Okay, first you need to determine at what voltage you are going to run your DC side of the power house. The higher the voltage the greater the level of efficiency of your components and the smaller the cabling needed to connect your power grid together. Now one factor that plays into your battery set up is cost, what can you afford? The better the battery and greater the capacity as well as the better the efficiency the higher the cost.
When it comes to batteries, 2V is the best way to go. They have the highest capacity and can handle deeper discharges than other batteries of greater voltage. They also have a longer life expectancy if you take good care of them and don't regularly discharge them past 60%. BUT they are also the most expensive so most people opt not to get them. Most off grid systems use 6V batteries in series / parallel.
So essentially you have the following choices (best first):
- 2V
- Pros: Best capacity available of the lot and the best
longevity, estimated 10-15 years of service.
- Cons: Extremely costly. Very heavy
- Pros: Best capacity available of the lot and the best
longevity, estimated 10-15 years of service.
- 4V
- Pros: Also have very excellent capacity available.
- Cons: Extremely costly. Very heavy. Lifespan is estimated to
be about 2-4 years shorter than 2V.
- Pros: Also have very excellent capacity available.
- 6V
- Pros: Nice balance of capacity and cost.
- Cons: Life span is estimated between 6-8 years (although
with proper maintenance and not to deep of cycling they have been
known last up to 10 years).
- Pros: Nice balance of capacity and cost.
- 8V
- Pros: Not as many are required to build a system.
- Cons: Cannot be used for 12V systems.
- Pros: Not as many are required to build a system.
- 12V
- Pros: Least expensive and smallest of the batteries
available.
- Cons: Very short lifespan and limited in capacity.
- Pros: Least expensive and smallest of the batteries
available.
Pretty much all residential system will use the 6V battery in a series parallel set-up. The cost to use benefits are the primary reason for this.
Next you have the following battery manufactures to choose from. While they all have quality products you really need to read the fine print and reviews of each product before you deside who you will use for your battery source.
- Surrette
- US Battery
- Trojan
- Sun Xtender
- Full River
- Crown
- Rolls
- Sportsmans
- Lifeline
- MK Batteries
I have decided that for my base system (start small and work your way up) I will start with 8 Trojan L16HC; 6 volt 435 AH deep cycle batteries wired in parallel. This will give me the time I need to run my equipment I have specified in my worksheet with only a small buffer for poor solar days. But I have an alternative solutions for that which I will cover in another post sometime later.
These batteries are about $400.00 a piece once you factor in tax, shipping and handling. Now here I am lucky as there is a local seller so I can save on shipping and handling and the batteries will cost me around $365.00 each after taxes and fees/fines (core and so forth)
Alright, now an area I would like to touch on is understanding capacity and how it translates to something understandable. As I may have noted elsewhere, I am a mechanical engineer by training and trade so a lot of the electronic and electrical mojo is not in my vocabulary. But after working with my simple base system (which I will describe later) I now have a good understanding of what it all means and now have a way to help translate it to you, the layperson that is like me... just an average Joe that wants to not be on the grid.
So the first thing I will do is create the dictionary of terminology you will encounter. Then I will define it and lastly I will break it down so you “Get it!”. This section will also try to make it so you can understand how to convert power types from one to another
Amp Hour – This is the rating
assigned to deep cycle batteries and how it works is if a battery is
rated at 100AH then in theory is should deliver a constant 5Amps for
20 hours. This is theoretical as there are many conditions that
affect this (temperature, humidity, battery age, specific gravity...)
State of Charge – This is
essentially a level or percentage of charge on the batteries based on
specific gravity. It’s critical you are aware of the state of
charge of your batteries, this is due to the fact that when the
specific gravity of a battery falls below 1.225 or the voltage for a
12v battery falls below 12.4v or for a 6v, 6.2v then the batteries
will start sulfation.
Specific Gravity – This is the
ratio of density of a given substance compared to the density of
fresh water at 4oC (39oF). At this temperature
the density of water is at it’s greatest value and equal to 1 g/ml.
Before you can obtain specific gravity
readings on your batteries you need to do a couple of things.
*** NOTE: Don’t perform this test if
you just added distilled water to your batteries. Wait several hours
to allow the newly added water to mix with the existing electrolyte
fluid. ***
- Take the load off your batteries. (This is one reason why it’s a good idea to have two banks of batteries. One bank on load and the other on charge.)
- When you start your readings, fill the hydrometer several times. The idea is to stir up the electrolytes so as to increase the accuracy of your reads.
- Fill the hydrometer enough to float the indicator.
- In your battery log write down the specific gravity reading then repeat for the remaining cells in each battery in your array. As a note a 12V marine deep cycle batter will generally have six cells and a 6V deep cycle battery will have 3 cells. Each of these cells produces about 2 volts.
