Time to continue my ramblings with, as promised, starter versus deep-cycle batteries.
Note: "Starter" batteries are often called SLI (starting, lighting, ignition) batteries in an apparent effort to refocus the descriptive name applied to these batteries.
Starting an engine requires a very high rate of discharge spanning a brief period of time.
As we've seen, initial demand on the battery can be as high as 1500 amps for a period lasting up to two tenths of a second. Demand then tapers off to several hundred amps, for a total discharge period of 10-15 seconds. (10-15 seconds may not seem very long but imagine holding the ignition key in the start position for 15 seconds to get an idea how long it really is.)
The size of the engine and its compression ratio are the two primary factors determining the size and duration of discharge demanded by the starter. Other factors include type and condition of the fuel and ignition systems, ambient air temperature, and oil viscosity,
The size of the battery, the electrolyte formula, and the battery's internal design and construction determine how well the battery can meet the demand placed on it by the starter.
The initial surge of electricity comes from the electrochemical process that occurs where the lead and electrolyte molecules are in direct contact with each other. Manufacturers design their SLI batteries to have the largest possible lead surface area to increase the size of the lead/electrolyte boundary, resulting in the highest possible amount of amps during the initial surge.
Ideally, very thin lead plates, separated by thin layers of electrolyte, would allow more surface area in each cell. While this would create a very large initial surge, the battery's charge would be quickly depleted. Thin layers of lead and electrolyte also create issues in terms of battery durability.
As a result, manufacturers have to balance a battery's physical size, durability, and magnitude of the initial surge, along with the amount of time it takes to deplete the battery's charge.
As the available electrons in the lead/electrolyte boundary are depleted, the process spreads outward, drawing upon molecules located deeper in the lead and electrolyte. This secondary electrochemical process takes longer and produces less amps than the initial surge, effectively serving as the battery's reserve capacity.
It's worth noting that the curve on a graph of a SLI battery's output in terms of number of amps over time is very similar to the demand curve of a starter.
The design and construction trade-offs needed to produce a SLI battery results in limited reserve capacity. Therefore, SLI batteries are designed to produce intense burst of energy for a short period before being recharged. They are not designed for continuous draw over an extended period.
The typical starting cycle draws less than 3% of the battery's charge. Deeper discharges shortens a SLI battery's lifespan, with a 30% depth of discharge limiting the lifespan to 130-150 discharge/recharge cycles. 50% discharge reduces the lifespan to 100-120 cycles and 100% discharge shortens the lifespan to as little as 12-15 cycles, which is one-tenth the 30% discharge lifespan.
Whenever I hear somebody cranking their engine until all they hear is the relay clicking, I can't help thinking, "Another battery's just been murdered." Growing up in northern Minnesota and living in Alaska since 1980, I've seen quite a few batteries being murdered, as well as murdered a few myself. It didn't take me long to learn how to keep from murdering batteries in subzero temperatures, which are especially hard on batteries. (And, I'm grateful to the old-timers who helped me learn this.)
In subzero temperatures, you quickly run into the law of diminishing returns. Limit your cranking to 5 to 10 seconds. If the engine hasn't tried to start (i.e., fired at least one or twice), wait 3-5 minutes to allow the battery to build up its surface charge before trying again. After 5 o 6 unsuccessful attempts, hook up a battery charger and go get a cup of coffee because you've probably flooded the engine.
This assumes you've prepared your vehicle for cold weather with an ignition tune-up, winter weight oil, and fuel system and choke adjustments if necessary. In high school, my buddies and I made hundreds of dollars jump-starting college student vehicles, which were probably fine in Minneapolis but not tuned-up well enough for the -20°F and below temperatures common in Bemidji.
It's 1:00am so I think I'll stop here and leave deep cycle battery differences for my next rambling.
This is necessarily a significant generalization. Many (most?) professional electrochemical scientists and engineers don't fully understand what goes on in lead/acid batteries. As a result, these batteries tend to be magical mystery to the average person, which leaves the door open to a flood of misinformation and questionable gimmicks. One place that seems to have reliable information is the
Battery University website.
