Solar Power for MP2

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Author T. Gillett

The MP2 devices and similar routers consume only small amounts of power, typically 1 - 2 Watts at 12 Volts, and so may be readily operated from solar and battery power in circumstances where it is not possible or convenient to provide mains power.


Solar Panels

Solar Panel 10W
Solar Panel Connections

Solar panels are generally rated in terms of Watts at a given Voltage - eg a 10 Watt, 12 Volt panel.

The Watt rating is determined by the product of the open circuit voltage (typically 20V for a 12V rated panel) and the short circuit current, both measured when the panel is fully illuminated by sunlight. So it is a guide to the size of the panel rather than an actual measure of the power you can obtain from the panel in normal use.

A 10W, 12V rated panel will generally deliver 0.5Amps of current when simply connected to a 12V battery, and a 20W panel will deliver 1 Amp.

Solar panels suitable for external use sell on-line typically for $15-20 for 5 Watt 12V, $30-40 for 10 Watt, and $50-60 for 20 Watt.

The efficiency of solar panels falls off with rising temperature and with dust accumulated on the glass surface. Locate panels so that they have good air circulation to the back of the panel, and where they can easily be cleaned.

When locating panels, be aware of shadows from trees and buildings that may obstruct sunlight getting to the panel over the course of the day and through the annual seasons. Panels need to be fully illuminated - a shadow on just one part of the panel will generally result in no output from the whole panel as all the cells of the panel are connected in series.

The period of charging is greatly affected by the orientation of the panel in relation to the sun. Changing the orientation between morning and afternoon can significantly increase the amount of energy collected. You may choose to use two panels which are oriented differently to catch morning and afternoon sunlight in order to extend the charging without needing to move the panels.

Solar Charger / Regulators

PWM Solar Regulator

Common low cost solar regulators are generally meant for use with lead-acid batteries and are of the Pulse Width Modulation (PWM) type which control the amount of current delivered to the battery being charged by rapidly switching the current on and off, and monitoring the voltage of the battery, reducing the current when the battery is charged.

Use of specialised solar regulators such as as Maximum Power Point Tracking (MPPT) devices, can improve the amount of power delivered from the cell by adjusting the voltage/current operating point of the cell using a dc-dc inverter to deliver current to the battery being charged. (

Both the above types of solar regulators are readily available on-line with prices starting at $10.

There are also dc-dc converter modules available which are capable of constant voltage and constant current operation, and these may also be used for simple battery charging as discussed in a later section. These sell for a few dollars as PCB modules without an enclosure.


The amount of sunlight received at a particular place on the Earth's surface obviously varies quite widely, due to both the angle of the sun at a particular latitude and longitude, and due to climatic conditions i.e. how many sunny days occur over periods of time during the year.

For the purposes of this discussion, we will assume a moderately sunny climate. In this situation, you can expect that a solar panel which has a fixed orientation, and set at the optimum angle for the latitude of the location, will receive full illumiination for about six hours on a cloudless day. Either side of this time window, the panel output will be reduced as the angle of the sun is increased relative to the panel, and also because the light intensity is less in the early and late parts of the day.

You may need to adjust the calculation figures to account for the situation at your location. There is a lot of information available on-line about both the optimum positioning of panels and about the weather patterns at particular locations. (Ref, also local weather bureau sites for "sunny day" counts.)


 Please note that even small batteries hold a lot of stored energy and and are capable of delivering high currents.
 They must be handled with a lot of care. 
 Mishandling batteries can lead to serious injury due to fire and explosion.
 Particular care must be exercised to ensure that batteries are suitably fused, are not short circuited, 
 nor over-charged or over-discharged.
 The transport of batteries, particularly in aircraft, is tightly regulated and penalties apply for non-compliance.

Two battery technologies of particular interest for this discussion are Lead Acid and Lithium-Ion (Li-Ion).

