A voltmeter is the mostbasic system instrument.Battery voltmeters areinexpensive, easy to install, and canprovide a wealth of system informationto renewable energy users, RVers, oranyone who depends on a battery.Why a Voltmeter?Ten years ago, voltmeters were all we had forinformation about our systems. Ampere-hour metersthat calculated battery efficiency were a pipe dream.Even now, small systems cannot justify the additionalexpense and complexity of the new sophisticatedbattery state of charge (SOC) instruments. I still use ahomebrew battery voltmeter in addition to our hi-techinstruments like the Cruising Amp-hour meter, the SPM2000, and the Power Monitor 15. The voltmeter isalways there, consumes virtually no power, and tells meat a glance what’s happening with our system.Reading a battery voltmeter and turning that informationinto a reliable assessment of the battery’s state ofcharge is like tracking an animal by its footprints.Tracking requires noticing small details andextrapolating information from these details. A trackeruses his knowledge of the animal’s habits. A trackerconsiders the weather and season. A tracker’sknowledge of his subject and its environment allowshim to predict the actions ofhis subject.After watching the voltmeter for a few of the battery’scharge/discharge cycles, the user gets a idea of hisbattery’s voltage profiles. After watching the voltmeterfor a season or two, the user learns how to relate the effects of temperature and current on his battery’svoltage. Just like the behavior of animals vary with typeand location, the behavior of batteries differ with typeand operating environment.What Kind of Voltmeter?It really doesn’t matter what type of voltmeter you useto measure your battery’s voltage. Better instrumentsyield more accurate measurements with higherresolution. Differences in battery voltage of 0.1 VDC aresignificant, so the instrument should have a basicaccuracy at least 0.5% or better. Accurate analogbattery voltmeters can be purchased for under $40.Digital multimeters cost from $40 to $300 and performhighly accurate voltage measurements and much morebesides. Or you can homebrew an expanded scaleanalog battery voltmeter, see HP #35, Page 92. Youcan homebrew an LED Bat-O-Meter, see HP #10, Page26. Any of these instruments will give you the voltagemeasurement you need.Installation of a battery voltmeter is easy. Just connectit to the battery’s main positive and negative buss orterminals. Be sure to get the polarity right becauseanalog meters can be damaged by reverse polarity.Since the battery voltmeter consumes very little power,the wires feeding it can be small (18 gauge copper orsmaller).
Reading the CurvesThe data presented here on the graphs was generatedfrom our set of Trojan L-16W deep cycle lead-acidbatteries. Each Trojan L-16 battery is composed ofthree series connected, 350 Ampere-hour, lead-acidcells. The graphs and the data here relates to six ofthese lead-acid cells in series forming a 12 Volt battery.Those of you using a 24 Volt system with twelve leadacid cells in series must multiply the voltage in the textand on the charts by two. The voltage versus state ofcharge (SOC) profiles will match those of similarlyconstructed cells. Other types of lead acid cells, like carbatteries, lead-calcium cells, and “RV deep cycle”batteries will have different charge/discharge curves. Ioffer these graphs as examples of what to look for withyour battery. While specific voltage vs. SOC points willvary from battery type to battery type, the shape andrelationship of the curves is similar for all deep cyclelead-acid technologies.Current and Batteries and Ohm’s LawBattery voltage can be affected by three factors — stateof charge, current, and temperature. State of charge iswhat we are trying to find out, so that leaves currentand temperature as factors to reckon with.Current means the rate of electron flow through thebattery caused by either charge or discharge. Every electrochemical cell has internal resistance. As currentmoves through the cell, the cell’s voltage changesbecause of this internal cell resistance. When the cell isbeing recharged, current flow causes the cell’s voltageto rise. The higher the recharging current the higher thevoltage rise. As the cell is discharged, the dischargingcurrent causes the cell’s voltage to drop. The higher thedischarging current, the greater the battery’s batterydepression. This holds true for all electrochemical cellsregardless of type, size, or environment. While absolutevalues vary widely between different acid and alkalinetechnologies, the relationship between current flow andcell voltage remains constant.
The graphs show a variety of recharge and dischargerates from C/5 to C/100. This C/XX number is actually arate of charge or discharge in Amperes proportioned tothe capacity of the battery. For example, consider abattery of 100 Ampere-hours. If you divide this Ampere hour capacity by 10 hours, then you get a charge (ordischarge) rate of 10 Amperes. Ten Amperes is a C/10charge (or discharge) rate for a 100 Ampere-hourbattery. Consider another battery of 500 Ampere-hourscapacity. Here a C/10 rate would be 50 Amperes. Whilethe absolute values of the charge (or discharge)currents is different between the two batteries ofdifferent capacity, their effect on the battery’s voltage isthe same. The currents are in the same proportion tothe batteries capacity.If voltage is to be related to battery state of charge, thenwe must compensate for voltage variation due tocurrent movement through the battery. Hence there area variety of curves on both the charge and dischargegraphs.Included on the charge graph is a gray curve entitled“Rest”. This rest curve is a generic representation of six lead-acid cells in series and at Rest. “At Rest” meansthat no current is moving through the cells, i.e., thatthey are neither being charged or discharged.Determining a battery’s state of charge from voltagemeasurement is vague enough if current is movingthrough the battery. The vagaries increaseexponentially if no current is moving through thebattery. This is why this curve is gray.Temperature and BatteriesThe lead acid reaction is temperature sensitive. Coolingthe cell changes its voltage vs. SOC profile. As thelead-acid battery cools, its internal resistanceincreases. This means that voltage elevation underrecharging is increased in cold cells. The same internalresistance increase produces increased voltagedepression in cold cells when discharged.At 32°F (0°C), the effect of temperature becomes pronounced enough to distinctly change not only thebattery voltage vs. SOC profile, but also its usefulAmpere-hour capacity. The discharge voltage curvesmay be depressed by as much as 0.5 VDC from thoseshown on the graph. Charge voltages will be elevatedby as much as 0.5 VDC for a cold 12 Volt lead-acidbattery.Lead-acid Internal Resistance and SOCIn lead-acid cells, the electrolyte (sulfuric acid)participates in the cell’s normal charge/dischargereactions. As the cells are discharged, the sulfate ionsare bonded to the plates — sulfuric acid leaves theelectrolyte. The process is reversed when the cell isrecharged.A fully charged lead-acid cell has an electrolyte that is a25% solution of sulfuric acid in water (specific gravityabout 1.26). A fully discharged lead-acid cell has virtually no sulfuric acid in its almost pure waterelectrolyte (specific gravity about 1.00). As the sulfuricacid concentration in the electrolyte changes so doesthe electrical resistance of the electrolyte, which in turnchanges the internal resistance of the entire cell.The bottom line is that the internal resistance of alllead-acid cells changes with the cell’s state of charge.This characteristic gives the lead-acid reaction itsparticular shape or signature on the voltage vs. SOCgraphs. This signature is unique — very different fromalkaline cells whose electrolyte resistance remainsconstant regardless of SOC. The shape of the lead-acidcurves makes it possible to use a voltmeter todetermine a battery’s state of charge.