Batteries
There are three
classes of
batteries.
Deep Cycle
These
Batteries are built with fewer large heavy plates. They are not good at producing sudden High discharges of
current, which is required for Starting applications. They are better at producing the slower steady discharge required in house use for powering
lighting and inverters etc. A good design tends to have a large gap between the bottom of the plates and the base of the casing, to allow sediment to collect without shorting the plates.
The main concern for as users, is that this type of
battery can withstand deep cycling many times greater than starting batteries can. In the order of maybe 1500 deep discharge/charge cycles
Starting
These are built with many more lighter plates. This creates a larger plate surface area, giving the ability to produce high surge demands of
current for starter
motor operation. These batteries do not like deep discharging and will accept only as little as maybe 30 deep discharge cycles before they fail.
And a combination of both.
These can be found in a couple of designs. Either a mid size plate design or a twin design of both deep cycle and a smaller starting section joined on to the end. Pretty much self explanetry. It is a compromise and will suffice in small vessels where use is not high and room critical.
Combination Starting/deepcycle.
These can come in a range of forms. Either a seperated cell design, one end being a deepcycle and a smaller end being a start. OK on small
boats with perhaps a light duty
outboard where high starting current is not a major issue. That should allued to the fact that the reserve capacities of these batteries are not huge, but they do suit a market.
Or...a cell desing that fits between the two seperate Deepcycle and start designs. These are a compromise and not really suited to larger vessel systems. But once again, they do suit a particular market.
Battery Designs
Flooded Lead Acid (FLA)
These have Plates submerged in an liquid Electrolyte of Sulfuric Acid.
Advantages: They tend to be the lowest cost/Ahr, or stated a different way, “the best bang for your buck” type
battery. A little more robust at being badly treated in regards to
charging, thus less expensive
charging methods can be used. They tend to be the better at deep discharge duty and tend to have the highest number of discharge cycle rates, providing the 50% minimum cycle
rule is employed.
Disadvantages:
These require regular
inspection to assure the battery is topped off, so as the plates do not dry out. They can spill Acid and thus can only be installed upright. They can be more quickly damaged if left discharged. They are not as robust in regards to Vibration as Gell and
AGM . They have a higher discharge rate and require constant
maintenance charging to keep them in good condition.
Gel Cell
The plates of these batteries are submerged in a thick Gel type paste of Sulfuric Acid.
Advantages:
They have the distinced advantage of being able to be placed in any orientation, having no liquid that can leak out. They are also very good at withstanding high vibration environments. If charged correctly, they do not vent much in the way of Gas. This also means that some of these batteries can come totally sealed, or at the very least, have a relief vent in case of pressure build up.
They have a very low self discharge rate.
Disadvantages:
More expensive than FLA. As the Gel dylutes the Acid, the battery has to be made in a way that makes it larger and heavier. So although these batteries are nuggity, don't let that fool you into thinking it is more powerful. The electrolyte can not be replaced, although in saying that, it should never need to be. It requires special charging needs and can be damaged or even become dangerous if charged over 14.2V. Thus unregulated chargers should not be used.
Can require a special
charger to bring them up out of a very deep discharge.
AGM
These have a glass mat with the Acid absorbed into it, sandwiched between the plates.
Advantages:
These are also considered “maintenance free”, having no user replaceable electrolyte. Thus they can also be installed in a variety of angles, but not upside down. They do have the distinct advantage of being able to be used for both Starting and deep cycle duties, although once again a compromise and most likely at the detriment of their over all life span. However, They are also made in all three specialised types as Start, Deepcycle and Dual, which. Good shock resistance and very low gas release when charged properly. They have a very low self discharge rate.
Disadvantages:
These tend to be the most expensive batteries. They also tend to be the heaviest/Ahr. Once again, you can not replace Electrolyte, but also once again, you should never need too.
A few common rules for Batteries:
Never mix old batteries with new in the same bank. The older battery will pull the new one down to it’s poor operational level.
And even more importantly, never mix different types of batteries (FLA, Gel, AGM) either in the same bank. Thus also different types should not be charged by the same
charger. Unless the charger is truly special and can charge each bank to their own specific needs.
