12 Volt Solar System to charge Cordless Tool Batteries

It's that time of year when relatives start to ask what I want for Christmas. I am pretty content with what I have right now. But there is a project I have been wanting to start but haven't put any money into yet. I would like to set up a 12 volt solar system to charge my cordless drill batteries. That way, when the power goes out, I will still have torque at the pull of a trigger.

It would be built around the Dewalt DCB119 20 volt car charger. Just one for now, they are spendy. It looks like that could be driven by a 20 watt 12 volt solar panel. I think my batteries are 1 amp hour. 1 amp hour x 20 volts = 20 amp hours. But the charger is probably trading amperage to step of the voltage. So it would probably take more than an hour to charge on that kind of power supply. Maybe up to two hours? I think that would be acceptable. This would be a very bare-bones charging system. If it works, I have the option of scaling it up in the future.

My family's shop is scheduled for some major repairs next summer. If I build a small system that works, maybe we will build in a 100 watt panel just dedicated to charging the cordless tools.

Jeremy VanGelder wrote:It would be built around the Dewalt DCB119 20 volt car charger. Just one for now, they are spendy. It looks like that could be driven by a 20 watt 12 volt solar panel.

Jeremy, a direct-drive from a solar panel to the battery charger will simply not work.

You will need to put the "big bin" capacity of a vehicle or deep cycle battery in between.

If you already have enough chargers, what you need is a pure sine wave inverter to connect them to the 12V source.

None of this should dissuade you from asking for practical gear for Christmas.

I would double check the batteries you have. If you have the 1.5Ah 20V battery then the battery is nominally 5 x 3.7V or 18.5V or 1.5Ah x 18.5V = 27.75 Watt hours. With some conversion inefficiency from the charger it would be closer to 30 Wh. "20V" is marketing talk for batteries that are 5 cells in series. It mentions the charger will charge one of those batteries in 45 minutes, so I would imagine it expects to see a minimum amount of power in order to initiate charging as well as to not kill the car battery since it is made for a car. So to meet that requirement you would need at least 40Wh of power on the supply side.

A smaller panel might be able to work to trickle charge a battery that can supply the minimum 40Wh. It would probably take at least a 8Ah 12V lead acid battery or hopefully greater to supply the power you need without excessive voltage drop. You could use a battery like they use for a computer UPS or older style automotive boost pack. You could also just use a car battery. It might not be perfect, but they are cheap enough and easy to source locally. Even a used one that barely starts a car would still be strong enough to charge a couple batteries with solar keeping it topped up. You could also use a Li-Ion charger and 4S pack to then charge the 5S batteries, but that's a bit more complex and expensive to set up.

Given the higher cost per watt for smaller solar panels, I would go with a 100 watt panel if you can. It won't put out 100 watts unless it is perfectly pointed at the sun and the UV index is 10 or greater. The UV index is actually a measurement of how many hundreds of watts per square meter, and panels are generally measured at 1000 Watts per square meter, or UV index of 10.

A cheap solar charge controller like you can get from Amazon, Harbor Freight, or similar would work fine. It looks like the cost of a cheap charge controller plus a cheap 100 Watt panel would be about the same as the DeWalt car charger. I bought a 140 Watt panel and cheap controller at least a decade ago and they still work just fine. My batteries finally died and I'm going to move to Li-Ion, but I'll eventually grab another lead acid battery just because it's handy. I used it for emergency lighting with LED's, charging my cell phone, USB boost pack, and laptop, as well as occasionally grab one of the batteries to jump start cars. It's really handy to have around.

Jeremy,

There have been some great answers so far, and I think you are on to a great project.  I started one of these projects about 2 years ago during lockdown.  I made a thread you can find of my build HERE;

https://permies.com/t/135729/permaculture-projects/Building-pair-battery-generators

My thoughts for you are that if this is the first time you have tried one of these projects, try building one based on an SLA battery.  My first one was a mere 15 amp-hour battery, but you could go larger if you wanted.  My build fit into a plastic.50 cal ammo box for convenience.  And again, for reference, mine is powered at present by a 28 watt solar panel or a home-modified A/C wall wart charger.  

