Lithium batteries will measure more than this. When they come off the charger they should measure somewhere around 4 to 4.2 volts. If you are able to measure 1.5 volts, then that battery should have shut down, assuming it is a protected battery. If it is not a protected battery, be careful. A lithium battery should not be discharged that low! If you are unable to measure a voltage that is closer to normal, the battery should be replaced (I assume the charger is good). Contact the vendor you obtained the battery from for further instructions or to obtain replacements.
Also, be sure it is a lithium battery you are testing. If your mod was designed to use NiMh batteries, then this will be considered a normal voltage. Lithium batteries 3.7-4.2 volts, NiMh batteries, 1.0 to 1.4 volts.