Pass-through charging and UPS mode explained: leaving the battery plugged in so the outage finds it full

Every appliance page on this site says the same thing about the units it ranks: plug the battery into the wall, plug the refrigerator into the battery, and leave it. That is pass-through charging with a UPS mode, and it is the difference between a battery that protects a refrigerator with nobody home and one that protects it only if you are there to carry it downstairs. Here is what the mode is, what it is not, and the numbers the makers state for it.

What pass-through charging is

Plugged into the wall, a power station in pass-through mode feeds its outlets from the grid and keeps its battery at full charge. When the grid fails, its inverter takes over the outlets from the battery. When the grid returns, it hands back and recharges. The load sees a single brief gap at each transition, measured in milliseconds, and that gap is the specification that matters.

What the makers state for the gap

Unit	Switchover stated	Page
EcoFlow DELTA 3, DELTA 3 Plus, DELTA 3 Max, RIVER 3 Plus	under 10 ms	1 kWh, 2 kWh, CPAP
Anker SOLIX C1000 Gen 2, C2000 Gen 2, C300	10 ms	1 kWh, 2 kWh, CPAP
EcoFlow DELTA 2 Max	under 20 ms	2 kWh
Anker SOLIX F3800, F3000	under 20 ms	expandable
Bluetti AC180	20 ms	refrigerator
Jackery Explorer 1000 v2	under 20 ms	1 kWh

Figures from the makers’ pages on the dates printed on each card. For a refrigerator, a sump pump or a freezer, any of them is fine: a compressor does not notice 20 ms. For a desktop computer or a router, 10 ms is inside what nearly every power supply rides through and 20 ms usually is. For anything life-critical the CPAP page’s note applies: ask the equipment’s maker what gap it tolerates, and treat the answer as the specification.

What it is not

A true online UPS, the kind a data centre uses, has no switchover at all: the load runs from the inverter continuously and the grid only charges the battery. Power stations are line-interactive at best. The makers’ own language is careful about this, “UPS mode” or “EPS,” and the honest reading is: fast enough for almost everything in a house, not a guarantee of zero interruption. Two other differences: a power station’s outlets are the ones on its face, not the whole circuit, so it protects what is plugged into it; and it does not condition the grid’s voltage the way a good UPS does.

Setting it up

1. Plug the unit into the wall outlet nearest the appliance, on its own circuit if possible.
2. Enable the UPS or pass-through setting in the unit’s app if it has one; on most units it is on by default when plugged in.
3. Plug the appliance into the unit’s AC outlet. For a refrigerator, that is all; the battery is now a buffer between the wall and the compressor.
4. Check the unit’s display or app once: it should show grid input and full charge.
5. Once a season, unplug the wall side for a minute and watch the appliance keep running. A rehearsal is worth more than a spec sheet.

The two trade-offs

Holding at 100%. A battery kept full ages slightly faster than one at 50%. LiFePO4 tolerates it far better than the NMC cells of a few years ago, and every maker designs its UPS mode around it; EcoFlow’s guidance to store between 30 and 85%, in the lifespan guide, is for a unit on a shelf, not one on float. For a backup battery, full when the outage arrives is worth the trade.

Heat and fan noise. Passing a refrigerator’s load through the unit all day warms the electronics, and some units run a fan. Keep it out of a closed cabinet and off carpet, and for a bedroom CPAP prefer the units the CPAP page notes as quiet.

What it changes about sizing

Nothing about the watt-hours, everything about the starting point. A unit that lived on a shelf arrives at the outage with whatever the shelf left it; a unit in UPS mode arrives full. When you size from the runtime tables or the calculator, the figures assume a full battery, and this is how you get one.