You can certainly build larger specs than I did.  I  was followed a YouTube build that was mostly designed for charging USB devices plus a few car cigarette lighter powered devices.  It was a simple build for a first timer and cost about $100 before the solar panel.  It does have a cheap solar charge controller that is the brains of the system.

My unsolicited advice to you is to basically build a similar device, maybe with a larger SLA battery and in a larger case.  I bought a 5.56 cal magazine case that is almost twice the volume of the .50 cal case. This is far a future project.  My further unsolicited advice is to use your existing battery chargers plugged into a little inverter that itself is plugged into a dedicated car 12v cigarette socket.  This is not the most efficient way of doing things but is probably the simplest, especially if this is your first try.  I am building a second, much larger device which theoretically has much more capacity but is a much, much more complicated build.  I am running into problems because I bit off more than I could chew.  I will get it done, it will just take time.

But I do think that this is a very good project for you!  Some basic numbers look like the following :

A single 1amp hour 20 volt (really it’s 18 volts) has a total energy that looks something like the following:

 1amp x 18 volts = 18 watt hours

My single 12 volt, 15 amp hour battery looks like the following:

 15 amp x 12 volts = 180 watt hours

But since it is an SLA battery, we can only get about half the energy without damaging the battery so figure 90 watt hours.  There are further losses to something called the Peukert effect which deals with the loss of power as the voltage drops.  The bottom line is that I think that my single 15 amp battery would charge about 3-4 of your 1 amp batteries before itself needing a recharge.  Do this in the sunlight while being attached to a large solar panel and the picture gets even better.

If you felt really ambitious, you could go for a LiFePo4 battery chemistry, but that gets complicated quickly.

Bottom line:  I think this is a great project and I would love to hear how yours works out!

Eric

Jeremy VanGelder wrote:It's that time of year when relatives start to ask what I want for Christmas. I am pretty content with what I have right now. But there is a project I have been wanting to start but haven't put any money into yet. I would like to set up a 12 volt solar system to charge my cordless drill batteries. That way, when the power goes out, I will still have torque at the pull of a trigger.

It would be built around the Dewalt DCB119 20 volt car charger. Just one for now, they are spendy. It looks like that could be driven by a 20 watt 12 volt solar panel. I think my batteries are 1 amp hour. 1 amp hour x 20 volts = 20 amp hours. But the charger is probably trading amperage to step of the voltage. So it would probably take more than an hour to charge on that kind of power supply. Maybe up to two hours? I think that would be acceptable. This would be a very bare-bones charging system. If it works, I have the option of scaling it up in the future.

My family's shop is scheduled for some major repairs next summer. If I build a small system that works, maybe we will build in a 100 watt panel just dedicated to charging the cordless tools.

Jeremy I've found the 12 volt chargers less than impressive in the past. I prefer the much cheaper and better route of using my plug in chargers and a pure sine small inverter. https://www.amazon.com/s?k=200watt+puresine+inverter&ref=nb_sb_noss
You will have to choose one. Use the car as your 12 volt source, get a 12 volt splitter for the accessory plug and directly plug in a solar panel trickle charger. You could also add a small 12 volt battery with the savings. The battery makes the system much more versatile for about the same money as the single function in car charger...

Cheers, David

David was much more succinct than me regarding the actual charger.  I like that little 12v inverter and I think that you could build a 12v battery system that could run that inverter plus your charger fairly easily.

At one point I really wanted to build a Buck converter adapter that would attach to a special built fitting for my batteries (Ridgid in my case).  You can find these attachments on Amazon easily enough.  Essentially I wanted to plop my tool battery into my battery pack and charge away.

Of course, this was a needlessly complicated system.  I have read that when one just starts a project, one tends to make it as complicated as possible.  As one gets experience using and especially building these projects and realizes how unnecessarily difficult they become, one wants a simpler project.

