I don't have electricity outside. One friend suggested adding salt to the water, but that doesn't sound healthy for the chickens. Any other ideas that keep water liquid without the use of electricity? Maybe something with batteries??
Unless you have a huge bank of big (car battery or larger) you will not be able to keep any volume of water liquid under battery power for long, but it can be done...
To give you an idea, a standard car battery has about 45 amp/hours of power (bigger ones about 75 amp/hours) a very small bird bath heater is about 50 Watts @ 110 Volts or about .45 watts, while a decent sized heater is about 200 Watts @ 110 Volts, or about 1.8 Amps...
Now if you are using an inverter that converts the 12 Volts DC to 110 Volts AC to run the heater there is a loss, a good estimate is about a 25% loss...
So with that said using an inverter and a car battery and a little math, assuming best case scenarios (rare in the real world)...
A 45Ah car battery running a 50W heater should last about 3 days...
A 75Ah car battery running a 200W heater should last just over a day...
Now that assumes the heater is on all the time, one with a thermostat will last longer, but you also have to factor in cold batteries never perform as well as warm ones, and batteries progressively lose capacity with age, so to be blunt I would halve the run times above if you were designing a battery system to be on the safe side...
So realistically with two car batteries, an inverter and a smaller 50W heater you could keep a small volume of water liquid as long as you swapped out (bringing one in for charging and taking a fresh one out) the batteries every two days or so...
An extension would would likely be a better option...