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Batteries

For our purpose there are two distinct categories of 12V storage battery,
- wet or flooded cell
- gel and AGM (absorbed glass mat)

Flooded lead acid  types

These batteries are known as flooded or wet due to the nature of the electrolyte (diluted H₂SO₄) and are available in,
- Starting or cranking batteries in a serviceable and pseudo "maintenance free" styles.
- Deep cycle batteries in a serviceable and pseudo "maintenance free" styles.
- Marine batteries, means different things to different manufacturers and often a  compromised combination of both. 

Gel and AGM types

Gel are similar to flooded batteries except the electrolyte is in a gel form not a liquid.
AGM are basically the same as flooded batteries except the liquid electrolyte is held in a porous mat separator between the plates.    Also referred to as VRLA (Valve Regulated Lead Acid) batteries due to a relief valve in each cell to relieve pressure in the advent of over charging (float volt >2.4VPC).

- Also available in deep cycle, cranking types and hybrid types
- They have the advantage of being truly maintenance free and usually the ability to be used in any physical position, (inverted is not recommended) and don't need to be used in a externally vented enclosure but check the manufacturer's specific instructions.

What is the difference between a starting and deep cycle battery ?

Starting batteries are designed and constructed to deliver a short burst of high current, for example 350A for 5 seconds when starting an engine and then be immediately recharged by the vehicles alternator.
Because of this starting batteries are rated in CCA (Cold Cranking Amps) capacity.
CCA is usually defined as the current (Amperes) the battery can deliver for 30 seconds in an ambient temperature of -18degC without falling below 1.2V per cell (ie. 7.2V for a 12 V battery or 14.4V for a 24V battery).

Deep cycle batteries on the other hand are designed to deliver a lower current for a longer time and be more deeply discharged before being recharged.  They generally feature fewer but heavier plates and subtle differences in the plate material and electrolyte composition. 
These batteries are rated in AH (Amp Hour) capacity.
Ah rating is a measure of a battery's ability to deliver a current over a defined time period, usually 20 hours (referred to as C20 rating, for the type of battery we are interested in), without the cell voltage falling below 1.75V (ie. 10.5V for a 12 V battery or 21.0V for a 24V battery).
For example, a 12V 100Ah battery using a C20 rating would deliver 5A for 20 hours and not fall below 10.5V.  (It is not a linear relationship, typically, you would sustain a 100A discharge for only 1/2 hour, conversely, you could discharge at 1A for much greater than 100 hours).

It is common for many deep cycle AGM batteries to also have a reasonably high CCA rating as well making them a genuine dual purpose maintenance free battery.   This often applies to a lesser degree with gel batteries.

Flooded lead acid Vs AGM & Gel batteries.

Gel  and AGM batteries as I mentioned earlier have the advantage of being 
- maintenance free and 
- able to be used in any position (unless manufacturer advises otherwise).
- no gassing unless over charging therefore do not need to be in externally vented area (again, unless manufacturer advises otherwise).    However even Gel and AGM batteries should not be in a tightly sealed enclosure since they do have safety valves which will vent if over charging.

Flooded lead acid deep cycle batteries have the advantage of being 
- cheaper, around half the price of gel and AGM
- nothing else I can think of . . . . 

However they do require some maintenance, you must keep the water level topped up and they must be installed in a vented area.
Some water usage is a normal function of a correctly charged and equalised battery.
I would not recommend any flooded (wet) battery that does not have ready access to the cells, in other words it should have accessible screw or snap in filler plugs for each cell.

Things you should know before choosing a battery.

My choice for a "house" battery is an AGM type, gel batteries are much more demanding in their charging requirements for optimum life (just one example, 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%") and flooded types need electrolyte levels to be frequently monitored and topped up and with increasing frequency as they get older.

In my opinion AGM and gel batteries are superior to flooded types in all caravan and 4x4 and marine applications.    
The only down side is initial cost but this is more than offset by longer service life.
Flooded have the potential to last in floating use but inevitable lapses in maintenance which increases as they age usually means a premature end.


The "house" battery in your RV or boat should be a genuine deep cycle battery, not a cranking battery.
(Make sure you deal with a reputable supplier too.  I knew of one dealer a few ago who stocked a limited number of battery types but had lots of different labels to put on them.)

While deep cycle AGM batteries are initially more expensive, in this application, they will not only perform much better, but will also last much longer and will end up costing you less - if maintained and charged correctly.


