New Tools for Managing Your Battery Bank
In Part II of this three-part series, we learn that upgrading your DC electrical system will require an assessment—and the likely replacement—of its components. "Systems" from our March 2012 issue.
The complexity of modern battery technology dictates that more sophisticated charging regimens are in order. As a result, some of the older charging systems still in use on many of the boats built in the 1970s and the 1980s may not be up to today’s tasks. Some of the older systems pose potential safety risks, while in other cases upgrades are necessary to ensure that you get that maximum return on the more efficient—and expensive—batteries that we discussed in the first installment of this series. (See “Replacing Batteries? Do Your Homework First” in CW’s February 2012 issue.)
Shore power-supplied battery chargers aren’t our only concern here. Most engine-driven alternators found on boats employ constant-rate internal voltage regulators that are really designed and intended for conventional flooded-cell lead-acid batteries, and preferably those with serviceable cells. To truly maximize your DC system, you’re probably going to need to update your alternator and voltage regulator. Let’s walk through an assessment of these two devices one step at a time.
Assessing an Engine-Driven Alternator
If we’ve learned anything over the years, it’s that any alternator that spends the majority of its time at or near its full output potential is probably not going to last too long. Why, you ask? Simple. Alternators get hot—really hot! I’ve personally measured alternator temperatures that exceeded 200 F. Heat is the ongoing enemy of all things electrical. Too much heat causes insulation to break down, semi-conductors to fail, and, ultimately, equipment to be ruined. So it pays to overrate your needs if equipment longevity is a concern.
What that means is that your replacement alternator could easily cost well in excess of $1,000. And if you’re taking the leap, it also pays to consider alternator-temperature sensing and voltage regulation that provides automatic output step-back if the alternator reaches a conservative defined temperature. The Sterling Pro-Digital regulator, for example, will automatically step back high charge rates when the temperature exceeds 194 F. This capability becomes all the more important in light of what we know today about battery-recharge acceptance rates.
In earlier times, the inherent internal resistance of your boat’s batteries would help to keep things in check by limiting the amount of current that could be delivered from the alternator. That’s less the case today with A.G.M. and, most especially, the thin-plate pure-lead, or T.P.P.L., versions of A.G.M. batteries. Those batteries can take all the amperage you can possibly supply at a very rapid rate. While this makes for fast recharge times, it puts a heavy burden on charging equipment unless your system has temperature-sensing capability both at the batteries and the alternator. You’ll also need a voltage regulator or battery charger that reduces output when temperatures get out of hand.
To control the alternator’s temperature, choose one with a higher output rating so the alternator doesn’t work as hard. It will indeed run cooler if it’s not constantly running at full output. I typically like to size electrical-system components once a DC load analysis has been done for a boat to ensure that an alternator is running most of the time at no more than 70 percent to 75 percent of its rated output.
Installing New Components
Now that you’ve decided to upgrade to the 200-amp-hour-rated alternator of your dreams, what else do you need to consider?
Alternators consume engine power. A rough guideline used in making calculations is that 1 horsepower equals 746 watts. Amperage multiplied by voltage equals watts, so using the 200-amp example at 12 volts, we see that it’s going to take a little over 3 horsepower to run the alternator.
If your boat is running a 7-horsepower, single-cylinder diesel engine as an auxiliary, you may want to think of another way to generate more electric power. Most modern cruising boats run engines of 30 horsepower or more, so this isn’t as much of a problem these days. What may be a problem, however, is that once you get over about a 100-amp alternator, you’re going to need to run a double pulley system to prevent belt slippage. This requires you to replace all of the pulleys on the front of your engine. Check with your engine manufacturer to be sure this isn’t going to be a problem. Also, you need to carefully compare mounting-bolt locations and spacing to make certain that you’re not going to need a custom mounting bracket for the new alternator and that the new unit will actually fit on your engine without interfering with other engine-mounted components. Don’t forget the clearance for the front crankshaft pulley, either. Is there enough room in your engine bed to accommodate a wider front pulley without interfering with the engine bed and any crossbar that may be installed as part of the engine-mounting arrangement?
If you have a newer engine with a flat serpentine belt, you should be OK with the stock pulley system up to an alternator rated at about 150 amps. After that, you may be out of luck if you want more output, because finding an oversized serpentine pulley set and matching belt of the right length isn’t going to be easy. Check with both the alternator and engine manufacturer on the availability of the needed parts. Balmar, for one, offers a fairly comprehensive line of pulley and custom alternator-mounting kits as well as optional temperature-sensing, programmable voltage regulators and wiring kits for its alternator product line.
Next, you need to consider the wiring used in your original alternator/regulator system. It’s undoubtedly going to need to be up-sized to accommodate the increased amperage that your new system is capable of providing.
One last potential problem concerns your engine’s tachometer. Often, a diesel engine’s tachometer is driven by a signal generated at the original-equipment alternator. You must determine if there’s compatibility with your new high-output unit or if you’ll need a different tachometer-rpm signal source.
Clearly, upgrading to a new high-output alternator may not be a simple Saturday-morning D.I.Y. project; rather, it’s one that comes with a price. You really should think about your cruising needs and whether the payback is going to be worth it.
