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John Drake Services, Inc.
1427 E. 68th Street
Long Beach, CA 90805

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Battery Types and Sizing
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.  


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.


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
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


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).


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

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

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

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

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

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.


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

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

 Back to Top     

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

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.