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Batteries

Batteries. Storing Renewable Energy “Chemical engines used to push electrons around”. Basic Terms. Voltage – Electronic pressure Current – Flow of electrons Power – Amount of energy being generated. How it Works. Cells Contain Electrochemical Couples

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Batteries

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  1. Batteries Storing Renewable Energy “Chemical engines used to push electrons around”

  2. Basic Terms Voltage – Electronic pressure Current – Flow of electrons Power – Amount of energy being generated

  3. How it Works • Cells Contain • Electrochemical Couples • Two materials which react chemically to release free electrons • Electrolyte • Transfers the electron between electrochemical couples • Sometimes electrolyte participates in reaction (lead-acid) sometimes not (nickel -cadmium, nickel Iron)

  4. How it Works • Polarity – One part of couple is electron rich and other is electron deficient • While discharging electrons flow from the electron rich negative cathode pole to the electron deficient positive anode pole • While recharging process is reversed

  5. Lets Look at the Atom • Chemical bonding is the sharing or exchange of electrons • Sodium and Chlorine are chemical elements • When combined they become something different – salt • Chemicals made during the discharge process are broken by the charging process

  6. Battery Capacity • Measured in ampere-hours (amp-hours) at a given voltage • Depends on two factors: • How much energy is needed and • How long the energy is needed Example 350 amp-hour battery can provide: 35 amps for 10 hours or 100 amps for 3.5 hours

  7. Important!!! A battery based alternative energy system will not be effective if it is not sized correctly

  8. Life Expectancy and cost • At least 5 years • Often over 10 years or 1500 deep cycles • Shipping is expensive

  9. State Of Charge • Percentage which represents the amount of energy remaining in the battery • A battery is “deep cycled” when it reaches 20% or less state of charge • A shallow cycle (car battery) will withdraw less than 10% • State of discharge is opposite so a battery is “deep cycled” if it is at discharged to 80%

  10. Rest Voltage vs. State of Charge

  11. Temperature • Batteries get sluggish at cold temperatures • Usable capacity drops radically below 40° F • Self Discharge happens rapidly above 120° F • Keep them between 55° F 100° F

  12. Hydrometer • Measures density of liquid with respect to water • The electrolyte has greater specific gravity at greater states of charge • So voltage can be an indicator • Careful opening cells, contamination of the electrolyte solution is possible

  13. Rates of Charge and Discharge • 50 amp load for a 100 amp battery is large • But for 2000 amp battery – no problem • So we combine current pulled (or added) with capacity to get a rating scheme • If it take 10 hours to fill a completely drained battery then – C/10 charge rate • If it takes 5 hours to drain a battery then C/5 discharge rate

  14. Rates of Charge and Discharge • Recommended rates are C/10 – C/20 • Using a C/5 rate will cause much more electrical energy to be loss as heat • This heat can damage battery plates • Example – • 440 Ampere-hour battery • How many amps added for a C/10 • How many amps added for a C/20

  15. Equalizing Charge • After time individual cells vary in their state of charge • If difference is greater than .05 volts – equalize • Controlled overcharge at C/20 rate for 7 hours • Turn off voltage sensitive gear before equalizing

  16. Self Discharge • Temperature greater than 120° F results in total discharge in 4 weeks • At room temperature loss is 6% and will discharge in 16 weeks Storage Fully charged 35 ° F - 40 ° F

  17. Capacity vs. Age If a battery is supposed to be good for 5 years – This means it will hold 80% of its original capacity after 5 years of proper use

  18. Battery Care • Don’t discharge beyond 80% • C/10 – C/20 rate • Always fill up when recharging • Keep batteries at room temperature • Use distilled water • Size batteries properly • Equalize every 5 months or 5 charges • Keep batteries and connections clean

  19. Connecting Cells • Power in battery can be increased by arranging the cells in two ways • Series • One path for electrons to follow • Connect + to –’ • Increases voltage • Parallel • Multiple paths for electrons to follow • Connect (+ to +) and (- to -) • Increases amperage

  20. Series • Each cell in lead acid battery is 2.1 volts • Nickel-Cadmium is 1.25 volts • Flashlight batteries are 1.5 volts each • A lead acid battery is typically 6 volts • This is 3 – 2.1 volts cells wired in series

  21. Parallel • Increases Capacity • Trojan L-16 are 350 amps and 6 volts • Wire them in parallel and you will get 700 amps • Wire two of these “700 amp batteries” in series and you get one 12 volt, 700 amp battery

  22. One Trojan L-16

  23. Where to connect? .

  24. Right

  25. Where to connect? . . .

  26. Right

  27. How to connect? . . . . .

  28. Right

  29. Wire Sizing for DC Applications • Voltage drop is caused by a conductors electrical resistance • This voltage drop can be used to calculate power loss

  30. VDI Voltage drop Index • Easier method for determining wire size • What you need to know • Amps (Watts/volts) • Feet (one-way distance) • Acceptable % volt drop • Voltage

  31. How to Use Formula and Chart • Example: 1 KW, 24 volt system, 60 feet, 3% drop • Amps = 1000 watts/ 24 volts = 41.67 amps • VDI = 41.67 amps * 50 feet = 28.9 3% * 24 volts

  32. VDI Chart 2 AWG wire That’s pretty big wire What if we make it a 48 volt system?

  33. How to Use Formula and Chart • Example: 1 KW, 48 volt system, 60 feet, 3% drop • Amps = 1000 watts/ 48 volts = 20.8 amps • VDI = 20.8 amps * 50 feet = 7.23 3% * 48 volts

  34. VDI Chart 8 AWG wire That’s better

  35. Practical Considerations • Lighting Circuits • 10% drop in incandescent leads to 25% drop in light output • 10% drop in fluorescents results in 10% loss in light output • Suggested acceptable loss 2-3%

  36. Practical Considerations • DC Motors • Operate at 10-15% more efficiently • Minimal surge demands • Some motors will fail to start if drop is too great (Sun Frost)

  37. AC Motors • Exhibit high surges when starting

  38. PV Battery Charging Circuits • Need to be higher than battery voltage so they are wired to be around 16 volts • A voltage drop of 1 or 2 volts is significant • A 10% drop will result in 50% loss of power in some cases • 2-3% loss is recommended

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