As a battery discharges, its voltage falls, reducing its ability to push out power. While charging, the voltage must be set correctly in order to properly charge your batteries. Monitoring a battery's voltage is crucial. Preventing each cell from dropping below 1.9V will avert unnecessary battery damage.
Voltage provides the 'pressure' or 'force' to move electrons (current) around a circuit. The voltage you see on a battery description is nominal. It is not a static figure but one that varies according to the battery's activity, its surroundings, and its present condition. It is the electro-chemical potential of a battery.
Voltage is affected by:
1) The level of current passing through it
High rate of discharge = Rapid voltage drop
Low rate of discharge = Slow voltage drop
2) The temperature of the battery
High temperature = Lower charging voltage
Low temperature = Higher charging voltage
3) The state of charge of the battery
Fully charged battery = High voltage
Fully discharged battery = Low voltage
Here we see the voltage drop (left axis) as a battery discharges from a full state of charge - 0% DoD (bottom axis). The graph contains five different rates of discharge. C/3 shows the fastest current discharge, and C/100 the slowest. Discharging at the C/3 rate causes rapid, deep and damaging decrease in voltage. On the other hand, the slow C/100 rate of discharge has a stable decline in voltage, preventing the voltage from dropping off dangerously.
Significantly, discharging a battery past 80%, even at the C/100 rate, should be avoided completely. Similarly, so should discharging a deep-cycle battery rapidly, such as the C/3 rate. As the red line shows, rapid discharges should be brief at best. Common advice and usual discharge cycles would use the C/20 rate and would not go beyond 50% discharge. As you can see, the voltage stays at a constant 'safe' level, reducing the damage to the battery and ensuring a longer lifespan (more cycles).
It is important to note that the above graph is only an example, and in fact voltages will vary. These voltages are also affected by the fact that the battery has been put under load (it has been discharged). To obtain more accurate voltage readings it would be necessary to let the battery rest for a few hours. Below are the approximate voltages of a battery that has not experienced a recent load.
As we can see, the no-load voltage when at 100% DoD is 10.5V. Your battery should never be allowed to reach this level of full discharge. Essentially this shows the battery is exhausted and unable to provide any more voltage 'force' as the specific gravity of the electrolyte has dropped and the plates are covered in lead sulphate.
The graph indicates that to best preserve your batteries, you should keep discharges between 10% and 50%. Lower that 5% discharge can damage your batteries as the short chemical reaction distributes the lead sulphate unevenly on the plates, damaging them. Obviously keeping discharges at only 20% would require an enormous battery bank, and in most cases is unrealistic. Therefore, 50% represents a good trade-off between battery life and cost/size of the battery bank. The yellow region past 50% is not overly damaging to your batteries, but consistent cycling to this level will make the batteries less cost effective and will shorten their lifespan. Clearly, the red region past 80% should be avoided unless absolutely necessary, say in case of emergency.
The voltage (no-load, no-charge) of a battery is largely unaffected by temperature. There is little effect on the electro-chemical potential between the plates. However, the speed of the electro-chemical reaction is affected, but this affects the capacity (current) of the battery.
The voltage under charge is affected by temperature. Higher charge voltage is required at lower temperatures and a lower charge voltage at higher temperatures. A temperature sensor for your charge controller is necessary to adjust the charging voltage.