Battery care -sealed lead acid types
The actual voltage used to charge a sealed lead acid battery depends on the temperature. It can be a precision operation, and this is one reason why a good battery charger is costly. Don't use the $50 battery chargers found at wal-mart or other stores, unless you are willing to greatly sacrifice life of the battery due to overvoltage, overtemperature, and ripple current (explained in the references below).

If the intent is to fast-charge the battery, it's best to refer to the manufacturers' data sheet for that model, usually found online.

If the intent is to operate your equipment from an AC-operated power supply, and also keep a battery attached to the power supply's output in order to 'float charge' it as well as provide fail-over for your equipment, then two things should be kept in mind:

1. The DC voltage output of the power supply should be adjusted based on the maximum temperature the battery will be exposed to while in operation. Most store-bought regulated power supplies (so-called "12 volt" or "13.8 volt" regulated power supplies) have an internal adjustment which can be used to set the voltage fairly precisely, within a range of 12.5 to 14.5 volts.

2. The initial time required to float-charge a discharged 90 amp-hour battery is about 7 days. Float-charging this manner is very gentle on the battery, and is preferred to fast-charging for users desiring maximum service life. A battery which is float charged at no more than the reccommended voltage will have several more years of life than one which is repeatedly fast-charged. The reasons for this are chemical rather than electrical. It should be kept in mind that a battery is a chemical device first, and an electical device second.

In a field-portable system, 13.5V was chosen as a compromise of 2.25 volts per cell (6 cells in a '12V' battery) so that the battery may be operated in the float charge mode continuously at temperatures of up to 86 degrees F, and for short periods at higher temperatures. It's easy to keep the battery temperature in an acceptable range by keeping it in the shade. It follows that for a given voltage, colder temperatures lead to undercharging, and warmer temperatures lead to overcharging. It's worth the extra efort to use a regulated power supply and make the adjustment to it, so as to keep your emergency battery in the best condition.

If you are ambitious, you can substitute a 10-turn precision potentiometer for the internal adjustment of the power supply, and add a digital panel meter, so you can set the voltage according to the temperature (use a chart). Expensive battery chargers provide clean DC power for equipment and also have temperature compensation built-in, for hands-off operation.

These are only tips. there's much more to know: Refer to the references, for charts showing float voltage versus temperature, and other useful knowledge for those who maintain sealed lead-acid batteries.

General information on saturated glass mat sealed lead acid batteries
General information on lead-calcium (sealed lead-acid) batteries
Operators manual for sealed lead acid batteries
High-detail information for those who specify batteries