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What are the 3 Stages of Smart Chargers?

 Tech Tips & Tricks

You may have heard it said "you need a 3 stage charger." We've said it, and we'll say it again. The best kind of charger to use on your battery is a 3 stage charger. They are also called "smart chargers" or "micro processor controlled chargers". Basically, these types of chargers are safe, easy to use, and will not overcharge your battery. Almost all of the chargers we sell are 3 stage chargers.

Okay, so it's hard to deny that 3 stage chargers work and they work well. But here's the million dollar question: What are the 3 stages? What makes these chargers so different and efficient? Is it really worth it? Lets find out by going through each stage, one by one.

Stage 1 | Bulk Charge

Bulk StageThe primary purpose of a battery charger is to recharge a battery. This first stage is typically where the highest voltage and amperage the charger is rated for will actually be used. The level of charge that can be applied without overheating the battery is known as the battery's natural absorption rate. For a typical 12 volt AGM battery, the charging voltage going into a battery will reach 14.6-14.8 volts, while flooded batteries can be even higher. For the gel battery, the voltage should be no more than 14.2-14.3 volts. If the charger is a 10 amp charger, and if the battery resistance allows for it, the charger will put out a full 10 amps. This stage will recharge batteries that are severely drained. There is no risk of overcharging in this stage because the battery hasn't even reached full yet.


Stage 2 | Absorption Charge

Absorption StageSmart chargers will detect voltage and resistance from the battery prior to charging. After reading the battery the charger determines which stage to properly charge at. Once the battery has reached 80%* state of charge, the charger will enter the absorption stage. At this point most chargers will maintain a steady voltage, while the amperage declines. The lower current going into the battery safely brings up the charge on the battery without overheating it.

This stage takes more time. For instance, the last remaining 20% of the battery takes much longer when compared to the first 20% during the bulk stage. The current continuously declines until the battery almost reaches full capacity.

*Actual state of charge Absorption Stage will enter will vary from charger to charger

Stage 3 | Float Charge

Float StageSome chargers enter float mode as early as 85% state of charge but others begin closer to 95%. Either way, the float stage brings the battery all the way through and maintains the 100% state of charge. The voltage will taper down and maintain at a steady 13.2-13.4 volts, which is the maximum voltage a 12 volt battery can hold. The current will also decrease to a point where it's considered a trickle. That's where the term "trickle charger" comes from. It's essentially the float stage where there is charge going into the battery at all times, but only at a safe rate to ensure a full state of charge and nothing more. Most smart battery chargers do not turn off at this point, yet it is completely safe to leave a battery in float mode for months to even years at a time.


It's the healthiest thing for a battery to be at 100% state of charge.

We've said it before and we'll say it again. The best kind of charger to use on a battery is a 3 stage smart charger. They are easy to use and worry free. You don't ever have to worry about leaving the charger on the battery for too long. In fact, it's best if you DO leave it on. When a battery is not at a fully charged state, sulfate crystal build on the plates and this robs you of power. If you leave your powersports in the shed during off-season or for vacations, please connect the battery to a 3 stage charger. This will ensure that your battery will be ready to start whenever you are. Use the link below to browse our selection of smart changers and 12v/24v trickle chargers.

Choose your Smart Battery Charger

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  • Donna Dora Hi I’m using a Shop Force battery charger to charge an ACDelco 75S and the charger is flashing red lines and Battery Recondition is flashing yellow. I do not have a quide. What does it mean? Thank You, Donna

    Reply  •  November 7, 2021 at 6:39 pm
    • BatteryStuff Tech Unfortunately, we do not carry the Shop Force brand chargers so I'm unable to advise. I would suggest pulling a manual from manufacturers website or contact the dealer you purchased the charger from directly.

      Reply  •  November 9, 2021 at 9:15 am
  • Carlos L Fuentes Have to say it, learned more in a five minutes reading than I did in a four hour seminar. Plain and simple. Thanks...

    Reply  •  July 8, 2020 at 4:04 pm
  • Nathan So I recently installed my 3 stage charger into my LiFePO4 battery bank in my RV. When I plug it in, initially, the charger works as it should through all 3 phases. But once it gets to phase 3, and I'm using my batteries, it won't go back to stage 2 or stage 1... I have to physically unplug the charger and presto is starts back in the phase I need based off the condition of the battery? I'm using a PowerMax 30A charger. Am I doing something wrong? Is it faulty? Is it not designed to go backwards? If it's a smart charger, and I'm using my batteries, shouldn't it be able to pick up the condition of my battery usage and charge accordingly?

    Reply  •  May 11, 2020 at 3:41 pm
    • BatteryStuff Tech The PowerMax's are designed for lead acid batteries, so their charging profile is meant for those batteries out of the box. The charger may be capable of charging lithium batteries initially, but for maintenance purposes it may have issues restarting the charging cycle. The reason is lithium's sit at a higher voltage than lead acid batteries, so in affect it doesn't know when to restart the charging cycle for your batteries. In an RV this probably isn't going to be a big issue as they PowerMax acts as a power-supply when pluged in, so amperage is supplemented by the unit as request are needed. When you unplug and plug into shore power it will complete a charge cycle, but for long-term maintenance charging you should talk to PowerMax about adjusting the profile. The PowerMax voltage can be adjusted in the field in order to customize the charging profile. You may have to work with your battery manufacturer and PowerMax in order to customize the charger to work correctly with your batteries.

