Under certain limited circumstances, you can get away with using a standard car battery. The circumstances are: where you have a small boat, with a fractional h.p. motor (i.e. one that doesn't draw a lot of current) and where you are only travelling short distances between charges. The reason for saying this is that, as we will see later, you will be relying on getting up to 70% of the charge out of your battery(ies) and into the motor on any extended trips, and standard car batteries will not survive for long when being regularly discharged to that extent. So, you can use relatively inexpensive batteries, but only if you strictly limit the depth of routine discharge, i.e limit the speed and duration of use.
For the general situation, where you want the maximum range and/or speed, the advice is to go for 'deep-cycle' batteries - ones that will allow up to 70% discharge on a regular basis, and provide a reasonable number of charge/discharge cycles (~250 - four to five years use, if only used once a week)
Another choice is between open cell (the convential design) and sealed (sealed come in two type: gel and glass-mat). Unless you really need the unspillable and service-free characteristics of the current generation of sealed batteries, the open cell deep-cycle design should give you more charge/discharge cycles and a longer life. (Battery manufacturers may wish to differ on this.).
As you might expect, the bigger the boat (and hence the motor) the bigger the 'battery' needed. If you are powering the Small category of craft then just one 12 volt battery will be sufficient (you might have trouble carrying more than one!). As the boat gets bigger and the motor more powerful, so does the size of battery needed. The two factors governing battery size - more accurately battery voltage and energy capacity - are the motor voltage and the motor power output.
In order to drive 24, 36 or 48 volt motors it will be necessary to connect two, three or four batteries in series to meet the voltage required by the motor. Then depending on the motor power, the propellor efficiency, the hull efficiency, the intended average boat speed and the desired range (plus a few variables like wind and current), it may be necessary to connect one or more batteries in parallel with each series one to provide sufficient power to meet the speed and range requirements. Terrapin is a typical example, with three parallel-connected sets of two series-connected units giving the required combination of voltage and power.
So, in order to reduce the guess-work, it is only neccessary to define some rules of thumb and set up some simple formulae that will connect volts, h.p., current etc., etc. so that you can plug in the numbers for your own situation and get some idea of what battery capacity will meet your requirements:
Rule 1. Average power demand should not exceed 20% of battery capacity.
Rule 2. Peak (short-duration) demand should not exceed 40% of capacity
Rule 3. The battery discharge limit should be no more than 70% of its capacity. (limiting maximum discharge to 50% will measurably extend the battery life)
Taking a 1 h.p., 12v motor as our example, the current (I) taken at full power is:
I = Power(in Watts)/Volts = 746/12 = 62 A
To meet rule 1, the capacity of our battery should be a minimum of:
62/0.2 = 310 A
To meet rule 2, the capacity of our battery should be a minimum of:
62/0.4 = 155 A
To meet rule 3, with a four hour cruising duration (for example), the battery capacity should be:
(62 x 4 hours)/0.7 = 354 AH
In satisfying rule 3, we have, in this example, met the other two rules as well.
But we would be unlikely to run our motor at full power continuously. If we have sized the motor to suit our boat and cruising conditions, we would hope to cruise at no more than 50% of maximum power, thus leaving plenty of reserve power (and hence speed) for any emergency situation. So in practise, we can satisfy rule 3 with a maximum battery capacity of:
(31 x 4)/0.7 = 177 AH
...and this still meets both rules 1 and 2, i.e. our average current consumption is less than 20% capacity and our peak (at 1 hp) is less than 40%.
We can now see that our 1hp, 12 volt motor would need two 12v 95AH deep-cycle batteries wired in parallel. This battery bank would not be discharged by quite 70%, if we average 0.5 hp for a total of 4 hours, which would help to maximise the potential charge/discharge cycles it could support.
It is also possible to see how we can ring the changes using these simple formulae:
What total duration could I expect by adding a third battery in parallel?
- almost 6.5 hours
What average horse power would still give a four hour duration with a 12 volt motor but with only one 95 AH battery?
- 0.25 hp - a typical trolling motor output
Two points are worth making before leaving this topic:
- These calculations assume no losses in the system at all. There is no loss of energy in the wiring, the connections or the power controller. The motor is 100% efficient. The propellor also transmits all its energy to the water. And there are no currents or headwinds. All of these things will actually have an effect on these simple calculations, but the results are accurate enough to provide a practical basis for battery selection.
- David Williams' Terrapin/LEMCO combination achieves a cruising speed of 3 knots at only 10 A consumption, hence the estimated 20 hours cruising duration.
I hope that this page, lengthy though it is, gives some insight into how to size a battery bank to suit your particular requirements. If you need any further help, there is an email address over on the guestbook page.
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