What's All This Battery-Charging Stuff, Anyhow?

This article is in Bob Pease on Analog Volume 2 in the Analog section of the Electronic Design Library. 

Members can download the PDF ebook.

On October 4, at noon, I sat down at my breakfast table, and plugged in my soldering iron. I was going to build the circuit shown in Figure 1. I had one hour to put it together, which was enough time, and then I was going to drive down to the airport to fly to Kathmandu.

As the soldering iron was warming up, I looked for the collection of parts that I needed for this circuit and had shoved into an envelope. Rats! Where were they? I knew I had left the parts in a safe place. I searched in every reasonable spot, every pocket of my briefcase, and all around my house. After 10 minutes, I gave up and unplugged the soldering iron. (Later, 10 miles up the trail above Namche, I found the parts in an envelope in my trousers' left front pocket, which, of course, was a "safe place.")

Fortunately, the circuit that I was going to build was just a spare, a back-up, and we never needed it. So, it wasn't a big deal that it didn't get built.

About eight years ago, I explained in "What's All This Battery-Powered Stuff, Anyhow?" that I used a gear-motor with a hand crank, at about 40 RPM, to charge up the batteries for my new Sony camcorder when I was off backpacking or trekking. That was better than nothing. But the gear-motor's maximum output—barely 2 W!—was limited NOT by the motor, nor by the strength of your arm, but by the gears' maximum allowed torque, which was NOT a lot. So a couple of years later, I bought a small solar panel that could put out much more charge on a typical sunny day. Next, I bought a bigger, yet lighter panel. Then when I was in Kathmandu, I discovered that one of my panels had apparently quit (really, it hadn't), so I bought another panel from Lotus Energy (see the table).

Because my camcorder batteries have been mostly NiCads, I used a simple circuit and just let the solar panel's photocurrents flow into my batteries. The circuit shown in Figure 2 is merely a simple scheme with a Schottky rectifier to connect the solar panel's output to the battery, plus a detector to show if the battery isn't connected. If the battery is below 8 or 9 V, the LED will NOT turn ON, and that's GOOD. That means the battery is getting charged, and holding the voltage low. But if the voltage is above 10 V, the LED will turn on, indicating that the battery is NOT getting charged. This is a bad thing, so the LED signifies bad news. It's time to re-adjust the rubber bands! I mount the LED right near the banana plugs, which I keep outside of my pack's back pocket, while the battery rides inside the pocket. The solar panel is lashed on top of my pack.

The number of 1N4002s in series at D2, D2.5, D3, D3.5 should be perhaps two or three, but maybe more, depending on your actual battery. I recently found that one of my batteries has six NiCad cells in it, not the usual five, so I had to use a couple of extra diodes in series, or the LED wouldn't have gone out!

Many SONY and RCA camcorders have a simple flat interface to the battery, where it was easy for me to set up a couple of small blunt bolts or pins, to be pressed against the recessed terminals of the battery. The connector should be arranged and keyed in such a way that it cannot be applied BACKWARDS to the battery. The sketch of how I did mine is shown in Figure 3. I used tin snips to cut copper-clad 1/16-in. epoxy material into thin strips, such as 3/8-in. wide. And I used a hacksaw blade to saw a dozen gaps in the copper. I soldered three of these thin strips (at the places marked with S) to make a triangular frame, which is easy to strap to the battery with a few rubber bands. I used the isolated foil areas to solder up circuit nodes, such as the LM334 and various diodes.

How do I know how much charge to put into a NiCad battery? I have several two-hour NiCads (2000 mA-H). If the battery gets low, and the camcorder shuts off because it's low, then I can put in well over 1 A-H, or 0.3 A × 3 or 4 hr., before I need to taper off. Usually, if I'm charging up one battery, I'm using another one to record with, so rather than worry about EXACTLY how full it is, I just swap batteries and fill up the one I was using.

