When taking specific gravity measurements, it is important to correct for temperature to get a true reading. As a rule of thumb, specific gravity will change by 0.0003 points per 10ºF. See the table below:
The above table shows the actual hydrometer readings of acid at a specific gravity of 1.265 @ 77ºF. As the acid get colder it contracts and the apparent density increases and as it gets hot it expands and the apparent density decreases. This is important when determining the state of charge. If the electrolyte temperature is 100ºF and a density reading of 1.258 is taken and not corrected, unnecessary charging may occur. This will shorten the battery life, waste electricity and unnecessarily gas the battery. Further maintenance from extra watering will then be required.
The best way to measure the state of charge of a battery is to measure the specific gravity. This can been done under load but care must be taken and it is never recommended if the truck is in service. The following table shows varying state of a charge and the corresponding corrected specific gravities of a battery at full charge at 1.265 and 77ºF.
Full charge at 1.265 @ 77ºF
% Charged Specific Gravity
100% 1.255 – 1.275
75% 1.215 – 1.235
50% 1.180 – 1.200
Battery capacity and battery recharge times are all based on each cell having an electrolyte temperature of 77ºF (25ºC). Temperatures below the nominal 77 ºF reduce the batteries effective capacity and lengthen the time to restore the battery to full charge. Temperatures above 77ºF will slightly increase capacity, but also will increase self-discharge and shorten battery life.
Major problems occur at temperature extremes. This is the range below 40ºF (4ºC) and above 100ºF (38ºC). Electrolyte in a fully charged battery has a freezing point of about –85ºF (65ºC) but electrolyte in a fully discharged battery with low specific gravity has a much higher freezing point just below 32ºF (0ºC). A fully discharged battery, is endanger of freezing below 4ºC and it should be charged immediately or un-repairable damage will result.
Lower Temperature – lower capacity, more heavily cycled.
Higher Temperature – higher capacity, more self-discharge and battery wear.