A lead–acid battery does not reveal its state of charge through voltage readings alone.
Because battery voltage varies depending on charging or load current flow, measurements with a voltmeter tell you very little about the state of the battery except under very specific conditions. Under most circumstances, specific gravity measurement of the battery’s water and sulfuric acid mixture (electrolyte) is the best way to determine state of charge and cell condition.
A lead-acid cell has an active electrolyte that takes part in the electrochemical process while charging its sulfuric acid concentration. Because this change in concentration directly reflects the change in the cell’s state of charge (much more accurately than does cell voltage), measurements of the acid strength of the electrolyte can be used as a charge indicator.
While there are many ways to determine how much sulfuric acid is dissolved in water, the easiest, least expensive method for field use is the hydrometer. A hydrometer does not measure sulfuric acid concentration directly, but it answers that question by revealing the specific gravity of the electrolyte, which is its density compared to that of pure water.
False Readings
Density measurements provide an accurate indication of acid concentration, but density of the acid/water solution can vary for other, unrelated, reasons—and accuracy can be affected. Any density measurement is affected by temperature, and most hydrometers are calibrated to read properly only at a normal operating temperature of 77 F. For example: at an electrolyte temperature of 125 F (maximum for battery operation) a hydrometer reads 16 points low; at a 26 F, the reading is 16 points high.
The hydrometer method also assumes that we are measuring the density of only pure sulfuric acid in pure water. Anything else dissolved in the electrolyte would increase the overall density and give a false reading.
If the battery has just been watered, don’t expect an accurate hydrometer reading until after the battery has been charged. Charging will stir up the electrolyte and mix in the water. Otherwise, the lower-density pure water will float on top of the rest of the electrolyte and your hydrometer may only sample this portion.
Hydrometer readings while the charge is in progress lag behind the battery’s actual state of charge. Only when the battery is nearly to the end of its 8-hour charge does the hydrometer provide a good indication of the battery’s status. Batteries which have been repeatedly overflowed by overfilling with water will tend to have weakened, diluted electrolyte because of loss of sulfuric acid. (If no overflow occurs, only pure water is vented as vapor during normal use, and minimal acid loss occurs). Hydrometer readings of badly diluted electrolyte are not useful indicators of battery status.
An older battery that shows signs of acid overflow (heavy corrosion, deposits in the truck compartment) should be serviced by a battery specialist who can determine the degree of dilution of the electrolyte and bring the concentration back to original specifications. Once this is done, hydrometer readings will be a reliable guide to the state of charge and overall health of the battery.
Choosing and using
Several types of led–acid battery hydrometers are available, and even those intended for use with auto starting batteries will work for industrial batteries. It is best to use a hydrometer which shows a numerical specific gravity value. The typical range of values: 1.100 to over 1.300 (for easier reading, the decimal point is usually omitted on the scale, for example: 1100 to 1300)
The numbers shown by the hydrometer are specific gravity values, or the density of the liquid compared to that of water which has specific gravity designated as 1.000. The electrolyte in a led–acid battery is at its highest sulfuric acid concentration when the battery is fully charged. The typical full–charge reading for an industrial battery is 1.275, but many batteries operate with a stronger electrolyte. Check the manufacturer’s data on each battery to verify the recommended full–charge specific gravity reading—it can be as much as 1.300 or more. A lead–acid battery is ready for recharging when it has reached its 80% discharge point, and this is indicated by a hydrometer reading between approximately 1.140 and 1.180. Battery operation below 1.120 is not recommended.
The best time to use a hydrometer to spot the problem cell(s) in a problem battery is the time of trouble: at the time during the shift that lift truck performance starts to drop. Bad cells will usually stand out with much lower specific gravity readings than the rest after use, even if these same cells had acceptable readings when the battery was fully charged.
A fully charged battery with well mixed electrolyte should show hydrometer reading on all cells to be within 25 points of each other. If a discrepancy greater than this persists after an equalizing (extended) charge, the battery should be checked by an experienced battery technician.
Tips for Proper use of a Hydrometer
1) Measure more than one cell when checking for state of charge
2) Don’t expect an accurate reading right after watering the battery
3) Battery temperatures affect reading accuracy—electrolyte should be at normal room temperature
4) Wear eye protection when leaning over the battery
5) Electrolyte residue that remains in the hydrometer is corrosive—rinse out and store hydrometer in an acid–resistant container.
