Introduction: Solar Charged Power Shelf

About: There's always a better way, just keep recycling those brainwaves!

This project came about as an idea to extend my garage solar power setup to charge my battery packs, rechargeable tools and devices..

My requirements:

  • LCD display to observe solar charging voltage and current draw.
  • Charge my 12v nicad/nimh battery packs
  • Charge my 12v lithium ion battery pack
  • Charge my Bosch ixo
  • Solar daytime charging
  • USB charging from lead acid battery
  • Timer controlled output


The basics of a solar charging setup typically comprises of a solar panel, a charge controller and a lead acid battery. My solar shelf extends this basic setup using a combination of step down regulators, volt/current LCD panel and switches.

Step 1: Design

i need my solar shelf to produce the following outputs:

  • solar charge lead acid battery via controller
  • solar charge battery packs/tools directly during sunlight hours (auxilliary output).
  • provide timer controlled lead acid battery output primarily for garden lighting during the night at selected times.

All my battery packs are 12v or less, so will be using step down regulators, I also want to be able to switch on/off charging to the controller and auxillary output. This way i can monitor total solar current draw and individual output current draw.

see attached circuit diagram.

Step 2: Parts

see pics

Step 3: Solar Panel

see pics

Step 4: Storage Battery

I initially paired up two 6v gel lead acid batteries in series, but later switched to using a donated 12v car lead acid battery.

The car battery I was given to be recycled, was a sealed lead acid battery. I cracked open the top sealed cap to check the distilled water levels. one cell was half dry, which I topped up with distilled water.

"Sealed maintenance free" car lead acid batteries are nothing but a big con for consumers by the manufacturers. I hot glued back the top cell cap, and the battery appears to be holding its charge.

Battery terminal clamps were improvised using hose clips and wire soldered to lead strip which were clamped to terminals.

This battery is rated at 330 cranking Amps and has a capacity of 45 amp hours. I read that the trickle maintenance charge should be 3-10% of the max amps. I am seeing an average trickle current of around 0.33 amps which ties up with the theory.

if you would prefer a lower trickle current, you could switch to using a lower capacity battery.

That has just given me an idea, of being able to switch between two batteries. The higher capacity battery could be used during winter or when AUX power is not required.

During winter daylight hours are precious and sunny days are less frequent. This could be saved in a higher capacity battery for overnight use. When charging from AUX you could also switch to a lower trickle current by using the lower capacity battery, giving you more AUX power.

It would also give you the ability to switch to a backup supply should you need to.

i have since added the dual battery switch, see attached diagram.

When switched to the LOW capacity lead acid battery, the maintenance current fluctuates betwenn 0.0 to 0.1A.

When switched to the HIGH capacity lead acid battery, the maintenance current fluctuates between 0.2 to 0.5A.

By using a DPDT switch for both AUX and Low/High setting, the Low capacity battery setting is automatically selected when charging using the AUX setting.

Update on the Bosch Battery....

After about a month of usage, one of the cells on the bosch car battery stopped charging (no bubbles). Incidentally, same cell that was originally half dry. The voltage would rise to 13.3v, but drop quite sharply on applying a load. Strangely, after that it would charge back up to 14.4v. Tried a few attempts at reviving it. Not wasting any more time on this battery, will swap out, but for now switched back to sealed lead acid battery.

Step 5: Charge Controller

The charge controller settings were set to B1 for sealed lead acid batteries. Setting the float voltage doesn't seem to have any immediately visible effect.

From my research, thie float voltage should kick in if the charge voltage exceeds float voltage for 48 hours. Battery manufacturers recommend its dropped back to the float voltage, to prevent overcharging and heating. Not practically possible on a solar cycle, but could happen i guess on wind powered systems.

The charge controller always charges upto 14.4v atter which the flashing arrow (from solar panel to battery) goes steady (on reaching 14.4v).

However my current reading display shows the battery still drawing 0.3 amps. i guess that the PWM kicks in when the solar voltage exceeds 14.4v, as the battery voltage is never allowed to exceeds this.

The PWM must just limit the overall average voltage but let the current flow on demand to charge the battery.

Current denand from the battery will be high when discharged. and will drop as it reaches its fully charged state, after which a trickle current is used to maintain its charge.

On a bright day when the battery is sitting at 14.4v and solar panel output is 20v, the current appears to rhythmically cycle between 0.2 and 0.5 amps. I guess this is the PWM in action.

My load output is set to 24 hours, as i will be using my own programmable timer.

Step 6: Charging 12v Nimh/nicad Pack

i used my existing intelligent charger for charging nicad/nimh battery packs.
the battery crade is wired direct from dc socket to battery clip terminals. i removed the bridge rectifier.

Step 7: Charging 12V Lithium Ion Battery Pack

I used a step-down regulator adjusted to 12.6v for charging my 12v lithium ion battery back.

My 12v lithium ion battery pack, is a nicad battery pack that i converted to lithium ion using 3 lithium ion batteries and a 12v battery bns board. There are plenty of instructables showing you how to do this.

I added a power switch in series so I can turn this on/off when required, to minimize idle current draw.

I think using a CC/CV lm2596 buck regulator for this application is actually better, so its on order. it has 3 pots. 1 for adjusting voltage, 1 for adjusting max current, 1 for detecting fully charged trickle current and lighting up a fully charged status led.

The max current limiter is useful on cloudy days,. For example if the solar panel generates 14v at 0.7 amps, and yiur battery demanded 1 amp, you could set max current to say 0.5 amps. This prevents the solar voltage from dropping due to excessive current demand. In effect you are acting as a manual MPPT adjusting current according to the days sunlight conditions.

Step 8: Charging Bosch Ixo

This is the original bosch ixo that came with a charging cradle. Newer models are usb charged, but this has an internal transformer and mains lead.
The cradle makes it easy to drop in and leave to charge.

i removed the internal transformer and replaced with a step down buck regulator adjusted to 7v output (as per original transformer).

the green charging light on the driver shows when charging and turns off when fully charged.

Step 9: Charging USB Device

Easy one. Already provided by charge controller.

This uses stored lead acid battery power.

Step 10: Timer Controlled Output

This uses stored lead acid battery power. I use this output for garden lighting at preset times.

Step 11: Charging Additional 6v/12v Lead Acid Batteries

i know you could swap the existing charge controller lead acid battery with another battery for charging.

However, i need the option of being able to charge 6v batts and would like to do this via the AUX connection.

The simplest option was to add another mini 6v/12v lead acid charge controller to the AUX port.

Step 12: Charging AA/AAA NiMh/NiCd Batteries

My existing NiMh/NiCd battery charger has a 12v input. This also successfully charged AA and AAA batteries via the solar AUX port.

Step 13: Mount It, Use It !

The shelf housing was made from scraps of ply , chipboard, laminate, and a pair of brackets. i kept it open plan.

Now just add sunshine!

On a bright sunny day I am drawing 14.6 volts at 1.38 amps which matches the 20W rating of the panel (P=V*I).

i have not had to charge anything from my mains wall outlet as yet.

Make "charge" while the sun shines!