Lead Acid Batteries

Lead Acid batteries have been in use for many years, and are commonly available and economical. They are widely used in the automotive industry - motor vehicles commonly have 'wet' Lead Acid batteries with capacities upwards of 75 Ampere-Hours at 12 Volts.

Sealed Lead-Acid Battery 9AHr

There are also specialised versions where the electrolyte is in a gel form and the batteries are completely sealed (commonly referred to as Sealed Lead Acid (SLA) Gel, and other types which are optimised for deep discharge.

A key feature of Lead Acid batteries is that the number of discharge cycles (ie the 'life') of the battery is dependent on the depth of discharge in regular use. If the normal daily discharge cycle uses 25% of the battery's nominal capacity, then the number of cycles you will get before the battery capacity falls off significantly may be two or three times the number of cycles where the depth of discharge is 50%. And if the depth of discharge is regularly 75% or more, the life of the battery will be greatly reduced.

As a general guide for conventional lead acid batteries, keeping the normal daily discharge cycle to 25% will ensure a long battery life, while regular discharge beyond 50% will cause the battery to fail prematurely. Battery manufacturers publish a "Depth of Discharge-Lifetime" graphs to show this characteristic.

An automotive battery needs to be able to deliver several hundred amps of current to crank an engine during starting, and after a few years of use their capacity declines and this level of current is no longer available. However, even at this point, the battery may well have a significant ampere-hour capacity remaining, especially when used at low current levels.

Discarded automotive batteries generally have only scrap value, and are available in most areas of the world, and so may be a good way to provide economical solar and standby power for small electronic devices such as MPs.

Another readily available type of lead-acid battery is the Sealed Lead Acid (SLA) battery which is commonly used in electronic equipment such as alarm systems and UPS for back up power. These are available in capacities of a few Ampere-hours up to 100 AH or more. A common size is the 12 Volt, 9AH which is widely used in small UPS and alarm systems.

Charging lead-acid batteries for optimum performance and lifetime is a somewhat complex process.

Simple chargers supply a constant current until the battery reaches a terminal voltage corresponding to full charge, at which the current is reduced to prevent over-charging. This voltage point is quite critical and is temperature dependent. Simple chargers often over or under charge, resulting in reduced lifetime and performance of the batteries. It is generally better to err on the side of undercharging which stores less energy in the battery, but will not destroy the battery.

More sophisticated chargers supply current to the battery in a series of steps and have accurate, temperature controlled voltage sensing to accurately control the charging process and maximise performance and lifetime.

There must also be some facility in the system to prevent excessive discharging of the battery, as this will permanently reduce battery capacity and shorten its life. When the battery voltage falls to a designated point corresponding to the desired depth of discharge, the load must be disconnected until the battery has been at least partially recharged.

Lithium Ion Batteries

Li-Ion batteries are of more recent origin, but are now widely used to power portable electronic equipment like laptop computers, and recently we have seen the introduction of large scale batteries of 10kWHr capacities and more, used to power motor vehicles and residences.

Li-Ion Battery Type 18650 2500mAHr

Of particular interest for this discussion are the Li-Ion batteries commonly used in laptops. These batteries are often referred to as "18650" cells. These cells have a nominal voltage of 3.6 Volts, and are commonly used with three cells in series to give a nominal terminal voltage of 10.8 Volts. A simple three cell battery assembled from cells with a capacity of say 2250mAHr would also have a capacity of 2250mAHr, but at a nominal terminal voltage of 10.8 volts.

It is also common to have two or three sets of cells connected in parallel to increase the capacity of the battery. For example a "six cell" laptop battery made up from 2250mAHr cells would have a capacity of 4500mAHr, while a "nine cell" battery would have a capacity of 6750mAHr using the same cells. In these arrangements, the individual cells are usually connected in parallel so that each "cell" has two or three times the capacity of the single cell. In theory there is no real limit to how many cells you can connect in parallel this way.

The 18650 cells are readily available and come in different capacities ranging from 2500mAHr to 6500mAHr. They are available on-line for typically $3-$5 each.

