800 CCA it will produce up to 800 amps of current at 0 °F (−18 °C) for 30 seconds, maintaining at least 1.2 volts per cell. In the marine world this rating is less important than the amp-hour rating.
Amp-hours (Ah)
At the base level, Amp-hours tell us how long a battery can generate a set amount of current. For example, a 100 Ah battery should deliver 5 amps for 20 hours, 10 amps for 10 hours, and so forth.
However, Peukert’s Law (pew-kurt) explains that a batteries’ discharge capacity varies with the duration of the discharge. Using Peukert's Law on the same example as before, the 100 Ah battery would actually only last 7.6 hours under a constant10 amp load, not the assumed 10 hours, as we had stated earlier.
This law, and it's related equation, are not easy to comprehend. Luckily, there's an online calculator that does it for us. Click on the equation in the upper right to go to it.
So far we've only been discussing one battery scenarios. However, in most bowfishing applications there is sometimes a need to get either higher voltages, or more Amp-hours for your given load.
Trolling motors above 55 lbs of thrust typically require at least 24 volts to operate and motors over 80 lbs may require 36 volts for proper operation. To get these voltages the batteries must be wired "in series".
Connect the positive terminal on one battery to the negative terminal on the next battery using a jumper wire, then apply the positive and negative wires of the load (trolling motor) to the open terminals on each
battery.
A common misconception when running batteries in series is that the Amp-hour capacity increases with each added battery. In fact, the Amp-hour capacity for the battery bank stays the same, however the increase in voltage allows the amperage needed to run the load to be cut in half.
Alternatively, two batteries can be wired "parallel" (see diagram below). In this case the Amp-hour capacity does indeed double, but the amperage stays the same at 12 volts.
In regards to run-time, for example with LED lights, there is no advantage to running the lights at 12 volts vs. 24 volts in an effort to maximize run time.
If you want to maximize your load's runtime you need to select batteries, wiring, and circuit components with the least resistance. Also, look for batteries with the highest Amp-hour capacities you can find.
New, fully-charged batteries
will provide the longest run-times initially, but expect those times to decrease as the batt-eries age, or if the weather is extremely hot.
This article would be remiss if we didn't talk about the additional components that make up the circuit load. Wires, connectors, switches, and fuses/circuit breakers all contribute to resistance in a circuit, leading to voltage drop.
When calculating wire resistance make sure to factor in the lengths of both positive and negative wires. A 20-foot run from the front of the boat to the back is actually 40 feet of wire in the circuit.
Wire resistance will be the main contributor to voltage drop and running a larger gauge wire can help reduce that resistance. Here is a handy online tool that will recommend a wire size for you if you plug in a few variables.
(Continued on next page)
19
March, 2015