What if I'm going to leave camp and leave my battery charging in the sun? I will usually put the solar panel in a sunny place, and lay it out at an angle so that the solar radiation will get more oblique as the day goes on, and the rate of charge will taper off. I might just put the panel FLAT on the ground. At noon, the sun will come booming in, but in the afternoon, the panel's output will drop a lot. If the panel's output falls to 0.2 A, a 2-A-H battery can take that much current in for a long time with no harm (C/10 rate). My panels can put out about 0.4 A, which is NOT a horrible amount.

Many modern camcorders have a gauge to tell you if the battery is nearly full, or what. (Some batteries come with a "fuel gauge," and most of those are rather optimistic; after one-half hour of charge, they say that the battery is full, which is obviously malarkey!) Of course, the correct way to terminate charging on NiCads is to detect when the battery has a rapid rate of rise in temperature. But I have never had to do that when hiking. For a fixed installation, I would probably set one up.

One of my trekking friends had NiMH batteries for his newer camcorder. I checked it out, and NiMHs like to get charged the same way as NiCads. Just push in the amperes until the battery is nearly full, but be sure to taper off to C/10 when it gets full. I told him to do exactly what I was doing, and he made similar adapters.

But a couple of my trekking friends had new camcorders with lithium batteries. I knew that you have to be very careful with them, because over-charging a lithium battery can lead to RAPID DISASSEMBLY. Most of you guys know what that is...or you can figure it out.

So, it's important to have a reliable regulator that will charge your lithium battery up to 8.200 V (or 8.400 V) and no higher.

NSC makes two nice little ICs that can do that, the LM3420 and LM3620. These are nice, accurate series regulators, but they're NOT easy to turn into shunt regulators. I want a shunt regulator, and here's why:

A solar panel is a current source. It puts out an approximately constant current into any load that you connect to it—even a short circuit. Plus, even if you leave its output open-circuited, it isn't unhappy. When you think about it, it's fundamentally different from any ordinary voltage source. (For further notes on 50 good current sources, see the Web seminar I gave on Dec. 6, 2000, in the archives at www.netseminar.com.)

When I put the output of a 20-V solar panel into an 8-V battery, some power is wasted, but that's not a big deal. The battery and the solar panel are both happy about this situation.

If I want to get all of the energy possible from that solar panel, I feed that current into a series stack of two 8-V batteries. To do that, you need current-mode charging, and you need shunt regulators, not series regulators. Then if my batteries are low and I'm just coming into a period of sunshine after a long spell of clouds, I can simply stack two of them in series and shove the current through BOTH of them. Best of all, these batteries, with their shunt regulators, are mix-and-match, so I can stack any two in series. I can charge up one of my NiCads in series with an 8.2-V lithium, or a NiMH in series with an 8.4-V lithium, and the solar panel just kicks the charge into both. If one of the lithiums gets full, the charge is shunted through the power transistor and it keeps flowing through the other battery, so charging continues. This wouldn't be possible with a series regulator. Because we had about 12 batteries, four solar panels, and about 10 regulator modules on our trek, we weren't only self-sufficient, but we were inter-operable. As a result, the loss of any one regulator or any one panel wouldn't stop us. It would barely slow us down.

Could I use a switching regulator to convert 20 V at 300 mA into 8 V at 700 mA? YES, in theory I could. But I don't usually need quite that much efficiency. I built one, once, and it did work, but it wasn't a winner even though it weighed only 1 ounce. Usually, it's just fine to stack the two batteries in series. Simplicity is a great virtue—even though I have to carry around two batteries that tend to get a bit heavy!

For nine years, I have used as my standard convention, that the + wire from the solar panel has an orange (or red) banana plug, which mates with an orange (or red) banana jack, and an orange (or red) wire going to the battery. The ­ side is reversed in gender: a violet (or blue or yellow) banana jack from the solar panel, and a violet plug tied to the battery. This makes it easy to connect two batteries in series. (Yes, I know it looks funny when the two batteries are connected in series by plugging a yellow plug into an orange jack, but it's perfectly OK, and nothing can go wrong.)

