Do the quality lead-acid batteries on your boat last at least five years and preferably ten? Very few do. Most boatowners have come to assume changing batteries every two or three years is normal whereas this is, in fact, an indication of expensive improper systems design and battery maintenance.
Cranking Batteries
A battery that is solely reserved for cranking typically sees very little stress. Despite the fact a starter motor creates a heavy electrical load – potentially hundreds of amps – it is only for a few seconds. In total, very little energy is taken out of the battery. As soon as the engine fires up, the alternator kicks in and recharges the battery. The battery is discharged by perhaps five percent of its capacity and then immediately recharged. We call this micro cycling. A quality battery can do this thousands of times before failing.
House Batteries
Problems typically arise with ‘house’ batteries, especially on sailboats. These batteries are regularly substantially discharged, and often not recharged for some time, and frequently not fully recharged. This is a tough duty cycle for batteries. The deeper the discharge the greater the internal physical stresses and the fewer times the battery can be ‘cycled’ like this. If a battery is left in a discharged state, physical changes occur which make it increasingly difficult to fully recharge it. This is called ‘sulfation’. When it comes time to recharge, as the battery approaches a full state of charge it takes an ever longer time to complete the charge cycle. Charging is frequently cut short, leaving the battery partially discharged and at risk of sulfation.
Decades ago, we addressed these issues with what is called the ‘mid-capacity rule’, which we still use today. In normal use we try not to discharge a battery below 50% of its capacity (occasional deeper discharges are OK). We assume that many times we will not be able to recharge beyond an 80% state of charge. We ensure periodically (ideally, weekly in a cruising situation, but certainly at least monthly) that we carry out the extended charge (several hours) necessary to fully recharge the battery and reverse incipient sulfation.
Alternator Controllers
A conventional alternator (the type that comes as standard equipment with almost all engines) is designed to maintain a micro-cycled cranking battery and does not work well in this kind of situation. We need a more powerful alternator with an external multi-step (‘smart’) voltage regulator. These are not cheap, but frequently over time the cost is more than recouped through extended battery life and reduced replacement costs.
The multi-step regulator has two essential voltage control parameters. One is for what we call the ‘absorption’ cycle and the other for ‘float’. These two voltages, absorption and float, are the two single most important DC system voltages on any boat that works its batteries hard: they need to be checked to ensure they are within the battery manufacturer’s specifications. All boatowners should know how to do this. It requires a digital multimeter.
Checking Absorption & Float Voltage
For the test, a battery is first partially discharged. The multimeter is put in DC volts mode with the two meter probes on the battery positive and negative posts. This is the only place to get reliable data. The engine is cranked. The alternator will drive up the battery’s voltage, typically to something a little above 14.0 volts, and then the voltage will flat line at this voltage for some extended period. This is the absorption voltage. When the battery is fully charged (this may take several hours with a large battery bank) the voltage will trip to something around 13.5 volts. This is the float voltage.
If you plug into shorepower for extended periods of time it is essential to also check these two voltages with respect to the battery charger. If either is wrong, the battery will die a premature death. Get the absorption and float voltages right, and you will optimize battery life.
In our Boat Electrics 101 course, we teach you how to properly size, manage and care for your batteries to maximize their lifespan, ultimately saving money in the long run. If you're looking for more content on this topic, you should definitely check out Nigel's talk on Advances in Batteries & Solar!
