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Battery Basic PART 1

battery basic

How Battery works 
Battery has two terminals, one terminal is marked (+), or positive, while the other is marked (-), or negative. Electrons collect on the negative terminal and will flow to positive terminal when a load is connected between these terminals.  

Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power -- unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not take place. Once you connect a wire, the reaction starts.  


Battery Capacity
Batteries are rated in terms of their "ampere-hour" capacity. What that means is a battery’s capacity is expressed as the number of amperes of current that it can deliver per hour at its stated voltage. There’s an interesting phenomena here and it relates to a battery’s ability to deliver a low current versus a high current. A battery can deliver a low current over a longer time than it can deliver a higher current for the same number of total watts of power. Think of it this way:

If you were to draw 1 amp of current out of a 24V battery that could deliver that 1 amp for 100 hours, you could say that you had a 100 amp-hour battery rated at a 100 hour discharge rate.

Since the formula for determining watts is watts = current X voltage, you could say that the battery had a capacity of 1amp x 24V x 100 hours = 2400 total watts.

On the other hand, the same battery would not deliver 24V at 20 amps for 5 hours. Batteries are rated at their ability to deliver a certain amount of current over a certain amount of time. If you ask for less current than the rating, it will usually deliver more total watts of it. If you ask for more current than the rating, it will deliver fewer total watts. Battery capacity is usually stated in terms of how much power (current X voltage) it will deliver over 20 or 100 hours. It’s best to figure your calculations on the lowest rating in terms of power available over time. 

Let’s do a typical example. You are looking at a battery where the manufacturer says their battery is rated at 410 amp-hours at a 20 hour rate. (Actually, we’re talking about 4 - 6 volt batteries that add up to 24V). Ignoring for the moment the effect that deep depth of discharges has on the numbers of charge/discharge cycles, that would mean that theoretically, you could pull 20.5 amps for 20 hours out of that battery.

410 amp-hours / 20hours = 20.5 amps
20.5 amps x 24V = 492 watts per hour
20.5 amps x 24V x 20 hours = 9840 watt-hours.
or more simply,
410 amp-hours x 24V = 9840 watt-hours

The same battery is rated at 488 amp-hours over 72 hours and will deliver:
488 amp-hours / 72hours = 6.7 amps x 24V x 72 hours = 11578 watt-hours

Over 100 hours the battery is rated at 512 amp hours so you would get:
512 x 24V = 12288 watt-hours


Depth of Discharge 
In order to get the highest number of charge/discharge cycles out of batteries, you don’t want to suck out more than 40% of their capacity before you recharge them. That has an immediate effect on your calculations concerning how much power you have available per hour. If we figured that at the 20 hour rate, the battery could deliver 492 watts per hour but we only want to pull a maximum of 40% of the total capacity available, that means that we’re really talking about 492 x 0.4 = 197 watts per hour. 

There’s another problem! The inverter isn’t 100% percent efficient. With a 24V system it’s actually more like 90% efficient, so we take the 197 watts and multiply that by .9 and we get 177 watts per hour. That means that we can use 177 watts each and every hour for 20 hours running before we would want to recharge our batteries. Now you could use more, and you could use less, but we at least know what the minimum standard is. In other words, "Your mileage may vary" but you know that at a minimum you have 177 watts an hour to mess around with every hour for 20 hours. 

If you added a second bank of batteries "in parallel", you would double the capacity and would have 354 watts to use each and every hour for 20 hours. If you were to use the batteries for 6 hours a night, you could go three days before you had to recharge them although it would be better to recharge them whenever the generator was running for some other reason.


Types of Batteries 

There are several different types of batteries that can be use for storage of renewable energy such as solar and wind power. Without going off the deep onto detail, they are breakdown as follow: 

car batt

1) Ordinary car batteries

Ordinary car batteries were designed to produce a large amount of power for a short amount of time. Just enough to crank your starter and then go back into charge mode. A typical car battery, even the best grade car battery, won’t stand up to repeated charge/discharges. At best, you might get 75 cycles out of them, at worst, a lot less than that.  


2) Deep cycle "marine" batteries

These batteries are not really for serious use. They’re ok for running a trolling motor a few times a season or in the RV, but they just don’t have the ability to take repeated charge/discharge cycles like a battery that was designed for serious applications. They’re inexpensive, but their internal construction means won’t stand up in the long run and they quickly die if abused or repeatedly used to a deep level of discharge.  

In other words, while they look attractive on the surface because of their price, you can’t get as much power out of them because they can’t be discharged as deeply as a higher quality battery and you can’t get as many charge/discharge cycles out of them. In the best case, you might get 175 shallow depth of discharge cycles out of them. These batteries are to be avoided.  


3) "Golf cart" batteries 

There are golf cart batteries and there are golf cart batteries. The battery manufacturers that make most car batteries usually offer a battery for use in golf carts. While these are slightly better than ordinary car batteries or the "marine" batteries that you can get at your local tire and battery outfit, they still aren’t good enough to warrant their use in the application that we’re talking about. You want the grade of batteries that industry uses in electric fork lifts.

These batteries are not suitable for renewable energy application. 


