View as PDF
Ample Power Support
Batteries are Complex Mechanisms
If deep cycle batteries were as simple as the car battery, you could leave the dock with the standard automotive electrical system supplied by most boat builders and enjoy all the power you desire. The fact is, deep cycle batteries are constructed differently than car batteries and require specialized monitoring and charging equipment to operate efficiently for long periods. Expensive batteries are no solution to the power equation ...even the best batteries can be easily ruined by mistreatment.
Proper Battery Monitoring
Voltage indicators such as LEDs are worse than no indicators because they imply a sense of correctness, when in fact they are incapable of reporting battery state of charge. A simple voltmeter, and ammeter are not adequate as monitoring instruments unless you are willing to spend most of your on-board time staring at them and logging conditions. The fact that these instruments are widely used is testimony to the widespread misinformation about battery characteristics.
Some of the worst people to ask about batteries are people who work in general electronic disciplines. Before anyone can claim to be a battery expert, a comprehensive study program is required. General electronic knowledge just isn't sufficient. Even those working in battery distribution channels can't be relied upon to dispense correct and meaningful battery information.
Since deep cycle batteries are complex mechanisms, no short article about them will make you an expert. The following information hits a few of the high points. Maybe you'll recognize some of the ways you're killing batteries. As we point out, there are ways to enjoy reliable energy.
Limit Battery Discharges
The further a battery is discharged, the greater the mechanical stresses on its plates. Therefore deep discharges shorten the life of a battery. To avoid frequent replacements, you need to limit the depth of discharge. As a rule, it is best not to discharge batteries more than about 50% of their rated Amp-hour capacity. If you disobey this rule, expect to have battery problems including sudden death ...failure without warning. Most of us are painfully aware that things go wrong at the worst possible times, and battery failure isn't an exception.
Battery Capacity and Voltage
During discharge, battery terminal voltage is not a valid indicator of remaining capacity. This fact rules out the voltmeter as an accurate gauge that tells you when to stop discharging and start charging. Only when a battery has not been charged or discharged in the last 24 hours, can voltage be related to state of charge. A fully charged battery will show a voltage of about 12.8 after being rested for 24 hours. A battery with a 50% state of charge will exhibit a voltage of about 12.2. These values are general and do vary somewhat from manufacturer to manufacturer.
With only 0.6 Volts between a fully charged battery and one 50% discharged, it should be apparent that inexpensive analog voltmeters are not accurate enough to be meaningful.
Battery Capacity Measurement
Since the voltmeter isn't sufficient to indicate state of charge as the batteries are being used, Amp-hour instruments have been designed. Amp-hour instruments continuously measure the current into and out of the battery bank and compute the Amp-hours extracted. As the battery is being charged, Amp-hours extracted declines. While this sounds simple in principle, implementation methods differ significantly, and as a result, some instruments are much more accurate than others. The best Amp-hour instruments not only indicate Amp-hours extracted, but also display Amp-hours remaining. Measurement of Amp-hours remaining is a complex operation requiring computation of an exponential equation discovered by Peukert in 1897. Recent tests confirm the accuracy of this important equation. Some Amp-hour instruments try to fudge a solution without solving the equation, but such devices provide poor accuracy.
Prevent Battery Overcharges
While excessive discharge is a major cause of battery failure, overcharge is another significant killer. Batteries that are perpetually overcharged corrode their positive plates. As the plate gets weaker and weaker it becomes susceptible to damage from high current. Sooner or later, a load draw will snap the plate connection open and the battery fails. Overcharge seems like a simple problem to cure. Rather than run the risk of overcharge, why not just undercharge?
Avoid Battery Undercharges
If all you lost by undercharge were a few Amp-hours of capacity, then undercharge would be a solution. Undercharge destroys batteries in a different way, however, by a buildup of lead sulfate. Only a correct charge will obtain maximum life from batteries. Many people are completely unaware of the loss of capacity from their batteries due to undercharge. Even worn-out batteries will start an engine, the test used by many to gauge life left in their batteries.
What is a Full Charge?
To fully charge a battery requires that its terminal voltage rises to about 14.4 Volts. If permanently applied, this voltage would boil all of the electrolyte from the battery and might even cause a condition called thermal runaway. With thermal runaway, the battery gets extremely hot and may even explode, spewing acid everywhere.
Use Multi-Step Regulation
While 14.4 Volts is required to fully charge batteries, a voltage under the gassing potential of 13.8 Volts is necessary to prevent overcharge. Low cost regulators, such as those found in automotive alternators produce a voltage of about 14 Volts. This isn't quite enough to fully charge a deep cycle battery. On the other hand, it isn't high enough to immediately cause overcharge, but it does extract its toll. Did you ever notice water loss after long hours of motoring?
Use Temperature Compensation
Proper voltage regulation is only part of the charge characteristic required by a battery. Temperature compensation over a wide range is a must-have. At low temperatures, increased internal resistance and reduced chemical activity require a higher charging voltage. High temperatures have the opposite effect. To prevent thermal runaway, applied voltage must be decreased. Thermal runaway is a problem seen with fast charge regulators which don't use temperature sensing at the battery.
The Ample Power Solution
Since it takes a high voltage to fully charge a battery, and a lower voltage to maintain it after a full charge, multi-step regulators and chargers have come into existence. Such devices are a vast improvement over conventional single setpoint regulators. Ultimate performance is offered by regulators which sense battery temperature and compensate output as a result. Temperature compensation may be your only insurance against thermal runaway.
While solutions to the power equation are simple in concept, a significant and complex engineering effort is needed before devices which optimize battery performance can be produced. Ample Power Company has gained an international reputation by offering microcomputer based products which provide ultimate battery care in the harshest of conditons ...life at sea, under the hood of an RV touring the desert, and in a mountain top shelter buried in snow at sub-zero temperature. Specific performance, accuracy and reliability of Ample Power products is unequalled.