Professional BoatBuilder - December-January 2018

42 PROFESSIONAL BOATBUILDER

SYSTEMS: Battery Technology

degraded, creating a risk of insulation

failure and subsequent short circuits.

Any time you get a short circuit with

Cable Burnout

Even with temperature protection,

alternators can run extremely hot.

Some are rated to operate at temp-

eratures as high as 392°F (200°C). It is

not uncommon for the case on an alter-

nator to be at 230°F (110°C) or higher.

Most boatbuilders in the U.S. install

wiring with a temperature rating of

221°F (105°C). In Europe, much cable

found on boats is rated at only 90°C

(104°F). When this cable is attached to

an alternator with a case running at

230°F or higher, the heat from the alter-

nator will preheat the cable to close to

its rated temperature, at which point

the cable’s current-carrying capability

theoretically reduces to close to 0 amps,

but instead we frequently run well over

100 amps through it: many of these

installations unavoidably fail to com-

ply with ABYC standards. As a result,

the insulation close to alternators

regularly run hard can be seriously

the alternator is adequate to handle the

remaining heat. But as soon as we con-

nect that same alternator to a battery

bank capable of absorbing high charge

rates for extended periods of time, we

are potentially in trouble.

To handle this situation, the Yanmar

Technical Bulletin requires Yanmar’s

125-amp, 12V alternator to be limited

to 100 amps if connected to a lithium-

ion battery, and requires its 65-amp,

24V alternator to be limited to 50

amps. 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 tem-

perature threshold is reached. In a

warm ambient climate, this may be in

less than 10 minutes.

One way or another, all existing alter-

nators connected to high-charge-rate

batteries need temperature protection.

Hot cables are another consequence of

high-rate alternators. No malfunction,

such as a short circuit, damaged these

cables; they were melted by the normal

heat generated by the alternator.

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