Battery Types and Sizing

electricity from the sun by John Drake II | home

First off, when working on, around or near batteries -

Always wear eye protection.

Battery Types (electrolyte containment):

Today there are a number of different battery chemistries available.

As my only experience is with the lead-acid type, I will stick with that.

The other types, with the exception of Nickel Cadmium and Lead Calcium, have very

little history to expound on.

There are basically three types of lead acid storage batteries.

Each has its pros and cons.

Flooded Batteries

This is the oldest type, the kind you add distilled water to periodically, where the electrolyte

(water and acid solution) is in a free liquid form in each cell.

The lead plates are submerged in the solution.

These have a long history of use in automotive applications.

Pros:

The lowest cost per amp hour of storage capacity.

Can tolerate high charge rates better than the AGM or Gel batteries.

Lost electrolyte can be replaced.

A useable life span of two to twelve years, depending on the type and how they are taken

care of.

Cons:

As the battery is charged, hydrogen and oxygen gases are released from the water.

This can lead to an explosive environment.

Acid mists, which are corrosive, can be released through the vent holes in the caps.

The tops need to be kept clean.

The batteries must be kept in an upright position to prevent electrolyte loss.

Distilled water must be added periodically to keep the electrolyte at the manufacturer's recommended level.

Sealed Batteries or referred to as VRLA ( Valve Regulated Lead Acid )

AGM (absorbed glass matt) suspends the electrolyte in a fiberglass mat between the

plates.

As the batteries are being charged, the hydrogen and oxygen released from the electrolyte

collects under the top (deck) of the case, then re-combines into water which falls back into the

electrolyte.

Pros

These batteries do not need to have water added periodically.

They can be used indoors as long as all of the connections are insulated.

They can, in most cases, be laid on their side during use.

Many wheel chairs use two 12 volt AGM's in series (for 24 volt) due to the

above benefits and have a fairly long life.

Life spans of four to ten years when properly cared for.

These shine in applications where a flooded battery can not be maintained - due

to location in a vehicle or on a mountain top (not kidding).

Cons

A very high cost per amp hour.

If the batteries are subjected to too high of a charging rate, the one-way vents can

discharge electrolyte. These vents are designed to release pressure in the battery

to prevent bursting of the battery case.

Once electrolyte is lost, it is lost for good.

An AGM battery that is charged with too high of a voltage can experience a thermal runaway -

this is where heat is generated in the top of the battery and can continue even after the

charging source is disconnected. An explosion can result.

Most, if not all, AGM batteries have the charging parameters stamped or inked in the top

of the case (everyone I have seen do). Stay within the limits set by the manufacturer for

safety.

GEL Cell (the electrolyte is captured in a silica gel)

Pretty much the same as the AGM's in operation, just a little less tolerant when being overcharged.

A note on gel batteries, we have used a number of them and one of our 98 amp hour 12 volt

batteries just gave up the ghost - after over twelve years of use.

These, in most cases, will cost more than an AGM battery of the same storage capacity.

Battery Types (dis-charge depth):

Most sealed lead acid batteries are deep cycle batteries.

This means that they are intended to be drawn down to a low

state of charge and recharged as soon as possible.

When it comes to flooded batteries there are basically three

types.

The first is the automotive, or SLA (starter, lights & accessories)

battery.

These have lots of thin lead plates so there is a large surface area

of lead in contact with the electrolyte.

This allows them to deliver a high amperage output to power up the

starter.

These batteries are designed for a quick shallow discharge and immediate

recharging once the starter is dis-engaged.

As most of us have experienced, when you knock you car battery

flat a few times it is ready for the bone yard.

This type of battery really has no place in an alternative energy system.

The next type is the marine "deep-cycle" battery.

It is actually a cross between an automotive battery and a true deep

cycle battery.

These have thicker plates than an automotive battery and are usually

found with a rated storage capacity of eighty to one-hundred amp hours.

For battery storage capacity information please click onto the following

link:

Marine batteries, due to their size as well as most having carrying handles

(to lug between the boat and garage) can be used is some portable

alternative energy set ups.

If the battery bank is of any size, I would not recommend this type of

battery.

The third type is the true deep cycle battery.

It has less plates than the two previous types but they are thicker and in

most cases, more porous.

