How does an engine act when the ignition battery dies? Do different ignitions act different? Is it a spit sputter and die, an instant off, a slow drop in power or what?

It becomes silent

With a RCexl you will see the rpm drop and will not go to full power before it dies completly.

Tells you it is time to land and recharge.

Milton

Loss of peak power, rough running, won't throttle up correctly, silence.

The spit/sputter only lasts a few seconds before the engine quits

sput....sput...sput....silence.....thud!

If the ignition battery dies, so will the motor. However, if the voltage is low, but enough to keep the engine running, the engine will run very rough. You will initially think that it is running rich, buit any attempt to adjust the needles will have no effect. It will just cut out.

If you engine has these symptoms, check the ignition battery

+1
it really depends on the battery and the condition of it - anything from loss of power to sudden silence! your ignition battery is just important as the Rx battery, never use batteries that are even doubtful. it could cost you a lot more than a new battery pack,

Also on twins it usually shows up as one cylinder dropping out and then the silence follows a few seconds later.

sput....sput...sput....silence.....thud! ....*#$%^&!.....waaa!

Now it is not that bad.The engine will start missing and breaking up at full throttle.Like someone said sounds like it is going rich on carb.If and when this happens throttle back and head for a landing ASAP if you have to make a go around do not go to full throttle,sometimes battery will recover enough t o make a safe goaround. Do not try this unless you have to. Keep check on your ignition battery and do not every use a battery that you would not trust on your radio.
BCCHI