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.

  • What do the instructions for the charger say?
    – Solar Mike
    Commented Oct 5, 2021 at 17:10
  • Don't touch the live wire. Commented Oct 5, 2021 at 20:14

5 Answers 5


The slower the charging rate, the more efficient the battery charges. Faster charging is only used when time is essential.

  • By more efficient, do you mean that it uses less electricity to gain the same amount of charge? Commented Oct 7, 2021 at 15:31

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.

  • I have to -1 since most trickle chargers, which are different from battery tenders or float chargers, will keep the Voltage above 13.5 Volts which will ruin the battery fairly quickly. 14 Volts will usually ruin it within a year. If you have such a charger you want to put it on lamp timer so it only turns on for an hour per day or so. Even the float chargers put can put out too much Voltage (>13.3) and ruin a battery. A float charger is used after fully charging the battery with a normal charger. Commented Oct 6, 2021 at 10:42
  • @AlexCannon Happy to take a -1 if I am wrong, however, can you backup your comment with some real data?
    – HandyHowie
    Commented Oct 6, 2021 at 11:07
  • 1
    @AlexCannon I have just been out to check on my charger that I keep a couple of batteries on charge. It was supplying 13.7 volts. The battery was drawing 4mA because it was fully charged. I am pretty sure you will agree 4mA is not going to harm a large lead acid battery. So your 13.5 volt statement must be wrong.
    – HandyHowie
    Commented Oct 6, 2021 at 11:23
  • I destroyed a brand new lead calcium marine deep cycle battery in 4 years at 13.5 Volts and slightly above room temperature conditions. Perhaps some maintenance free or AGM batteries are different but your traditional battery will be ruined at that Voltage unless it's in freezing weather. I'll have to do a current test now since 4mA is hard for me to believe. Someone else here recently ruined a battery by keeping it an unregulated "trickle charger" for about a year. Commented Oct 7, 2021 at 0:59
  • @AlexCannon I did start by saying “A true trickle charger”. Cheap unregulated chargers may well have a higher output voltage than desirable and hence keep pushing a large current through a full battery. Once the battery voltage reaches the charge voltage, current will stop flowing.
    – HandyHowie
    Commented Oct 7, 2021 at 6:25

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.


I have an RAC charger similar to this one https://argos-support.co.uk/instruction-manual/6857800-rac-12-amp-battery-charger.pdf

The instructions state that if you are charging a deeply discharged battery it is better to run on a trickle charge for a couple of hours before switching to a fast charge. Fully charging on a trickle charge will take longer (possibly days), so perhaps it is better to switch up to a fast charge after a couple of hours. Fast charge will switch to float charge when fully charged.

If using trickle charge it is a more efficient way of charging the battery (cheaper). If you just want to top it up over night that is probably a good option. However this model of charger does not seem to support the float mode switch on the trickle mode so probably need to take care not to over charge the battery when using as others have mentioned here.


Fast charging a fully discharged lead acid battery can shorten its life. But your typical 6 Amp automotive battery charger is not a fast charger. Jumping the car and letting the alternator charge it is also fast charging.

  • This -2 down voting without stating the reason is abuse. It started after my other answer about how modern cars have not made a significant improvement in safety in highway speed head on collisions compared to older cars. Commented Oct 7, 2021 at 1:07
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    The reason I think this is wrong, is that you can’t say that 6 amps is not fast charging. If the charger keeps pushing 6 amps through a charged battery, it will damage it. It is the output voltage that determines whether it is fast charging or not. According to this power-sonic.com/blog/how-to-charge-a-lead-acid-battery fast charging is when up to 2.45v per cell is applied. The recommended float voltage is 2.25-2.3 volts for trickle charging. I also don’t fully agree that alternator charging is necessarily fast charging, but I understand the dangers.
    – HandyHowie
    Commented Oct 7, 2021 at 7:07
  • Does it matter that I have a higher amp rated charger? Commented Oct 7, 2021 at 15:31
  • @Aaargh Zombies It could matter. If your fully discharged battery starts charging at >20 Amps on the fast charger I think that would not be optimal for the battery's life. Does it have a slower setting to keep the charge current under 10 Amps? This is based on what I read somewhere else a long time ago about fast charging a fully discharged battery, but don't know how damaging it actually is. Commented Oct 7, 2021 at 17:11

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