Poor performance of the Internal combustion engine is a major contributor to hybrid battery failure. When more power is required then the ICE can provide the electric engine kicks in drawing on the battery. If the battery does not get the opportunity to get charge up before it is called upon for a discharge the capacity drops even further. If this continues then the point may be reached where the weakest cell in the series fails under load.
Causes for poor performance in the ICE are varied but the usual culprits are
a) MAF sensor
b) Spark plugs
c) ignition plug in coils
d) throttle body
e) air filter
f) oxygen sensor
g) engine oil and coolant
h) Clogged catalytic converters
Interesting enough we have never experienced a timing problem in these cars. Idling as in gas engine is not an issue as the ICE should shut down when the vehicle is stationary.
Quite unlikely for moisture to get inside the hybrid battery as the operating temperature can reach 50 degrees centigrade. No chance for moisture to accumulate. They will all be vented. Gasoline engine runs poorly for several reasons. It could be blocked throttle body, dirty are filters, damage oxygen sensors and dirty or damaged Air Flow Meter. Just to name a few. These can be eliminated with a good standard service from a knowledgeable workshop. If it still persist then it will quite likely be the main hybrid battery. A drop cell or just weak battery will cause the computer to force charge the battery which accounts for the engine working strangely.