Josh may know the battery as "two for $37.50" but we don't... The battery type will make a difference to the answer.

Hi Mel. I'm not sure which batteries we are talking about. Can you tell me which ones?

If you're talking about lipo or Li-On (3 x 3.7V cells)... I hope you're using a lipo/Li-On charger and a charge control/protection circuit... Continuing to hammer a charge (overcharging) into these types of batteries without overcharge protection won't be a good thing...

No Josh, they are the 12volt ones that they were two for $37.50 and you gave us a 10% discount code and those batteries seem to last forever on a single charge.

You know the ones POWER something or other.

Who is they and who are us? Where di you buy them from? Can you be more specific? What type chemistry are they? Lead acid, NiMH, NiCad, LiPo or Li-On? We need to know this so we can tell you their max charge... They are all different...

Those would be the Sealed Lead Acid (SLA).

Typically, this should be charged at over 12v, common voltage is around 14v, then drops if it's a trickle charger.

When fully charged the battery voltage should be 12.7v

You haven't said if that voltage is while charging, charger voltage or battery voltage after charge. What voltage is the charger? i.e. what voltage is reported on the charger if you have no battery connected? What voltage are the batteries when you take them off charge?

Ah , POWERSONIC , 12v 9ah batteries . I get 12.6 volts when I check mine with a meter after sitting for a while and some use. They may vary depending on charger used and what charge level they are at.

Here's some more info on SLA batteries and charging them, charge levels etc.