Battery chargers, for the purpose of this discussion, fall into two broad categories, Linear of which there are several sub-categories, and Switchmode.
Basic Chargers
This category in its most basic form consists of a stepdown transformer and rectifier.
It should only be used under supervision, left connected to a battery, it will either not fully charge or in most cases it will overcharge causing loss of water, either way, with detrimental consequences for
the life of the battery.
The only application for this type of charger that I can think of would be to charge your vehicle battery after inadvertently leaving your head lights on or similar. In other words a cheap charger laying around in your home garage to use occasionally.
Typically, this type is available in ratings from 2 Amp to 25 Amp. (Ones I have tested show these claimed ratings are usually optimistic).
I have noticed recently that many of these cheap chargers are rated in
milliamps (mA) instead if amps.
For instance a 5 amp charger is now a 5000 mA charger.
I assume it is done to make it sound like you are getting more for your
money. Personally, I would stay well clear of products that are
labeled in such a manner, I believe it says something about the ethics of
the manufacturer/distributor and quality of the product.
These things are usually around the $20 to $50 mark (or should I say 5000 cents).
An important thing to be aware of with these chargers is that the output
current rating is into a flat battery.
To give an example, a charger may be labeled as 5A (or more commonly these
days 5000mA) but even if this is achievable, it will be into a flat
battery. As the battery voltage rises the charge current will
fall to the extent that it will only be charging at maybe 1A once the
battery is half charged. In other words, to fully charge the
battery will take a BLT (bloody long time).
Some of these chargers will never fully charge the battery, the majority
will eventually boil them dry if left connected long term.
Characteristics – cheap to purchase, are heavy and bulky (for what they do) and hot in
operation, basically the majority are cheap and nasty battery wreckers.
Automatic Chargers
Next in this category is the same stepdown transformer and rectifier, but this time followed by a voltage / current regulator of varying degrees of sophistication.
They are considerably better than the non-regulated type above, they at least have the ability to decrease output or disconnect from the battery to prevent gross overcharging, but I have not tested one that I would leave connected to a battery for long periods.
Many of these "automatic" chargers, whilst they will partially or totally disconnect from the battery when charged, will not reconnect when the battery voltage falls, they rely on you to operate a reset button or switch them off and on again.
There are some rare reasonably good performers in this category, but they tend to be in a similar price range to the Switchmode Chargers described below.
Once again, the output of ones I have seen tended to be somewhat optimistic.
Characteristics – not quite so cheap (in fact, some are quite expensive), are heavy and baulky and hot in operation.
There is another in this category known as ferro-resonant but are less common and offer no practical advantages that I am aware of over the preceding type.
This is definitely the way to go if you want a charger to be permanently connected to your battery without abusing it as they typically have quite sophisticated charging algorithms with the ability to select charging characteristics to suit different battery types such as
flooded lead acid, lead calcium types as well as gel and AGM.
They are available with output ranging from 5 Amp to 100+ Amp (for general consumer models) and are mostly manufactured in Europe to very high
standards. I have yet to test one whose output did not conform
to that claimed for it.
They are often made by Quality Assured companies, this rarely, if ever,
applies to previous groups above.
Compared to linear chargers, they are a fraction of the size and weight for the same output and run far cooler.
The weight and size reduction is due to the elimination of the bulky 50 Hz step-down isolation transformer since this function is now performed at a higher frequency where the transformer design is quite different.
Several of these switchmode chargers have the ability to genuinely independently charge two and three banks of batteries and without having to use paralleling relays or diodes.
Also, the output voltage has very low ripple amplitude which means no audible 'hum' in your radio or stereo while your battery is charging, which is in marked contrast to many linear types.
These chargers with their sophisticated charging algorithms compared with typical linear chargers of the 'same' rating can recharge a battery far more quickly.
It is extremely important that a battery be fully charged but not overcharged.
Undercharging results in sulphation of the battery plates which is largely irreversible and causes a rapid loss of battery capacity.
