First answering Jim Lee's question: One implication of my post is that if you cannot guarantee starter operation in the "left-hand" efficient side of the power-current curve using a house bank then you should have a dedicated start-only-start battery. This technical position helps to derive a battery bank design philosophy for cruising. It IS possible to have a single
bank system, however, it would be prudent to have a backup like a dry-charged battery and bottle of acid properly stored. Another option is to have a minimum-sized SLA battery always on float (when a charge source is available. Note: with the advent of affordable properly designed battery monitors single
house battery banks are the most cost effective and energy efficient manner of configuring your house bank. Alternating two banks of batteries is only life-time effective if the "resting" battery is left fully charged while using the other bank...a caveat not bandied within the marine community yet known in alternative energy applications. This proper method of two bank usage is not practical on a cruising boat which has limited space and budget
Gord: The number "5" derives from dividing the Power (watts) by 10 Volts (12 V would be O.K only if one could guarantee that the manufacturer's power spec is rated at that number and not for a more practical sagging voltage situation) to achieve the peak load current. We then multiply the peak rated load current by two to reach the locked-rotor (stall) current value. Observing the parabolic curve it is easy to note that the locked-rotor current is always twice the peak power current.
Most batteriey specifications (for batteries intended for use other than consumer) include internal resistance because the buyers are more technically astute than consumer users or battery distributors. GNB, Optima, Hawker, Prevailer, and others include the Ri of their batteries...sometimes you have to search for the inclusive spec sheets
to get the values. Some manufacturers (like Prevailer) publish Ri versus state of charge which is VERY nice to have. These numbers vary with temperature, of course.
If you are fortunate enough to have a battery monitor
, a battery, a high power inverter
, and a heavy load such as a 1500W heater or microwave oven
(1200W input power or so) it is convenient to measure Ri by starting with a fully charged battery and appling the heavy heater load (or quart of water
in the microwave) and logging the battery terminal voltage and output current for 10 minutes. Calculate Ri as the voltage divided by the simultaneous current value. You will notice that the Ri starts very low then climbs to a value and flattens out (if the battery bank size is designed to carry the load and is in good condition). The average value of Ri in the "flat" part of the curve is the real initial value to use. This value is good to log down and compare to future values in order to determine battery condition and wiring
The starter output power parabola does NOT infer that it is desirable to operate at the peak power at all times. It merely means that the peak value is the most that you can get for a given (rated) terminal voltage. What I am pointing out is that it is desirable to operate ONLY in the left-hand portion of the curve and proper battery choice helps to guarantee this. Obviously when ambient temperatures are high and the battery is larger than a minimum size you will be able to operate closer to the left-hand origin of the curve thereby minimizing cranking times and unnecessary power loss.
Yes, we are interested in the torque required to start yet if we do not pay attention to the phenomenon of possible operation in the "right-hand" mirror image of the power-current curve we might fall into the trap of using an insufficiently sized power source (the battery). I've heard people say, "what difference does it make, the engine starts anyway"? Well when their starter fails permaturely or when very cold conditions conspire to prevent the engine from starting at all then, they will be more interested in this concept! Remember that the stall current value produces the instant cranking torque at zero engine speed under the most limiting drastic starting conditions...if you can't break through the starting stiction to get the engine rotating it just woun't turn. It is that value which is the beginning point for determining a starter size by an engine manufacturer engineer
Hope this helps,