For FLA batteries, *never* charge with a current exceeding 25% of the banks total Ahr rating. eg 100Ahr = 25A.
A sufficient current to charge the batteries efficiently is also required and for FLA batteries, a current of 20% is ideal.
*Never*= with new technology in charging
equipment these days, that
rule may not necessarily apply as "never" anymore. If proper monitoring, especially in temperature is carried out, much higher charge acceptance can be had. So alwasy ensure your charger is capable of monitoring the battery and like in any application,
read the manual!
AGM/Gel, are able to safely handle a current rate of 25% and with newer technology and revised information, some of these batteries can handle much much higher. With good temperature monitoring, some manufacturers state their batteries can be charged at 100% of the rating.
Regularly check the levels of FLA batteries. Always fill with distilled
water only and fill to the level indicator only. Do not overfill.
The rule of thumb of a 50% discharge applies to all battery types. This 50% mark determines the best over all life expectancy in relation to the overall cost of the battery. As soon as a battery begins to discharge, it also begins to degrade. The further it is discharged, the less life expectancy it has.
You can never fully charge a battery with a standard automotive charger. To fully recharge a battery, it must go through two stages. First is the bulk charge. The second is the Absorption charge and then the battery can be placed into long term trickle charge. However, even on trickle charge, the batteries capacity maybe still be reducing and cycling the batteries charge once a week is good exercise for the battery.
New generation
MPPT charging devices do this differently again and do not use the two distinct charge stages.
Battery State chart:
Voltage---state of charge--specific gravity
12.75 -----100% -------------1.265
12.70 -------95% -------------1.257
12.65 -------90%--------------1.249
12.60 -------85%--------------1.241
12.55 -------80%--------------1.233
12.50 -------75%--------------1.225
12.45 ------ 70%--------------1.218
12.40 -------65%--------------1.211
12.35 -------60%--------------1.204
12.30 -------55%--------------1.197
12.25 -------50%--------------1.190
12.20 -------45%--------------1.184
12.00 -------25%--------------1.155
11.75 -------00%--------------1.120
The following is from our Resident Battery Guru Rick.
It can be used in conjunction with above rules, but I would suggest you need to have a reasonable knowledge of electrical and batteries to use these suggestions.
Flooded start-only start batteries can fail with ONE deep discharge. Because one instance of this may not result in a failure is no justification to think that it will survive another cycle or that another battery will survive one. The old addage that if you want your car to start reliably and the battery has been run down by leaving the
radio or lights on then replace the battery!
All lead-acid deep-discharge batteries can be charged at any voltage as long as the resulting current falls within the Amp-hour law (even this may be exceeded for short periods of time). The Amp-hour law places any particular charge voltage limit (other than float voltages) or and particular charge current limit into the category of myth, not fact. The idea that one may not have the
equipment to charge a battery using the Amp-hour law does not mean that one cannot still use a
monitor and adjust the charge current to fall within the Amp-hour law limits with confidence and
safety.
There are caveats that allow mixing batteries within a bank safely, i.e. one CAN mix
AGM and GEl batteries. In general, one CAN mix old batteries and new ones. With good charging systems that cyclically bring upper limit acceptance voltages for sufficient times even old batteries will demonstrate a "Best" internal resistance and state of capacity even though those values have been degraded over time. With good treatment these old batteries will not "pull-down" a new one placed in parallel. In fact, they have been shown to track both during charge as well as discharge.
Tracking means that they proportionally contribute charge current according to their derated specifications and they charge accept proportionally as well.
The caveat for mixing battery types has more to do with their full state of charge specific gravity (this is how one can mix various agm brands and gel brands by calculating their internal specific gravities using their rated full charge open circuit volgages at the same temperature) than with their different other manufactured internals.
The problems normally associated with mixing different battery types and aging are large when they have been subjected to cyclical charge cycles with low acceptance voltages over time and when they have been subjected to mechanical deterioration like shock and vibration (even had AGM troubles with this but it is more of a problem, of course, with flooded batteries). Sitting for long periods of time in a state of deep discharge will statistically offer a better chance of a shorted cell when attempting to recover the battery and, therefore, a battery with such a
history should never be placed into
service with new ones in parallel.
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