When fully charged, these cells have a terminal voltage of 4.2 Volts, so a three cell battery shows 12.6 Volts.

Li-Ion batteries are relatively compact and light in weight compared to lead acid batteries. They do however require careful management and handling as over charging and excessive discharging will destroy the battery in short order, and may lead to physical damage due to fire and explosion.

Discarded laptop batteries may provide a source of 18650 batteries, as often only one cell has failed but this reduces the capacity to the point where it is not useful for laptop operation. Note that great care must be taken in recovering such batteries.

A Simple Solar Powered MP System

An example of this sort of system would be for a very simple, completely standalone, VT-RACHEL classroom router set up which is used for a few hours during the day, or perhaps for a few hours in the evening if a battery is included.

An MP router draws 75-100mA of current from a 12V supply. A 5Watt solar panel can deliver 250mA of current in full sunlight, and so could be used to power an MP directly, provided there is constant sun available.

Obviously there are some limitations around the use of such a system given that the MP will stop working if the sunlight is interrupted, and so some form of battery backup would be useful.

A simple system suitable for short term use would consist of just a small (eg 5W ) solar panel connected directly to a lead-acid battery and the MP router. A discarded automotive / motor cycle battery would provide ample capacity to provide power during short interruptions to the sunlight, or you might use a small Sealed Lead-Acid battery.

Given the small solar panel with limited current capability and the intermittent use of the system, a solar regulator is not required, particularly if the battery is a large automotive type. If the battery is a small SLA type, then some care should be exercised not to overcharge the battery by leaving it connected permanently to the solar panel without any load.

Powering an MP for Continuous Operation

Where an MP is used as part of a network, it is commonly required that the device be capable of continuous operation. The battery in such a system must have sufficient capacity to power the system overnight and during cloudy weather when limited sunlight is available. And the solar panel must have sufficient capacity to power the device load and to recharge the battery in the available window of sunlight.

Following is a simple analysis that gives an indication of the size of the solar panel and battery required for continuous operation in various situations.

Assuming a nominal load of 100mA for the MP, the system will consume 2400mAHr of energy per day. The solar panel must be capable of returning at least this much energy into the system.

A 10Watt panel will deliver 500mA for six hours on a sunny day, i.e. 3000mAHr. So if every day was a sunny day, this system would run indefinitely in theory. But of course this is not the case in most locations.

A typical situation might give an average of three hours of full sunlight each day, averaged out over the year. This implies that a 20Watt solar panel would be required to generate enough energy to keep the system running.

The battery in the system must have enough capacity to operate the load for the longest expected period of low sunshine. If this period is say, two days, then the battery would need to have a capacity of at least 2 x 2400mAHr or 4.8AHr. So a standard 9AHr SLA battery would easily meet this requirement.

Such a period of no solar charging would discharge the battery to around 50% of its nominal capacity, while the normal daily cycle of 18 hours of no charge and six hours of charge would result in the battery being discharged by 20% of its capacity. This level of depth of discharge would be expected to give a good service life for the battery.

If however the requirement was for regular three day periods of no charging, then the 9AHr battery would be discharged by 75% of its capacity during each such cycle, which would reduce its service life considerably. In this situation it would be worth considering using an 18AHr battery which would be discharged by around 33% of its capacity during the extended no charge period.

Note that adding a second radio (e.g. a USB wifi or 3G device), or even a USB memory to an MP router will increase its current consumption, possibly to 150% or 200% of its base consumption. This would require scaling up of the battery and solar panel accordingly.

It may be that in some situations, fully continuous operation is not required e.g. extended business hours of 12 or 18 hours per day. This will proportionately reduce the requirements for solar panel and battery capacity.

Practical Systems

SLA Battery Based System

A simple system based on a Sealed Lead-Acid battery and a commercial solar regulator may be assembled from components readily available on-line.

Based on the example discussed above, you would need a 20 Watt/12 Volt solar panel, a solar regulator and one or two 9 AH SLA batteries.

