STATIONARY BATTERIES ARE expected to run at normal room temperatures for their whole lives. For this reason, batteries are designed to operate best within a limited temperature range around room temperature, normally specified as 20 °C or 25 °C (68 °F or 77 °F). Their capacity and life specifications are based on that environment. But what happens if the temperature varies? We’ll discuss some examples below.
BATTERY CAPACITY INCREASES AS TEMPERATURE INCREASES
Stationary battery capacity in Ah is normally specified (at room temperature) for the eight-hour discharge rate. The battery is discharged at a constant current, chosen so that the battery reaches its end-of-discharge voltage in exactly eight hours. For lead-acid, that’s 1.75 VPC (volts per cell), and for NiCd, somewhere around 1.0 VPC. For example, a 100 Ah battery can be discharged at 12.5 A for eight hours. At the end of that time, the terminal voltage of a lead-acid cell will be 1.75 VPC. For a 60-cell battery, that’s 105 Vdc.
NiCd batteries are useful to about 1.1 VPC. At this point, if the discharge continues, the terminal voltage collapses quickly. A 92-cell battery, at the end of its useful discharge, will measure about 101 Vdc.
Battery capacity increases at temperatures above room temperature. But before you start installing heaters in the battery room, please note the downside to life and maintenance.
LIFE DECREASES WITH ELEVATED TEMPERATURE
For every 10 °C in elevated temperature, a lead-acid battery’s life decreases by 50%. If your battery is rated for a 20-year life at 25 °C, then at 35 °C (95 °F) it will last only 10 years. And that’s assuming you use a temperature-compensated charger (see Temperature Compensation in SECTION 5.3). Without temperature compensation, there will be excessive gassing due to electrolysis (see SECTION 220.127.116.11).
ADDITIONAL MAINTENANCE IS NEEDED
If you have a flooded type lead-acid battery, more maintenance is required, especially water replenishment. There is a double danger here: If the electrolyte level falls too low, plates may be exposed, making the active material that’s exposed to the air unusable. Also, the specific gravity of the electrolyte increases, accelerating grid corrosion. The only bright spot, if there is one, is that the increasing specific gravity will reduce the float current (by raising the open-circuit voltage of the cells) and slow down the effects of electrolysis.
Sealed lead-acid batteries will tolerate a moderate amount of overcharge over a limited range of temperature. But if your environment falls outside this range (above about 86 °F), the battery may experience a permanent loss of electrolyte, with additional corrosion, loss of capacity, and compromised life expectancy.
At the other end, as you might expect, you get a boost in battery life at lower temperatures. But, discharge capacity is reduced. If you’ve sized your battery to handle the expected loads only at room temperature, you may run short during a power emergency. Also, if you are using a non-compensated charger, there is a risk of undercharging, which over the long term may lead to permanent damage to the battery.
We noted above that flooded batteries, if operated outside the normal temperature limits, may require more frequent watering, due to gassing. Electrolyte loss can also cause additional corrosion at cell interconnections, and deposits of corrosive materials around vent caps. Evidence of corrosion can also occur for VRLA and other sealed batteries. Corrosion at terminals and connectors must be cleaned to maintain peak battery performance. There is also a risk of ground faults from large accumulations of electrolyte spray. Batteries with flame arrestor vents may clog due to excessive gassing.
NOTE: Battery manufacturers recommend cleaning solutions for corrosion (sodium bicarbonate for lead-acid, boric acid for NiCd). Be sure to use the correct solution for your battery type. Never let the cleaning solution get into the active electrolyte. Be sure to follow the manufacturer’s maintenance manual for your battery.
DISCHARGE CAPACITY DROPS AT LOW TEMPERATURES
One of the noted benefits of NiCd battery technology is improved low temperature operation compared to lead-acid batteries. At 0 °C, a NiCd battery might have 90% of its room temperature capacity, while lead-acid would be down to 80% (lower at high discharge rates). You might be tempted to say (about lead-acid), “Well, that isn’t so bad.” But, as usual, there’s another fly in the ointment. One of the reasons that lead-acid performance deteriorates at low temperatures is that the electrolyte's specific gravity decreases as the battery discharges. This reduces the activity of the electrolyte, leading to higher internal resistance and reduced capacity. It also raises the freezing temperature of the electrolyte. A fully charged battery won’t freeze solid at any temperature you’re likely to encounter, unless you work in Antarctica in the winter. But a discharged battery will have a freezing point above 0 °F (-17 °C) – still mighty cold, but possible in many areas for a battery in an unheated building.
The specific gravity of the electrolyte in NiCd batteries doesn’t change as the battery discharges, so the freezing point doesn’t change with the state of charge.
Just as temperature changes affect battery discharge characteristics, they also affect charging characteristics. High temperatures, in particular, can have a negative impact on battery performance and life.
Lead-acid and NiCd batteries both exhibit a negative on-charge temperature coefficient. That means that as the battery temperature rises, the battery terminal voltage decreases if the charging current is kept constant. It doesn’t matter if the temperature rise is due to increasing ambient temperature or internal heating caused by the charging process. In the other direction, the terminal voltage rises if the temperature goes down.
Since virtually all stationary chargers are the constant-voltage type, this means that as the temperature increases, the float current also increases. Increased float current can have serious negative effects on battery life, as we described above. If you want to keep the charging current constant, you must decrease the charging voltage as the temperature rises.
Since we normally can’t measure the float current, we depend on the battery manufacturer’s recommendation for float voltage, which is usually specified at 25 °C or 20 °C. The float voltage is a function of battery type and construction; for lead-acid batteries, it also depends on specific gravity. The float voltage recommendation is a balance between rated capacity, life, and maintenance requirements. Once we know the best float voltage at 25 °C, then we need to adjust the voltage for the actual battery temperature, either manually (you don’t want to do that), or automatically with a temperature-compensated charger.
A temperature-compensated charger is designed to adjust the output float voltage (equalize, too, of course) automatically as the temperature changes. You can read more about specifying and using temperature compensation in SECTION 5.3. Also, there are additional descriptions of temperature effects on discharging and charging batteries in SECTIONS 1.4.6 and 1.5.6.