The next step is to normalize your
readings. Remember specific gravity is a temperature based process.
So for every 10 degrees ABOVE 80 degrees, add .004 to the readings
and for every 10 degrees BELOW 80 degrees subtract .004.
The last step is to compare the
readings with what the battery manufacture says the specific gravity
should be. As an example, below is the specific gravity of
Rolls-Surrette batteries:
Charged Specific Gravity 100% 1.265-1.275 75% 1.225-1.235 50% 1.190-1.200 25% 1.155-1.165 0% 1.120-1.130
Parasitic Drain – This is
pretty self explanatory, but just in case. Basically any load that is
attached to your batteries is a drain. A parasitic drain is a small
drain caused by “ghost” loads. One example would be the battery
monitor you have installed so you can see how your batteries are
performing. I have a Trimetric TM-2025RV battery monitor that draws
about .1A. That is a parasitic load. I can control it by installing a
disconnect between the monitor and the battery array, otherwise that
drain is always there.
You need to really keep an eye out for
parasitic drains, also known as ghost loads. They can cause you all
sorts of headaches if you don’t keep a check on them. Anything
electrical that is plugged in has the potential of being a ghost
load. Your DVD player, your TV, your microwave... and so on. They all
draw power even when you turn them off. And it adds up pretty fast as
to how much power they will draw.
A good practice is to have an isolation
switch between your loads and source. Case in point, I use power
strips mounted to the wall in my cabin (we are still building our
house and the electrical is already designed to have a switch for
every outlet in the house, more on that later) and have all my loads
plugged into the power strip, when I am finished with a load I simply
turn the power strip off... no more drain.
So unless you have a lot of money to
put into regeneration (solar, wind, hydro, generator) you need to be
a power “nazi” so as to save yourself some serious headaches.
Equalization Charge – Hey
anyone that said off grid living is easy and without hassle is an
idiot. There is a lot of maintenance that has to be done, and the
larger your system... the more the maintenance and that is my segue
into what equalization charging is.
Over time battery performance degrades
and this degradation is due to each battery in an array reacting
differently to being charged. As time passes the difference will
become more pronounced and when that happens it’s time to perform
an equalization charge. Now the rule of thumb is once every 10 cycles
or at least once a month or when the voltage range across the
batteries in a bank is over .30 volts.
*** WARNING!!:
Equalization charging must be performed on
VENTED (not sealed) wet lead acid batteries! ***
To perform an equalization charge the
current is limited while the voltages are higher than normal. This is
in order to bring ALL the cells in all the batteries to 100% charge.
Most lead acid battery chargers use a fixed charge voltage around
13.6 volts in normal operations. When you perform an equalizing
charge the voltage is increased to 14.4 volts or higher (if you have
a 24V or 48V system then the values are higher).
Now when this is going on some cells
may already be at 100%, which means they will start venting as the
electrolyte boils. So exercise caution when performing this
maintenance action. It’s a good idea to wear acid resistant
protective clothing and make sure the space your batteries are in is
well ventilated. Another safety precaution is to ensure there are no
heat or spark sources close to the batteries as the gas that is
vented is volatile and can combust explosively.
This method of charging is the best way
to help ensure the longevity and efficiency of your batteries. A
final word of caution, don’t over charge for very long. I don’t
really have a time, but at least 1-2 hours of over charging should be
sufficient.
Absorption Charge – Absorption
charging is where the voltage is constant and will gradually taper
off as internal resistance increases during the charge cycle.
Float Charge – Float is when
the batteries are fully charged and the voltage is reduced to a lower
level (in a 12V system that would be around 12.8 – 13.2V). This is
done to help reduce gassing and prolong battery life. You know this
as trickle charging it’s sole function is to keep batteries that
are charged from discharging.
Sulfation – There are
theoretically three types of lead sulfate. The first is soft lead
sulfate which will generally decompose with regular charging. Second
is a hard lead sulfate that will decomposes during equalization
charging. And third is a very hard lead sulfate that fails to
decompose even equalization charging.
“Lead sulfate (PbSO4) is created
at both the positive and negative electrode plates during a
discharge. In principle, during the charging period, 100% of the lead
sulfate transforms to the positive plate (lead dioxide), the negative
plate (lead) and sulfuric acid. However, in real life, when PbSO4
(lead sulfate) is left in the battery for a period of time, it
crystallizes and becomes a hard sulfate that coats the surface of the
electrode plates. This phenomenon is called sulfation. Because hard
lead sulfate is a non-conductive material, when it coats the
electrode plates, it causes a reduction in the area needed for the
electro-chemical reactions. It also reduces the batteries' active
materials needed to maintain a high capacity.”
Source: Boat Electric Co., Inc. 2520
Westlake Ave N Seattle, WA 98109
Okay I
think that’s enough on batteries for now.