Battery life, assuming all are correctly charged and of similar manufactured quality, is determined by . . .
  *  the number of charge/discharge cycles
  *  depth of discharge of each cycle.
  *  temperature
When comparing the service life of AGM vs gel, there isn't much in it, some manufacturer's literature claims gel is better, others claim AGM but it is generally accepted that they will both outlast a flooded battery given the same deep cycling conditions often by a factor of 2 to 3 (on a charge/discharge cycle basis, not floating shelf life).

In my experience, assuming same quality and  providing you follow the makers directions, particularly in regard to charging, the AGM and gel will give the best life expectancy and lastly the flooded (wet) battery in cycling applications.   Here I am talking about a gel battery that will be about 1/3 more expensive than the same quality AGM battery.  I have found the AGM battery to be less demanding and easier to live with resulting in a longer life for most of us.
But in standby float applications, I don't believe there is a huge difference between the three of them in terms of potential life expectancy.
 

Irrespective of claims by the manufacturer, in the interest of long battery life, I would never discharge any deep cycle battery more than 50% of the AH capacity and only on an occasional basis, 30% being the maximum under normal conditions.

Different batteries will tolerate deep discharge better than others but irrespective of the battery type and manufacturer, life will always be tied to number of cycles AND depth of discharge in cycling applications.
Typical example for a "good" deep cycle AGM battery manufacturer's claims, 
  *   200 cycles @ 100% depth of discharge, 480 @ 50%, 1350 @ 30% in a cycling application and 
  *   10 year life in float applications (and as they say, conditions apply).

You will note from the above figures that the battery will last almost three times longer if you discharge to only 30% of capacity compared to 50%. 

It will be more economical to purchase a larger battery and discharge more shallow than a small battery and discharge deeper.

For example, if you needed to discharge 30 Ah on a regular basis before it could be recharged, you will be much better off to have a 100 Ah battery and discharge 30% of capacity than have a 75 Ah one and have to discharge 40% or worse still, 50 Ah battery and have to discharge 60% of capacity.

Based on prices that I have recently obtained where battery cost increments roughly .75 with capacity, then comparing the number of cycles from 30% and 50% depth of discharge is a factor of almost 3 (480 vs 1350 cycles in the previous example), you can see the larger battery would be just under half the price on an AH per $$$ ratio over it's life.   (That is comparing 100 Ah to a 50Ah battery.)
The other obvious advantage of the larger battery is the ability to get a few more Ah out of if on an occasional basis.

When deciding on a battery size (AH capacity) you need to be aware that the stated capacity only applies to a new battery (after a few charges), it's all downhill after that.
For example, that new 120Ah AGM battery with a 10 year float life will be down to 50% capacity if you are lucky and have done everything right after 8-10 years.
Typically, you will have a 90Ah battery after 7 years and this is for a quality battery that has been charged exactly as per manufacturers instructions and has been kept at or below 25^oC.

Remember that when battery life is given in years, that only applies in floating or standby use.
The life of a battery in frequent charge/discharge use is given by the number of charge/discharge cycles at a specified DoD (depth of discharge) not age.
For example, if you discharge 30 Ah every day from your 100 Ah battery and it has a 1000 cycles @ 30% DoD life, then if you do everything right with regards to charging and temperature, you can expect a life of around two years (not almost three years as you may expect from a quick calculation of 1000/365).
The only way to extend battery life in cyclic use is to reduce the % DoD and that means a bigger Ah battery.

Also 100% discharge doesn't mean the battery reads 0V, it means the cell voltage as dropped to a level which will be stipulated by the manufacturer but typically 1.75V per cell or 10.5V for a 12V battery.


Pay attention to the different charging voltage requirement depending on whether to battery is used in float (standby) or cycling (being constantly charged and discharged) application.
Typically, for an AGM, float charge is 13.6 - 13.8V and for cycle use 14.4 - 14.7V. (Always use the figures printed on the side of the battery, if it's not printed and just a stuck on label, in my opinion you have bought the wrong battery.)

In my situation where we use the caravan for 2-3 months of the year and the remainder it is idle, I re-program my charger accordingly.

Pay attention also to the maximum charging current that may be stipulated. 
To ignore this could easily convert your 10 year battery into a 1 year one in float applications or greatly reduce to number of cycles in a cycling application.


Also pay attention to temperature, that is battery temperature not ambient.   This will depend on ambient temperature and rate of charging.
 

As a general rule, for every 8^oC above 25^oC battery life will be halved.


