A battery's available capacity varies depending on the temperature. As the ambient temperature rises, a battery's ability to deliver current increases. As the temperature falls, so does the battery's ability to deliver current.
Temperature is a significant factor in battery performance, shelf life, charging and voltage control. At higher temperatures, there is dramatically more chemical activity inside a battery than at lower temperatures. Battery capacity is reduced as temperature goes down and increases as temperature goes up. This is why your car battery has reduced performance on a cold winter morning and why capacity needs to be considered when sizing your battery for use in different environments. The standard rating for batteries is at room temperature (25°C/77°F). At approximately -22°F (-27°C), battery capacity drops by 50%. At freezing capacity, it is reduced by 20%. Capacity is increased at higher temperatures. At 122°F, a battery's capacity will be increased by about 10-15%. As mentioned earlier, battery charging voltage also changes with temperature. It will vary from about 2.74 volts per cell at -40°C to 2.3 volts per cell at 50°C. This is why temperature sensing and compensating chargers are so important.
The Thermal Mass of larger batteries and battery banks leads to more discussion. Because some of these batteries have so much mass, they will change the internal temperature much slower than the surrounding air temperature. A large insulated battery bank may vary as little as 10° over 24 hours internally, even though the air temperature varies from 20° to 70°. In these circumstances, external thermocouples attached and insulated to one of the positive terminals are a good idea. The sensor will then read very close to the actual internal battery temperature.
Even though the battery capacity at high temperatures is higher, battery life is shortened. High temperatures affect the battery's service life according to a common “rule of thumb” or the law of “Arrhenius,” which states that the corrosion rate will be doubled at 10° C. Therefore, the lifetime will be halved per 10° C increase in temperature.
Example:
- Fifteen years at 20° C becomes reduced to 7.5 years at 30°C
NOTE: Even though a battery's ability to deliver current increases as temperature rises, prolonged operation at extreme temperatures may shorten the battery's life.
To calculate the approximate capacity correlation due to temperature, add or subtract the % adjustments shown in the following table:
|
Discharge Time |
0°C |
5°C |
10°C |
15°C |
20°C |
25°C |
30°C |
35°C |
40°C |
|
<30 Min |
-20% |
-15% |
-12% |
-8% |
-3% |
0% |
+5% |
+8% |
+10% |
|
30-60 Min |
-18% |
-13% |
-11% |
-7% |
-2% |
0% |
+4% |
+6% |
+8% |
|
>60 Min |
-16 |
-12% |
-10% |
-6% |
-1% |
0% |
+3% |
+4% |
+5% |
Typical self-discharge of VRLA DRY CELL batteries at different temperatures:
A fully charged battery's shelf life will discharge at approximately:
- 2% per month when stored at 8°C/46°F
- 3% per month when stored at 20°C/68°F
- 5% per month when stored at 30°C/86°F
- 10% per month when stored at 40°C/104°F
Typical self-discharge of quality Deep Cycle Flooded batteries at different temperatures:
A fully charged battery's shelf life may discharge at approx;
- 6% per month when stored at 8°C/46°F
- 9% per month when stored at 20°C/68°F
- 15% per month when stored at 30°C/86°F
- 30% per month when stored at 40°C/104°F
Batteries kept in storage while discharged will not perform as intended when put into service. Battery inventories should be constantly checked and recharged when necessary. A battery in storage should never be allowed to discharge more than 45-50% of its original capacity.
Discharging batteries depends on several factors. These include, but are not limited to, the initial state of charge, the depth of discharge, the age and original capacity of the battery, the load or rate of discharge and the temperature at the time of discharge. To estimate the cycle time at 70° F (21.1° C), divide the battery's ampere-hour rating by the load or discharge current in amps. For example, a new 72-ampere-hour battery with a 10-amp load should last approximately 7.2 hours. As the battery ages, this capacity will be reduced.
Some factors affecting a car charging system's ability to charge are;
- How much current from the alternator is diverted to the battery to charge it
- How long the current is available, and at what temperature the charging activity is taking place.
Generally, idling the engine or on short “stop-and-go trips” during cold or hot weather or at night will not recharge a battery. A long daytime trip in warm weather should recharge a battery.
This type of charge continually monitors and maintains a pre-set battery voltage, regardless of charge conditions. These chargers are used in stationary, emergency backup power, emergency lighting, and other similar applications. Most quality AGM and GELL chargers will have an alternative float cycle in their finishing charge algorithm. The frequency of discharge and temperature will dictate a more exact setting. For example, the more frequent the discharge, the higher the suggested recharge voltage should be to ensure that the recharge time is sufficient to maintain the battery's proper performance. The typical float voltage for monitoring and maintaining is between 2.25 and 2.30 volts per cell at 25°C/77°F.
