Battery Float Chargers

Types Of Battery Chargers           What Is A Float Charger?           How Does A Float Charger Work?           Battery Charging Safety

How Can Battery Life Be Increased?     What Is Battery Self-Discharge?      Battery F.A.Qs      Postive vs. Negative Ground Vehicles



Fast vs. Slow, Charge and Discharging

The storage capacity of a lead-acid (automotive type) battery is not fixed, but varies according to how slowly the battery is discharged.  Similarly, the charge capacity of a battery is not fixed, but varies on how slowly a battery is charged.  With batteries, the slower the charge the better!

When a battery is charged or discharged, this initially affects only the chemicals closest to the gap between the battery plates and the electrolyte. With time, the charge stored in the chemicals spreads by diffusion* of these chemicals throughout the battery.

* Diffusion describes the spread of particles through random motion from regions of higher concentration to regions of lower concentration.

Fast Charging
(Not the best for a battery)
If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a FAST charge for only a few minutes, then during the short charging time there will only be a charge nearest the battery plates.  Charging a battery too fast can also cause a dangerous condition called Thermal Runaway** If using a high-amperage battery charger (or even charger with monitoring electronics set to a high amperage setting), the charger's meter will falsely show that the battery is fully charged.  Why?  Because after a few hours this charge will spread to the volume of the plates and electrolyte, resulting in a charge that may be so low as to be incapable of starting a vehicle.

** Thermal Runaway is a very dangerous condition that can occur if batteries are charged too fast. One of the byproducts of Gassing are Oxygen and Hydrogen. As the battery heats up, the gassing rate increases as well and it becomes increasingly likely that the Hydrogen around it will explode.

Slow Charging
(Best for the battery)
On the other hand, if the battery is given a SLOW charge using a low-amperage float charger, the charge will certainly take longer, but the battery will become more fully charged. During a slow charge, the charge current has time to distribute to the entire volume of the plates and electrolyte, while continuously being replenished by the battery float charger. The battery voltage will remain below the charger's voltage throughout the charging process allowing charge current to flow into the battery until the battery is  truly fully charged, and the float charger stops charging the battery.  Article continued below...


...continued from above.
Fast Discharge
A similar occurrence happens when a battery is quickly discharged.  We have all experienced the situation of continuously cranking the engine,  trying to start our car.  Eventually, the battery dies - so it appears.  But when we come back an hour later, the battery has sufficient charge to crank the engine a few more times.

Why does this happen? If a battery is subject to a fast discharge (such as starting a car with a current draw of 100 amps or more) for a few minutes, the battery will appear to go dead. However, it may have only lost its charge nearest the plates. If the discharge is stopped for awhile, the battery may resume normal operation at the appropriate voltage and power.

Slow Discharge
On the other hand, if a battery is subject to a slow, deep discharge (such as leaving the car lights on, a current draw of 7 amps or less) for hours, then any observed reduction in battery performance will likely be permanent.

Float chargers can be used on both types of batteries:

Charging Batteries
A charging battery is used in automobiles.  These lead acid batteries are designed for starting automotive engines but not designed for deep discharge. They have a large number of thin plates designed for maximum surface area, and therefore maximum current output, but which can easily be damaged by deep discharge.

Repeated deep discharges will result in capacity loss and ultimately in premature failure, as the electrodes disintegrate due to mechanical stresses that arise from cycling. Starting batteries kept on continuous charge will have corrosion in the electrodes which will result in premature failure. Starting batteries should be kept on a charger that cycles on & off, with minimal amperage to prevent sulfation.

Starting batteries are lighter weight than deep cycle batteries of the same battery dimensions, because the cell plates do not extend all the way to the bottom of the battery case. This allows loose disintegrated lead to fall off the plates and collect under the cells, to prolong the service life of the battery. If this loose debris rises high enough it can touch the plates and lead to failure of a cell, resulting in loss of battery voltage and capacity.

Deep Cycle Batteries
Deep cycle batteries are used to power trolling motors, wheel chairs, forklifts, golf carts, marine and motorhome applications.   Specially designed deep-cycle cells are much less susceptible to degradation due to cycling, and are required for applications where the batteries are regularly discharged. These batteries have thicker plates that can deliver less peak current, but can withstand frequent discharging.  Battery float chargers also work ideally on deep cycle batteries.

