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:
Wh required (for a trip) = 1.25W (energizer power draw) x hrs (total runtime)
For example, for two 9 hour nights you will need 1.25W x 18hrs = 27Wh If you bring a set of those AAs you will have sufficient battery capacity, because 8 x (.95 x 3.5Wh) = 27Wh
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 (ex. in the chart below) that you could recharge with solar energy; this type of battery also maintains a flat discharge curve:
Choosing the right size for your trip will be a matter of converting the Amp-hours claimed on a battery to Watt-hours; 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 useful, so .95 x 84Wh = 79.8Wh
Given the energizer draws 1.25W, 79.8Wh / 1.5W = 63.8 hrs of energy to power the fence. 63.8 hrs / 9 hr nights = 7 nights
As for other types of battery, maybe half of their rated capacity will be available for the energizer to use 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 useful, so .5 x 84Wh = 42Wh
Given the energizer draws 1.25W, 42Wh / 1.25W = 33.6 hrs of energy to power the fence. 33.6 hrs / 9hr 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 M8 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 to DC 12V cable. With unregulated voltage, however, expect maybe half of their energy to be usable. If you already 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.
As we discovered, however, ‘bigger’ versions (for charging laptops in a hurry) that are capable of 12V power delivery 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 - so it might be possible to draw what you need from certain models (like the 6Ah Talentcell YB1206000).
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!