      Reply  •  May 21, 2020 at 7:38 am
  • Paul This is very useful, but i have a 4 stage charger and i wonder what the 4th stage does. Please help explain the 4th stage.

    Reply  •  January 13, 2017 at 1:49 am
    • BatteryStuff Tech There is no black and white answer, as it really depends on the manufacturer, and what they consider a stage to be. Your best bet is to consult the manual for your charger, or contact the manufacturer.

      Reply  •  January 16, 2017 at 2:00 pm
  • Hamed Great, thank you :)

    Reply  •  September 29, 2016 at 8:00 am
  • Jonathon I hooked up my supercheap auto 7 stage charger to my lead acid battery and it has been in the “bulk” phase for 2 hours now. This is the first time I’ve needed to charge the car battery (I let the car sit for 2 months)
    The “faulty battery” light is not on, but I’m wondering if there really is something wrong?
    Any advice would be nice!

    Reply  •  September 29, 2016 at 3:20 am
    • BatteryStuff Tech Car batteries generally start at about 50AH, so only being on a couple of hours could be normal depending on the level of the battery, and the chargers amp rating.

      Reply  •  January 16, 2017 at 10:49 am
  • Shabbir what should the absorption voltage be set up to? and the time? for AGM batteries

    Reply  •  September 2, 2016 at 5:43 am
    • BatteryStuff Tech If you are looking for the charging profile for an AGM battery I would refer you to the manufacturer of the battery you are trying to charge. Most smart chargers have certain charging profiles built into them, but if they feel resistance they will reduce the voltage. However in a manual setup, I would refer to the manufacturer to be sure you program the charger to their specifications.

      Reply  •  January 16, 2017 at 10:31 am
  • Arthur Cowgill wow? ive been trying to find out why my battery is still stuck on float mode for 3 days now and wont go into pulse mode, now I know why, thank you for this info, its put my mind at wrest, I was told that it could take up to 10 days.

    Reply  •  June 30, 2016 at 1:26 am
  • Greg W So i have a flooded deep Cycle marine battery.. I have a 3 stage smart charger. I can’t find any info on weather i should vent when charging. Also how do i tell when i need to add distilled water… Thanks a bunch

    Reply  •  June 22, 2016 at 12:26 pm
    • BatteryStuff Tech Generally there is no need to remove the caps unless you are checking the water level in the battery. Batteries today have vents built into the caps or the case to prevent gas build-up. As far as watering your battery I would consult with the manufacturer to be sure of the proper procedure… but generally there is a plastic inserts that go to the into the cells of the battery, and most have you fill the battery till the water just barely touches the bottom of the plastic insert.

      Reply  •  January 16, 2017 at 9:43 am
  • Vishu very good information its helps for new buyer s

    Reply  •  June 7, 2016 at 9:30 pm
  • Vikram Singh Very simple , clear and communicating language. It’s really help full and informative.

    Reply  •  June 5, 2016 at 2:01 am
  • Scott Just put a 3 stage 24v charger on my mobility scooter, plan on leaving 4 months. Any thoughts?  Without knowing all the details, as long as the system is a 24v system and the charger is sized correctly, should be good.

    Reply  •  July 6, 2014 at 12:27 pm
  • Abdul Khan Thanx a alot
    Gud info
    Way 2 go frnd;)

    Reply  •  May 12, 2014 at 9:42 pm
  • Andrew Grant Great explanation, was surfing around – this was the best I found. Thanks heaps, helps me with my RV charging issues.

    Reply  •  April 12, 2014 at 4:42 pm
  • Rob Moir I‘m just wondering if you can pull power while this is in use? Can I use lights(12v) and a water pump (12v) etc in this circuit without interrupting the charger?

    Reply  •  March 21, 2014 at 6:13 pm
  • Frank I now realize a charger is not just a charger but there are choices we should concider before buying.
    very good explanation, thank you.

    Reply  •  December 27, 2013 at 3:06 am
  • Igor Exactly what I was looking for. Thank you.

    Reply  •  December 9, 2013 at 9:57 pm
    • Dan B Testing nesting

      Reply  •  February 27, 2017 at 8:43 pm
  • Kamal very impressive article with visualization thanks a lot

    Reply  •  November 12, 2013 at 9:40 pm
  • Sharat Simple and informative. Very good explanation.

    Thanks a lot :)

    Reply  •  September 22, 2013 at 3:36 pm
  • Venuu Very nice article… informative…!!! thanks!!!

    Reply  •  April 15, 2013 at 10:41 am
    • Jeremy Fear Glad you found it usefull! :)

      Reply  •  April 15, 2013 at 11:58 am
  • A L Laoye Very useful information
    Thank you.

    Reply  •  February 8, 2013 at 8:21 am
    • BatteryStuff Tech You‘re welcome. Thanks for reading our article.

      Reply  •  February 8, 2013 at 10:22 am
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