BASIC PRINCIPLES
The VoltWatch design exploits the theoretical straight line relationship between the no-load-no-charging Voltage of a marine or automotive (i.e.lead-acid) battery and its State of Charge (the percentage of the rated fully-charged capacity): 100% at 12.8V, 0% at 11.8V (the straight blue line on the graph); i.e. just 1 Volt.
In practice a number of factors affect this simple relationship:
1. Batteries use a reversible electro-chemical reaction which is significantly less than 100% efficient (the difference is ‘lost’ as heat), and is time dependant.
2. Charging is a law of diminishing returns; charging rate is inversely proportional to the SoC, so a 100% charge is impossible.
3. Charging current is proportional the difference between battery and charging voltages.
4. It takes time for charge to physically percolate into (and out of) a battery, recent charging and heavy use of a battery respectively raise and depress the battery voltage. Battery voltage recovers very quickly from discharge, but less quickly from charging - a freshly (even partly-) charged battery will show a voltage far above that which reflects the SoC and will take many hours to settle unless some charge is used.
5. Above a sustained charging voltage of about 14.4V (depending on electrolyte strength and temperature), an increasing proportion of the charge current is dissipated by electrolysis of the battery acid (electrolyte) into hydrogen and oxygen - so-called ‘gassing’. The resulting mixture is extremely explosive!
6. Lead-acid batteries (unlike the NiMH or L-Ion rechargeable found in portable electronic gadgets) do not like being discharged below about 25% SoC (depends on type). The more deeply discharged, the shorter their life. They also deteriorate when left unused.
7. A common (and confusing) failure mode is for one [or two] of the ten separate cells in a battery to become shorted internally. It takes and holds a charge, but has become a nominal 10.8V [9.6V] battery with a fully charged voltage of about 11.4V [10V].
The chart gives an idea of a typical real-life relationship between voltage and SoC. That the straight line relationship doesn’t apply at the extremes is of little practical importance; no battery should be allowed to get into the orange/red area of plummeting voltage. Similarly, a battery (not charging) voltage over 12.6 indicates a ‘fully’ charged state.
However, while the voltage range over the maximum Usable Capacity range is a mere 0.70V (12.05V to 12.75V), the design had to accommodate the charging voltage rising to around 14.4V (momentarily as much as 16½V with some ‘smart’ charging cycles) and sinking well below 11V if the battery is unduly discharged or one cell is internally shorted.
For a diagram showing the voltages at which VoltWatch’s LEDs are lit Click Here