These can not deliver a high amp output to start an engine but they can

handle deep dis-charging better than an automotive battery.

The best choice of the three for alternative energy and emergency back-up

applications.

Battery Sizing:

This is where it gets a little more interesting.

There is an old saying that goes "You can not be too rich,

too thin or have too many batteries."

You want to size your battery bank to accomplish these goals:

Maximun battery life - batteries are expensive and heavy.

and

To have the least amount of power interruption to your loads when weather

reduces the power available to your batteries, this is called autonomous

operation where you are drawing power out but no power is being put back

into the batteries.

First off, let's discuss battery life.

When a lead acid battery is kept fully charged and the electrolyte level (on flooded batteries) is

kept where it belongs, you will get the most life out of a battery.

In real life this condition never happens - we use our batteries.

The lower you draw down your batteries and keep them in a low state of

charge, the shorter the life span will be.

This condition also reduces the battery capacity (the amount of power

available for use).

We try to keep our batteries at least at an 80% state of charge, this means

that 20% of the available power has been taken off the top.

All of our batteries are true deep cycle batteries - but we do this to get the most charge/dis-charge

cycles and life out of them.

The larger the battery bank, the less it will get worked with a given set of load.

Lets say you have a 100 amp hour battery.

Your daily loads add up to 20 amp hours (or approx. 250 watt hours).

Not counting recharging, the battery will be drawn down to 80 amp hours of storage - bettery

yet let's call it 80% State Of Charge.

Those same load running off of a 225 amp hour battery would draw it down to about 91%

State Of Charge.

The higher you can keep the State Of Charge of your batteries, the longer they will last.

Choosing a Battery:

The choice of battery type is dependent on a few variables.

A flooded battery is a good choice when its location is readily

accessible to add water and clean the tops off. If they are going

to be located indoors they have to be sealed off from the inside

of the room or building and have external ventilation.

An AGM or Gel Cell battery can be used indoors where ventilation

from the outside is not possible. These are also good for installations

where battery servicing is either not possible or very difficult.

Remember, if it is tough to do, we probably won't do it.

How batteries work.

This will be a very simple treatise on lead acid battery operation by a simple

minded battery user.

Basically a battery is a housing with partitians to seperate each cell.

In each cell are lead plates are submerged in an electrolyte solution consisting

of distilled water and sulfuric acid.

Connectors between the cells (each cell is about 2.0 volts) connects them

in series to increase the voltage of the battery. A 12 volt lead acid battery will have

six cells).

Each cell will have a vented cap. The vents allow hydrogen and oxygen released

from the water in the electrolyte to vent to the outside. The caps allow you

to replace the water lost during charging.

Batteries only out-gas and release mists during charging.

As the battery is dis-charged (power drawn out), the acid migrates from the

electrolyte into the pores of the lead plates.

The more acid in the solution, the denser (heavier) it is. This is why when you

test the specific gravity of the electrolyte in a fully charged battery, it is higher

(denser) than a battery in a low state of charge.

When the battery is being re-charged the electrolyte moves from the lead plates

back into the electrolyte solution.

If a battery is kept in a less than fully charge condition (low state of charge), the

acid in the lead plates breaks down. This allows sulfates to form and plug the

pores in the plates. The less open pores in the plates, the less capacity the battery has.

This is why a new or well maintained battery can store more useable

energy than an old or abused battery can.

Through the years I have heard many experts say that a lead acid battery should

be drawn down to a low state of charge and the recharged (over-charge) to

extend the life of it.

I may not be the brightest bulb on the tree, but this makes no sense.

Please see the next article.

Battery Equalization Charging.

As batteries go through charge/discharge cycles a couple of things can

happen.

The individual cells can go out of balance - they wind up with different

voltages which can cause over heating during charging cycles.

The electrolyte can become stratified. This means that the acid starts

to settle in the liquid so the bottom of the cell has a higher concentration

of acid compared to the top.

First off, Never give a sealed battery an equalization charge unless the

manufacturer of the battery recommends it. A sealed battery can over-heat

and over-pressure when subjected to the high voltages common with

equalization charging.

Make sure that the electrolyte level is over the the top of the plates and at

or below the high mark in each cell.

The high voltage equalization charge will help balance each cell in the

battery.

