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
    Oct 5 '21 at 17:10
  • Don't touch the live wire. Oct 5 '21 at 20:14

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? Oct 7 '21 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. Oct 6 '21 at 10:42
  • @AlexCannon Happy to take a -1 if I am wrong, however, can you backup your comment with some real data?
    – HandyHowie
    Oct 6 '21 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
    Oct 6 '21 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. Oct 7 '21 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
    Oct 7 '21 at 6:25

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. Oct 7 '21 at 1:07
  • 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
    Oct 7 '21 at 7:07
  • Does it matter that I have a higher amp rated charger? Oct 7 '21 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. Oct 7 '21 at 17:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.