Building a 12V Solar Power Battery Pack

There are large number of solar panels available on the market today, some promise to charge up your phone, while some pack enough power to charge an ipad. They have become quite affordable and highly portable. However, they all have one major flaw, and it has to do with how the charging circuit in the phone and tablet charges its internal battery.

So now you found your perfect solar panel that has the perfect USB connector for your phone, setting it under the full sun and the phone starts to charge as expected. However, unless you live in a desert, there is always things in the sky that will ruin your perfect charging system, that is... clouds, birds, shaking trees or even passing airplanes. The phone's charging circuit is designed to protect itself when the power source become unreliable, and it does this by cutting the current to the internal battery when it detects a droop in the power supply.

However, for solar power sources, sudden interrupts in power happens all the time. Just having somebody walk by and cast a shadow on the solar panel is enough to cut the charging process off. You though you got the perfect system going, only to come back an hour later and found the phone didn't pick up any charge, and sometime worse than no charge, the phone was firing up all its circuits to lock on the fluctuating power source, it actually end up using more charge from the battery.

This is an instructable on how to build a small power pack to store excessive charge during full sun condition, and use the excessive charge to ride out the time when there is a shadow on the panel. I designed the system for 12V operation because it is a common voltage commercial solar panel are designed to work with.

The basic specifications for my design are:

Nominal voltage................13.0v
Battery capacity.................3.3Ah
Spec energy capacity........ >40Wh
Battery chemistry............... LiFePO4
Max charge current ........... 3A
Max discharge current ...... 7A (continuous)
Pulse discharge current .... 27A (10s)
Charge drop out voltage ... 1V
Full charge voltage ............ 14.4V

The output of the battery end up to be a lot more powerful than I initially expected, and it was powerful enough to power a small inverter to run lights and other small appliances.

The LiFePO4 battery chemistry is selected for its good match with solar panel's output voltage, for its excellent power characteristics and for its long cycle life. A good battery should supply more than 1000 cycles of charge.

Material you'll need:

For battery: (All parts can be obtained from www.batteryspace.com)

4x LiFePO4 cells, either purchase it as a pre-assembled battery pack, or build your own pack

1x 12V LiFePO4 battery protection circuit. I use PCM-LFP7A4S for my own pack due to its low idle current drain

For battery charge controller:

TL431 - Bandgap regulator

VN2222 - Can be replaced with any small signal N-Channel MOSFET

2x Red LED - type is not important

LTV-816 - Optical isolator with BJT output, can be replaced with similar type

IRF9Z24N - High power P-Channel MOSFET, larger current device can be used to reduce loss

2A schottky diode - Any diode with low forward voltage will work here

100k Trim pot, multi turn unit is ideal, but regular trim pot works just fine

Resistors: 4.7k, 100k, 510k and 1k. Regular 1/8w through hole type works just fine, or SMD if you prefer

Connectors:

I use regular 5mm barrel connector for power connection to solar panel, and car power connector for output

Power meter:

Amazon sells a number of LED or LCD based voltage meter for fairly low cost, any one of the unit will work here

Step 1: Background Info: Knowing the Maximum Power Point of the Solar Panel

Here are some basic background information that will help drive the design of the battery pack.

Solar panels are constant voltage, constant current device. They have a specific design voltage that each panel is designed to work at. As the load draw more and more current from the panel, the output voltage dip slightly but not to greatly. At one point, the current draw exceed the amount of current the panel can produce (directly related to the amount of light falls on the panel.) That point is known as the maximum power point. Passing the maximum power point, the voltage of the panel start to dip and power output decreases.

Thus, in order to maximize the amount of power the solar panel generates, it is necessary to run the solar panel as close to the maximum power point as possible. This is illustrated by the graph of the solar panel I'm planning to use for this project. Although the final design will work with many different solar panels, but it will be the most efficient when the voltage of the maximum power point of the solar panel matches the design voltage.

For this project, I'm using the Mercury 27 foldable solar panel made by Instapark.

From the power plot, it look like the maximum power point of this particular panel is 14V

Step 2: Basic Design Blocks

The design consists of a charge controller, a battery pack, a voltage meter for observing the battery charge state and connectors for power input and output.

Step 3: The Charge Controller

The most complicated part of the design is the charge controller. There are several design requirements the charge controller has to meet:

1. Low dropout, since the solar panel voltage is a bit over 14V and nominal battery voltage is 13.4V (3.35V per cell), the allowed dropout voltage of the charge controller has to be as little as possible.

2. High current capability. At maximum power output, the solar panel will put out near 2A of current. Thus, the pass transistor should be able to pass through at least 2A of current with minimal dropout and should not overheat.

3. Minimal current drain on the battery when there is no current from the solar panel. This is necessary to prevent the charge circuit from discharging the battery while in storage.

4. No need for current regulation. Since the solar panel is a constant current device, there is no need to regulate the current flowing through, only the voltage need to be regulated.

5. Adjustable voltage regulation. Ideally set to the maximum charge voltage of the Lithium cell. For this design, it is 14.4V (3.6V per cell).

The schematic is shown at the top of the page.

The main pass though transistor is a power P-MOS. In normal operation state, the MOS transistor is driven above threshold to ensure minimal on-state resistance (Linear mode.)

Voltage regulation is accomplished though the use of TL431 bandgap regulator.

Output of the MOS transistor is connected to a schottky diode to prevent current back flowing from the Battery into the charge controller. Schottky diode is used to minimize the on state voltage drop.

An optical isolator is used to cut off the connection between the battery and the TL431 feedback circuit. Even though the voltage divider is fairly high in resistance (100kohm), it still presented an unwanted leakage current when the battery is not in use. Thus, using a optical isolator that is connected to the solar panel's supply voltage can effectively disconnect the voltage divider when solar power is absent to ensure minimal power loss.

Step 4: The Battery

The battery pack is made with four 26650 LiFePO4 cells connected in series. I used a 3.3Ah unit obtained from batteryspace.com. The battery pack is wired to a 8A battery monitor which will protect the battery from over charge, undercharge and short circuit condition.

There are other pre-made battery packs that can also be used. For people who are not experienced with battery pack building, I recommend buying one of the pre-made battery pack that has a build in battery monitor circuit. Battery pack building is dangerous as these are very high power lithium cell and can cause an explosion if short circuited.

The pre-made battery pack contains monitor circuit that will protect the battery when short circuit is detected.

Step 5: Putting the Pieces Together

The charge controller is build using breadboard technique with wire wrapping wire. The entire design is put into a box I got. The design is awfully similar to the minty boost, except the power level is about 40x larger.

A off the shelf voltage meter bought from Amazon is added on the top to allow the use to monitor the battery voltage during charging and discharge.

For initial adjustment, it is necessary to trim the output voltage to the designed set point prior to use. The best way to do this is to use a lab power supply to fully charge the battery to 14.4V, than let the battery rest for 5 minutes, the voltage should drop to around 14V. Attach the power source (solar or lab power supply) and adjust the trim pot until the battery is charged back to 14.4V again.

The P-MOS transistor does get a bit warm under use, I put a small heat sink on it to help cooling it down during a hot day.