Battery Butler
Battery Storage Chargers

How can I tell if a "new" battery is fresh?
Look for the small decal on the side of the battery housing.  This sticker will tell you the month and year that the battery was shipped from the manufacturer.  The more current the date, the fresher the battery.

The letter corresponds with the month, starting with "A" for January, "B" for February, etc. The number represents the year with "9" standing for 1999, "0" for 2000, and so on. A9, would be January, 1999. C0 would be March, 2000.( The letter "I" is skipped so the letter "M" would be December.)


A float charger (also called a storage charger, maintenance charger, or smart charger) will charge a battery at a similar rate as a battery self-discharges, thus maintaining a full capacity battery. However, the main difference between a trickle charger and a float charger is that a float charger has circuitry to prevent battery overcharging.

How does a Float Charger work?

Below are two examples of the internal printed circuit board of a battery float charger.

A. These wires carry the 6 or 12 volt DC output current that goes to the battery.

B. This is the printed circuit board that contains the "brains" of the float charger.  This circuit board will do many things.  One, it will reduce the input voltage of 110 volts AC (alternating current) to 6 or 12 volts DC (direct current)*.  Two, it will continuously monitor the battery's charge status; fully-charged or undercharged.  Third, if a battery becomes undercharged, it will automatically turn the charging cycle on.

* The output current to charge a 12 volt battery is actually around 13.8 volts DC. But for simplicity, we'll state 12 volts.

C. These wires carry the 110 volt AC (house current) into the printed circuit board.  The voltage will later be reduced by the circuitry.

D. These wires go to the selector switch to allow the user to switch between 6 or 12 volt battery charge.

E. These LED indicators let the user know the battery's charge or charging status.  Some float chargers use 1 indicator to show that there is power going into the PC board.  Other chargers us multiple or multi-color indicators to indicate power to the PC board as well as the on / off state of charger going to the battery.

Here is another example of the internal printed circuit board of a battery float charger.

A float charger senses when a battery voltage is at the appropriate float level and temporarily ceases charging; it maintains the charge current at zero or a very minimal level until it senses that the battery output voltage has fallen, and then resumes charging. It is important to note that the appropriate float voltage varies significantly with the construction of the battery and the ambient temperature. With the appropriate voltage for the battery type and with proper temperature compensation, a float charger may be kept connected indefinitely without damaging the battery.

NOTICE!  As with the use of any type of battery charger, it is best to periodically check the electrolyte (fluid level) of your battery.  On most batteries, you can remove the vent caps to add distilled water.  However, on some maintenance-free batteries, you can't.  If the water level should drop below the top of the cells, add sufficient distilled water to cover the cells.  Do not use tap water as it contains minerals and deposits that build up on the battery plates. 

Most maintenance-free batteries are sealed.  Water cannot be added to a maintenance-free battery - nor should you have to as the electrolyte usually does not evaporate when using a low amperage battery float charger.

Battery manufacturers define the end-of-life of a battery when it can no longer hold a proper charge (for example, a cell has shorted) or when the available battery capacity is 80% or less than what the battery was rated for. The life of lead acid (automotive-type) batteries is usually limited by several factors.  Some you can't control. Some you can.
Cycle Life is a measure of how many charge and discharge cycles a battery can take before its lead-plate grids/plates are expected to collapse and short out.  There's not much you can do about the life cycle of a battery.  When you need to use your battery, you should use it.  That's why you bought it.

Age also affects batteries as the chemistry inside them attacks the lead plates. As with battery life cycle, there's nothing you can do about the age of a battery - it is what it is.  But lead-acid batteries like to be kept fully charged, all the time.  If you don't plan to use your battery for weeks, months or years, try to keep it a cool/cold place. Since lead-acid batteries will not freeze if kept fully charged, store your battery on a float charger, in a cool/cold place to maximize its life.  A  battery that has frozen is usually void of its warranty!

Construction.  The thicker the lead plates, the more abuse, charge and discharge cycles they can take. The heavier the battery for a given group size, the thicker the plates, the longer it will last.  So you can use weight as one guide to buying lead-acid batteries. The heavier, the better.

Sulphation is a constant threat to batteries that are not fully re-charged. A layer of lead sulphate can form in these cells and inhibit the electro-chemical reaction that allows you to

charge/discharge batteries. Many batteries can be saved from the recycling heap if they are kept fully charged.  A battery float charger will keep your battery fully charged, without over charging.  Also, make sure you periodically check the fluid level of your battery (if possible). The fluid level should always cover the internal plates.

