Charging AA & AAA Batteries From Solar - an Addendum

This project is actually an addendum to the main project here:

This is part of my mad strategy to go "off grid" (slowly)!

It is also part of a strategy to have a neat and capable location in the house for all the gadget and battery charging needs. I must admit I hate all the clutter with cables and wall warts all over the place, do you?

This addendum covers the inclusion of an AA/AAA battery charger. This charger came with a wall wart but as I said above I prefer not to use this and the instrucable includes information on a replacement its replacement with a 12V to 3V step down (or 'buck') power supply.

Not the most exciting instructable but I hope it encourages you to do likewise!

Step 1: The AA/AAA Battery Charger

After some considerable research I choose the Technoline BL-700 charger from Battery Logic.

BatteryLogic link to BL700

It with charge both NiMh and NiCd AA and AAA batteries. Each of the 4 battery locations are independently controlled.

The unit will perform 1 of 4 functions:

1) Charge - simply charges the battery

2) Discharge - this just discharges the battery and lets you know its capacity in mAh

3) Refresh - this function will repeatedly charge and discharge the battery to improve the capacity (if possible). This function is useful for batteries which have been worn out by continual charge and discharge cycles such as solar garden lights. It won't make a new battery out of an old one though!

4) Test - this charges, discharges then recharges to let you know the current capacity of the battery.

The charge and discharge currents can be selected so you can fast or slow charge your batteries as required. (always slow charge them unless you are short of time)!

Step 2: The Power Supply

The BL700 is supplied with a mains AC power supply that is rated 3V 2.8a This is quite an unusual (high) amperage supply at such a low voltage. As I wanted to charge my batteries from a 12VDC solar/battery supply I needed to find a suitable alternative substitute power supply.

Note: The power supply is terminated with a 2.1mm 'bullet' style connector (see photo)

I had a power supply lying around which I'd bought on e-bay and decided to use it. However, it is close to the rated output (3A), so you may want to choose something with a bit more capacity (amperage). Another consideration is that the maximum current requirement will be demanded when 4 batteries are in the charger and are charging at the highest rate each (700mA each). Usually I will be charging at 200mA or possibly at 500mA as I prefer to extend the life of the cells by treating them well so this power supply should be suitable.

Note: I added a small heat sink for the LM2896 chip. Apparently this allows up to 3A operation.

I checked a typical datasheet for the LM 2596 and the efficiency from ~12.5V to 3.0V is about 74% so pretty low but not unreasonable given the 'turndown' ratio from the input to the output voltage

e-bay link for LM2596 power supply

a higher capacity (much higher) power supply you might want to consider (I have not purchased or used this one so cannot comment on its performance)

e-bay alternative power supply

I've used the power supply pictured above for a few months now with no problems - it does quite hot though when I tested it with 4 x AA batteries at the 700mA setting!

If the links above become bad, try web searching for "LM2596 power supply" there are loads of options out there and these power supplies are great value.

Step 3: Prepare the BL700 Base for Fixing to the Box Lid

I wanted to locate the battery charger on the box lid permanently. To do this I decided to bolt the base of the BL700 to the box lid using a couple of M5 countersunk bolts.

Open up the BL700 and remove the base from the BL700. This is easily done by removing the 4 screws from the underside of the unit.

....there goes the warranty - you have been warned!!

Drill two holes in the base, I choose to make the 2 holes down the centreline of the base. 2The 2 injection mould marks decided the exact locations!

NB: make sure that any bolts you use do not touch (and/or damage) the printed circuit board when the two halves are rejoined together. IE they probably need to be very low profile or countersunk, as you will see in a few next steps I also cover the screw heads with insulation tape.

Step 4: Drill Holes in Box Lid

To hold the BL700 charger in position drill 2 holes in the lid of the box to correspond with the 2 holes drilled in the BL700 case in the previous step.

Tip - Use the already drilled base as a template and mark the hole positions.

Step 5: Ensure the Base of the BL700 Fits.

Before reassembling the BL700, ensure that the base fits the 2 holes drilled in the box lid.

Step 6: Lock the M5 Bolts to the Base

Cover the heads of the countersunk screws with some insulation tape.

Now lock the M5 bolts in position with a M5 nut. The height of the nut needs to be less than the 4 rubber feet on the base of the BL700.

Step 7: Reassemble the BL700 and Fit to Box

Reassemble the BL700 with the 4 case screws removed in an earlier step.

Fit the BL700 to the box lid and retain in place with an additional 2 M5 nuts and M5 washers underneath the box lid. Don't over-tighten these nuts as you may overstress and crack the BL700 base.

Step 8: Install a DC Wire

Rather than cutting off the 2.1mm bullet connector from the supplied power supply I purchased one from a local electronics shop. You can do this or even purchase a separate plug and wire your own up.

Drill a 5mm hole and feed a DC wire through and into the box. This needs to be terminated (connected) to the 2 output terminals of the power supply.

NB the +ve (positive) goes to the centre of the connector and the negative to the outer.

Step 9: Mount the Power Supply

Mount the power supply inside the box using the 4 holes in the PCB.

Connect the PCB up to the supply - take care connecting +ve to +12VC and -ve to 0VDC!

Step 10: Set-up the Voltage (before Connecting to BL700 Battery Charger)!

Ensure the BL700 is NOT connected to the power supply (disconnected at the bullet connector)

Switch on the power supply (I used a 5A fuse to protect this circuit)

Press the button on the left hand side of the PCB to switch on the voltmeter. Toggle the button on the right hand side of the PCB to display the output voltage

Adjust the potentiometer to achieve a voltage of 3.0-3.2 volts

Plug in the BL700 and ensure that the voltage does not dip significantly (more than 0.1- 0.2V), when batteries re charging. If you charge 4 batteries at the same time the heatsink does get quite hot!

Step 11: Check the Output

Finally, double check that the output voltage and polarity is correct. It should be 3.0V +/- 0.2V and the polarity should be positive to the centre, and negative to the outside (refer to photo).