The solar regulator has three sets of connections:

- Solar panel
- Battery
- Load

The regulator will control the charging of the battery, and will also manage the load so that if the battery reaches the designated point of maximum discharge, the load will be disconnected to prevent excessive discharge of the battery.

Some points to keep in mind when setting up such a system:

- Ensure that system components are housed in suitable weather proof enclosures if they are located 
- Locate the battery so that it remains as cool as possible to maximise its service life.
- Locate the solar panel so that it has a good flow of air around it for cooling, and where it can be easily 
- Ensure that the load is connected via the regulator, not directly connected to the battery.

Following are some links to on-line suppliers of typical components.

Operation of SLA / PWM Regulator System

The graphs below show the operation of a simple system with a 9 AHr SLA battery and a PWM regulator connected to a 20 Watt solar panel and a load of 0.16 Amps. The graphs show the battery voltage and current during a typical day / night cycle.

During the day the battery is charged in bursts of up to 1 Amp when sunlight is available, and the battery voltage rises at times to almost 14V at which point the regulator will limit the charge current.

During the night, the current drawn from the battery is constant and the voltage slowly falls.

System Voltage
Battery Current

Li-Ion Battery Based System

A simple system for powering a MP continuously may be constructed from a solar panel, a number of 18650 Li-Ion cells, a protection circuit module, and a DC-DC converter module.

For this system you wil need the following components:

- A solar panel
- Three 18650 Li-Ion cells (or six or nine depending on the battery capacity required)
- A battery charger/protection circuit module
- A DC-DC converter circuit module
Li-Ion battery based solar power supply schematic

Charger/Protection Module

A key component of a system based on Li-Ion batteries is the protection circuit to manage the charge and discharge of the batteries. There are modules available to provide this function for common battery arrangements such as a three cell battery.

Below are links to some examples of these modules:

The protection modules generally require a connection to each terminal of the individual cells making up the battery in order to be able to monitor the state of charge of the cells. There are thus four connections from a three cell battery to the board. The module controls the charge and discharge current and the depth of discharge in order to maximise battery performance and life.

Below is a diagram of the connections between the cells and a typical Charger/Protection module.

Li-Ion Battery Charger/Protection Module Connections

DC-DC Buck-Boost Converter Module

The DC-DC Buck-Boost converter circuit is used to maintain a constant output voltage to the load as the battery is charged and discharged.

When the battery is being charged, the voltage at the Output terminals of the Charger/Protection module will be essentially the same as the battery voltage, and will rise to 12.6 volts as the battery approaches full charge.

When the battery is full charged and the solar panel is illuminated, the battery is disconnected and the voltage at the Output terminals of the Charger/Protection module will rise to that of the solar panel (typically 18 - 20 Volts).

When the Solar Panel is not illuminated, the battery is connected through to the Output terminals of the Charger/Protection module so that it can provide power to the load. The battery voltage may fall to less that 10.8 Volts as the battery discharges.

The DC-DC Converter circuit will take input over this range of voltages and convert it to a steady output voltage (e.g. 12 Volts) which you can set with an adjustment control on the module.

DC-DC Buck-Boost Converter with adjustment control

Below are links to some examples of these modules:

Operation of the Li-Ion System

The graphs below show the operation of the system over several days with a 20W panel and 4500mAHr three cell Li-Ion battery.

On Day 1, the battery has reached full charge and the system voltage rises to near the open circuit voltage of the panel as it is only supplying the small current to operate the MP2.

On Day 2, there is only limited sunshine and the battery does not reach full charge, so the system voltage remains at the battery voltage.

On Day 3, there is full sunshine and the battery becomes fully charged after several hours, and the system voltage rises to near the open circuit voltage of the panel.

At all times, regardless of the system voltage, the voltage supplied to the MP2 remains at a steady 12Volts as set by the Buck-Boost Converter.

System Voltage
Solar Panel Current