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Batteries

Learn the fundamentals of batteries, including chemical processes, voltage, capacity, and care tips for optimal performance in storing renewable energy. Discover how to connect cells and calculate power loss in DC applications.

eugenegabriel
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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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