4) Industrial deep-cycle lead-acid

Now we're getting to the type of battery that we’re interested in. These types of batteries are used for everything from commercial marine applications, fork lifts the telecommunications industry (ever wonder why the phones still work when the lights are out?), to home power systems. These batteries are designed to put out lots of power, recharge quickly, and provide many charge/discharge cycles.  

These batteries are designed with heavier components so they withstand heavy duty use. Given an estimated 40% depth of discharge, i.e., you use up 40% of its capacity before you recharge it, you should expect better than 800 charge/discharge cycles. The better quality batteries will provide around 2000 cycles. If you charged and discharged them every day, they would last for 2+ years!


5) Industrial deep-cycle maintenance free batteries

These can either be called "gel-cells" or "sealed lead-acid" types.

Gel cell is a lead-acid battery which has a thixotropic gelled electrolyte in stead of a liquid electrolyte. As result, the battery in non-spillable and can be operated in virtually any position. However, operation upside down is not recommended. Gel batteries typically have up to three times the cycle life of a wet cell antimony alloy deep cycle battery.

The gel -cell type use a gooey binder to hold the acid so they don't slosh around like the standard lead acid types but be aware that they're very sensitive to overcharging. What that means is that you must use an inverter/charger that knows what type of battery it is dealing with. The reason for this is that when these types of batteries are overcharged, the gel moves away from the plates and you can never get it to move back.

The advantage to these batteries is that there is no maintenance to be done, they won't gas unless you do something wrong (which won't happen if you use the right inverter/charger). If you don't want to have to set up a vented battery box, take hydrometer readings, fill battery cells, etc., use this type. Like the AGM's, you can install this type of batteries at any place. You also eliminate having to deal with a lot of nasty battery terminal corrosion and maintenance. These batteries also usually have a lower self-discharge rate over time when sitting idle than "flooded" batteries do.


6) Absorbed Glass Mat/VRLA 

These batteries use a type of construction where the acid is bound to a glass mat and so are like gel-cells in that they don't spill if you drop one. The advantages to these are that they have an extremely low internal resistance which means they recharge much faster, they provide an excellent number of charge/discharge cycles, they don't leak if cracked, they don't gas while charging (well they gas a tiny amount but it's lower than the point of being explosive) and are heavily constructed so that they are much less sensitive to shock.

The differences between gel cell and AGM (starved electrolyte) batteries

Both are recombinant batteries; both are sealed valve regulated. The major difference is that the "starved" or “absorbed electrolyte" battery contains an amount of liquid electrolyte added at the factory that soaks into the special separators. Therefore, it is non-spillable since all the electrolyte is trapped in the sponge-like separator material. There is no "free" electrolyte to spill if tipped or punctured. 

Because of this "acid-starved" condition, this type of battery does not normally perform well in heavy, deep discharge applications. The gel cell has more electrolytes available; therefore it is better suited to deep discharge applications and can accept occasional overcharging.



The differences between gel cell and traditional wet batteries 

Wet cells do not have special pressurized sealing vents, as they do not work on the recombination principle. They contain liquid electrolyte that can cause corrosion and spill if tipped or punctured. Therefore, they are not air transportable without special containers. They can not be shipped via UPS or Parcel Post or used near sensitive electronic equipment. Installation has to be upright to prevent spillage of the liquid electrolyte. 

Wet cells can lose capacity and become permanently damaged if:

Ø  left in a discharged state for any length of time. This is especially true of antimony and hybrid types.
Ø  continually overcharged, due to active material shedding. This includes specially designed deep cycle wet cells, but is especially true for antimony types.

The advantages and disadvantages of both types of sealed valve regulated batteries Gel Battery Advantages
• Totally maintenance free
• Air transportable
• No corrosion
• Spill proof/leak proof
• Installs upright or on its side
• Superior deep cycle life
• Very low to no gassing (unless overcharged)
• Compatible with sensitive electronic equipment
• Superior shelf life• Rugged and vibration resistant
• Very safe at sea with no chlorine gas in bulge (due to sulfuric acid and salt water mixing)
• Will not freeze to -20°F
• Lowest cost per month (cost / months of life)
• Lowest cost per cycle (cost/ life cycles) 

Gel Battery Disadvantages
• Higher initial cost
• Heavier weight
• Water can not be replaced if continually overcharged
• Automatic temperature sensing, voltage- regulated chargers must be used
• Charge voltage must be limited to extend life(13.8 to 14.1 volts maximum at 68°F) 

Absorbed Electrolyte Advantage
• Totally maintenance free
• Spill proof/ leak proof
• No corrosion
• Installs upright or on its side
• Lower initial cost than gel batteries
• Compatible with sensitive electronic equipment
• Very low to no gassing (unless overcharged)
• Excellent for starting and stationary applications
• Will accept a higher charging voltage than gel batteries 

Absorbed Electrolyte Disadvantages
• Shorter life cycle than gel in deep cycle applications
• Automatic temperature sensing, voltage regulated chargers must be used
• Water can not be replaced if continually overcharged
• Charge voltage must be limited to 14.4 to 14.6 volts maximum at 68°F