How long will the battery pack last?
Roughly 24 continuous hours (three 8-hour nights) with fresh alkaline batteries in optimal conditions. Don't forget that batteries are affected by the cold, so bring spares if out for more than two nights.
Gotta start with these:
Power (Watts) = Current (Amps) x Electromotive Force (Volts)
Energy (Watt hour) = Power (Watts) x Time (hours)
A standard alkaline AA battery is rated for 1.5 V with about 2.8 Ah of charge. That means it has an energy capacity of roughly 4 Wh. When connected in series, you add the voltage and the energy rating of each battery, so a Bear Sentry battery pack (with alkaline batteries) provides 12 V and holds 32 Wh of energy.
The Bear Sentry energizer runs on that 12 V and is rated to draw 100 mA of current when the fence is operating (even when it's not being discharged). As such, it draws 1.2 W of power when electrified. For every hour it operates, it consumes 1.2 Wh of energy, so the pack can therefore charge the fence for 27 continuous hours. Assuming you only have the fence running while you sleep, the pack should do you for up to three 8-hour days.
The energizer releases a full 1.0 Joule pulse of energy when the hot wire is grounded. A Joule is a Ws, or 0.00028 Wh of energy. As such, shocks delivered by the energizer make no appreciable dent in the battery pack's capacity.
The fence works with such a small release of energy, because the fence energizer transforms the voltage of the system up to around 10,000 V while limiting the pulse to only 3/10,000 of a second. 1.0 J of energy delivered in such a tiny fraction of a second is an extremely painful experience, but does no damage.
Here's the rub: the type of batteries and the ambient temperature can make a fair amount of difference to the fence's longevity. We tested the system using standard alkaline batteries with temperatures around 20C and got results in line with the theory presented above. At colder temperatures however, longevity will be reduced.
Long and short of it is that if you're camping out for two nights or more, you should bring a spare set of batteries with you.
An alternative would be to bring along a solar charged power bank such as the one we offer. Ours has a rated 50 Wh capacity and can recharge in full sunlight at 1 W.