Lightweight battery charging

[David Thomas]

A gear-head, electronics-geek question:

There seems to me to be a play to recharge batteries with a small solar cell array. No voltage regulator. Just the array’s wires clipped to your cell/camera battery

Let's say your battery can charge at 1 amp (read it off your charger's output rating). And you have a solar cell array of sufficient voltage that maxs at 0.5 amps. Then the voltage will be defined by the batteries charging voltage. UNTIL THEY ARE CHARGED. So if you don't over-charge them, you could have a much lighter (and actually more efficient) set-up.

It would involve more attention and a bit more labor to check the charge level and a willingness to accept a 70-80% charge as good enough. Because it would be risky to go for 95% and over-voltage the batteries.

A smart phone, especially, has a very detailed battery-charge indicator/app. But even a digital camera will show you 3 to 6 bars for battery charge, giving you enough info to know how many hours you can safely charge, unregulated, before checking battery level by reinserting it in the camera.

Has anyone tried this?

I like the potential of the iPhone (maps, GPS, travel documents, guidebooks, recreational reading, flashlight, vibrator, etc) to lighten the load, and if 2 ounces of solar cells could give unlimited charging on the trail – that would be sweet.

[Tyler T]

Li-ion batteries, outside of a device, no way. Li-ion and Li-Po are VERY sensitive to overcharging (as in, they either catch fire or explode) and needs dedicated circuitry to safely charge. Even if it doesn't flame or pop, you'll reduce it's lifespan dramatically by direct-charging.

Li-ion / Li-Po batteries, while installed inside the device, generally yes. Devices that charge via USB already have the charge circuitry inside them, just feed it 4.4v to 5.25v at up to 500mA and it will probably be happy.

NiMH batteries, again, sure. A small solar cell is unlikely to overcharge the cells (it's simply unable to deliver enough current) so as long as you're feeding 1.4V to 2V per cell, you should be fine.

[Bob Gross]

"Has anyone tried this? "

This is a fairly bad idea. You seem to be completely oversimplifying the battery-charging-solar panel relationship.

All batteries are a little different, but a general rule of thumb is that most batteries can be charged optimally at a rate of about 1/10th or 1/20th the maximum discharge rate. You can go faster than that, but you will screw up the battery chemistry. So, somewhere between the solar panel and the battery you need to have a reverse-battery blocking diode. That keeps the battery from trying to "charge" the solar panel when the sky goes cloudy. Also in there you have the charge rate controller. Once the battery comes back up to full charge, you really want something to sense that voltage and then cut back on the continuing solar current.

And then there is a little more to it than that.

Solar panels become practical on a weight basis if you can transport them to some remote area where you need power, then set up the panels semi-permanently for use. The perfect spot for that is Mount Everest Base Camp. Incidentally, a Sherpa I know runs a lodge a few miles from there, and he has a great solar panel-charge controller-battery system. I shipped him the parts.

–B.G.–

[drowning in spam]

Have you looked at this?

http://www.cpfmarketplace.com/mp/showthread.php?229002-FS-Solar-Powered-amp-USB-Chargers-2-panels-added

[James holden]

i have always wondered about the feasibility of one of the cottage manuf making a top cover or backpack cover with solar panels and an integrated battery to store the charge

i think that would be quite in demand if it worked, and you could generally use off the shelf parts

that would definately be "innovative" IMO ;)

[Jerry Adams]

There's some voltage drop across a reverse-battery blocking diode so you lose some charging capacity, especially if the solar panel voltage is just barely high enough.

If you're there keeping an eye on it, you can just disconnect it when the sun goes down.

[Bob Gross]

"If you're there keeping an eye on it, you can just disconnect it when the sun goes down."

That might get pretty old after the first three or four times, especially if you have to take off a backpack to get to the solar panel or battery.

To make this practical, you have to have a serious demand for electrical energy. For the rest of us, it is more weight-efficient to carry extra batteries.

–B.G.–

[Jerry Adams]

"That might get pretty old after the first three or four times, especially if you have to take off a backpack to get to the solar panel or battery."

You really only have to worry about it at the end of the day. If it's daylight, you'll probably get enough light to offset any discharge. And you have to discharge for many hours to make any difference.

"To make this practical, you have to have a serious demand for electrical energy. For the rest of us, it is more weight-efficient to carry extra batteries."

Quick analysis – these numbers may or may not be valid, just what I saw quickly online

1 Watt panel weighs 1.6 pounds, maybe 8 Watt Hours per day if you're lucky.

Lithium AA battery 4.5 Watt Hour at 0.5 oz.

It would take 25 days to charge 1.6 pounds of Li AA batteries.

If you're gone less than 25 days, weighs less to just carry batteries

[David Thomas]

Tyler: Agreed, it is easiest if the device has a charging circuit inside of it. And I understand that over-charging lithium batteries is BAD. (Had a classmate who went to work at a battery firm. There were several reasons to evacuate the building and a couple more ways to light the garbage truck on fire and they all involved lithium batteries.) But are there situations where you know the battery can take, say, 100 mA for 10 hours. Then could you give it an unregulated 100 mA for 6 hours only, disconnecting after that time?

