We understand that all this terminology can be a bit confusing at times but once you know how it works it is quite simple. Below we will try to explain what it all means.
Volt or Voltage (V)
The number of volts is the amount of energy given to an electronic circuit. By a circuit we mean, for example, an electronic device. With a 12V device, 12 volts are always “given” from the battery. A battery always has a fixed voltage (e.g. 12, 24, or 72 volts) and a device always works at a certain voltage. For example, a device that works on 12 volts obviously needs a battery that also supplies 12V.
Current – Ampere (A)
When we talk about amperes (or amps), we are talking about how much electricity "flows" per second. If the number of amps goes up, then current flowing through the device per second also goes up. An electrical device usually works on a fixed voltage, but the amount of amps it draws can vary depending on, for example, the position of your trolling engine (a trolling engine at full throttle draws more amps than in half throttle for instance).
Example 1: Suppose I have a ERIDER PUSA Li3. The trolling engine runs on 72V and currently draws 15A. I decide to go a little faster. The engine still runs on 72V but now pulls 25A. The voltage has remained the same but the number of amps has gone up.
Power – Watts (W)
Power is the voltage multiplied by the number of amps, or W = V x A. This is the amount of energy consumed by a device and therefore an indication of how powerful it is. This goes up when the number of amps also goes up.
Example 2: Suppose I have a 72v motor that draws 15 amps. So the power consumption is 72 times 15 = 1080W.
Example 3: Suppose I have another 72v I am running in gear / speed setting 2. The motor runs on 72V and draws 15A and thus has a power consumption of 1080W (72 x 15). When I switch to gear / speed setting 3, the motor draws 25A and still runs on 72V. The power consumption of the trolling motor is now 1800W.
Capacity – Amp hours (Ah)
Battery capacity is measured in Ah, or Amp-hours. As the name suggests this means how many amps the battery can deliver in an hour. For example, a 72V lithium battery with a capacity of 100Ah can deliver 100A to a 72-volt device for one hour. The same 100Ah battery could supply power for 4 hours (100/25=4) to a 25 ampere device. If a battery has 72V50, this means that the battery works on 72 Volt and has a capacity of 50Ah. A 24V100 battery works on 24 Volt with a capacity of 100 Ah etc. In practice for lead-acid batteries the nominal capacity (how many Amps hours the battery can deliver according to specifications) differs greatly from the effective capacity (how many Amps the battery can actually deliver during use).
Example 4: I run my ERIDER drawing 15A at 72V. I have a 72 volt battery of 52 ah. My total run time is now 52/15 =3,4 hours. When I switch to faster speed the engine draws 25A. My total runtime is now 52/25 = 2.08 hours.
Capacity – Watt-hour (Wh):
Another way to measure the capacity of the battery is in Watt-hours (Wh). Wh is calculated by multiplying the number of Amps with the battery voltage. For example, a 72V100Ah (a 72 volt battery with a capacity of 100Ah) has a capacity of 72 x 100 = 7200Wh (7,2kWH). A 24V50Ah battery has a capacity of 24 x 50 = 1200Wh (1,2kWH).
Example 5: I have a 2000W HUB motor and a battery with a capacity of 7200Wh. My runtime at full throttle is 3,6 hours with this battery (7200 / 600 = 3,6). I do not even need to know how the HUB motor or battery voltage to calculate this (as long as they work at the same voltage obviously).
The attentive reader notes that the runtime of a battery with a device can be calculated in two ways. Either by dividing the number of Amps of the battery by the power draw in A of the trolling motor or by dividing the number of Wh of the battery Wh by the number of W of the trolling engine.