The gas bubbles formed in the electrolyte help force the acid from the

bottom of the cell up into the solution. This helps keep the concentration

of acid even from the top to the bottom of the cell.

This is the most important time to be concerned with oxygen and explosive

hydrogen gas that is released through the vents in the battery caps.

Once the equalization charge is finished, and the batteries have cooled down,

it would be a good time to check the electrolyte level in each cell.

Battery Maintenance:

First off, set up your battery bank so that it is easy to access the tops of

the batteries.

If the batteries are not easy to get to, it is unlikely that they will be maintained

properly.

If the batteries are not taken care of, they will need to be replaced sooner than

necessary.

Money aside, the Trojan T-105 batteries we use here weigh about sixty-five

pounds each. They are tough to slug around but if they were difficult to get

to, they would really be hernia makers.

I have come to these conclusions from personal experience.

The most obvious thing is to keep the electrolyte levels on flooded batteries

where they belong.

The frequency that you water your batteries will depend on the ambient temperature and

how agressively they are charged.

Always use distilled water in flooded batteries.

Another important part of maintenance is to keep the battery tops clean.

Flooded batteries release acid mists through the vent caps and some

sealed batteries can release liquid around the terminal posts when

recharged using a high charging rate.

Battery Installation:

First off, remember that batteries release oxygen and hydrogen when they

are being charged - this is an explosive combination.

The vent caps can also release acid mists when the batteries are being

charged at a high rate - the mist can corrode exposed metal connections

on the battery tops and control equipment nearby.

Most manufacturers rate battery life on charge/dis-charge cycles and the

operating temperature of the battery.

A common base line for battery temperature is 77 degrees F.

Above this battery life is reduced, the electrolyte and lead plates breakdown.

Below this battery life is extended but the ability to charge and dis-charge is

slowed down.

A quick note: the higher the state of charge, the lower the freezing point of the

electrolyte will be. Never charge a battery that is frozen.

As batteries are being charged they generate heat.

The batteries in a battery bank should have at least 1" spacing between them,

more if possible. This helps the battery dissipate heat.

Battery location:

The site that the batteries will be installed in will dictate their location.

If you are in an area that has freezing weather, you would want to keep them

shielded from low temperatures.

In hot areas you would want to locate the batteries where they will be out

of the sun and able to dissipate heat.

If you are going to locate flooded batteries indoors you should place them

in a sealed container with two vermin screened vents to the outside.

A low placed vent on one end of the enclosure will be for incoming air and a high

placed vent on the other end will be for the exhaust air.

A battery box ventilating fan, especially for large numbers of batteries, is

highly recommended.

You would want one that only comes on when the batteries are being charged.

In freezing weather it is recommended that the batteries be placed on one to

two inch rigid foam. This will reduce the chances of the battery freezing as well

as cut down on electrolyte stratification.

In an outdoor installation you would want plenty of room around the batteries as

well as raintight vents.

Whether your battery bank is indoors or outdoors, it is a good idea to keep it

locked up.

Children and unwary adults have no business having access to a battery

bank.

Battery storage and capacity:

First off we will discuss automotive (SLA) batteries and get them out of the way.

Automotive batteries are usually rated in Cold Cranking Amps.

This means that a specific battery (when fully charged) at a specific temperature

will deliver so many amps (cranking amps to crank, or run the starter).

These numbers hold no meaning for those of us setting up a storage battery bank for an alternative energy system.

Now that we are past that, lets go over amp hour ratings of deep cycle storage batteries.

Deep cycle batteries have a storage capacity rated in amp hours at a specific amp draw rate.

The following is an example of the Trojan T-105RE (renewable energy) Premium

Line deep cycle battery:

The 5 hour rate is 185, the 20 hour rate is 225 and the 100 hour rate is 250 amps.

This means that if your amp draw is five amps, you can pull 185 amps out of the

battery and so on.

The slower you draw power out of a battery, the more you will get out of it.

They also rate this battery at 1,600 cycles (charge/discharge) when brought

down to a 50% DOD (depth of discharge).

Please note: that if you regularly draw your batteries down to a low SOC

(state of charge), they will lose their storage capacity.

This is due to sulfation (sulfate crystals forming on the surface of the lead

plates) and breakdown (sloughing) of the plate material.

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