You will need a DVM (digital volt meter) to check each batteries.
1. 1.5VDC(regular) should be 1.65V fresh
2. 1.2VDC (NiCd or NiMh) should be 1.29 -1.34V fresh
Minimum using 4AA is 4.47VDC
Maximum voltage is 8VDC
Using external battery like the ATG Elite Gold Battery Pack or Quantum Turbo is 330V. Can't use on 430EX's.
Anyone one of the batteries fail, you can't turn on the flash.
If you can open the flash, check the On/Off Switch using the DVM in Ohm's Scale.
The above information was provided to me by ATG.
Truly,
Bill
onebill58@yahoo.com