POWER STORAGE

POWER STORAGE

Deciding on battery type:

What are you going to use it for?

The main point of energy storage is to smooth out the fluctuations in energy availability creating a constant source which can be calculated and depended on.

The rechargeable battery in an application with an intermittent power source is closer to how you would use a capacitor.  Intermittent power calls for the immediate storage of as much of the available energy at that moment and then storing it for long term use. This produces a functioning, which is more like a reservoir (to be filled and emptied at random) than a battery.

This is not how classical batteries are used, most applications call for a full charge to be applied to the battery maximizing the time the battery can spend away from the charger. The battery will then be taken and used in a device till complete drainage, at which point you can recharge the battery again.

Super capacitors do this the best but have down sides below is a comparison of rechargeable battery types and capacitors in terms of intermediate power supplies.

Before you can choose a battery you have to understand how they are described. Below is a description of the key points that I use to choose a storage source. If you want more info try these links first:

BATTERY TUTORIALS:

Lead acid charging
http://www.chargingchargers.com/tutorials/charging.html

all the rest
http://www.powerstream.com/BatteryFAQ.html
http://www.batterystuff.com/tutorial_battery.html

BATTERY TERMS:

for describing batteries:

The power available in batteries is described in AMP/HOURS (AH or mah (milliamp Hours) ), which is a description of energy like joules, it is power (watts) used over time. A 1500mah marking tells you the battery can source (give) 1500ma constantly for one hour.

There are many different ways to charge a battery but they all deal with the voltage of the battery in comparison to its charge rate or "C" rate . The "C" rate is the amount of stored amperage that can be sourced (produced) by the battery in 1 hour:

-So a battery with a "C" rating of 1500ma if discharged suddenly at a rate of 10 C would give 15 amps (if the battery could source that much current). This rating is used a lot in the charging specifications of batteries and defines the charge method used. Here is a rough overview of the charge methods in relation to C:

+ C/.02-.05 trickle charge can be kept on the battery continuously without harm.

+  < C/10 slow charge or overnight charges are usually at or below C/10 for NICAD and NIMH. Takes a long time but does not need to be monitored, can just be timed.

+ C/3-C/5 faster charging for NICAD and NIMH. (Can be used with a timed charger.)

+ 1C Fastest possible charge for NICAD and NIMH must be monitored closely. (needs more advanced charge completion techniques)

**+Lith-Ion get about (0.2C-0.7C).

**LEAD ACID just need 2.15 volts per cell don't have to worry about C.

>>The Coulometric charging efficiency is also mentioned a lot and describes the amount of energy you have to put in to get X amount out. For example:
 50 amps Hours out of a battery with a 50% coulometric charging efficiency would mean you would have to put in 100 amp hours of charging into it. It basically describes the rate of charge a battery can take and store.

The four main battery types and uses:

NICAD:

Positives:

 -In general better at giving large amounts of amperage fast. Good for remote motor applications like servos or electric drills.
*-More charge discharge cycles than nimh (* depends on use* has memory effect that reduces the number of real cycles)

- Good voltage slope at full charge easy to detect to stop charging.
-Accepts trickle charge well.
-Retains charge better than NIMH.

Negatives:

-Has a memory effect if not fully discharged before recharging. (Bad for intermitted power source charging like solar).
-Does not store as much charge per weight as NIMH or LITH-ION (25%-40% less).
-Very toxic concentrates a lot of cadmium in one place which the environment and your body is not use to (DO NOT EAT).

NIMH:

In my opinion the best for most solar applications.
Used a lot for cameras and other high long-term power electronics.

Positives:

-Higher energy storage rate compared to NICAD. (25%-40% More)
-Has no memory effects can be trickle charged or fast charged (good for solar and intermittent power sources).
-Is robust temperature wise (-20c to 60c).
-Is tolerant of slight overcharge.
-Eco friendly (no cadmium and no feeling bad when you throw them into the field you were just collecting solar from).

Negatives:

-Faster self discharge rate- it doesn't hold on to a charge as well as the other chemistries.
-Costs a little more than NICAD.
-Harder to detect voltage slope on full charge a little more work to make sure it has a full charge (not so important in our application).

Lithium-ion and lithium polymer batteries:

Not good for solar or intermediate power source charging although it can be done.
Used in computers and other high-end self-regulating electronics.

Benefits:
-High power to weight ratio.

Downsides:
-Can't be charged at below 0c or above 45c. (Bad for outside applications)
-Can blow up, has a greater likelihood of explosion than NICAD or NIMH.
-Can't be trickle charged.
-Complicated end of charge sequence.

