2600mAh AA battery? This is either an 1,2V NiMh-Battery (Not supported from the charger) or a fake "2600mAh" 3,6V LiIon battery. In order to enable the charging circuity in the OSTC plus it's required to set the battery type to "T2" (In front of the battery voltage in the main surface mode screen). So, in your older firmware, it's 3,6V rechargable.
Then, the OSTC assumes an AA 3,6V LiIon battery with 800mAh capacity.
It's strongly recommended to bring your unit up-to date, since there have been important safety updates in the OSTC firmware, especially concerning the battery usage in the OSTC plus.