Shamelessly ripped-off excerpt from a CRUZ PRO columnhttp://www.cruzpro.com/ahmonitr.htmlMost people are aware that their batteries should be sized so that they are discharged about 50% of their capacity before recharging. If you regularly discharge your batteries much more than 50% or much less than 50% you will not be obtaining optimal value for your dollars. Given we accept this to be true - how do you know when the batteries are 50% discharged and when they are fully charged?
Voltmeters and battery acid specific gravity (S.G.) testers are not good indicators of battery charge unless the battery has "rested" for a while. This is especially true for deep cycle batteries with their thick plates. Battery voltage and specific gravity will continue to change long after you have stopped charging and removed all loads from your battery. Battery voltage indicators or LED bar graphs are notoreously error prone and specific gravity testers can be messy and a nuisance. Sealed gel batteries and AGM (Absorbed Glass Mat) don't allow access to the electrolyte to test them with specific gravity testers. So how do we quickly, easily and accurately determine the state of charge of our batteries?
The answer lies in the use of a special battery gauge called an amp-hour monitor. This is a smart electronic instrument that uses a computer to track and calculate all the energy both entering and leaving the battery bank. It can be likened to a fuel gauge for your fuel tank - only much more accurate. The CruzPro VAH30 and VAH35 Amp-hour monitors use a current shunt to measure the current entering the battery and automatically apply an efficiency factor to compensate for losses due to less than perfect charge storage. For most wet batteries the efficiencies range from 70 to 85% (you get back 70-85% of the energy you put in). For AGM batteries the charge efficiency factor is closer to 90% and both CruzPro models enable you to specify the charge efficiency (which you learn by trial and error). That takes care of battery charging, but what about when we discharge a battery?
When a battery is discharged, things get a lot more complicated. The discharge effieciency is not a straight percentage like charge efficiency but a rather complicated equation that the amp-hour monitor has to calculate continuously. Mr. Peukert figured out why a 100 A-H battery discharged at a five amp rate will last 20 hours (5*20=100), but the same battery discharged at 20 amps will not last 5 hours but might only last 3 hours and 20 minutes. Through careful measurement he was able to determine an equation that accurately relates the size of the battery, the discharge rate and the amount of energy remaining. Peukerts law mathematically states the relationship between effective current and actual current. The difference can be amazing as the graph in Figure 1 shows. Amp-hour monitors that do not use Peukert's law can be out by a whopping 300-400% percent!
