After the engine starts the charging system will provide a very high charging current. This hard on the charging system, can damage radio`s but most importantly is absolutely death in the aircrafts` battery,
Bit of a sweeping generalisation, that!.
The charging apparatus, wether Dynamo or Alternator (and some are designed with residual magnetism , so are self-exciting even with a totally flat battery) In either case, are speed-dependent for output.
Assuming you have hand-propped (and, yes, you
can do it without an impulse mag) your initial charge rate will be
the maximum the apparatus can deliver at those revs....so, keeping the revs down will reduce the charge-rate....meanwhile, the voltage on the battery-plates will rise rapidly as they take a surface- charge.....this opposes the charging voltage, (which is governed to prevent overcharging and "boiling" away the distilled water content of the sulphuric acid electrolyte)
Unfortunately, this charging circuit is normally a compromise and it will
NOT deep-charge a "flat" battery....to preserve battery -life, do not deep-cycle apply a regular float-charge and, yes, avoid high-current charging ,other than the surface-charge which is removed during a "normal" easy start.
Radios should not be affected if you minimise voltage-spikes usually caused by disconnecting a slave"jump" battery prematurely....about 2 minutes of fast-idle should see a drop in charge-rate as both batteries are linked and their surface-charges equalise.
Above, of course, applies to Lead-Acid types, sealed,Gel or "wet".