The effect of adding an additional cell will be to increase working time by about 1-1.5 hours.
As for the extra cell. Here's what to look for:
1. Button-top, 3.7 volt lithium ion 18650 size
2. At least 2600mAh capacity (beware of cells which claim 3400mAh plus - this is likely inflated as the known limit at present is around 3400mAh) for 18650
3. "Protected" cells are better since they will prevent over charging and over discharging. Protected means they have a PCB circuit within the cells itself which monitors the cell chemistry
4. ICR, IMR, INR cell chemistries are all good. Note with IMR they do not need to be "protected".
Incidentally, another option is to obtain two 3400mAh capacity cells. This will give you longer working times on a single charge due to the higher capacity. Estimation is 5-6+ hours on a single charge.
These are good examples:
The higher the mAh (capacity) figure the longer they'll last on a single charge. But beware many false vendors inflate the figures, so anything above 3800mAh beware unless it's from the likes of Panasonic, LG, Samsung, Sony, etc.
Two of these in the V3 will give you roughly 5-6+ hours on a single charge. So you may not wish to use the extension barrel. Also the good thing with lithium-ion is that they self-discharge very slowly, so when kept in the V3 for a while they won't lose much charge. Also li-ion don't leak and corrode.
Good outlets are (as well as the above) Battery Bros, Orbtronics, Battery Junction, and similar outlets.
Li-ion Battery Safety Notes
Use caution as misusing or mishandling the battery may cause a FIRE or EXPLOSION which may result in personal injury, property damage, or death. The user must have an appropriate understanding of the potential dangers of LITHIUM ION BATTERIES before purchase and usage. No express or implied guarantee of compatibility, suitability, or fitness for any particular purpose or device can be made. Any recommendations herein of batteries is intended for the stated use. It is the buyer’s responsibility to check whether purchased batteries are suitable for the stated use as given on this website. The recommended battery is neither designed nor intended to be used with an E-CIGARETTE, VAPORIZER, or similar device and MUST NOT be used with such devices due to RISK OF FIRE or EXPLOSION.
DO NOT USE WITH E-CIGARETTE, VAPORIZER, OR SIMILAR DEVICE
Additional battery manufacturer and battery industry association warnings for using lithium-ion batteries in e-cigarette or vaping devices can be viewed here: Be-CigaretteSafe.org
DO NOT STORE BATTERY/CELL LOOSE OR IN A POCKET, PURSE, ETC. ALWAYS USE A PROTECTIVE CASE OR BOX FOR STORAGE AND TRANSPORT
WHEN NOT IN USE, ALWAYS STORE LITHIUM ION BATTERIES IN THE PROTECTIVE CASE/BOX IN WHICH BATTERIES WERE DELIVERED
Misusing or mishandling lithium ion batteries can pose a SERIOUS RISK of PERSONAL INJURY, PROPERTY DAMAGE, OR DEATH
BATTERIES MAY EXPLODE, BURN, OR CAUSE A FIRE IF MISUSED OR MISHANDLED
Usage of batteries is AT YOUR OWN RISK.
DO NOT short circuit intentionally or unintentionally
KEEP AWAY from metal/conductive objects to prevent short circuiting
DO NOT use if PVC wrapper or terminal insulator is damaged or torn
DO NOT use if battery is damaged in any way
DO NOT over-charge or charge above the maximum voltage rating
DO NOT over-discharge or exceed the continuous discharge rating
DO NOT modify, disassemble, puncture, cut, crush, or incinerate
DO NOT expose to liquids or high temperatures
DO NOT solder onto battery
DO NOT use force to install or install in reverse/backwards
ONLY use within manufacturer’s specification
KEEP AWAY from pets and children
ALWAYS charge in or on a fire-proof surface and never leave batteries charging unattended
ONLY use a charger designed for this specific type of battery
DO NOT mix and match brands and models, old and new, used and unused batteries
STOP immediately if while charging/storing/using the battery it emits an unusual smell, feels hot, changes color or shape, or appears abnormal in any way
It is your responsibility to determine that your charger or device is functioning properly
If exposed to battery electrolyte, flush with water immediately and/or immediately contact a physician or emergency services
DO NOT throw away in trash; contact your local jurisdiction for proper recycling or disposal