12v inverters are simple, cheap and effective.  My suggestion is to simply build a system that has 1-2 car 12v DC outlets and use one to power the inverter that will power the charger.  These will be components already built and easily connected.

Again, this is all just my unsolicited advice so take or leave as you see fit, it is after all, your project.

Eric

Thank you so much, guys. I knew that it would be worth my while to run it by you all and avoid disappointment. And I just replaced the battery in my minivan, in a quest to diagnose all kinds of problems. (It wasn't the battery, the van needs a new computer) So I could bring the old battery in to get the $12 core charge back, or I could build a solar system around it. I think I will build a solar system.

I took a look at my cordless tool batteries today and they are in fact the 1.5 amp hour ones. They are even marked 30 Watt hours. So I can see that a 20 watt solar panel would be undersized. That size of panel is probably aimed at charging cell phones. And I live in the Northwet, so I can only count on a high UV index in July, August and September. I will keep reading what you guys have recommended and iterate my plan.

In my dreams, I walk into a garage sale someday and see an old NiFE battery with a $5 price tag on it. But I will start with what I have.

I made one to charge dewalt and ryobi batteries 20v and 18v lithiums. .  I have found 100w is not enough in Ohio.  The 12volt chargers I am using pull 6 solid amps at 13 volts.  I used 2 x 100 watt panels, a 5 farad capacitor "battery" and a 30 amp mppt controller.  It works great.  I wired the panels in series so this gives it about 40 volts open circuit.  This seems to work well on the half cloudy days.  Your old battery will be fine as long as you have enough solar coming in to maintain 6 amps at 13v.  Good luck and happy experimenting.

Here is a short video I did that shows the run on sun lithium battery charger.
 https://youtube.com/shorts/6pKJNR4ql6s  

I need to double check the conclusions I drew last year when I designed my system. I think when you take a twelve volt battery and step up the voltage to 110 just to bring it back down to 18 (or is it 30?) to charge the tool battery, you are losing something like ten or fifteen percent of your Wh in the process. Isn't stepping directly from 12V to 18 much more efficient?

The 110 inverter I bought for my car was much more expensive than the 'cigarette' adapter I bought to attach directly to my battery bank at home. I'm using my DC charger for my cordless tools at home so that I don't need to power up my inverter. I've been assuming that is making more efficient use of (home) battery power, am I wrong? If doing a lot of work with the tools, I take the DC charger with me when I drive somewhere, charging along the trip and using the excess charge from the car battery by leaving it on for a while after stopping. If I were using the AC charger with the inverter for this I'd be introducing more drag to the alternator and taking more out of the battery when stopped, wouldn't I?

[edit: sorry, mixing up car chargers with the USB bypass on my inverter. The 'cigarette' lighter is 12V to start with, so no down step, just going from 12 to 18 which is why I thought it was much more efficient.]

Hey, good questions. Some quick thoughts FWIW:

There is no free lunch. The energy comes from somewhere.

The more conversions, the more losses. Yup, the numbers are ugly.

It's not about volts, it's about amps. So, grossly oversimplified, the volume of Amps delivered over time is the firehose of electrical energy that powers big-ass tools. My 2c.

David Baillie wrote:
Jeremy I've found the 12 volt chargers less than impressive in the past. I prefer the much cheaper and better route of using my plug in chargers and a pure sine small inverter

What was it you found lacking in the 12V charger? I can't tell any difference when charging between my AC and DC chargers.

I have a Ryobi one plus charger purchased earlier this year. It was less expensive that an inverter, so in this case it is cheaper, simpler, and I think more efficient. Dewalt seems to charge a premium on their product, more than twice as much, but I'd hope it would be at least as good of quality.

Douglas Alpenstock wrote:Hey, good questions. Some quick thoughts FWIW:

There is no free lunch. The energy comes from somewhere.