LEAD ACID:

The workhorse of the battery world
Used for high amperage storage/use applications like:
Motor ignition
Electric vehicles


TWO TYPES of lead acid battery:
Starting (cranking motors)->gives many amps fast less overall power storage

And deep cycle (marine/golf cart)->better long-term storage less on demand amps.

Three types of construction:
Wet Cell (flooded), old style you refill and spill

New sealed more expensive more robust and efficient.
Gel Cell, new
And Absorbed Glass Mat (AGM)

Positives
-Very robust you just need to give it 2.15 volts per cell to make it happy.
- A good cheap choice for applications that don't need to move, and can take up space.

Negative:
-soooo heavy and big.

More info;
http://www.johnhenryshammer.com/ePres/batteryPage.html

Super capacitors Overview:

wikipedia overview: http://en.wikipedia.org/wiki/Supercapacitor This is what wikipedia has to say:

Electric double-layer capacitors, also known as supercapacitors, electrochemical double layer capacitors (EDLCs) or ultracapacitors are electrochemical capacitors that have an unusually high energy density when compared to common capacitors, typically on the order of thousands of times greater than a high-capacity electrolytic capacitor. For instance, a typical D-cell sized electrolytic capacitor will have a capacitance measured in microfarads, while the same size electric double-layer capacitor would have a capacitance of several farads, an improvement of about four orders of magnitude in capacitance, but usually at a lower working voltage. Larger, commercial electric double-layer capacitors have capacities as high as 5,000 farads.

Electric double-layer capacitors have a variety of commercial applications, notably in "energy smoothing" and momentary-load devices. Some of the earliest uses were motor startup capacitors for large engines in tanks and submarines, and as the cost has fallen they have started to appear on diesel trucks and railroad locomotives.More recently they have become a topic of some interest in the green energy world, where their ability to soak up energy quickly makes them particularly suitable for regenerative braking applications, whereas batteries have difficulty in this application due to slow charging times. If the LEES or EEStor devices can be commercialized, they could make an attractive replacement for batteries in all-electric cars and plug-in hybrids, as they are quick charging, exhibit temperature stability, and have safety properties suitable to such applications.

CAPACITOR
STORAGE CAPACITY

How to figure out the storage capacity of a capacitor:

A gerat reference, below charts are taken from here: http://www.antonine-education.co.uk/physics_a2/module_4/Topic_7/Topic_7.htm


- Capacitance is a size measurement of how much charge (or electrons) at what pressure (the voltage) a capacitor can hold. Caps are often used in terms of their charge discharge times, so it is like having a bucket that you measure the space of in terms of gallons AND seconds in order to fill those gallons.

-The Capacitance is measured in Farads: A farad is the amount of ampere flow whichis required to change the voltage by one volt in one second. wiki ref: http://en.wikipedia.org/wiki/Farad

Farads:
are just a constant to relate how much charge C can happen (in columbs) at voltage V for a set of plates:
1 FARAD = (1 Columb)(1 volt) in one second

* a Columb is 6.2415 times 10 raised to the 18th electrons. *

-AMPERAGE is measured as 1 columb passing a point in one second -> so the charge C is the potential Amps.

1 Farad = 1,000,000 uF (microFarads)

-The Capacitance changes with the size of the plates, 1 volt difference with two huge plates would give more Capacitance or amperage than smaller plates with 1 volt.

Image and good tutorial on magnetic forces here: http://www.physics.sjsu.edu/becker/physics51/mag_field.htm

-More voltage on the same size plates will give you more amperage (Q) or "CHARGE" .



So you can find the charge or relative stored POWER (volts*amps) by:
CHARGE (Q) (potential POWER) = (Capacitance (in Farads) )*( voltage (applied) )

SO:

What is the charge held by a 470 microfarad capacitor charged to 8.5 V?        
 
Formula first:  Q = CV (from above)
 
Q = 470 ´ 10 -6 F × 8.5 V = 4.0 × 10 -3 C (*answer rounded*.004 watts)
 

Energy in a Capacitor
When we charge up a capacitor, we make a certain amount of charge move through the application of a certain voltage.  We are doing a job of work on the charge to build up the electric field in the capacitor.  Thus we can get the capacitor to do a job of useful work (see here for more on energy and work).

  We know that:
1.      Energy = charge × voltage
2.      Q = CV.
This second relationship tells us that the charge – voltage graph is a straight line:

*Amps out are not equal to power out because the voltage falls linearly with the amperage.*

The capacitor is charged with charge Q to a voltage V.  Suppose we discharged the capacitor by a tiny amount of charge, dQ.  The resulting tiny energy loss (dW) can be worked out from the first equation:
dW = V × dQ   

        
This is the same as the area of the pink rectangle on the graph.
If we discharge the capacitor completely, we can see that:
Energy loss = area of all the little rectangles
 = area of triangle below the graph (because it is a linear slope)
 = ½ QV
  By substitution of Q = CV, we can go on to write:  
E = ½ CV^2

Stored energy:
Which is saying -> E = (1/2 capacitance (in Farads)*(voltage*voltage)).