All battery charging voltages are for 25^oC battery (not ambient) temperature therefore when selecting a battery charger or solar regulator, preferably choose one with a remote temperature sensor, usually an extra option.
Some charger/regulators rely on inbuilt temperature sensing (many others have none at all) therefore, depending on the location of the battery in relation to the charger, hence difference in temperature, this can be a very worthwhile feature.  Keep in mind also that battery temperature will rise during charging.
Charging voltages are specified at 25^oC and will always specify charging voltage temperature compensation.
Typically this can be -30mV/degC for cycling application and -20mV/degC in float use for a 12V battery. This is usually given as mV/degC/cell.
 

An advantage of AGM over some gel batteries may be seen when using an inverter.  Many inverters just love to shut down too soon when encountering high start up loads due to momentary low battery voltage.   The lower internal resistance, hence greater current capability of the AGM battery can mean this is less of a problem when compared to some gel batteries.


To have a trouble free and long battery life it is absolutely imperative that a good battery charging system be used and set for that particular type of battery.   Can't stress this enough.    See my "Battery Charger" page.

A good switch mode battery charger should always be used in preference to a (cheap) linear charger particularly for gel and AGM batteries, and set to the appropriate voltage and charging sequence.
(There is a suggestion from one 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 battery life.  However no other manufacturer of batteries or battery chargers that I have come across makes this claim and personally I believe this may be just of academic interest.   Switch mode chargers have very low ripple content and at frequencies around 25 Khz whereas linear chargers have comparatively higher levels and at 100 Hz because it is much more difficult to filter out at the lower frequency.)

(If you are going to use a supermarket type charger, you may as well waste some more money and buy one of their batteries too.)


Unfortunately , the warranty offered on a deep cycle battery is often far less than for a cranking battery. Suppliers claim this is due a high failure rate experienced by some end users.   
Deep cycle batteries are understandably very popular with the 4x4 fraternity (in winch applications and for running that fridge in the back) and are receiving abuse for which they were never designed usually discharging until absolutely flat.
To make matters worse, they are almost always charged at the same voltage as the starting battery which may be quite inappropriate depending on the chemistry of the deep cycle battery.   And then there is the problem with the elevated temperature under the bonnet.    Most manufacturers advise against using in temperatures over 45^oC if you read the fine print and as I said above, for every 8^oC above 25^oC battery life will be halved.

Similarly, they are being used, correctly, in marine house battery applications, but once installed they are forgotten about for months on end, users believing that because they are a deep cycle battery they require almost no attention, and that is exactly what they get.
You may get away with that if you have an AGM or gel but not a flooded (wet) one.

As I have said, provided they are used in the correct application and are correctly maintained and charged, deep cycle batteries will significantly out perform and outlast a cranking only battery.
If you do this, a service life of seven or more years is realistically attainable.

If you are not prepared to do this then it will be cheaper to purchase supermarket cranking batteries and replace them every six to twelve months. 


Is it worth paying 150 to 250% and more for the US and German AGM & Gel batteries instead of the far more common Chinese one?  
  Consider this . . .
-  *warranty can be more than double, (from as low as 6 months to 36+ months in one case)
-  *claimed charge/discharge cycle life can be double and more
-  *claimed float life can be 2-3 times longer
   * Based on manufacturers own data (if you dig deep enough to find it, sometimes very elusive and contradictory).
However, definitely not worth the extra if you fail to charge as per the manufacturers strict instructions, 
you can ruin a $650 one as quickly and easily as a $300 one.
Batteries from different manufacturers, while they may look the same can have quite different charging requirements and limitations, particularly in terms of maximum charging current and whether it is to be used in a floating or cycling application.


Recently (Mar 08), I needed a replacement for my caravan's Trojan deep cycle flooded battery, it's just over 8 years old.

The most suitable replacement for my purpose is the Deka Seamate 8A31DTM, Vision 6FM120S, Fullriver HGL120-12 and the Remco RM12-100 on a price vs specifications basis.
The Deka is from the USA, the others are of Chinese manufacture and all are AGM types and claim ISO 9001 quality control.

All of these batteries manufacturers had an adequate amount of performance data available if you take the time to find it as opposed to the more common tell-you-nothing-useful sales brochures from the majority of others.
Unfortunately I believe opinions of sellers, even if you can find one with even a remote understanding of the product they are selling, is too biased, contradictory and self serving to be taken into account.

All are in the same $300-400 price range (Mar 08) once you have had a serious talk with the supplier and discounts from the fabled "recommended retail price" between 10% and 20% are the norm (don't be put off by their sobbing).
For example, Remco RRP $399 down to $299, Deka Seamate RRP $523 down to $385.

