SAFE CHARGING
Nigel Calder looks at managing the heat created by alternators
One way or another we now have batteries in the marketplace that can absorb very high levels of charging current, enabling us to optimize electrical systems in ways not previously possible. However, this high charge acceptance rate has the effect of forcing charging devices (alternators and battery chargers) to maximum rated output for extended periods of time. It also subjects the cables in the system to high continuous currents, creating some installation challenges.
No charging device is 100 percent efficient, nor is any cable 100 percent efficient at conducting charging currents. Inefficiencies translate into heat, which in turn translates into damage to sensitive electronics and a potential fire risk for windings, cables and connections. Conventional alternators are the worst, because the average alternator is, at best, only 60 percent efficient at converting mechanical energy into electricity (although there are some specialized alternators that are considerably more efficient than this), with the remaining 40-plus percent of the input energy converting to heat. If the heat is not removed, the windings will fry and the diodes will fail.
In a “traditional” charging situation, the batteries accept the alternator’s full rated output for only a short period of time, after which the battery charge acceptance rate declines, the alternator’s output reduces and the fan within the alternator is adequate to handle the remaining heat. However, as soon as we connect that same alternator to a battery bank capable of absorbing high charge rates for extended periods of time, we are potentially in trouble.
In fact, to handle just this kind of situation, a (June 2017) Yanmar Technical Bulletin requires that its 125-amp, 12-volt alternator to be limited to 100 amps if connected to a lithium-ion battery, and that its 65-amp, 24-volt alternator be limited to 50 amps. Similarly, Balmar, a major supplier of high-output alternators, has for years provided a temperature sensor that attaches to the back of the alternator, with the voltage regulator cutting the alternator’s output in half if the temperature threshold is reached. One way or another, all existing alternators connected to high charge rate batteries need temperature protection.
CABLE BURNOUT Even with temperature protection, alternators can run extremely hot, with some rated to operate at temperatures of almost 400 degrees F. It is also not uncommon for on an alternator to be at 230 degrees F or higher in the course of normal operation. Most boatbuilders in the United States use wiring that has a temperature rating of 221 degrees F, while in Europe much cable found on boats is rated just under 200 degrees F. As a result, when this cable is attached to an alternator with a case that is running at 230 degrees or higher, the heat from the alternator will pre-heat the cable to close to its rated temperature, at which point the cable’s current carrying capability theoretically reduces to close to zero, even as we will frequently run well over 100 amps through it. Under these circumstance, the insulation close to alternators that are regularly run hard can be seriously degraded, creating a risk of insulation failure and subsequent short circuits. Any time you get a short circuit with high levels of current flow, you have a significant fire risk.
Given the scarcity of cables with a temperature rating above 221 degrees F, there is no good antidote to this situation other than to make the alternator cables as large as is practicable; mount them in free air to maximize cooling; and ensure that the positive cable is not run in contact with or adjacent to any grounded surface until the cable is well clear of the alternator, giving the cable the opportunity to dissipate some of its heat.
SHOREPOWER MELT DOWN
The shorepower cord and its end fittings are another weak link in our high-charge-rate systems. For one of my experiments, we had a massive lead-acid battery bank that was capable of absorbing for hours on end any charging energy I could throw at it. We were operating in Europe where the standard shorepower outlet is rated for 230 volts and 16 amps, which equates to 3.6 kW (pretty much the same as a U.S. 120-volt, 30-amp shoreside supply). I had a 3.5 kW battery charger that would have run at full output for hours if I had let it, but instead
I set it to 2.8 kW (around 80 percent of the shorepower outlet rating) in order to protect the shorepower circuit. I still melted down the dockside outlet.
The problem here is the friction-type connectors we have at both ends of a shorepower cord, as opposed to the bolted connections we normally have with high-current circuits. If you go around any large marina and inspect the shorepower pedestals you will see signs of burning; similarly, if you look at the two ends of shorepower cords. We are ever more often pushing these circuits to continuous high current levels for which they are not well suited. At the boat end, there is a connection available from SmartPlug in Seattle that takes care of the connection issues. Other than this, the only antidote is to be aware of the problem, not to push the shorecord to its rated current for extended periods of time and to regularly inspect the connections at both ends of the cord, on the dock and the boat.
IMPERFECT CONNECTIONS
In truth, we should have bolted connections wherever we have high currents. But even here we run into another potential problem. On boats, we commonly use stainless steel to bolt these connections together. However, stainless steel has very poor electrical conductivity. If the stainless becomes part of a conducting circuit with high continuous currents, it can become hot enough to create a fire risk. Not that there is anything wrong with using the stainless: you just have to make sure that it is only being used to clamp one conducting surface directly to another. Even a stainless washer between the two can cause problems.
At high amperage levels the terminals themselves can also be problematic, as these are invariably crimped. For a given stranded cable size (gauge) there are variations in the diameter of the copper depending on the standard used for sizing (SAE, AWG or ISO) and the number of strands in the cable, and there are also a have half-dozen different types of copper crimp-on terminals, varying in copper thickness and diameter. If the cables and terminals are not properly matched both to each other and the dies in the crimper, poor crimps will result. For example, I have pulled 1/0 terminals off their cables by hand after they had been crimped with a mismatched die in a hydraulic crimper.
Finally, heat shrink is often added to these terminals. If even a tiny corner of heat shrink extends into the conducting surface, and even if the connector is torqued down hard in a bolted connection, the connection can be resistive enough to generate a great deal of heat. You get the idea. Bottom line, on circuits carrying continuous high currents, all connections need to be electrically perfect and extremely tight.
USEFUL TOOLS
An effective means to check for potential heat problems in high-current DC circuits is to switch a multimeter into its DC volts mode and place one probe of the meter at one end of the circuit (e.g. the positive output terminal in the back of an alternator) and the other at the other end of the circuit (e.g. the positive terminal on the battery being charged), using extension leads if necessary. That done, fully load the circuit by running your charging devices flat out). Any voltage that is registered represents voltage drop caused by resistance in the circuit. In principal, the voltage drop on fully-loaded charging circuits should not exceed 3 percent of the rated voltage, which would be 0.36 volts on a 12-volt circuit. In any case, you should never exceed a 10 percent drop, or 1.2 volts on a 12-volt circuit.
If you perform this test and find that the voltage drop is too high, it may be the result of the cumulative resistance in undersized cables, but is frequently the result of resistive connections, which in high current circuits will form hot spots. Fortunately, we now have some tools that are particularly useful for detecting these kinds of things. One is an infrared laser heat gun, which will give precise temperature readings from very specific locations. Another is a thermal imaging camera, which will show temperature gradients within a larger area. The latter is a particularly useful device. These cameras can now be bought for under $200 and will plug into any smart phone.
STEP CHANGE IN DC SYSTEMS
We are entering a new era in terms of energy systems on boats, with extraordinarily high rate charging devices and batteries that can absorb pretty much anything we can throw at them. Together, these will give us greatly improved efficiencies and lifestyles, but along with this comes new installation challenges. Fortunately, we also have the tools to help us through these challenges and ensure safe and reliable installations. The pieces are coming together rather nicely! s