Before going on, I'm going to recap what I've said about batteries so far.
Almost all motor vehicle batteries currently on the market are lead-acid batteries, with only relatively minor differences between the various types. All brands are effectively the same, with differences mostly limited to price and quality control.
Although based on old technology, with minor improvements over time, lead-acid batteries still provide the best available balance of availability, functionality, performance, and price. Less common batteries with different types of chemistry likely provide a less than favorable price/performance balance. (We are, after all, just trying to get to and from places that are, at most, a few hundred miles away. In particular, we're not trying to get to the moon and back.)
Gel batteries are best in certain specific circumstances but are general not suitable for motor vehicle use. (I will get to discussing where gel batteries might be best in a motor vehicle … and what you need to use them there.)
AGM batteries cost more but are arguably your best choice.
Flooded batteries are just fine, with a caveat related to VRLA and non-VRLA versions.
Non-VRLA batteries, which are basically limited to flooded batteries, are fine if you're willing to faithfully perform the routine periodic maintenance they require. (Non-VRLA batteries are usually called open-cell batteries.)
If you're the type of person who only checks their tire pressure once a year or less, you're better off with a VRLA batteries. (VRLA batteries are usually called maintenance-free or sealed batteries.)
Size (i.e., BCI Group designation), price, and ego are important considerations when choosing the battery that's best for you.
There is one difference that's very important when choosing batteries for our motorhomes: Starting versus deep-cycle batteries.
Side note: In the process of writing my electrical system ramblings, I've been researching the subject to make sure the information I provide is reasonably accurate and up-to-date, instead of relying solely on experience and memory. (Actually, this research is directed towards designing an electrical system for the boat I'm building that is reliable and does what I need … this research provides a sideways benefit to my ramblings.)
In this research, I've been relying heavily on marine electrical systems information, recommendations, and standards. Motorhomes and boats have similar electrical system features (such as dual battery banks) and share many common needs and issues. The reason I'm relying on marine information is boats operate in a somewhat more dangerous environment than motorhomes, which presumably results in more precise and reliable recommendations and standards. As a result, we can take "lessons learned" from the marine industry to improve our motorhomes' reliability and operation. This is especially important for our older motorhomes because they were produced close to the dawn of the motorhome industry, when there was very little experience and even less standards. (
see https://en.wikipedia.org/wiki/Travco)
One very interesting bit of information I ran across was issues with the current standards for rating batteries, such as cold cranking amps (CCA). CCA, for example, is a measure of the number of amps a battery can deliver at 0°F for 30 seconds without dropping below 7.2 VDC. Average people cannot be faulted if they think this measurement would show up as a flat, horizontal line on a graph. (And, unfortunately, this may very well represent the test less than reputable battery manufacturers use to arrive at their CCA ratings.)
People with a good understanding of engine starting systems know an actual chart of starting system demand is a curve, starting with high amps and dropping down as the engine begin rotating. As it turns out, this view of the curve is also somewhat erroneous. (This included my prior understanding of starting system demand.)
Blue Sea Systems used refined test equipment, capable of very rapid time-slice sampling, to measure actual starting demand on a variety of marine engines under real world conditions. In spite of my understanding of variables in scientific measurement, I was very surprised by the results.
This demonstrates an ongoing problem with all fields of engineering and standardization. To avoid a lengthy (lengthier?) explanation, I'll use gas consumption as a metaphor. My 1990 Ford E150 van, with an inline 300 cu. in. engine, uses an average of less than five gallons of gasoline per week. Before you get jealous, I should point out the van only gets used once or twice a week for trips totaling less than ten miles and trips once or twice a month around 70 miles each. (This is why motor vehicle ads say, "Your mileage may vary.")
While Blue Sea used a variety of engines, it's my impression they used a large diesel marine engine as the most extreme example. (Makes sense, large diesel engines are notoriously difficult to start.) Blue Sea's tests indicate the greatest demand occurs almost immediately, when the starter has to overcome the engine's inertia. Their tests show this initial spike in peak starting current can reach as much as 1500 amps. This huge spike is part of what they call the Inrush Period, lasting approximately 1/4 second.
The graph they provide on their website shows an immediate spike of 1250-1300 amps, dropping quickly to 800 amps in a few hundredths of a second. The curve then slows, dropping to 600 amps in roughly 0.16 seconds, before showing a secondary spike reaching almost 800 amps that may represent engine resistance due to a cylinder compression stroke.
After the secondary spike is a relatively smooth curve representing a rapid decline in demand changing into a slower decline. Blue Sea calls the period following the secondary spike the Cranking Period, comprising the time from when the engine starts rotating freely and is finally running on its own, which averages 9-3/4 seconds.
WOW!!!
(And, "Whew!" Sorry, my scientific background got the better of me, leading to a description that might be a bit more detailed than necessary.)
At this point, you're probably wondering how a battery rated, for example, at 800 CCA can produce that much current. Remember, CCA rating is a measure spanning 30 seconds, which is roughly 750 times as long as the initial spike described above. The arithmetic mean (average to non-mathematical folks) for the first 10 seconds of Blue Sea's graph is roughly 800 amps, with the graph dropping to 200 amps a little before the 11 second mark. Also, the alternator has probably taken over and started recharging the battery well before the end of the CCA rating's 30 second period.
Simply put, motor vehicle batteries, as well as most electrical wiring and components, can handle short duration surges in excess of their rating. (Blue Sea's largest battery switch, designed for diesel engines, is rated at 600A continuous and 900A intermittent, using the current UL standard for these switches.)
Blue Sea calls the combination of the Inrush and Cranking periods the Engine Cranking Cycle. Based on their research, they established an Engine Starting Standard, which is being consider for incorporation into American Boat and Yacht Council (ABYC) standards and is already being referenced by a significant portion of the marine industry.
Okay, what started out as a side note turned into a description of what happens in a engine starting system and became longer than expected. So, I think I'll have to put off starting vs deep-cycle batteries for my next post. (At least, this post laid the groundwork for that discussion.)
BTW - I'm writing these using on-the-fly, stream-of-consciousness. I try to do some proofreading and editing before posting but typos and confusing sentence syntax does slip through. (It doesn't help I grew up in northern Minnesota, where "throw the cow over the fence some hay" syntax was common …
uff-da!)
