I have an electric engine on my boat that is less than ½ a horsepower (314 watts). To make it useful I need to attach it to a battery so the question of the size of battery arises. The boat in question is a sailing boat, so weight is a problem. I thus need to answer the question
"What is the lightest battery I need to get full power out of my engine for a period not exceeding 30 minutes?”
I only know 3 electrical equations
Ohm’s Law V=IR
Power Law P=VI
Peukert’s Law C=InT
where
V = Voltage in Volts
I = Current in ohmns
C = Capacity in Amp/hrs
N = Peukert number
P = Power (watts)
R = Resistance in amps
T = Time in hours
Known
Volts V 12.00
Watts P 314.00
Capacity C 110.00
Peukert n 1.30
I can thus deduce
calculate amps I 26.17
calculate ohms R 0.46
Calculate time T 1.58
I think that the above is a correct answer to the question
"What is the battery I need to get full power out of my engine.”
To get the size of the battery down I played with the Capacity as this seems to be the limiting factor with weight. I also noticed that most battery web sites seem to imply that the full 12 volts may not be available so I played a little with the Volts.
I got the following results
Fixed
Peukert n 1.30 1.30 1.30 1.30
Watts P 314.00 314.00 314.00 314.00
Variable
Volts V 12.00 10.00 12.00 10.00
Capacity C 110.00 110.00 30.00 30.00
Calculated
amps I 26.17 31.40 26.17 31.40
ohms R 0.46 0.32 0.46 0.32
time T 1.58 1.25 0.43 0.34
I know nothing goes according to theory so I took the lightest battery indicated in the last column (30 amp hours) and added 30%. So I went out and bought a 40 Amp Hour battery safe in the knowledge that I could get half an hour of flat out motoring with my engine.
WRONG!
I doubt I’m getting half the power I should have.
So what am I doing wrong?
1. Is there a limitation to Peukert’s theory?
2. Is there another equation my teachers forgot to tell me when I was at school?
3. Some other reason?
This is not a theoretical question as the sailing season is fast approaching and I need an answer soon!
Any suggestions will be appreciated (but No - I will not return to a stinky old petrol outboard!)