A filtered battery charger uses aluminum electrolytic capacitors in the dc output filter. Capacitors typically have a maximum service temperature of 85 °C. This doesn’t mean that it’s OK to put the charger in a hothouse. Like other practical components, capacitors have self-heating effects: They get warmer than the environment just by doing their job. Since the charger is rated to operate in a 50 °C environment, and has an internal cabinet temperature rise of 5 °C to 10 °C, this gives us a comfortable margin of about 25 °C. The expected capacitor temperature rise is between 10 °C and 15 °C, including the internal cabinet rise.
Low temperature operation doesn’t limit the life of electrolytic capacitors, but the capacitance may be reduced, leading to higher ripple voltages in the dc output. At higher temperatures, there’s no significant change in capacitance or internal resistance, but life is reduced. Here’s that pesky number again: For each 10 °C increase in temperature, life is reduced by 50%. Cooler is better.
Carbon Resistors (SCRF Series)
The dc output voltage of the charger is controlled by a feedback circuit that uses carbon resistors to sample the output voltage. Their resistance usually decreases as temperature rises. For a non-temperature-compensated charger this could lead to undesirable changes in the output voltage. The sampling circuit is potentiometric, dependent on the ratio of two resistances instead of the value of one resistor. This means that any change in the sampled output voltage depends on how well the values of two similar resistors track as the temperature changes. As long as the resistors are in the same temperature environment, the deviation in output voltage will be within the charger’s ±0.5% specification.
Standard thermal-magnetic circuit breakers are calibrated for rated performance at 40 °C. This means that they will carry their rated trip current forever at 40 °C ambient temperature. The 40 °C value is mandated by UL in the circuit breaker standard, UL489.
Above 40 °C, the trip current of a thermal-magnetic breaker may be reduced. The trip value at 50 °C may be reduced by about 10%. Because of this, one manufacturer specifies circuit breaker ratings according to the NEC requirement that the breaker must be rated no lower than 125% of the expected load current; that is, the breaker will carry no more than 80% of its rated trip current. This provides a margin of about 10% to prevent nuisance trips.
If you expect your application to spend a lot of its lifetime above 40 °C, circuit breakers can be supplied custom with high temperature calibration.
HOW CAN I SAFELY USE A BATTERY CHARGER OVER ITS 50 °C RATING?
A battery charger can be operated at its full power rating up to 50 °C. Obviously, it would work at 51 °C, and for a range of temperatures above that. But what happens to it? Why is there an upper limit? As we noted above in the section on filter capacitors, electronic components have upper temperature limits; beyond those limits, performance degrades, and there may be permanent deterioration, component damage or catastrophic failure, which you probably wouldn’t like.
Fortunately, there is a way to use a charger at elevated temperatures, by derating the output current. Operating at a lower output current reduces the stresses on temperature-sensitive components, which allows you to use the charger in environments above 50 °C. The absolute temperature limit, though, is 70 °C, since that’s the upper limit for some internal components. See the section on derating in Appendix B.
WHEN IS IT USEFUL TO INCLUDE AN OVER-TEMPERATURE ALARM?
Well, by now you know that letting things get too hot is a no-no. You’re probably asking yourself, “How can I keep tabs on this charger, to make sure that nothing goes wrong and causes it to overheat?” I hope you are.
There are lots of ways that a charger could overheat. If you have a fan-cooled model, the fan could become blocked by dust or debris, or simply wear out. Unfortunately, they do; fans are probably the least reliable part, which is why an over-temperature sensor is supplied with a fan-cooled, or forced convection, model. Other causes of overheating could be blocked heat sinks, blocked cooling vents, or installing other equipment too close, resulting in inadequate air space around the charger. Failure of an internal component or maladjustment of a setting such as current limit could also cause overheating.
If you’re concerned about overheating in a natural convection-cooled charger, you can order an optional over-temperature alarm. The alarm uses a thermostat mounted on one of the semiconductor heat sinks. In case of an over-temperature condition, the thermostat, which is a normally closed bimetallic switch, opens, triggering an alarm. The alarm annunciator may be a local audible or visual indicator, or a relay for remote indication.
What do you consider to be too hot?
In a charger that’s cooled by forced convection, any heat sink over 70 °C is too hot. In natural convection, we trigger the alarm at 100 °C.
Why is there a difference? Sounds like it should be 70 °C all the time.
In a fan-cooled charger, a heat sink over 70 °C probably means that a fan has failed, and the charger needs attention. While we could allow the heat sink in a fan-cooled unit to run at a higher temperature, it’s designed to have a lower temperature rise, to take advantage of the fan. Running it long term without the fan could spell disaster. If the alarm signals, you need to service the charger.
Couldn’t we just use the over-temperature relay to turn off the charger when it gets too hot? Then it would go back on when the charger cools down.
I’ll pretend you didn’t ask that. First, the charger still has a problem; shutting it down doesn’t fix it. Second, and more important, you will be cycling the battery repeatedly, which could drastically shorten its useful service life. Third, the thermostat may be only a signal-level contact and can’t handle the power required by the charger.
IS THERE A LOW TEMPERATURE LIMIT FOR CHARGER OPERATION?
A charger operates fine down to temperatures well below 0 °C. The charger specifications are rated over a range of 0 °C to 50 °C; thus, characteristics like voltage regulation may not be within their guaranteed values outside this range.
Most internal components are rated for operation at -20 °C, and some at as low as -40 °C. Although I’ve never seen it happen, it’s possible that SCRs might not turn on at extremely low temperatures.
Optional cabinet heaters are available. While these are primarily designed to prevent condensation damage during storage, they may help if performance is degraded in the big chill. We also urge you to acclimate a charger to a warm room for 24 hours before energizing, if it has just come in from the cold.
HOW SHOULD I STORE A CHARGER AT LOW TEMPERATURES?
Those heaters that we mentioned in the previous paragraph won’t necessarily keep you toasty, but if the charger is to be stored in an unheated building, they’ll prevent condensa-tion from forming potentially damaging ice crystals inside delicate components. Heaters are available for operation from 120 Vac or 240 Vac and have their own circuit breaker.
The heaters are thermostatically controlled to turn off at about 21 °C (70 °F). We do not recommend outside storage, but if you have to do it, keep the charger in its original packaging, and energize the heaters.