When taking specific gravity measurements, it is important to correct for temperature. As a rule of thumb, the specific gravity will change by 0.0003 points per 3ºC (~6ºF). See the table below:
The above table shows the actual hydrometer readings of acid at a specific gravity of 1.265 @ 25ºC (77ºF). As the acid cools 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 38ºC (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 may be done under load but care must be taken and it is never recommended if the truck is in service. The following table shows the varying state-of-charge and the corresponding corrected specific gravities of a battery at full charge at 1.265 and 25ºC (77ºF).
Full charge at 1.265 @ 25ºC (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 25ºC (77 ºF). Temperatures below the nominal 25ºC (77 ºF) reduce effective capacity and lengthen the time to restore the battery to full charge. Temperatures above 25ºC (77 ºF) 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 4ºC (40ºF) and above 38ºC (100ºF). The electrolyte in a fully charged battery has a freezing point of approximately –85ºF (-65ºC). However, the electrolyte in a fully discharged battery with low specific gravity has a much higher freezing point; just below 0ºC (32ºF). A fully discharged battery is in danger of freezing below 4ºC (40ºF) and should be charged immediately. Damaged caused by freezing is typically not recoverable and will require replacement.
Lower Temperature – lower capacity, more heavily cycled.
Higher Temperature – higher capacity, more self-discharge and battery wear.