How much battery capacity will you need?
To calculate the energy required for your trip in Watt-hours (Wh):
A Watt-hour (Wh) measures the total energy supplied if electrical power of one Watt is maintained for one hour:
Watts x hours = Wh
In practical terms for us, it is a measure of the total energy stored in a battery for powering the fence; for example, each of the 1.5V AAs below has a rated capacity of 3500 mWh or 3.5Wh. Eight of them in series supplies 12V:
We recommend rechargeable Li-Ion cells because they are voltage-regulated: each has a chip inside that lets it maintain a steady 1.5V as it discharges. Because the energizer can enjoy the constant voltage it needs, it can deplete almost all their stored energy to power the fence longer (whereas other batteries ‘quit early’ once their discharge voltage drops below the energizer’s 12V threshold). As you can see below, most of the 3.5Wh in the Li-ion AA is usable energy:
1.25W (energizer power draw) x hrs (total runtime needed) = Wh required (without recharging)
For example, to run the fence for two 9 hour nights, 1.25W x 18hrs, you will need to bring 27Wh of battery capacity.
Now hunt for the Wh listed on batteries, keeping in mind, however, that you will have to factor in how much of their total energy will be usable for the energizer. For instance, a set of the AA’s above will be sufficient, because:
8 x (.95 x 3.5Wh) = 27Wh
For a longer trip with the Hixons you could bring an extra set or recharge with a solar panel (charging them evenly lest you fry something in the case). Or bring a bigger battery.
Most batteries designate their energy capacity in terms of Amp-hours (Ah): how long they can deliver a certain amount of current (A).
LiFeP04 batteries are rated in Ah (and Wh in smaller print). If you are planning more than a weekend in the backcountry, consider a relatively small one that you could recharge with solar energy (see our options below); this type of battery also maintains a flat discharge curve:
Figuring out if the one you select has the right capacity for your trip will be a matter of converting its claimed Ah into Wh; then taking into account its usable energy, then factoring in the power consumption of the energizer. For instance, a 12V (LiFePO4) battery with 7Ah capacity:
Ah x V = Wh
7Ah x 12V = 84Wh stored in the battery, of which est. 95% will be usable, so .95 x 84Wh = 79.8Wh
Wh / W (power consumption) = h (run time)
79.8Wh / 1.25W = 63.8 hrs
63.8 hrs / 9 hr nights = 7 nights
As for other types of battery, maybe half of their rated capacity will be usable because of voltage drop. For example, a 12V (SLA) battery with 7Ah capacity:
Ah x V = Wh
7Ah x 12V = 84Wh stored in the battery, of which est. 50% will be usable, so .5 x 84Wh = 42Wh
42Wh / 1.25W = 33.6 hrs
33.6 hrs / 9 hr nights = 3.7 nights
18V-20V cordless tool batteries are a bulky yet powerful option when stepped down to a regulated 12V with a ‘buck DC voltage converter’ and adapter for the brand. (Test results for some of our own 18V Milwaukee M18 batteries are below). If you already own one of these expensive batteries, you might consider using it as a power supply for the fence. Dewalt and Milwaukee converter-adapters can be found with More Good Kit.
Power Banks
5V power banks (for cell phones) can be used with a USB 5V to 12V cable. But this “boosted” voltage will be unregulated, so expect maybe half the energy to be usable. If you have one, test it with the energizer to see what’s possible.
5V power banks that can output regulated 12V are a more viable option. In our tests the mini Veektomx model VT103 powered the fence for an impressive two nights (considering it weighs only 165g). So we made it available here. You can use the USB 5V to 12V booster cable above, but a USBC Type-C 12V PD cable is the better bet because it can “trigger” the battery to deliver 12V power, no conversion needed; this gives you 40 mins more power, at least with the VT103 model. So try a USBC trigger cable as long as your power bank is Type-C PD (power delivery) capable @12V:
As we discovered, however, ‘bigger’ versions (for charging laptops in a hurry) that are capable of 12V PD may not be worth the extra weight - they’re not designed for low wattage applications and so they may be less efficient in this mode. But if you have such a multi-voltage power bank (check the back), test it to see what it can do; if you’re considering buying one, look for reviews that reference tested capacity @12V.
12V (Li-Ion) power banks or battery packs (ex. for CCTV cameras or LED strips) have unregulated voltage and may not supply ‘true’ 12V in the first place (by design, to keep size and costs down). However, our energizer is somewhat resilient and can take voltages slightly above or below 12V. We tested the 6Ah Talentcell YB1206000, for instance, and it performed well. 12V power banks could be an economical option then, but make sure before you head out.
12V (LiFeP04) power banks (ex. for astronomy field work) are a better, albeit beefier option. The Talentcell 4100 in the chart below does output constant 12V, and it’s packed with usable energy.
In summary
Whether you calculate in terms of Wh or Ah, the result should be considered a rough estimate. And remember that battery capacities are almost always overrated. Choose the one for your trip based on published test results. Most important, test it with your Bear Sentry yourself to see if it lives up to its rating, reputation, and your math!
