Hi,
A 5-cell NiMH battery needs to be charged to 7.0V to 7.25V for a full charge and the charger has to put out a slight bit more to negotiate contact resistance.
That's not your (entire) problem though, as 3.107V indicates either two overcharged cells, 3 flat cells or 4 to 5 seriously discharged cells.
If one cell has been pole reversed (by being discharged way too low and/or being very unbablanced to start with) it will take down the measure by two cells.
Another possible reason is a too high cell impedance (eg. due to old age and/or overcharge). If you have one cell pole reversed and a bit of high impedance (which will make a flat battery seem fully charged very quickly, without it being the case), the 3.107V is quite possible.
But... This is all guessing (though with decades of practice
), so first you have to check each cell one by one, by piercing through the isolation material - sewing needles may help, but don't short any cells!
When you have the exact voltage of each cell and its polarity (neighboring cells will be placed with the positive poles facing opposite), come back with the numbers for a solution.
While you're at it, measure the chargers output, unloaded and while charging.
I do fear the battery is beyond a full life as a 5-cell battery, but if that's the case, the good ones may still be used for other purposes.
Never trust a sales persons word