The more conversions, the more losses. Yup, the numbers are ugly.

It's not about volts, it's about amps. So, grossly oversimplified, the volume of Amps delivered over time is the firehose of electrical energy that powers big-ass tools. My 2c.

There is such a thing as a ten dollar lunch and a fifteen dollar lunch. Inverters make less efficient use of your precious Amp hours. Question is are the relative differences between 12>110>18 and 12>18 that significant? Yep, ugly numbers, so hoping permies has some genius ready to step out of the shadows and save the day...

Coydon Wallham wrote:

David Baillie wrote:
Jeremy I've found the 12 volt chargers less than impressive in the past. I prefer the much cheaper and better route of using my plug in chargers and a pure sine small inverter

What was it you found lacking in the 12V charger? I can't tell any difference when charging between my AC and DC chargers.

I have a Ryobi one plus charger purchased earlier this year. It was less expensive that an inverter, so in this case it is cheaper, simpler, and I think more efficient. Dewalt seems to charge a premium on their product, more than twice as much, but I'd hope it would be at least as good of quality.

For myself I found the ryobi in car slow. I found the 1.5Ahhr batteries closer to the 45 minute mark in car versus 20 minutes on my plug in. One battery a day not a big deal  and I do travel with a number of them. Next would be cost I Have some ryobis, some ridgids a laptop for field work and rechargeable batteries for my laser level off the top of my head all of which would require a dedicated car charger with one function only. An in car charger is a one trick pony. On my small inverter I have run led worklights, provided power in an outage to a cordless landline for a client and run a wireless internet setup on top of charging tools. Efficiency wise I accept the conversion losses for the gain in function. I might be an outlier of course as I use a wide variety of items.
cheers,  David

Thanks everyone. Since starting this thread I have taken my bit and brace out of my big toolbox and set it on the shelf under my workbench--right next to my corded and cordless drills. I find out of sight really means out of mind when it comes to my tools.

For the solar system I have assembled a small list of parts:

100 Amp  MPPT Charge Controller

100 watt monocrystaline solar panel

Cigarette Lighter Output panel

I use my cordless drill for about 20 minutes at a time. And then I don't use if for a week or so. So charging speed is less of a concern for me. If I was a contractor I would want to be able to charge quickly.

I should also add on some alligator clips so that I can use the system to charge/maintain additional car batteries. If I build this system, I am sure that additional uses will come to light.

Jeremy VanGelder wrote:Thanks everyone. Since starting this thread I have taken my bit and brace out of my big toolbox and set it on the shelf under my workbench--right next to my corded and cordless drills. I find out of sight really means out of mind when it comes to my tools.

For the solar system I have assembled a small list of parts:

30 Watt MPPT Charge Controller

100 watt monocrystaline solar panel

Cigarette Lighter Output panel

I use my cordless drill for about 20 minutes at a time. And then I don't use if for a week or so. So charging speed is less of a concern for me. If I was a contractor I would want to be able to charge quickly.

I should also add on some alligator clips so that I can use the system to charge/maintain additional car batteries. If I build this system, I am sure that additional uses will come to light.

Just a small note that charge controller is not a true MPPT type. As long as you stick to the 12 volt panels it will be fine though. It bugs me they get to name it "mppt" even if it does not do that function as we have come to understand it. I had a near identical one in my trailer. Good choice on the panel and the accessory plug/USB setup...

It's actually a bit funny how it's harder to design a good charger where the voltages are really close together, but the source voltage is a bit variable. For instance, many so-called solar panels actually output around 19V-21V or so in full sun. The one linked above has a Vmp = 20.3V. But trying to get a buck/boost converter to reliably output 21V to fully charge a 5S battery directly from a solar panel isn't so easy. It seems like it would be cheaper to just use a simple filtering circuit and directly charge the batteries through an appropriate BMS and deal with it only charging to ~86%. That comes with its' own pro's and con's and I don't know if there are any quirks to the various BMS's used on power tools, so I wouldn't recommend doing that.

