DIY Off-Grid Battery Charger – Ask a Prepper


In a grid-down scenario, batteries will be almost worth their weight in gold. So many of our devices and bits of high-speed gear rely on batteries to do their work for us.

As we all know, there will not be a source of disposable batteries post-SHTF, so many of us will switch to rechargeable batteries. The problem is, how do you charge these batteries without the grid?

It is true that we can use our generators or renewable energy systems to run the battery chargers, but we should also be prepared to be able to charge our batteries independently of these systems.

The good news is that you can build a small DIY solar battery charger with readily available materials to trickle charge your AA or AAA batteries.

Special Considerations

Before I get into the building of this DIY solar battery charger, a few things need to be considered.

This charger will only be used for nickel metal hydride (NiMH) batteries, not lithium-ion or LiFePO4. If you want to build a circuit for lithium batteries, you’ll need to include a battery protection circuit.
This is not a rapid charger.
Only use rechargeable batteries for this charger. Never put alkaline batteries into it.

Understanding How We Need To Charge The Batteries

On the side of the rechargeable batteries, there should be a capacity rating in milliamp hours (mAh). This is the number of milliamps that the battery can produce for one hour before becoming depleted.

Knowing this number is essential because, with a circuit like this, we should not apply more than 10% of the battery’s rated capacity. If you want to build a rapid solar battery charger, many more components will be required.

The next important thing to note is how many milliamp hours the solar panel will put out under full sun. The maximum capacity should not exceed 10% of the battery’s capacity. For batteries of 1300 mAh, you’ll want to find a solar panel with an output of around 130 mAh.

The panel’s voltage needs to be a few volts higher than that of the batteries you are trying to charge. This is to ensure enough voltage is available to top up the batteries fully.

Related: Turn a Car Battery Into an Emergency Power Source For the Home

The design that will be shown here uses two AA battery holders that each holds 2 batteries. These holders connect the batteries in series, meaning the capacity stays the same, but the voltage will double.

In this case, each holder has a capacity of 1300 mAh and a voltage of around 2.6 volts when fully charged.

I connected the two battery holders in parallel, which means that the capacity is 2600 mAh and about 5.2 volts between them.

The solar panel I used was rated 250mAh and 6 volts which should work well for this charger. Ideally, I would like the panel’s voltage to be a bit higher, but this was all I had available at the time.

I could have only installed a battery holder for two AAs, but I felt the charger would be more useful if it could charge four batteries.

Building The DIY Solar Battery Charger

The construction of this charger took less than half an hour, and all the components were easily found on Amazon. The only special skills that you will need are basic soldering skills.

Related: 18th Century Skills That Will Become Life-Saving When SHTF

For equipment, you will need a soldering iron with solder, wire strippers, a multimeter, and a hot glue gun.

As for materials, you will need a 5V solar panel, a blocking diode, AA battery holders, rechargeable AA batteries and a piece of wood to mount the charger on. 

Instructions

1. Attach the solar panel to the wood using the hot glue gun. Run the wires around the edges of the board to the opposite side. Secure in place.

2. Attach the battery holders to the opposite side of the board that the solar panel is on. We are doing this to keep the batteries out of direct sunlight. We don’t want the batteries to get too hot.

3. Solder the negative wire from the solar panel to the negative wires from the battery holder.

4. Solder a diode to each of the positive leads from the battery holders. Orient the diode so that the line on the diode faces the direction of the battery holder.

5. Connect the diode’s other end to the solar panel’s positive lead. I used a jumper to keep the diodes separate from each other.

6. Glue the wires in place with hot glue.

7. Install batteries and place the panel in the sun.

Why Is The Diode Important?

Solar charging circuits like this one have a diode on the positive side between the solar panel and the battery.

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This diode serves to provide a one-way street for current to flow. We need this because when there is no sun shining, the panel will try to suck the electricity from the batteries.

The diode can also be used to drop the voltage of the panels if needed. Each diode will drop the voltage by a certain amount which you should be able to find on the diode’s specification sheet.

Testing The Solar Battery Charger

I took four dead NiMH AA batteries and installed them in the charger. I placed the charger in direct sunlight for the entire day, occasionally checking the batteries.

With the components I was using, it took about ten hours to get the voltage back up to 1.3V.

This is not the fastest charger, but if you were stuck in an SHTF situation where you were scavenging for parts, this is the kind of basic solar charger design you could cobble together. While it is not the best, it will do the job.

Although this is not a fast way to charge your dead NiMH AA or AAA batteries, it is a good thing to keep in mind for after SHTF.

All the materials for this can be easily scavenged or repurposed from other devices, and it is simple to build.

However, suppose you want to build faster and more reliable solar battery chargers. This can be an excellent place to start before you get into the more complicated circuits.

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