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What's the need for car battery chargers that I often see on display in auto parts shops?

I mean a battery is sold charged (and if it discharges too much it just dies and can't be restored) and every car has an alternator for charging the battery and if the battery is empty and can't start the engine then jump-starting is not a problem. So any usable battery can be installed into the car and fully charged there.

What are the scenarios for using those chargers? Why and when would one use such charger?

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Why and when would one use such charger?

To my mind, the major needs fall into two major categories:

  1. You have a vehicle that does not get driven enough to keep the battery up to charge against its passive drain. Common examples would include the summer-only car or the utility vehicle that you only use for heavy hauling. In either of those cases, I would tend to hook up the battery charger at least an hour before I really needed to drive the vehicle, just to avoid the potential frustration and delay.

  2. You have ever had a battery fail on you (or worse, your spouse). Traditionally, this is when most of us tend to buy a charger: immediately after a battery failure in a high stress environment. It's important to remember that a sudden dip in temperature (e.g., below freezing) can suddenly illustrate that a battery you thought was fine is actually not able to cope.

I have a relatively cheap charger (item 2 above applies to me). It was a relatively cheap purchase, especially when you consider the stress avoidance factor.

It is not a more expensive battery conditioner that a higher income person than me would leave connected to their summer car's battery through the winter. It is perfectly suitable for bringing my parent's old truck out of their garage after a few months of disuse, however.

EDIT: responding to sharptooth's questions in the comment:

I'm particularly interested in the 2nd scenario. Suppose the car won't start and I jump-start it and then buy a charger. How do I use the charger then? Do I charge the battery once and forget or what do I do with it?

You're going to have to make a judgement call. Is this a fairly new battery that died after a month of sitting out in the cold? Or is this an older battery that should have worked but for some reason did not have enough juice to get you going? Or somewhere in between?

In general, the time when I'm most likely to use a charger is when I am in something more like the second situation. I charge the older battery up to full, get the vehicle started and drive straight to the battery vendor of choice.

The times when I am willing to charge up a battery and then run it without concern are focused on the "disused vehicle" situation. In the case of the specific vehicle in question, I bought that battery and I know it's fine. The truck just never gets run unless there's some heavy hauling that needs to be done. In those cases, my procedure is generally:

  1. Hook it up to the charger
  2. Eat a sandwich
  3. Drive away
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  • You can also use a battery charger to periodically recharge a battery in a vehicle that's in storage. I would suggest that this is better than letting it drain to the point that it needs to be charged up before it can be used, particularly if we don't know how long it
    – Mathieu K.
    Commented Feb 8, 2017 at 7:23
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E.g., driving only short distances may make it difficult for the alternator to keep up (low running-engine-time to number-of-starts ratio.) Connecting a battery charger when the car is parked can keep the battery fully charged and eliminate the need for occasional jump-starts.

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  • This is especially true through cold winters
    – Rory Alsop
    Commented Feb 7, 2012 at 16:02
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If a battery is allowed to sit and partially discharge, it's wise to charge it with a battery charger rather than putting all that load on the alternator of the car. Using the car's alternator to "charge" rather than just topping off the battery leads to premature failure. With the ridiculous price that quality alternators cost these days, you do NOT want to have to replace it any more often than you have to!

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  • First, I respect your wisdom. Second, if I may make so bold, what sorts of loads are we talking about, here? What's the current draw of a charged battery vs. a discharged one? How many amps are alternators rated for, and how much of that rated current is already being drawn by a car under full load (A/C, headlights, etc.)?
    – Mathieu K.
    Commented Feb 8, 2017 at 7:02
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Many new and inexpensive chargers come with a conditioning option. This will help reverse sulfation and bring the battery to a like new condition. I have a fairly new battery that I would charge but it would still fail after a couple starts. I tried the recondition button and good as new.

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Electricity is always cheaper than petrol.

Battery charges may be useful for seasonal equipment -- if you store certain equipment with lead-acid batteries during the winter without resorting to the native charging system of the equipment (lawn mower, for example), then doing trickle charging during the winter will help maintain the full charge capacity of the battery, and prevent sulfation and premature failure.

Likewise, if your battery has been fully discharged 100%, it'll take several hours of driving in order to fully charge it back up. Consider that a normal car battery is rated, say, 61 A·h, then even if your alternator could poke it with 50A for a whole hour, it'll still be left uncharged; in reality, 20A charges are often called "rapid", 10A is "fast", and the normal charges are more likely to be just 6A to 8A, which means that a fully discharged battery of a compact car will take something like at least 8 hours to reach the full charge!

As such, if you leave a 100% drained battery to a jump-starting alone, and drive less than 4 hours per week, and have it charged during the normal use of the car in a small city with little driving over the course of several weeks or months, the battery might sulfate prematurely due to having a partial charge for such several months.

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In my case the battery charger is kinda like snow tires and synthetic oil; I like the extra assurance, but I can get by without it. Also, my wife owns a robot-fly-by-wire-electric-everything van that is particularly hard on the battery, and after replacing a third battery in 8 years I thought this might help with reliability. Given that the charger I bought has a de-sulfation feature I expect the battery to last a bit longer. Worth the money? Well, I do value reliability a lot, so for me, yes.

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The firmware on the navigation system of my Honda takes over an hour to update. It's much "greener" to clip a battery charger onto the battery than it is to idle the engine for that long. I'm not sure I'll actually need it to charge the battery, though. Oh...I also use it to keep my extra gel-cell batteries charged, too.

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Sometimes not everyone can just buy parts... So if I can just charge my battery up to start my car then it's cheaper than getting the battery. If the alternator fails before I can "find" a battery then I deal with that when it comes. Also maybe more actual use is the do-it-yourself type because we learned that if I'm prepared I'm ready. Never know who may ask for help.

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