Blogpost by Founder Joe Grez
We use the terms energy and power all the time to describe personal attributes etc. But these are
also scientific quantities. Energy and power are universal; meaning that they mathematically work
for all physical realms including mechanical, fluid, propulsive, electrical etc.
- Energy: Energy is always power x time. For example 20 Watts x 2 hrs is always 40 Watt
hours (Wh) which is true for all forms of power and energy including electrical, mechanical
propulsive etc. While there are many units in use to quantify energy– I chose Wh, you could
also use Joules, eV, Erg, Watt seconds, BTUs etc.
- Power: Power is always = a force-related thing x a
time related thing.
Here's what I mean.
- Mechanical power is in a rotating shaft is torque x
- Fluid motion power is pressure x flow rate,
- Propulsive power is thrust x velocity.
- Electrical power is voltage x current.
The first term is always force–related. Referring to the above examples,
Torque is a force x a distance. Pressure
is a force over
an area. Thrust is just a force. And
voltage is an electromotive force. So you can
think of voltage as an electrical version of torque, pressure or thrust.
The second term in all these examples is something / time;
Again, referring to the above examples, rotational velocity is
radians / second. Flow rate is cubic meters / second.
Velocity is meters / second. And current
is coulombs / second. So you can think of electrical current as a speed-related thing,
and the electrical equivalent of rotational velocity, flow rate or velocity.
This was a sneaky way to try and slide you into an understanding of something
about Voltage (Volts) and current (Amps). A volt is a kind of force and current is a flow rate of
electrical charge. Hope it worked, but if this was just plain confusing, Wikipedia has a great
page on the subject. At least some vague understanding of the nature of Voltage and current will
be useful as we continue…
One more thing: When the industry uses kW or kWh, the k indicates 1000x…
So 1.2kW is equal to 1,200W. For smaller devices you will see mW which is 1/1000th of a Watt.
Electrical Power and Energy in an outboard:
- A Watt, abbreviated by W, is a measure of power that is controlled by your throttle. If you
set your EP Carry throttle at max power, you are using the max-rated 220W into the motor, or
220W "input power". The input power to a motor controller is the output power output
power from the battery.
- If you keep your throttle set at max power for ½ hour, you have used some energy.
Energy is measured in Watts x hours or Wh, so in this example, 220W x ½ hr = 110Wh.
The energy-power relationship is easy to remember because the units, Wh (Watt hours), contains
the power unit (Watts), and the time unit (hours).
- The EP Carry battery stores 288Wh of energy (says so on its label). Dividing the 110Wh of
used energy from the above example, by the max stored energy in the battery gives you the percentage
of the battery's capacity used. So 110Wh / 288Wh = 0.38 = 38%. That's what we
used, but if you want to know how much remains in the battery, subtract what you used (38%)
from what you started with (100%), and the result is 62%. This percentage is called
the battery's state of charge.
- Let's do another one. Using EP Carry's max power setting (220W) and the stated battery capacity
(288Wh), how many minutes of runtime are provided? This is a simple problem if you know where to
begin. I tend to look at the units first, just to make sure I'm starting off right. In this case
we want to end up with time as an answer. One of the numbers given is in W, and the other is in Wh.
If we start with Wh, we know that we want to end up with hours. So the Wh term should be divided by
W, causing the W items to cancel out leaving just hrs. So let's do it. 288Wh / 220W = 1.3 hrs.
Because the problem calls for minutes instead of hours, multiply that result by 60 min/hr, and the
result is 78 minutes. We promise 70 minutes so why are there an additional 8 minutes in this result?
The answer is that we see 77 minutes in run-down testing. And the other minute is from de-rating of
the battery at a 220W draw. We state 70 min because it's easier to remember, and assures us our
advertised runtime is always provided even after the battery starts to age.
Voltage and current:
- The power of one Watt is made up of two parts, Voltage and current. Voltage is measured in
Volts, and current is measured in Amps.
- Only use deep cycle or Lithium batteries:
- Traditional engine starting batteries are rated in "cold-cranking Amps" (CCA). CCA
ratings in no way reflect the performance of a battery when powering an outboard. Use batteries
with an Ah rating instead.
- There are small lithium batteries rated by Ah "equivalence". If you see this, it is a
cranking battery, not a deep cycle battery. The usable capacity is only a fraction of the equivalent
capacity. Only use batteries that list an Ah rating (or a Wh rating) with no use of the word
- The stated Ah rating (also called Amp capacity, or "Ampacity") of a battery = Amps
(A) x time (hours) = Ah.
- Traditional deep cycle batteries must be "de-rated".
- The higher the Amp draw, the lower the Ah a deep cycle lead battery can provide. As a result,
the industry has standardized on what is called a 20 hr discharge rate to measure their Ah ratings.
The 20-hr Amp draw = stated Ah value / 20 hrs. So the 20 hr discharge rate for a 50 Ah battery
= 50 Ah/20h = 2.5 Amps. At higher currents, a deep cycle lead battery will not release
the Ah number stated. To some extent, this is true for all batteries, but the de-rating of lead based
batteries is much higher than for Lithium batteries.
- Lithium batteries have a comparatively high efficiency compared to traditional deep cycle batteries.
The EP Carry battery stores 11.25 Ah at a 20 hr discharge rate. The current that results in a 20 hr
discharge time is 11.25Ah / 20h = 0.56A. The current drawn by an EP Carry is much higher, yet
as we have seen in testing, our battery provides the advertised capacity when used at these higher
currents. In our application, de-rating is too small to impact most calculations for Lithium batteries.
But if we used a deep cycle battery with the same stated Ah rating, we would be able to use only
84% that capacity. (Wikipedia has a description of Peukert de-rating used for lead-based
- As a battery charges the voltage generally goes up. As it discharges, it generally goes down.
For our batteries, the max charge voltage is 29.4V and the low cutoff voltage is 20V. These
limits are reinforced by internal battery circuitry as well as by the EP Carry charger and motor.
Redundancy is good.
- When removed from the charger and allowed to rest, our fully charged voltage is around
27V-28V. In use, voltage quickly drops to ˜26V. It drops slowly thereafter until around 23V and
then drops quickly till the end. Meanwhile, the battery warms up during use. Because voltage
increases with temperature, and because most of the discharge cycle voltages are naturally flat,
we have seen cases where the voltage increases for part of the discharge cycle due to heating.
That's why voltage is not an indicator of remaining capacity with a Lithium battery.
- The EP Carry battery lists a voltage rating of 25.4V. Let's see if this and the Ah capacity
equal the stated Wh rating. 25.6V x 11.25A = 288Wh. The math checks out.