The added expense of buying an overpriced car charger, or finding a well built DC-DC charge controller not only makes more expensive than a cheaper, less energy efficient solution, but it's just a lot easier to grab another cheap inverter should it go bad and keep using an AC charger. Plus you can find hundreds of tutorials of people doing that vs the few designing custom solar charge controllers. It's only a matter of time before some of these things get worked out and more common. The massive flood of lithium batteries about to hit us will surely have inventive people out there finding new ways of utilizing them.

With the charger I put together it takes 6 amps at 14.4 v about and hour to charge a 4 amp hour 18v ryobi battery.  I put a meter in line to see how many amps were being pulled at what voltage.

Jeremy, I used a series of 6 x 2.7v x 30f capacitors in the electric box to act as a battery.  The controller needs a place to maintain voltage.

Here is how I made the super capacitor.

https://youtube.com/shorts/WGVlE7Bk9tA?feature=share

Christopher Shepherd wrote:With the charger I put together it takes 6 amps at 14.4 v about and hour to charge a 4 amp hour 18v ryobi battery.  I put a meter in line to see how many amps were being pulled at what voltage.

Jeremy, I used a series of 6 x 2.7v x 30f capacitors in the electric box to act as a battery.  The controller needs a place to maintain voltage.

Very interesting.  Is there some sort of amp limiting circuitry in it? I would be concerned you could short out the tool battery bms.

My understanding is that volts are pushed, while amperage is drawn. So if you plug a 12 volt item into a 48 volt circuit, you are going to fry your item. But if you plug a 0.5 amp item into a 100 amp power supply, your item is only going to draw the amperage that it uses.

Jeremy VanGelder wrote:My understanding is that volts are pushed, while amperage is drawn. So if you plug a 12 volt item into a 48 volt circuit, you are going to fry your item. But if you plug a 0.5 amp item into a 100 amp power supply, your item is only going to draw the amperage that it uses.

Sometimes yes on simple resistance circuits but sometimes no. Lithium can charge very fast but can also overheat or fry the control circuitry. A device like a tool battery pack would be regulated by the charger with the company never selling a charger for it able to supply more amps then it can handle.  

Thanks David, that makes sense. I intend to only charge the Lithium batteries with the manufacturer's charger. And my understanding is that Christopher Shepherd is doing the same. But we are calling two different things "chargers." The one is the 12 volt system for charging a 12 volt battery (or supercapacitor bank) and the second is the manufacturer's lithium charger. Or, really, we are talking about three things, cause the entire assembly could also be referred to as "the charger."

Sorry about the confusion.  I used a ryobi 12v to 18v charger to charge the lithium battery.  I used the super capacitor to trick the mppt controller into thinking a battery was hooked to it.  This make it so it will charge more directly with the sun and I don't need a battery bank.  The mppt controller I set to stop charging at 14.4v.  I set the output work port on the mppt control to shut off at 11.8v. The panels I put in series and put out about 40v together.  This starts charging with less light as long as there is 6 amps available for the ryobi charger.

I hope this clears things up.  I only have charged a few hundred times with the 12v to 18v ryobi charger, so I don't know if it will last a long time or not.  I am experimenting.

Christopher Shepherd wrote:Sorry about the confusion.  I used a ryobi 12v to 18v charger to charge the lithium battery.  I used the super capacitor to trick the mppt controller into thinking a battery was hooked to it.  This make it so it will charge more directly with the sun and I don't need a battery bank.  The mppt controller I set to stop charging at 14.4v.  I set the output work port on the mppt control to shut off at 11.8v. The panels I put in series and put out about 40v together.  This starts charging with less light as long as there is 6 amps available for the ryobi charger.

I hope this clears things up.  I only have charged a few hundred times with the 12v to 18v ryobi charger, so I don't know if it will last a long time or not.  I am experimenting.

Thank you for the clarification. That makes a lot more sense.
David