What is the energy held by a 50 000 mF capacitor charged to 12.0 V?

Use E = ½ CV^2
 
E = ½ × 50 000 × 10-6 F × 12.0V^2
 
= 3.6 J (P) total energy stored in joules

Super capacitors VS batteries:

VS.

            •            Charge Cycles: Ultracapacitors can be charged and discharged hundreds of thousands (and millions) of cycles without losing performance. A battery is only good for a limited amount of charge and discharge cycles. You probably notice this now with your cell phone or if you have a cordless phone at the house. The longer you have and more you use the less effective the battery holds the charge.
            •            Charging Time: As we know, batteries rely on chemical reactions and take more time to charge...unlike ultracapacitors which charge and discharge very quickly.
            •            Size / Weight: Batteries are larger and heavier where ultracapacitors tend to be smaller and lighter.
            •            Energy Density: Typically ultracapacitors hold one fifth to one tenth the energy of an electrochemical battery. This will be changing though as the development of ultracapacitors continue.
            •            Energy Release: Batteries release energy on a slower longer period of time while capacitors release stored energy very quickly.
           

            •            ******COST!!! *******
            •            An average NIMH battery costs $4.00 for 1600mah
            •            $4.00 for 1600mah @ 1.2v= 1.92 watt hours =6912 joules
            •            gives 17.28 joules per cent
            •           
            •            100f cap 2.7v =364.5 joules
            •            15 dollars
            •            gives .243 joules per cent
            •           
            •           superCAPS are ~71 times more expensive (2008).
            So for a comparable cap I would have to pay ~$284.00 on the low end

Battery charging methods:

Pulse width modulation (can be used in conjunction with delta Temp and delta Voltage)

Is a method of changing the frequency of a square wave in a manner that the Voltage over time gives a different voltage than the peak of the square wave.  The effective voltage OUT, is the original voltage IN times the Duty cycle (which is the ratio of time that the signal is high over x amount of time). For example:
An original voltage of 12V IN pulse width modified at a 50% Duty cycle yields an effective 6V OUT voltage.
Good explanation below:
http://www.oreillynet.com/pub/a/network/synd/2003/07/02/pwm.html
Why is this a good method for charging a battery?
It allows a circuit or chip to vary the rate voltage and current a battery needs at the specific mode in its charging cycle. Instead of guessing at when the battery is fully charged or what part of its life cycle it is in, the charge control circuit can dynamically change its output to fit the need of the battery.
Also good at using intermittent sources of power semi intelligently.
Further explanation below:
http://www.morningstarcorp.com/en/support/library/8.%20Why%20PWM1.pdf

Timed charge
The timed charge approach consists of discharging a battery completely then applying an X amount of charge for a specified time period. It works ok with NiCad but the battery needs to be fully discharged or problems can occur. (There are chips/chargers that do this).

Trickle charge

Trickle charging is when a current is applied permanently to the battery at a very low rate. Basically the internal resistance of a full charge is enough to stop the overcharge of the battery. Usually the rate is  ~C/.02-.05.

Delta Temp

This method is widely used and very reliable. When a battery is charged the chemical make up changes causing a rise in temperature at full charge. Usually a thermistor (http://en.wikipedia.org/wiki/Thermistor) is used to gauge the temperature of the battery casing and the difference in temp/time determines when a full charge state has been achieved -> page 20 (http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf).

Delta Voltage

This is the other widely used method it consists of measuring the voltage change at the terminals of the battery. When a cell is fully charged a drop or DIP in voltage occurs (smaller in NIMH than NICAD) this signals the end of the charge sequence and the controller should stop charging (see data sheet for the specification of your battery).

What I am using
Based on the 555 chip
Pulse width modulation

Circuit by Bill_Bowden
http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm#lithium.gif

I basically used the same circuit and added a pot to adjust the voltage and a regular zener diode and took out the current limiting diode. It works pretty well. Below is a step by step.

1. The Lm339 compares the battery voltage to the reference voltage. This brings the trigger pin (2) on the 555 chip low starting a pulse.

2.This takes the output of the 555 chip high and starts to fill the timing capacitor.

3. This triggers the two transistors applying voltage to the battery.

4. This reapplies voltage to the comparator, setting the output high.

5. This brings the trigger high, which turns on pin 7 draining the timing capacitor and brining the 555's output low.

I thought this one was very elegant also
Solar Battery Charger With LM317T
http://www.reuk.co.uk/Solar-Battery-Charger-With-LM317T.htm