There’s little evidence that phone radiation is higher when the battery is low but then there is little evidence about mobile phone radiation in general. It wouldn’t really make sense for phone radiation to increases with low battery. When the battery depletes, your phone will -simply do what it needs do to stay powerded on for that little bit longer. Most phones enter tow battery'” “Power saver’ modes to try and compensate. If anything, radiation could be reduced when phone batteries deplete as your phone’s screen dims, etc, but this world be negligible.
The thing is, when a phone tries to get a signal, it will do so with the greatest efficiency possible. so, if a signal is readily available, e.g. you’re in a major city, then your phone’s antennae won’t need to work so hard. Then, if you enter a subway or ride. the tube, your phone will sense it’s signal deplete and try and ramp us its efforts to recapture it. This will vastly increase. phone radiation.
This process would be the same regardless of whether your phone is fully charged. or very low on charge. It is always working for a signal, no matter where you are provided airplane mode isn’t turned on.