Table 9 - AGM Charge and Float Voltage vs. Temperature
|
Temp. |
Charge |
Float |
Temp. |
||
|
°F |
Standard |
Maximum |
Standard |
Maximum |
°C |
|
≥ 120 |
13.60 |
13.90 |
12.80 |
13.00 |
≥ 49 |
|
110 – 120 |
13.80 |
14.10 |
12.90 |
13.20 |
43 – 49 |
|
100 –110 |
13.90 |
14.20 |
13.00 |
13.30 |
38 – 43 |
|
90 – 100 |
14.00 |
14.30 |
13.10 |
13.40 |
32 – 38 |
|
80 – 90 |
14.10 |
14.40 |
13.20 |
13.50 |
27 – 32 |
|
70 – 80 |
14.30 |
14.60 |
13.40 |
13.70 |
21 – 27 |
|
60 – 80 |
14.45 |
14.75 |
13.55 |
13.85 |
16 – 21 |
|
50 – 60 |
14.60 |
14.90 |
13.70 |
14.00 |
10 – 16 |
|
40 – 50 |
14.80 |
15.10 |
13.90 |
14.20 |
4 – 10 |
|
≤ 40 |
15.10 |
15.40 |
14.20 |
14.50 |
≤ 4 |
|
Discover AGM |
|||||
|
77°F |
14.20 |
14.72 |
13.40 |
13.80 |
25°C |
Table 10 - GELL Charge and Float Voltage vs. Temperature
|
Temp. |
Charge |
Float |
Temp. |
||
|
°F |
Standard |
Maximum |
Standard |
Maximum |
°C |
|
≥ 120 |
13.00 |
13.30 |
12.80 |
13.00 |
≥ 49 |
|
110 – 120 |
13.20 |
13.50 |
12.90 |
13.20 |
44 – 48 |
|
100 –109 |
13.30 |
13.60 |
13.00 |
13.30 |
38 – 43 |
|
90 – 99 |
13.40 |
13.70 |
13.10 |
13.40 |
32 – 37 |
|
80 – 89 |
13.50 |
13.80 |
13.20 |
13.50 |
27 – 31 |
|
70 – 79 |
13.70 |
14.00 |
13.40 |
13.70 |
21 – 26 |
|
60 – 69 |
13.85 |
14.15 |
13.55 |
13.85 |
16 – 20 |
|
50 – 59 |
14.00 |
14.30 |
13.70 |
14.00 |
10 – 15 |
|
40 – 39 |
14.20 |
14.50 |
13.90 |
14.20 |
5 – 9 |
|
≤ 39 |
14.50 |
14.80 |
14.20 |
14.50 |
≤ 4 |
|
Discover GEL |
|||||
|
77°F |
14.10 |
14.50 |
13.35 |
13.75 |
25°C |
Table 11 - Flooded Charge and Float Voltage vs. Temperature
|
Temp. |
Charge |
Float |
Temp. |
||
|
°F |
Standard |
Maximum |
Standard |
Maximum |
°C |
|
≥ 120 |
14.10 |
14.40 |
12.80 |
13.00 |
≥ 49 |
|
110 – 120 |
14.20 |
14.50 |
12.90 |
13.20 |
44 – 48 |
|
100 –109 |
14.25 |
14.55 |
13.00 |
13.30 |
38 – 43 |
|
90 – 99 |
14.30 |
14.60 |
13.10 |
13.40 |
32 – 37 |
|
80 – 89 |
14.40 |
14.70 |
13.20 |
13.50 |
27 – 31 |
|
70 – 79 |
14.42 |
14.75 |
13.40 |
13.70 |
21 – 26 |
|
60 – 69 |
14.45 |
14.80 |
13.55 |
13.85 |
16 – 20 |
|
50 – 59 |
14.48 |
14.80 |
13.70 |
14.00 |
10 – 15 |
|
40 – 39 |
14.50 |
14.80 |
13.90 |
14.20 |
5 – 9 |
|
≤ 39 |
14.55 |
14.85 |
14.20 |
14.50 |
≤ 4 |
Equalization voltage for flooded batteries should typically be maintained at a maximum of 15.60 to 16 volts at 25°C/77°F. Make sure to correct the charging voltage to compensate for temperatures above and below 25°C/77°F.
There is no fixed voltage setting or current that will work with certainty for applications with wide temperature fluctuations if a temperature-sensing charger is not available. A temperature-sensing, voltage-regulated charger must be used. Anything else will damage any battery and cause premature failure! It may be possible to limit this potential by using an ambient temperature sensing charger and assuming that the battery temperature is similar to the surrounding temperature. NOTE, however, that the battery's temperature will fluctuate during discharge and recharge, which will eventually damage the battery and cause premature failure if not controlled. If the recommended charger is not available, then limiting the charge current and extending the charge time will reduce the chance of damaging the battery.