Use the proper size battery for your vehicle.  An undersized (or under-amperage) battery will prematurely fail because of the constant excessive load placed on it.  Consult your vehicle's owners manual for proper battery sizing.

Don't deep-discharge your battery.  This will occur if you leave your headlights on for an extended period of time which will quickly drain your battery, or if you leave your battery uncharged for an extended period of time (5 months or more).

Excessive vibration due to a loose hold-down clamp.  Lead acid batteries don't like to be bounced around as the internal components can become loose.  However, if you have one of the new OPTIMA BATTERIES, they are impervious to vibration.


Battery self-discharge is when a battery's charge slowly decreases over time, with no load on the battery.  "No load" means that there is no power being consumed from the battery as would a vehicle's clock, radio display, computer, newer alarm systems, and the like.

The battery self-discharge rate is a measure of how much batteries discharge on their own. The self-discharge rate is governed by the construction of the battery and the metallurgy of the lead used inside.

For instance, flooded cell batteries as used for automotive applications, typically use lead alloyed with Antimony* to increase their mechanical strength. However, the Antimony also increases the self-discharge rate to 8-40% per month. This is why flooded lead-acid batteries should be used often, or if this is not possible, be left on a float-charger.

* Antimony is a metallic element having four allotropic forms, the most common of which is a hard, extremely brittle, lustrous, silver-white, crystalline material. It is used in a wide variety of alloys, especially with lead in battery plates.


Did you know....
Charge current from a battery charger, needs to match the ability of the battery to absorb the energy. Using too large of a charge current on a battery can lead to boiling and venting of the electrolyte*.
In the above graphic, the battery case has bulged due to the high gas pressure developed during overcharge.

Use of a low amperage float charger would have avoided this problem.

* Electrolyte is the water and sulfuric acid mixture in the battery.



Did you know....

There is a safe way to connect a float charger (or any battery charger) to a vehicle's battery.

First of all, what do we mean by "safe way"?  What we want is to avoid a spark near the battery which could ignite hydrogen gas emitted by the battery, and possibly cause the battery to explode. A spark could be generated when the charger's clamps are connected to the battery.

Typically, float chargers of less than 1 amp would not cause a spark, but chargers of 2 or more amps could - and it pays to be safe.

First, connect the RED clamp from the float charger to the positive terminal of your battery.  Second, connect the BLACK clamp from your float charger to any negative grounded location in your engine compartment (except the negative terminal of your battery cable)*.  In the above photo, the negative clamp from the float charger is connected to a nearby nut & bolt (click to enlarge).

Lastly, plug the float charger into the wall outlet.

In this case, if there were a spark from the float charger, there is a good chance that the spark would not be at the battery's terminal.

* These steps would be for negative grounded vehicles as were most vehicle built after the 1950s.


Do you know....
Do you know the voltage of your battery?  How can you determine if your battery is 6 volt, 8 volt or 12 volt?

Simply count the number of cells. You can do this by counting the number of round individual cell caps, or removing the 'strip' type cell vent covers and counting the access holes for each cell. Each cell accounts for 2 volts (actually 2.2 volts).

3 cells =   6 volt battery
4 cells =   8 volt battery
6 cells = 12 volt battery
and so on.

Do NOT use an 8 volt charger on a 6 volt battery. Bad things will happen. Do not use a 6 volt charger on an 8 volt battery, it will actually discharge it.

 (blue color hex: 006699)
Explain Battery Gasses
When electrical current flows through water during the charging cycle, it breaks the water down into its original components, a mixture of Hydrogen & Oxygen. These two gasses are extremely flammable and can cause an explosion if the battery is not properly vented to the outside of the RV. This normal conversion of water into hydrogen and oxygen is part of the battery recharge cycle and is another reason you should check the battery water level at least once a month.

What does CCA mean?
Cold Cranking Amps is a rating used in the battery industry to define a battery's ability to start an engine in cold temperatures. The rating is the number of amps a new, fully charged battery can deliver at 0 Farenheit for 30 seconds, while maintaining a voltage of at least 7.2 volts, for a 12 volt battery. The higher the CCA rating, the greater the starting power of the battery.

What is a CA or MCA rating?
This is a rating used to describe the discharge load in amperes which a new, fully charged battery at 32 degrees F (0C), can continuously deliver for 30 seconds and maintain a terminal voltage equal or greater than 1.2 volts per cell. It is sometimes referred to as Marine Cranking Amps or Cranking Amps.




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