It can be possible in some instances to revive an old NiMH-battery at your own risk, but the success rate may vary depending on the current condition of the battery. If there is any physical damage like corrosion, bulging or leaks, it may not be safe to revive it.
Sometimes charging the battery can help. When the NiMH battery has been discharged for a long time and is completely empty, it might enter a deep discharge state. In some cases, cycling the battery might be the answer, by fully charging and discharging several times.
Some NiMH battery chargers, also known as a smart charger, come with a reconditioning or trickle charge feature. This applies a low current to the battery for a longer period, which can revive batteries that have been deeply discharged.
An empty NiMH-battery will have a low voltage. You can verify this by using a voltmeter. If the battery has 0V, it cannot be charged in a with a smart charger.
However, a NiMH-battery can be revived by using cables and another fully charged one. Connect one of the cables from positive of the fully charged battery to the positive of the empty battery. Keep them connected for three seconds and then disconnect them.
With the voltmeter you can see if the empty battery has now some voltage in it. Now you can put it in a smart charger again and it will start charging.
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