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 applied in electronics. Such as energy storage for portable/backup power.

Warning: 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. Follow their safety precautions. 

Solar charge controller basics illustrated with battery diagram by Electronzap
Solar charge controller basics illustrated with battery diagram by Electronzap

You might build a battery bank for backup power that can be charged with solar power. Solar charge controllers are not intended for use with generators that need a way to burn off excess energy. However, a regulated voltage can supplement battery charging while using a solar charge controller.

Below is a nice looking solar charge controller with detailed product descriptions. An affiliate link ad that supports this page.

Basic components for backup power:

  • Battery (bank) – Provides the needed amount of power, for the needed amount of time, to the desired load and other components that use up power. Watt is the unit of power. Watthours is the unit of power over time.
  • Inverter or buck/boost converter – Takes in power from the battery, and outputs a different voltage (and thus current) that works better for the load.
    1. Convert power (voltage versus current). Therefore you get close to the same power out as is put into it, even though the voltage changes.
  • Battery maintenance system (BMS) – Reduces the chances that batteries will be damaged. They are typically built into larger batteries these days.
  • Connectors and cables/wires – Transfers power from one component to another. Must be able to safely handle the current that it needs to carry. Care must also be taken so that connectors don’t short circuit. Short circuits are a direct positive to negative connection, or other connection made earlier than desired.

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Keep in mind that there are lots of premade battery systems that you can buy. But it is still a good idea to learn how they work. Doing so will help know you select a system or better build/modify a system that meets your needs.  

I bought the portable power supply above to be able to power a higher wattage appliance for a short period of time. Primarily to be able to use a 1000 Watt microwave for more than an hour. Lower power devices can also be powered for longer periods of time, but there’s cheaper options out there for them. That is an affiliate link ad.

Some important battery system topics:

Battery selection:

Technically speaking, a battery is multiple cells connected together to get the desired power. However, single cells, such as the commonly used AAA, AA, C, D and 1865o packages, are also called a battery in every day speak.

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

  • Cell chemistry: Nominal voltage, fully charged  voltage, and fully discharged voltage.
    1. Range of voltages (fully charged versus fully discharged).
      • Nominal voltage. Average voltage you can expect from a cell, as the voltage changes while charging (if applicable) and discharging.
    2. Rechargeable (secondary) or not (primary).
    3. Capacity. How much energy is stored for a given battery size.
  • Series connections of cells to increase voltage.
  • Parallel connections of cells to be able to provide more current/capacity.
  • Amp/milliamp hour capacity of cells.
  • Wattage/Power.
Electronic power math examples with batteries and an LED circuit learning electronics lesson
Electronic power math examples with batteries and an LED circuit learning electronics lesson

A single cell (nominal) 1.5V battery probably won’t be very useful by itself. So, you will probably connect them in series for more voltage.

Only some LEDs can light up slightly at 1.5V. To make matters worse, after an alkaline cell provides power for a while, it’s voltage actually drops well below 1.5V.

By connecting 3 fresh alkaline batteries in series, you can provide 4.5V to power an LED and protective resistor. Enough voltage for an LED with a forward voltage of 3 volts.

If 1.5V happens to be enough voltage for a circuit, but you want to power the circuit for three times as long as that a lone battery could provide before it’s chemistry is too weak, then you can use 3 alkaline batteries connected in parallel.

Of course, cells are often connected in both series and parallel in order to meet the voltage and total current needs. Only make series/parallel battery connections when they have the same chemistry, are at the same voltage, and have the same capacity (current over time).


Electronic power math examples with batteries and an LED circuit learning electronics lesson

  • 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).
    1. 1C is 1A of charging or discharging current for a 1Ah battery.
    2. 2C is 2A of charging or discharging current for a 1Ah battery.
    3. 0.1C is 0.1A of charging or discharging current for a 1Ah battery.
  • Purpose: to power something –  USB devices, RC boat/car/drone, automobile, store solar/wind/backup energy, etc.
  • Price/weight/efficiency/temperature range/cycling/etc.

Premade battery packs, like the one in the affiliate link ad above, are common and affordable these days. They often look similar to a car battery, and usually have safety circuitry contained with the batteries inside of the case. Always read all of the specifics and warnings for the battery carefully. They are all dangerous if they are abused. I like the one above because it has the versatile LifePO4 chemistry, stores a lot of energy (watthours) for the price, and has good reviews.


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 get you more current. The following lists some safety precautions to be aware of.

Connecting batteries in series:

The batteries should be at the same voltage, and they must all have the same capacity. Different capacities quickly become unbalanced. Their voltages will not stay the same.

Connecting batteries in parallel:

Batteries must be the same voltage (less than 0.1V difference) when first connected in parallel. Dangerously high levels of current will flow as a higher voltage battery directly charges a lower voltage battery.

Once batteries are connected in parallel, they will change voltage together.

High voltage:

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

  1. Electrocution is when voltage forces current through the human body. Possibly causing tissue/nerve damage or stopping the heart.
  2. Fire occurs when combustible material heats up. Moving current creates heat. Materials that don’t conduct well heat up rapidly when higher voltage forces current to flow through them. Plastic will burn or create toxic smoke under the right conditions.
High current:

High current produces a lot of heat based on the amount of resistance that it has to go through. Therefore, high currents need larger components and connections, which provide less resistance, and an increased surface area for heat dissipation. 

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

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

  • Inverter turns DC into AC. Typically 120VAC in the United States.
  • Rectification (adapter ) turns AC to DC. Usually household 120VAC (in the US) into any lower DC voltage.
Stepping up voltage

Converting a low voltage into a high voltage (boost converter) requires a lot more current in than you get out. The current is converted into voltage. Some current is also lost though. A 95% efficient will output 0.95 watts of power for each watt of power that was input.

Inverters take a lower DC voltage and output a higher AC voltage.

Stepping down (buck) voltage

Circuit wise, it’s generally nice to have a higher voltage being supplied than is needed to power a load. This is 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 with high voltages that you can get electrocuted, or other damage can be caused by a short circuit through an accidental short circuit (direct/unintended positive to negative connection).

Therefore, buck converter can take a higher voltage and convert it into more current for charging a relatively low voltage battery with relatively low power losses.

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

(Topics only, more to be added later)

  • Cable.
  • Connectors.

Managing/converting the power from the Battery.

(Topics only, more to be added later)

  • 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 as low priced (but less effective) 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: 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 charger.

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 labeled as being 3.7V. Whichever one of those is used to indicate the nominal voltage, the batteries still usually need to be limited to 4.2V max.
  • 3V discharge voltage.
  • Never drop below 2.5V.
  • 4.2V is fully charged. Never charge above that voltage unless using a specially made li ion battery that can do so. Those are rare.
  • Undesirable to keep fully charged as that will make the battery age quicker. Current keeps trickling in, causing chemical reactions. Let the li ion battery self discharge a bit over time after it was charged. If possible, give it a charge right before you need it to get the most stored energy. It will self discharge the most when it is close to fully charged but will level off quickly and barely self discharge after that.

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 not desirable 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 more quickly.
  • 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. Always plan for losses and misleading claims.
  • 1,000W microwave = 1,000W/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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