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I have a Hisense portable air conditioner. It's one of those awful units that exhausts the hot air out a flexible duct, and draws air for the condenser coil from the conditioned space.
The evaporator coil starts forming ice almost immediately after turning it on.
There is good airflow across both coils, and both blowers are running.
I think that leaves low refrigerant charge as the only other cause of the problem. Is there anything else I'm overlooking?
You are correct that a low refrigerant charge is the most likely cause since you have verified air flow.
One additional check would be to see if it has a throttle valve or capillary metering system, a capillary is an inexpensive metering method with a very small tube if this kind of metering and your problem a recharge would be needed.
If there is a throttle valve it may have stuck open I have seen a couple of mid sized units with throttle valves and a tap may get it to cycle but once they start sticking they get worse with time.
I will note if R134a I have seen small 1-2 lb recharge kits at auto stores, if your system has a service port you could get a can and recharge the unit. If you don’t have a service port there are bolt on ones that can be found on line for under 10$ but the ones I have are 1/4 flair fittings not the automotive R134a I think those are a 1/4” square thread or similar to acme and there are adapters for flair to acme that’s what I use for automotive.
I don’t know how they get away with selling the r134a but it is the real thing I checked it out a while back but the cost is quite high per pound compared to what certified people get it for in 30 lb cylinders .
Make sure that the fan is running and is turning the right way and that the air tube is not blocked or restricted. If this is OK then I would say it is low on refrigerant charge. A little icing at the entrance of the "cap tube" into the coil tubing is OK but icing of some or most of the coil is not OK.
