The factory specs for the Electro 36 tell that any speed lower than 1/30 are indicated as underexposure, but the user doesn´t have any way to check for the selected speed as the camera chooses it automatically. However this maximum time wouldn´t be true because I had an exposure of about 4 minutes stopping down to f16 under moonlight (during moonrise over the Rio de la Plata).
If the camera doesn´t react in a predictible manner upon low lighting conditions, there would be some problem with the electronics inside.
What would most affect exposure is the "memory" capacitor condition. This cap sets the shutter speed and if leaky or it´s capacitance value altered in any way, the set time can be wrong.
The battery voltage doesn´t affect exposure to any measurable way: the original battery is 5.6 V and what can be obtained is 6V. I have a 6 V akaline cell and exposure is OK both indoors in poor light or outdoors under bright sun, at least in my camera.
IMHO the best way to check for inaccuracies or troubles is to shoot an entire roll under the suspicious conditions, and check if the results are OK or not.
As an aditional info, I never had exactly the same exposure metering when using different SLRs, (Miranda Sensomat RE, Fujica ST 801, Praktica MTL50) nor with the Gossen Luna pro under same conditions, however all pictures were OK.
Hope this helps.
Many 35 mm and MF film cameras, some weird cameras... and nothing digital yet!