, compare sessions in your charge history at a similar battery level and in similar weather. A summer session against a winter one shows you temperature, not age.
Stalls that share one supply
At some sites, stalls share a supply: two sockets on one AC unit splitting a single circuit, or a group of fast-charging stalls drawing on one power cabinet. Alone, each can run at full rate. Together, they share what the supply has.
So a session can start fast and drop when someone plugs in beside you, then pick up again when they leave. On your curve it shows as a step down to a flat line rather than a slope.
The station page doesn’t show which stalls share a supply, so the drop itself is the clearest signal you’ll get. Where the site has a free stall away from other cars, moving is often worth a try, though the stalls that share aren’t always the two next to each other, and at a large site the whole row can be drawing on one cabinet. Moving costs you a few minutes, and it won’t help if your battery is already full enough that the slowdown is what’s limiting you.
If the app shows 0 kW
Two different situations show 0 kW when nothing is broken.
The session says charging has stopped. While a session is still running but not actively charging, the power reading is set to 0 kW rather than left at the last number. Your car may have finished, or stopped accepting power on its own.
The charger hasn’t reported yet. Some chargers take several minutes to send their first reading, and until then a session can sit at 0 kW while your car charges normally. Where we know a charger does this, the session screen tells you so. Until the numbers catch up, the unit’s own lights are the better guide.
kW and kWh, and what you’re billed for
The two units answer different questions. Power is the rate right now, in kW, and the session screen shows it while you’re charging. Energy delivered is how much has gone into the car, in kWh, and it stays with the session afterwards.
Most stations bill for energy, per kWh. On those a slow charge costs you time rather than money, because the same energy costs the same whether it took 20 minutes or an hour.
Some stations bill for time as well as, or instead of, energy, at a rate per hour. There a slow charge does cost more, because the hourly part keeps running for as long as you’re charging. Aalto marks those prices with a ~, because the per-kWh figure shown is an estimate: what you actually pay for each kWh depends on how fast your car charges, so charging slower than the station is rated for works out higher than the price on screen.
Idle fees are a separate thing, and they start after charging finishes rather than while it’s slow. How do I stop my charge session? covers those.
See your charging speed in the app
Most charge sessions have a power curve chart on the session screen, on the web and in the Aalto app on iPhone and Android. It’s there while you’re charging, and it stays with the session afterwards.
The vertical axis is power in kW. The horizontal axis is your battery percentage where the network reports it, and the time of day where it doesn’t. Hover over the line on a computer, or touch and hold it on a phone and slide along it, and you’ll see Power output for that moment, along with Battery and Energy delivered where those were reported.
A rate that falls as the battery fills is normal, and so is one that steps down in stages. What points at something outside your car is a step that lines up with an event rather than with your battery level: a sharp drop the moment another car plugs in nearby, or a rate that never climbs.
Until there are at least two readings you’ll see Waiting for charging data... in place of the line, and a session that finished without enough of them shows No charging data was recorded for this session instead.
When it really is a fault
Slow is worth reporting when it doesn’t match anything above. The clearest signs:
- The rate is still far below your car’s own maximum five minutes in, with a low battery, a pack that has had a drive to warm up, and no charge limit set close to where you are. Check what the stall is rated for as well: an older 50 kW unit running at 48 kW is working perfectly.
- The session keeps dropping to zero and picking up again.
- One stall is much slower than the one beside it in the same conditions, when both are rated the same and nothing is plugged in next to you.
Before you report it:
- Unplug, look at the connector and the socket for ice, grit or damage, and plug back in firmly. Poor contact makes a charger hold the current down.
- Start the session again, which clears most one-off states.
- If nothing changes, try a different stall. If there isn’t a free one, the map will show you what’s nearby.
One stall behaving differently from its neighbour points at that unit. Two stalls behaving the same way tells you less. It can be your car or the conditions, but it can equally be the site, which may be sharing a supply, holding down the total power it draws at a busy hour, or running hot. If every stall you try is slow and your car charges normally elsewhere, that’s still worth telling us about.
If you’re still at the charger, use Need help? on the session screen. It prefills the session ID we need, which is faster than describing the site from memory.
To report the charger itself, use Report issue at the bottom of the station page, sign in if you aren’t already, and pick Charging is slow or keeps stopping. That reaches our team rather than changing what other drivers see on the map, and we pass hardware faults on to the operator, who is the only one who can send a technician. You can also contact support with the station name, the stall number printed on the unit, and roughly what rate you were seeing.
Most stations bill for energy, so unless the price carries a ~, a slow charge costs you time rather than money.