Overcharging causes excessive gassing which results in a excessive loss of electrolyte and overheating, both of which result in irreversible battery plate damage. Also the gas generated - hydrogen and oxygen - is highly explosive.
Switchmode chargers provide correct charging voltage over a wide range of mains input voltage and frequency variations, typically 200-260VAC - 40-60Hz, whereas typical linear chargers require fairly precise input voltage and frequency. Although mains
frequency remains constant, the voltage that finally gets to your battery charger, particularly in remote areas or at the end of an extension lead, can be
anywhere in that 200-260V range.
The use of this type of charger compared to a typical linear type could increase the life of your batteries by a factor of 2 to 5.
It's not so much that a switchmode design is superior to a linear design, it's more because the more advanced switchmode design also incorporates much more advanced micro-processor controlled charging parameters and often these parameters can be user
adjustable to suit the battery type.
This is not new technology, it is well proven, every desktop PC has had a switchmode power supply since the XT model
nearly 30 years ago along with many other domestic appliances.
Every device with a name plate stating "Input 100-240VAC" has a switchmode
power supply (unless it is an extremely cheap and low powered device).
Battery voltage sensors to compensate for charger to battery cable voltage drop, remote battery temperature sensing, over temperature and over current shut down as well as the ability to select battery types, are features of most models.
These are really a different class of battery charger and are well worth the extra purchase price.
Be careful though, I have seen some switchmode chargers being sold at highly inflated prices.
Characteristics – initially usually more expensive, light, compact, cool operation, very reliable and very kind to your batteries.
A good switch mode battery charger should always be used in preference to a
linear (cheap) charger particularly for gel and AGM batteries, and set
to the appropriate voltage, since
there is some evidence from one particular USA battery manufacturer that high levels of ripple at frequencies below 250 Hz
where a battery is continuously cycling (charging and discharging) may
have a adverse affect on VRLA battery life. (Switch mode supplies have very
low ripple content and at frequencies around 25 Khz whereas linear supplies have comparatively
higher levels and at 100 Hz because it is much more difficult to filter out at
the lower frequency.)
Save you pennies until you can afford a decent switch mode charger with
various output options to suit a range of batteries types.
Whether you choose a linear or switchmode charger, invest in one that uses a three phase charging sequence.
Almost all switchmode types do, most linear types do not.
The three phases are as follows.
Boost phase
In this phase all the available current is applied until the battery voltage reaches a predetermined level for that battery type. Typically 14.4-15.0 V for a flooded lead acid type.
Absorption phase
The charge current will now be constantly adjusted to maintain a constant voltage for a predetermined time. This typically would be 14.0V for 1-2 hours for a flooded lead acid type.
Float phase
The charge current is now constantly adjusted to maintain a constant voltage of typically 13.2-13.8V for a flooded lead acid type.
When the battery voltage falls to a predetermined level, around 12V, due to loading that exceeds the charger output, or in some cases, also after a predetermined number of days, the sequence will start again.
Some 'smart' chargers employ a fourth phase, equalisation, however this phase is disabled if you select the gel battery option.
In the interests of battery longevity this feature is highly desirable.
Equalisation phase
This phase is used to reduce cell sulphation and stratification by periodically overcharging the battery for a short time. Typically, this phase will adjust the charge current to maintain 15.5-16.0V for an hour or so, once every 30-60 days.
Use the charging data supplied by the battery manufacturer or supplier, the above is typical only.
If your battery supplier can't give this data, I suggest you change supplier since this is basic but
very important information.
(Just one example from my battery
page, USA Deka who make gel and AGM say
of gel "Consistently charging at 0.7 volts above the recommended level
reduces life by almost 60%")
My battery charging set up.
The battery charging set up I use in my own caravan utilises a basic unregulated 240V power supply (output is 20 Volt DC 12 Amp) in conjunction with a very 'smart'
PL20 solar regulator although will the imminent change to an AGM battery
that will be changed to a switch mode supply with the same PL20 regulator .
Click here to see a brief description and block
diagram.
Also see Battery, Solar,
Batteries
& 3-Way Fridge Problems
and Generator pages.
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