[David Thomas]

Bob, My last installation was an off-the-grid environmental remediation system powered off PVs and a 12-volt AGM battery back. I wanted / needed it to be topped off and I used a charge controller with multiple charge modes, depending on battery charge state and history. And which had the blocking diode to avoid night-time losses. I've got a decent grasp (but not lots of experience) in how to size and configure an off-the-grid home or sailboat.

But in this thread, I was looking to have a really bare-bones set-up, accepting that it requires for oversight and attention. So as reduce PV weight from something like 10 ounces to something like 2 ounces by skipping the circuitry, housing, and using only for taking batteries from, say, 25% to 75% and never allowing them to get near full charge.

[David Thomas]

Jerry, Thanks for the calc. I was figuring the break-even with off-the-shelf bits would be 3 weeks or so. I guess it depends on your route – I'd hate to go into town just to charge up batteries but if you're going to post office, food store, restaurant anyway, what the heck?

With young kids, I'm probably not out for that long unless it's a float trip on the Yukon. And then weight doesn't matter.

[Bradley Danyluk]

IMO I think at the very least you need a voltage regulator. Output from solar arrays varies wildly. I have a 10-watt / 12-volt folding array that, even in what I'd consider to be "full, direct sunlight" swings from 6 to 16 volts seemingly on a whim. It's not on a whim, of course, it's in response to the temperature of the cells (they are less efficient the hotter they get), the time of day, altitude, angle towards the sun, things like that. And good luck if you're passing in and out of shade at all.

Therein lies another problem: if the connection stays on when you're passing through shade, and charge voltage drops below battery voltage, the batteries will actually start to drain.

As for charge rates, NiMH can take a rate of 0.5 x capacity. So you can theoretically charge a 2000mAH cell at 1000mA. It will get hot and you must not overcharge at that rate. It's be hard to find a portable solar cell that could keep that up, though, in practice. Impedance increases as the cell nears capacity.
You can safely overcharge NiMH cells at around 0.05 x capacity (in the 2000mAH example, that's 100mA – though I'd aim slightly low to be safe). They will dissipate the excess energy as heat and it will not damage the cell unless you do it for a really long time.

In my experience, smartphones which charge via USB can take some pretty wild swings on voltage and keep charging. They have pretty good internal regulators, though I'd imagine if you fed them 20%+ excess voltage for a sustained time the regulator would eventually burn out. For short bursts, I've tested my Android phone as accepting anywhere from 3.5v to 7v from a hand-crank charger that didn't have a regulator.

[David K]

I hate to even post this as I can already hear the groans. The ULer I ran it by said it has taken him X years to get his base weight down and there was no way he was hauling that thing.

http://solarjoos.com/

My way of thinking is this. It has a powerful onboard battery, 5,400 mAh, and a solar panel with hard to believe capabilities. There are tons of great reviews on it. Yeah, it weighs an honest 24 ounces but a battery alone this powerful weighs 16 ounces. It seems bullet proof as they also claim.

I am taking it to the desert in a couple of weeks for initial testing. Standby.

[Bradley Danyluk]

David,

No, just no. That is so far beyond "hard to believe," it is well into (and past) the realm of fantasy. There is no solar technology on earth that can deliver 18 watts with a cell that size. Not even half that. Not even a quarter that.

There are a lot of these types of things on the market. They rely on people charging them up at home, then they think the solar panel is doing something useful in the field, when really, it's just a portable battery pack. The solar panel is trickle-charging its pack, but it always takes WAY longer than claimed, even in ideal conditions.

I had something like this, from a more reputable company, and about twice the surface area (though it only claimed 5 watts panel output… closer to reality). It couldn't keep my iPhone charged in the middle of July in Italy.

[Bradley Danyluk]

This is what I have now.

http://www.amazon.com/Sunlinq-Portable-Solar-Panel-Charger/dp/B001TI6SP8

It works, just barely, to keep my stuff charged even when a bit overcast.
In comparison to those little hybrid gadgets, it has serious surface area. I wouldn't go for anything less. Probably more, really.

[Bradley Danyluk]

Here you go. Buried in another page of specs, David's JOOS panel is only capable of 2.6W in full sunlight.

Photovoltaics
Highest efficiency commercially available, monocrystalline solar cell with silicon nitride, anti-reflective coating (125x125mm, 2.6W max)

[Bob Gross]

For many years, the rule of thumb price for a bare solar panel has been $5 to $10 per watt of output. If you get into the big 100-watt panels for homes, the price can go below $5 per watt. As you get down on the low power scale, the price is $10 or a little more. Monocrystalline silicon tends to have a higher open circuit voltage and higher efficiency, so higher price per watt. Amorphous silicon tends to have a slightly lower open circuit voltage and lower efficiency, so lower price per watt. Now you know how much you are paying for the solar panel. Amorphous is what you typically find in a flexible panel, and monocrystalline is typically what you find in a rigid panel.

The overall price of the thing tends to be much higher, and that is from the fancy housings, fancy electronics, fancy intermediate batteries, fancy connectors, etc. The trouble is that some people are paying several times more for the fancy stuff than what the basic panel is. You can cut a lot of the cost and weight out of the thing with some home-brew electronics.

