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Temperature Considerations for Solar Batteries

Batteries are heavy, rugged, dumb animals! Ture! But not quite. Environment affects even a stone. And temperature affects everything living or non-living. Since batteries operate on chemical processes, temperature affects them seriously. Solar batteries are especially exposed to outdoor environment. “Why not keep them in a cool, dry, ventilated room in a building?” you may ask. The fact is, indoor space is always limited, and maintaining such environment indoors costs heavily. Commercial solar projects are necessarily large and outdoors. With increase in sizes of solar projects battery banks demand more space. Locating batteries close to the panels reduces the low-voltage, high-current copper run and hence, reduces copper losses. That means batteries will be exposed to large variations in mean and cyclic temperatures.

It is important for the solar designer and installer to know how temperature matters to batteries. Now different types of batteries react differently to different temperatures. But there is one trait applicable to all types of batteries. All have a range of optimal temperatures. The range of temperatures over which a battery can be charged is less compared to the temperature range over which it can operate. Outside these ranges, the batteries will not perform well. Knowledge of these temperature considerations is important in selecting a battery type for a given application. Extreme temperatures WILL reduce performance and life of the battery. Read more here

Cold temperature

Cold temperature affects in two major ways. It increases the internal resistance and reduces the capacity.
Internal Resistance-Cold raises the internal resistance of the battery. This will reduce the charging rate for fixed voltage chargers. The generation of heat inside the battery will alleviate the problem somewhat as charging proceeds. Smart chargers will cater for these variations. The effect on different chemistries is different, and the manufacturer’s specification must be honored.
Battery capacity– A battery will exhibit a lowered storage capacity at lowered temperatures. The variation also depends on the charge and discharge rtes. Again, the effect is different for batteries made with different chemistries. A lead acid battery may provide just half the nominal capacity at -18° C.

Elevated Temperatures

The common perception that batteries misbehave only in cold is not right. High temperature can cause damage and even fire to the battery. Rise of temperature lowers the voltage required to maintain a given charging current. Thus, for a given fixed charging voltage charging current will rise as the temperature rises. Since charging produces heat increasing the charging rate further, a run-away condition can exist if not controlled. Charging voltage should be reduced approximately 3mV/°C for lead acid if constant current charging is desired.

Capacity Loss at High Temperatures

Operating the battery beyond the permissible temperatures results in a capacity loss. This capacity loss is cumulative as the battery is cycled. An important thing to note is that capacity loss due to cycling at high temperatures:
Increases with increase of temperature
Is higher when battery is cycled at higher level of charge.

Permissible Temperature ranges

Lead Acid– Lead Acid batteries have wide tolerance to temperature variations. Charging and discharging operation is possible between -20°C and 50°C. The normal charging is at 0.3C (C is the capacity in AH. For a 200AH battery charging at 0.3 C means charging at 60 A) which should be reduced gradually to 0.1C below 0°C. A discharged battery is more likely to freeze and get damaged at low temperatures because the electrolyte now contains more of water.
Nickel-Cadmium and Nickel-metal Hydride batteries the normal operating range is 0°C and 45°C (which is 32°F to 113°F). At -18°C charge rate is limited to 0.1C. Above 45°C (113°F), reduce rate gradually to half the rate till 65°C (149°F). Discharged is possible between –20°C to 65°C which is –4°F to 149°F.
Li-ion– These can be charged 0°C to 45°C (32°F to 113°F) and discharged between –20°C to 60°C (–4°F to 140°F). Operating at the higher temperatures reduces life.

Charging Batteries at Low Temperatures

In order to charge at normal rates in low ambient temperatures batteries can be enclosed in some enclosure which maintains near normal temperature. Otherwise charge acceptance will be less because ion combination will be slower. Forcing high current can build up pressure causing explosion of sealed batteries.

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