Pass-Through Charging Explained

This is one of the most reliably misunderstood specs in the category, and the confusion costs people money. Someone buys a unit advertised as supporting pass-through charging, plugs a desktop computer and a router into it, plugs the unit into the wall, and assumes they now have a battery backup. Then the power flickers and everything reboots.

Both features exist and both are useful. They are just answers to different questions.

What pass-through charging actually is

Anker's published definition is the clearest one: pass-through charging means "the power station can receive power from the wall while supplying power to connected devices." That is it. Energy comes in through the AC input and goes out through the sockets at the same time, with whatever is left over going into the battery.

The practical value is straightforward. You can leave the unit plugged in at a campsite with a mains hook-up and run a fridge off it, and the fridge does not stop when you connect the charger. You can plug it into the van's shore power at a site, run everything from the unit, and arrive at the next stop with a full battery. Without pass-through you would have to choose: charge, or use.

BLUETTI publishes "Pass-through Charging: Yes" directly on the AC180's specification page, which is the kind of plain disclosure that makes a spec sheet useful. Others are less explicit, and absence of the phrase does not always mean absence of the capability — it often just means the maker lists it elsewhere, or not at all.

What UPS mode adds on top

A UPS — uninterruptible power supply — is about the handover, not the charging. When mains power disappears, there is a gap before the battery takes over, and the length of that gap decides whether your equipment survives it. Anker's guidance notes that "UPS support requires a fast battery switchover during outages" and warns that "a 20-50 ms delay can shut them down" for sensitive devices.

That is the whole reason switchover time is published in milliseconds. A desktop PC without its own internal buffer, a NAS mid-write, or some network equipment will drop out on a 30 ms gap. A fridge compressor, a lamp or a phone charger will not notice 100 ms.

The published switchover figures in our set

Every one of those figures sits inside the band Anker's guidance describes as potentially problematic for the most sensitive equipment, which is worth sitting with for a moment. A 10 ms unit will carry almost everything in a normal house. A 20 ms unit will carry almost everything too, but if you have one device that absolutely cannot blink, test it rather than assume it.

What to look for, in order

A published switchover time, if the unit is going to sit between the wall and something that matters. This is the figure that separates a backup from a battery.

Continuous output above your connected load, not your peak load. Anker's guidance is blunt about this: "the best results come from staying within the continuous output rating." A UPS is a sustained duty, not a brief one, and running near the ceiling permanently is a different proposition from doing it for ten minutes.

Pure sine wave output, which Anker lists among the requirements for genuine UPS behavior. Every unit we cover provides it, but it is worth checking on anything cheaper.

Somewhere ventilated to leave it. Anker's guidance notes that "good ventilation is essential because charging and inverting power both create heat" and recommends keeping units "cool, uncovered, and ventilated" for long-term standby. A permanently plugged-in unit in a closed cabinet is the most common way people shorten one of these.

Does leaving it plugged in wear the battery out?

This is the question everyone asks second, and the honest answer is that it depends on how the unit is designed and that the makers mostly do not document it. Some units bypass the battery when mains is present and only top it up occasionally; others cycle the battery continuously. The first costs you almost nothing; the second costs you cycles.

What we can say from published specification is that the LiFePO4 units we cover are rated between 3,000 and 4,000 cycles — Anker and EcoFlow to 80% remaining capacity, BLUETTI to 80% at 3,500+, Jackery to 70% at 4,000 — and that this is a large budget to spend. Anker's standby guidance is that "LiFePO4 models with smart management handle standby better" and to keep them cool and ventilated, which is a recommendation about heat rather than cycles. Heat is the variable you control.

We are not going to assert a percentage for pass-through wear, because we have not measured it and no maker in our set publishes it. How long power stations last covers what the cycle ratings do and do not tell you.

Where this matters most

For a router and a modem, this feature is the whole product — a sub-20 ms handover means the internet simply does not drop when the street does. Using a power station as a UPS for wifi and router is the specific build.

For a sump pump, it is the difference between a backup that works while you are asleep and one that needs you to go and plug it in. For a camping fridge on a site with a hook-up, pass-through alone is enough and the switchover figure barely matters.

And if you are sizing a unit for whole-house backup rather than a single appliance, what size power station for a power outage does that arithmetic, while the home backup roundup ranks the units.