Portable (-ish) Solar Briefcase!

You will probably find it more efficient and portable to skip the inverter and lead acid batter and make your own charge controller to charge the battery pack directly.

There is already an Instructable that discusses this approach:

Solar Battery Charger For Your Cordless Power Tools

However if you are working with different tools each with different power packs then using their original charger is probably more convenient and safer but will be less efficient.

You perhaps could help Karen/Ben out though by giving them some indication of the voltage and capacity of your battery packs, and the battery chemistry. Also an indication of how quickly the batteries need to be charged in order to make this solution more worthwhile than say filling the briefcase with pre-charged power packs or a precharged deep cycle leisure battery and inverter.

I suspect that this will be difficult to do with 'cheap' solar panels though. You would probably need to be looking at 60W+ caravan type panels in order to get enough energy to fast charge a power pack.

This site suggests expecting around 42Ah charge per day for a 12v battery in _summer time_ from a 60W panel

Sunstore - 60w 12v Flat Roof Solar Battery Charger Vehicle Kit

I'd guess that your power tool batteries are probably rated at around 3000mAh @ 18v and you are expecting to 'fast charge' them swapping them every hour over an 8 to 10 hour period so you are probably looking at something around 30Ah output minimum even on cloudy days just to keep one drill operational in summer time.

You didn't say which part of your attempt didn't work ? Was it the charging of the lead acid battery from the solar panel or the charging of the power pack from the inverter ? Note that few inverters provide a pure sine wave output, and that some chargers may not work with modified sine wave output varieties.

beacon_dave wrote:

 

 

You perhaps could help Karen/Ben out though by giving them some indication of the voltage and capacity of your battery packs, and the battery chemistry. Also an indication of how quickly the batteries need to be charged in order to make this solution more worthwhile than say filling the briefcase with pre-charged power packs or a precharged deep cycle leisure battery and inverter.

 

The batteries are both 18.8v, but we've had the drills for about 10 years and the labels have rubbed off. I think that they are Ni-Cd.

beacon_dave wrote:

 

 

 

You didn't say which part of your attempt didn't work ? Was it the charging of the lead acid battery from the solar panel or the charging of the power pack from the inverter ? Note that few inverters provide a pure sine wave output, and that some chargers may not work with modified sine wave output varieties.

We brought a cheap inverter which was not powerful enough plus we don't get much sun mostly rain.

I'm surprised that the drill batteries have lasted 10 years on a construction site.

Do you find that your solar panels manage to recharge the lead acid batteries ok on a typical day ?

Only just.

That sounds quite promising then.

If you take two 12v panels and wire them in series then you should be able to use the basis of that Instructables design to charge the 18.8v pack directly without using the inverter or lead acid batteries. May need a DC-DC convertor and a current limiter to help regulate the variance between rainy day and full on sunshine.

Here in Lancashire I think a hydro power version would probably work better - funnel for rain water and a turbine...