When using a modern battery charger to recharge a basic car battery that is in good condition, but is run down to the level that it is unable to turn an engine over, under what circumstances should you use the trickle charge setting as opposed to the fast charge setting, and vice-versa.
A true trickle charger can be left connected to the battery indefinitely and should not cause any damage to the battery from over charging. It will take longer for the battery to charge.
A fast charger will have a higher output voltage which will therefore pass a higher current through the battery. Care must be taken not to leave the charger connected for too long as this can cause damage to the battery. The battery can overheat and the electrolyte can boil away.
A modern charger may have a built in timer or sensing circuitry that will switch from fast charging to trickle charging when the battery is full.
A good microprocessor controlled charger is a 3-state charger. That's nearly all we have today because older chargers that used heavy transformers were unreasonably heavy. Today switched mode power supplies and microprocessors are so cheap practically every charger is a microprocessor controlled charger.
The stages are:
- Constant current (usually at most 25%-30% of the amp-hour rating of the battery in amperes)
- Constant absorption voltage (14.5 - 15.0 V, higher when cold)
- Constant float voltage (13.5 - 13.8 V, higher when cold)
Usually cheapest chargers such as 4-amp or 7-amp chargers have so small charge rate compared to the battery amp-hour rating so that the current is never too high. However, when using a really large charger such as 25-amp charger then you may need the ability to limit the charge current. Such big honking chargers usually have a second setting that puts the charging current to a far lower value.
The absorption voltage would ideally be temperature controlled, but cheapest chargers may have only one absorption voltage and slightly more expensive chargers may have a second "cold" setting to be used when the temperature is freezing. Only the very best chargers actually measure the temperature and adjust the voltage accordingly. Yet, nevertheless, since the absorption voltage time usually is limited and when the charge current gets really low, the float stage starts, it's not fatal if the temperature is slightly incorrect. In cold the voltage should be higher and if a charger with no "cold" setting and no temperature measurement is used, the voltage is too low, which only increases charging time and doesn't damage the battery.
Ideally you would have the ability to adjust the current at which the absorption time ends, to be useful in all sizes of batteries from the smallest motorcycle batteries to the largest truck batteries, but usually chargers unfortunately don't let you adjust that.
The float charging stage is usually implemented by a constant low voltage, but it may also be implemented by letting the battery rest and after a certain amount of time, starting a new absorption stage that will end very soon due to the current falling very low. Continuous float charging tends to degrade batteries slowly if used for a period of many years continuously, so the "rest and start a new absorption stage" strategy leads to longer battery life.
So, the question is, should you use the trickle charge setting? Usually you don't want to use that. The trickle charge setting only starts the charging from the float stage, letting the charger to be in that stage forever. If you don't do that, most likely the charger will start from constant current (if the battery takes that current, if not then directly to absorption voltage), then move to absorption voltage, then move to float voltage. If the battery is full, then it would move to the float stage anyway very fast. If the battery is not full, then you usually don't want to float-charge it because that's slower than absorption-charging.
However, if your charger is not a microprocessor controlled three-stage charger then my advice would be to not leave the charger connected to the battery for a long time. Thus, you don't want to use the trickle mode. You only want to charge batteries that are not full, and then estimate when the charging is done, then disconnect the charger. A bad charger such as one that is not based on a switched mode power supply can destroy a car battery.