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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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4 Answers 4

up vote 9 down vote accepted

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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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? –  sharptooth Feb 6 '12 at 15:02
@sharptooth, I'll add some words to the answer –  Bob Cross Feb 6 '12 at 15:42
This answer is wrong in so many ways! Batteries discharge much faster when sitting in hot climate, "freezing" a fully charged battery will only preserve its performance once it's back at a room temperature; charging the battery after several months of disuse is likewise the wrong way to go -- it better be charged all along to avoid sulfation etc etc. –  cnst Dec 7 at 20:56
@cnst, I am unclear about what you are objecting to. Are you saying that a car that sits unused all winter shouldn't expect to have a dead battery come summertime? Or you conflating a battery charger with a battery conditioner? –  Bob Cross Dec 7 at 21:09

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 Feb 7 '12 at 16:02

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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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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