I agree with the others who pointed out as you did, that a capacitor may be the sypmtom of failure. When capacitors (condenser) are under-rated for temperature or surge current, they wear out faster than normal. A healthy margin for voltage, max current & temp. allow for normal long life of 25yrs. Otherwise these stress factors can reduce it to less than a year such as the huge PSU recalls from major PC companies that are no longer in business.
The normal diode charge surges with internal ESR creates heat. Heat accelerates leakage aging and also increases ESR aging. The same is true for car batteries.
Some electrolytics standardize at 85'C and offer higher reliability types at 125'C. This would be a wise choice if considering one cap. Also consider replacing all big Caps.
When the electrolyte exceeds it's operating temperature by some margin, the pop-corn lid expands to offer safe containment of the nasty chemical. (read MSDS sheet)
An inverter consists of;
- a battery charger and battery
- an inverter and
- a controller
The controller and user front panel options decide when to charge, when to invert and when to switch from inverter output to mains output.
Symptoms and Analysis
Flip Flop switching in Auto mode
If the line voltage is detected as too low, it switches over at the Brown-Out(BO) threshold before a line failure occurs.
If the charger current is high and that loads the line voltage BOT then the time it will take longer for the battery to return to idle charge current and thus toggle rate will be slow. As the charger current reduces, the toggle rate will increase.
So it appears the battery charger could be overloading the BO threshold and the BO threshold detector may need re-calibration. If the BO threshold comes from the same transformer(XFMR) secondary from the AC line, that might explain some interaction, but not completely.
Something could be loading down the XMFR secondary low voltage out.
Could it be the battery is weak and lossy overloading the charger? Test S.G. in each cell. Replace battery if necessary. Test Charger voltages not to exceed 14.2V on battery (if 12V)
If the battery was the root cause then it might explain the results, but it also indicates an under-sized charger or at the least a poor way of detecting BO trip point... and poor method of detecting battery failure.
The next possibility, is there was a change in BO threshold and/or your AC line levels due to some other component failure.