Introduction to Battery Energy Storage Systems and Backup or Portable Power basics

This page is mostly a quick introduction to basic battery principles and how they are used in electronics and energy storage to supply portable/backup power.

Batteries and their circuits/systems become more dangerous as they get larger and/or can provide more power. Always consult with experts that specialize in building the same type of systems you are interested in building and follow their safety precautions. 



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Keep in mind that there are lots premade battery systems that you can buy. It’s still good to know this material though to help know which system to select, or to build/modify a system that better meets your needs.  

Some important battery system topics:

Battery selection:

There is an unlimited number of uses for battery systems and each battery type has different strength and weaknesses that must be taken into account when selecting what type of battery to use.

  • Cell (chemistry) nominal voltage/max charge voltage/max discharge voltage .
  • Cell series/parallel connections.
  • Amp/milliamp hour capacity.
  • Wattage/Power.
  • Primary (non rechargeable)/Secondary (rechargeable) batteries.
  • C-rate, the electrical current (amp) input (charge) or output (discharge) rate in relationship to the total current capacity (Ah)
  • Purpose: to power one or more of the following –  USB devices, RC boat/car/drone, automobile, store solar/wind/backup energy, etc.
  • Price/weight/efficiency/temperature range/cycling/etc.

Cautions

Rarely will you use a single cell battery to power something.  Connecting them in series will provide more voltage while connecting them in parallel will let you provide more current than one cell alone, power the same load for longer, or both.

High voltage:

High voltage transfers energy/power efficiently through conductors. But it runs the risk of forcing current through something not made to pass much, if any current. Therefore, adequate insulation must be provided. 

High current:

High current produces a lot of heat based on how much resistance it has to go through. Therefore everything needs to be larger to reduce resistance and increase surface area to dissipate heat. 

Conversions: Buck -Boost – Inverter – Rectification (adapter)

Often it is a good idea to have your battery system at a different voltage than your load will need. Luckily, you can easily buy (not much point making) circuitry that converts current to more voltage or

  • Inverter turns DC to AC.
  • Rectification (adapter ) turns AC to DC.
Stepping up voltage

Converting/inverting a low voltage into a high voltage (boost/inverter) will require a lot of input current to be able to provide a much lower output current.

Stepping down (buck) voltage

Circuit wise, it’s generally nice to have a higher voltage being supplied than is needed to power a load. Especially true when the power source is far away from the load and power converter.

Series batteries however, can easily get unbalanced (different voltages) if they aren’t perfectly matched and otherwise protected. The more series batteries you have, the trickier it is to keep them balanced. There is also a greater chance that you can get electrocuted while working on them, or other damage can be caused by a short circuit through an accidental connection (dropped wire/connector onto the circuit), or by failure of part of the circuit that creates a short.

Connecting cells/batteries to each other and to the rest of the system:

  • Cable.
  • Connectors.

Managing/converting the power from the Battery.

  • Cell balancing (of series cells).
  • State of charge.
  • Inverter (DC converted to AC).
  • DC voltage buck/boost converter. (Converts input voltage into more output current/Converts input current into more output voltage)
  • AC voltage step up/down.
  • Low voltage cutoff.
  • Over voltage protection.
  • Portable power bank/vehicle jump starter.
  • Solar charge controller.

Important topics awaiting assignment as new pages are added:

  • High voltage – Good for long distance power transmission over relatively small cables/wires. More dangerous to people and lower voltage electronics.
  • High current – Good for  getting lots of electrical work done (Lighting more LEDs, turning more/bigger motors, etc.). Needs relatively large cables/wires, connectors. Turns into a lot of waste heat over longer distances, especially in smaller cables.

Lead acid batteries:

Good for starting vehicles (typ. 12V) and low priced backup storage. Generally best t0 keep fully charged and to be used to provide power as little as possible. Different chemistries offer different flexibilities.

  • Cell nominal voltage (typically) 2V but should be kept charged above 2.1V.
  • Usually 6 cells are connected in series to make a nominal 12V battery (12.6V or more charged).
  • Charging and maintaining is not as simple as lithium based batteries. For now this site will just say to use a proper charge.

Lithium ion (Li-ion) batteries

As always, special chemistries may differ a bit. Always consult the manufacturer specifications for the particular batteries you are using.

  • 3.6V nominal voltage, sometimes 3.7V. Whichever one is used to indicate the nominal voltages, the batteries are still usually limited to 4.2V max.
  • 3V discharge voltage.
  • Never drop below 2.5V.
  • 4.2V fully charged. Never charge above that voltage unless using a specially made li ion battery that can do so, which is very rare.
  • Undesirable to keep fully charged as that will make the battery age quicker. Let the li ion battery self discharge over time after it was charged, and if possible, give it a charge right before you need the most stored energy. It will self discharge the most when it is close to fully charged.

Lithium Iron Phosphate (LiFePO4) batteries

As always with batteries, you may come across one with adjusted chemistry that differs a bit. Read the specs carefully and follow the manufacturer recommendations whenever you buy batteries.

  • 3.2V nominal cell voltage.
  • 4 series cells are commonly combined to make a 12.8V nominal battery when a 12V battery is desired.
  • 3V cell discharge voltage
  • Never discharge a cell below 2.5V
  • 3.65 maximum charged cell voltage
  • 2000+ cycles.
  • 4 series LiFePO4 cells have a close voltage range to 6 series 2V lead acid cells.
  • Unlike lead acid, it is undesirable to keep LiFePO4 fully charged all the time. Like li-ion, LiFePO4 slowly self discharges the closer it is to fully charged. Best to top it off right before you need them to power something for a longest amount of time. Steady current to keep lithium batteries fully charge ages them quicker.
  • Chemical symbols: Li = lithium, Fe = iron, Phosphate = PO4.

Some basic battery math examples:

  • 1V x 1A = 1W
  • 10V x 10A = 100W
  • 1V x 1Ah = 1Wh
  • 10V x 10Ah = 100Wh

Examples of power needed for various loads:

  • 120W incandescent light bulb = 120VAC (household voltage) x 1A
  • A 120Wh battery would power a single 120W light bulb for one hour, or 2 parallel 120W light bulbs for 30 minutes (1/2 hour). The battery voltage would have to be perfectly (100%) inverted (DC to AC) to 120VAC, which is impossible.
  • 1,000W microwave = 1,000/120VAC = 8.333A of current while the microwave is on at full power.

LED protected by a resistor:

  • 100Ω/1V = 0.01A (10mA) – 1V x 0.01A = 0.01W (mW)
  • 2V (forward voltage) LED @ 10mA = 0.02W (20mW) – Remember that a resistor usually limits current for an LED. It also uses power (heats up) from the voltage across it.
  • 2V x 0.01A = 0.02W

A red LED with a 2V forward voltage, connected in series with a 100 ohm resistor, and being powered by a 3V power source, will consume about 30mW of power.

  • 3V (Vf) LED @ 10mA = 0.03W (30mW)
  • 100Ω/1V = 0.01A (10mA) – 1V x 0.01A = 0.01W (mW)

A green or blue LED with a 3V forward voltage in series with a 100Ω resistor being powered by 4V will consume a total of 0.04W of power.

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  • Information on this site is not guaranteed to be accurate. Always consult the manufacturer info/datasheet of parts you use. Research the proper safety precautions for everything you do.
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