Solar HT Charging using Photovoltaics

Hooking up the HF rig and HTs to photovoltaic panels and a battery bank was easy. Charging the HT batteries from this solar setup was a little more tricky.

The HT

If your HT has a battery pack voltage of under 12 volts, the following information will help you in charging your HT from the sun. Basically, the photovoltaic panels coupled with a battery bank will create about 12.5 volts. The Alinco DJ-580T uses a 7.2 volt nickel cadmium pack. This battery pack can be charged because the charging voltage (the 12.5 volts) is greater than the HTís battery pack. Now, Alinco also offers a 12 volt battery pack. Charging this pack with our solar charging process will not be as successful. Because there is a voltage differential of only half a volt, the amperes needed to charge this 12 volt HT pack will not be delivered with enough force. The result, a fully charged 12 volt HT pack could require 2 to 3 days to charge! HT batteries that are rated above 12 volts (I am not aware of any) cannot be charged this way.

Nicad Batteries

Nicad batteries are pretty easy to figure out. A battery pack that is rated at 7.2 volts and has a capacity of 700 mah (milliamp/hours) is usually charged using an AC adapter. The convention is to charge the pack 110% because of losses inherent in the charging process. So, we would charge our pack with 770 mah of power. The other convention is that the charging time is usually 14 hours. Charging in less time makes for a faster, but less efficient charge. If the pack gets 770 mah delivered in 14 hours, then it receives about 55 milliamps in one hour (770 mah divided by 14 hours = 55 milliamps). So, the AC adapter should be rated around 55 mah. If you use an ammeter to check across the AC adapterís output terminals, you will find a wide range of actual outputs versus rated output. Furthermore, if you place the ammeter in series with the charging circuit youíll get an idea for the actual charge the battery is receiving. Letís say this actual rate is 100 milliamps, then you know the pack will be fully charged in 7.7 hours (770 mah/100 milliamps = 7.7 hours).

The Panels and Battery

If you plan to use your panels and battery only to charge your HT battery, then the system can be very small. A panel that produces 14 volts at 200 milliamp/hours will charge the battery bank fully (say a battery with 5 ampere/hours of capacity) with about 25 to 30 hours of sun. A panel such as this would create enough electricity in 4 hours of sunlight to fully charge our 7.2 volt HT pack mentioned above. Check out the Solar Electric link on the home page here for good deals on photovoltaic panels and batteries. If you already have a larger photovoltaic/battery system in place or are planning on assembling one, the same principles apply.

Dissecting an AC Adapter

The easiest way to create a solar charging port for your HT is to take apart an existing AC adapter. You want to be able to feed the DC power from your battery bank into the charging circuit of the HT charger. An AC HT charger is composed of two main sections. The first is a simple step down transformer plus a rectifier. This will produce the DC power the HTís battery needs to be charged. The second portion is a charging circuit. This circuit makes sure the battery is not overcharged and damaged. We are replacing the first section with our solar powered DC battery bank. Youíll need to determine where the step down transformer plus rectifier section is (usually all in the transformer block that plugs into the wall) and cut this away. Now, you may hook the charging circuit to the solar powered battery bank. BE CAREFUL TO OBSERVE PROPER POLARITY! Match the output wires of the transformer with the polarities they represent.

I cannot speak for all HT battery chargers, but I am familiar with the charger for the Alinco DJ-580T. You may simply cut away the wall transformer and hook the charging circuit up to your solar powered DC battery bank.

Charge Controllers

People who are unfamiliar with photovoltaics often ask why they cannot hook their HT charging circuit directly to a photovoltaic panel. For the same reason you shouldnít hook any battery (including the solar powered DC battery bank mentioned above) directly to a solar panel without a charge controller. A photovoltaic panel can create voltages around 24 vdc. This is too much voltage for a 12 volt battery. Furthermore, the solar panels might be sending too much current to the battery which would overcharge it and shorten its life.

A charge controller limits the voltage and current flowing to a battery, keeping it at a safe level. You may use a solar panel, plus a charge controller as a feed to your HTís charging circuit. I suggest a solar panel, plus a charge controller hooked to a small 12 volt storage battery (a small motorcycle battery will do) in the 2 to 10 ampere/hour range. Now, you may hook your HT charging circuit to this 12 volt, solar powered battery bank. This system has one great benefit. The 12 volt battery bank will charge when the sun is out. You may then charge your HT at anytime. However, with no 12 volt battery bank, you may only charge your HT when there is adequate sun.

Final Check

Once your solar charging setup is complete, use an ammeter to repeat the initial measurements taken. You may find that the current flow to your HT is higher or lower than when the HT charger was being run from the wall outlet. Adjust the charging time appropriately, using the above formulas.


Unfortunately, I have to place a disclaimer here. This article is for informational purposes only. I am not responsible for any damages that result from following the above information. If you are unsure of what you are doing, find someone with a little more experience than you have.