Still, if you want to fix this onto the upper back panel on a backpack, then you had better be going in the right trail direction to keep the sun on this.

–B.G.–

[Terri Wright]

I used this charger on mt JMT trip. It weighs 7oz and charges very fast. I have been meaning to post a review, but am a novice at electrical matters. Can someone suggest what measurements/data would be helpful to evaluate this charger?

[Bob Gross]

With the panel in full mid-day sun, measure the voltage and current output. Lots of people have multimeters that can measure the voltage easily, but the current is more difficult if it is high, or unless you have a good meter. Often you can use a fixed resistor as a test load. It's all low voltage DC, so you don't have to worry about getting shocked.

Edit: I copied this off their web site:

sCharger-5 USB Solar Charger Specifications
•Standard USB "A" connector
•USB output: 5 volts, 1 Amp (1000mA)
•Open size: 6-1/8" x 11.5" / 15.5 x 29.2 cm
•Folded size: 6-1/8" x 5-3/4" / 15.5 x 14.6 cm
•Indicator LED, surface mount circuitry
•State of the art flex circuits
•Weight: 7-8 oz / 200 g
•Handles temps up to 160F
•Water and weather resistant
•Patent Pending

=====================================
So, if you measured 1000mA at the output during a sunny day, that would mean something.

–B.G.–

[David Thomas]

A more qualitative assessment would be, How does it compare to your regular wall charger?

Many solar chargers are really trickle chargers – you wouldn't want to wait for them to finish the job. But if it really puts out 1 amp at 5 volts, that should charge your iPhone as fast as your wall charger.

More quantitative would be to record the %charge that your phone reports before and after putting it on the charger for 30 minutes. Compare that to the same period of time for the factory wall charger.

Ammeters are cool and all, but a lot of people are hesitant to cut into a perfectly good charging cord. I'm not, and I suspect Bob's not, but many people are.

[Bob Gross]

"Ammeters are cool and all, but a lot of people are hesitant to cut into a perfectly good charging cord."

Nor would there be any reason to do so.

–B.G.–

[Nick Gatel]

A = W / V

So if the panel is truly putting out 5 watts/5 volts then you will get 1 amp at the USB port. You will need perfect orientation to the sun for approximately 1.5 hours to charge a dead iPhone, assuming there is 100% efficiency.

Around Father's Day this year I posted a mini-review on the Goal Zero Nomad 7.

After that discussion, I did test it and output was just under 1 full amp. I have not taken it backpacking due to a "philosphical" problem with the concept. Bob and I have had several discussions about solar and we both agree it is generally not practical for the typical UL backpacker.

I do have a fair level of experience and understanding of solar systems. I installed a 250 watt system on my tent trailer. We have been using solar on our tent trailers since 2003, which includes over 1,000 camping days; if anyone wants to continue the discussion. If I were to integrate solar into a multi-week trip I would probably want an aux battery back so I could collect reserve capacity for those times my phone was fully charged.

If this post sounds dis-jointed it is because I am using my iPhone which is difficult to type on.

[Terri Wright]

Bob, I knew you would come back with something — something I can't handle! Where do you live?! I'm sure you have a super-duper multimeter and you can help me!

David, I think I can handle your suggestion. The charging times are very comparable to a wall unit, I would not consider it trickle charging. I kept 3 iphones charged on my trip — only charging when we set up camp. Granted, only my phone was used for GPS, but the others were ipoding away!

I'll see if I can put something together this weekend. Thx.

[Tyler T]

> But are there situations where you know the battery can take,
> say, 100 mA for 10 hours. Then could you give it an unregulated
> 100 mA for 6 hours only, disconnecting after that time?

For NiMH, sure. A AAA NiMH will not be seriously damaged by a few hours of 100mA even after it has been fully charged; an AA will tolerate 200mA for a while even after being completely topped off.

I'm not brave enough to try it with Li-ion, though. You'd really need a multimeter to determine the starting state of charge and then do some calculations to see how long at what mA charge rate it could accept. And you'd need to recheck voltage frequently during charging since a solar cell's output is not constant nor fully predicable. Too much hassle and still a bit risky IMHO, best to stick with Li-ion devices that have integrated chargers.

[Nick Gatel]

I would think that if you are charging a battery that is inside a phone or other device that has a USB charge port you would not need to worry about a charge controller, it is probably built into the device. Charging batteries directly is another matter. For wilderness travel (if I were so inclined to use solar), I would want a larger panel say 7 watts with a charge controller. This way in less than optimal conditions you have more solar collecting capability.

To measure amps with a digital multi-meter (DMM), it has to be capable of measuring amps, or you will need a special amp probe, which are expensive. If your DMM can measure amps you need access to two points in the charge line, which you do not have with a USB charging cord.

Here is a simple solution. Buy a very short USB extension cord with the heaviest gauge wire you can find. Now strip two access points in the charge line. Connect your probes to the points with the device connected (make sure the device is less than 1/2 charged).

[Author]

Posts