My final choice was the Vision 6FM120S but there wasn't much in it.    The cost, within reason, was not a deciding factor.   After all, when you amortise the cost over 5 to 10 years, there's little difference.
The thing that swayed me to the Vision Battery was the useful amount of no-nonsense data available from the Australian distributors, YHI Power Pty Ltd.   They seem to be a professional outfit, not just an importer of assorted products.   Good to see in a world of hype and glossy brochures.


Be very cautious when reading stories and advice on technical matters in on-line chat forums, 99% of the time it's the blind leading the blind.

And as I caution elsewhere on this web site, when shopping for a new battery or anything for that matter, don't just accept the sales pitch of the seller, if they can't back up their claims with documentation, they are most likely just telling you what they need to make the sale, whether it is fact or fiction, it makes no difference to most of them, a sale is all most are interested in.   Don't let them get away with "we have run out of data sheets" or "the supplier can't supply any documentation" etc, it just shows their contempt for the purchaser.    Fluent uneducated guesses and outright lies are often all too often a substitute for facts and many are very good at it.

Just recently, one large chain are selling a gel battery they promote as "made in Germany", pressed to confirm that, they reluctantly changed to "made with German technology", turns out it was just another Chinese battery.
Surprisingly they were quite unrepentant, they couldn't see anything wrong with promoting it as German made.
There might not be anything wrong with Chinese batteries but it certainly wasn't German.
I have yet to see a German or USA AGM or gel battery that wasn't boldly marked with that fact, unbranded are usually Chinese, Indian, Korean and Indonesian for flooded batteries. 


Some final words of warning, make sure that any battery you buy has the battery type & model number and charging info printed or stamped on the battery, never ever buy a battery with just a stick on label.

A short but true story - 
Several years ago I specified five N200 "marine" batteries and three N70 deep cycle batteries for a 20 metre game fishing boat whose electrical and electronics system I had designed and was overseeing.   Fortunately I happened to be on site when the delivery truck arrived, the driver asked me which batteries did I want the "deep cycle" labels on and did I want a "marine" label instead of the present "truck" label on the others.
Since I doubt there is any real difference between a battery designed for a boat and a truck, the labels on the N200s didn't matter other than for appearance but not so for the N70s, they needed to be genuine deep cycle not rebadged cranking batteries.
I had had suspicions for a while that they did this and had questioned them about it previously but they had denied the practice.   On this occasion the supplier told me that if I wanted "real" deep cycle batteries, he would have to order them in for me.
That was end of my association with that company and the last time I ever accepted a battery with only a stick on label.

When you are choosing a battery, make sure that the Ah capacity described by the vendor is at a C20 rate (a 20 hour discharge rate).   Some like to deceive by quoting an Ah capacity for the C100 rate (a 100 hour rate) but without telling you that or hiding it away in the fine print.  This makes a typical 100Ah battery look like it is a 120 Ah one.  There is nothing wrong with quoting C100 rate providing they make that clear but by default battery capacities are described at the C20 rate.
If the retailer is doing this, I'd go elsewhere and place little value in anything else they have to say. 
Sadly though, this type of deception may eventually become the norm in an effort to snare sales as it was with modified square wave inverters that gradually became modified sine wave inverters (see my Inverter page) and more recently your el-cheapo battery charger that used to be (for example) a 5 Amp one is now a 5000 mAmp one.    In the case of the inverter, this practice was originally perpetrated by those at the bottom of the food chain but in time even many reputable manufacturers and retailers found the need to follow suit, battery chargers will most likely follow.


And, if you are using flooded (wet cell) batteries, make sure your batteries are installed in a externally ventilated enclosure, and not just stuck under the bed or in a cupboard.   The risk of an explosion due to the ignition of the hydrogen and oxygen by-products of the charging process is very real, not to mention the potential damage any escaping sulfuric acid may do to the interior of your RV.  Even the acidic vapour often associated with charging will cause rapid deterioration of any steel and some fabrics in the vicinity.  
This applies to all flooded battery types.
Many caravan and motorhome manufacturers ignore this at your peril.
But as mentioned before, even Gel and AGM batteries should not be in a tightly sealed enclosure since they do have safety valves which will vent if over charging.


To be continued .......... a more in depth look at the battery types and their characteristics with typical specifications and graphs .......... soon.


Also see battery charger, solar charger and generator pages. 

 

 Ray's Caravans, Campervans & Motorhomes