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Problematic Systems ...do you have one?
Most people recognize the value of a competent doctor or dentist, but many of those same people are willing to trust the design of their electrical systems to people who don't have the engineering background to recommend proper equipment, and the connection of that gear. Anyone who can find their way to the local copying center can become an electrical system designer and thereafter promote the scuttlebutt du jour.
How does one unskilled in electrical systems know when they're getting bad advice? Read on.
An isolator which splits a charge to two or more battery banks should never be used. Anyone recommending one is not aware of the problems introduced by isolators. Dual output alternators have the properties of an isolator internally. An isolator introduces power losses, generating heat. There is also a voltage drop through an isolator which prevents fully charging the batteries. If remote sensing is used to overcome the voltage drop on one battery, say the house bank, then the starter battery is overcharged because of the peak ripple coming from the alternator.
There's no such thing as saving money by using an automotive alternator. Modifying a stock alternator for external regulation will result in a meltdown unless the current output is substantially limited. In other words, to prevent alternator burn-out, output current has to be limited to much less than the rating on the alternator. This means that charge times are greater, with a corresponding cost in engine wear and fuel consumption.
What about sensing temperature and stopping alternator current if the alternator overheats? This can be done, but on the average the alternator will probably be running about 50% of the time ...once more translating into more engine wear and fuel costs.
Attaching a boost converter to the output of an automotive alternator, which will load the alternator at its capacity, is a sure way to kill the alternator and/or the internal regulator. Internal regulators protect themselves and the alternator by reducing output to control temperature. Anyone who has tried to charge a large, deeply discharged house bank is probably familiar with alternator/regulator failure, if not during the first charge cycle, but shortly thereafter.
There is no substitute for a hot rated alternator built to withstand the rigors of continuous high output. Hot rated alternators are always externally regulated.
Battery Selector Switches
High charge current flowing through a battery selector switch with rotary contacts is a lurking time bomb. It will overheat, warp the plastic housing and eventually open circuit. That will smoke the alternator, alternator regulator, and maybe all the electronic gear which happens to be on at the time. This is a several thousand dollar oops.
Even if the familiar 1-2-both switch was reliable, it's just waiting for someone to turn it off with the engine running.
Multiple House Banks
More is always better. Bigger is always better. That may be true if you're talking about apple pie, but it just ain't so with batteries. Every battery added to a system reduces it reliability. Separate those batteries into different banks and reliability and useful life of the batteries is reduced once again. Mistreat a bank by deep discharge because you have another bank in reserve knocks more life from the system.
The most reliable and long-lived system is comprised on a single house bank and a single starter bank.
Excessive Electrolyte Loss
Electrolyte is boiled off when batteries are overcharged. Some amount of overcharge is necessary to restore full capacity, but that amount of charge does not require much water to be added on a regular basis. Adding water more than a couple times annually is evidence that the batteries are being killed from overcharge. Either the charge control is poorly designed, and/or temperature compensation is incorrect.
Old batteries beyond their useful life will use excessive electrolyte.
Some people recommend against using gel batteries. True, there has been, and may still be, brands of gel batteries that are very poor quality, but a gel battery built properly will survive more deep discharge cycles than any other lead-acid type. Contrary to some wisdom, a good gel battery will charge very fast. It will also tolerate very deep discharges, including those where recharge takes place weeks after the discharge.
AGM batteries are best suited for systems that maintain a long term float voltage and occasional demand full capacity discharges. They don't deep cycle on a regular basis very well, and in cycling service require frequent equalization. People recommending AGM batteries don't know that gel batteries are better suited for systems with regular deep discharges.
Thick Plate Traction Batteries
Batteries constructed with thick plate grids are built to deliver service for many years. Unfortunately, those thick plates which provide longevity also restrict charge rate to about 10% of Amp-hour capacity. Recharging a thick plate battery from empty to full may take eleven hours. If you plan on fast charging, forget about thick plate batteries.
A must read for anyone specifying, installing or using battery systems is the book Living on 12 Volts with Ample Power. There are many good books about batteries, most of them included in the references to Living, but none with the application specifics. Anyone who has not read Living on 12 Volts with Ample Power is probably not qualified to design or install a performance electrical system.