Further, there's hardly ANYTHING more repairable than a banana plug (which works great with no solder) or a banana jack (wrap the wire around the tab and crimp it or put on a minigator clip). We had zero trouble on the trek with our wires or connectors. (When the porter's RCA plugs failed, we managed to coyote them up—lashed them in parallel—and the lamp kept working every night.)

Now let's look at the circuit of Figure 1, the critical one for Lithium batteries. The LM4041-ADJ is basically OFF if the battery voltage is below 8.2 V. The circuit draws only 140 µA. This means that all of the current from the solar panel flows into the battery. When the voltage gets up to 8.2 V, the LM4041 sees 1.24 V at its ADJUST pin, so it turns on, and it turns on the big NPN to shunt off all current necessary to hold the battery voltage at 8.2 V. When this happens, there's enough current to turn on the red LED. So, if the battery is fully charged, the red LED turns ON and tells you this. Or, if the battery isn't connected, the red LED will also turn on. You may have to check your connections to tell which is happening. Still, it's a good two-mode indicator. It only wastes 10 mA in the LED when the battery is NOT getting charged. But the big NPN has to carry as much as 400 mA, and it can get hot, so be sure to provide an adequate heat fin.

This circuit is set up to be trimmed to 8.2 V, but if you disconnect the link L1, the voltage goes to 8.40 V. Which one should YOU trim for?

Connect a couple of small wires to your lithium battery's terminals, and monitor the voltage with a DVM as you charge it. If it stops at 8.2 V, that's what you need. One of our guys had an 8.2-V battery in his Canon Elura. The other guy had an 8.4-V lithium in his Sony. Most people would set up the regulator for just their battery. The circuit of Figure 1 was going to be trimmed for BOTH voltages, with a link to snip to get the higher voltage. How do we know for sure? We brought two DVMs to Kathmandu, and then we brought the lighter one along on the trek. We just trimmed the basic circuit of Figure 1 to 8.2 V by adding various high-value resistors across the 12.4 k in order to get the voltage to 8.2 V ± 0.25%. Then if we needed 8.4 (see the table), we would just undo the link. (Because we had low-temperature solder from Radio Shack, we could reconnect any wires using a match for heat.)

The 8.2-V battery for the Elura clipped onto its regulator by rubber-bands. The 8.4-V battery for the Sony had two small sockets set into the battery, and a couple of MINI banana plugs matched those sockets perfectly. I used to be nervous about lithium batteries, but now I'm perfectly comfortable with them. When you charge the battery with this circuit, it pulls the battery right up to 8.2 V, and then it keeps it at a full state. The LED tells you that it's full. Even if something FALLS OFF, the battery cannot be over-voltaged or overcharged.

What's the right way to charge lead-acids? (Fig. 4). That circuit can put out 14.4 V to bring a "12-V battery" up quickly. But after it gets up there, hysteresis is added through the 18 k, to bring the voltage down to a float voltage of 13.4 V. This circuit does have temperature compensation, because on a hiking trip or trek, you could easily have a working temperature range between +120°F and 0°F. You wouldn't want to over-charge the battery when hot, and you wouldn't want to under-charge it when cold, which is what would happen if you charged it to a fixed 13.4 V at all temperatures. (The other types of batteries don't need temperature compensation.)

So trim that pot to get 13.4-V DC at no load at 25°C, and (13.4 V ­ 22 mV/degree) at temperatures away from +25°. In this circuit, the LM334 is NOT used as a current-source, but as a low-voltage comparator. This circuit is a series regulator because you won't be stacking two of these!

(The town of Namche Bazaar has good reliable 220-V AC power. But some of the innkeepers have learned to charge tourists and trekkers high prices—as high as $4 to $7 for charging one battery. When they get you over a barrel, they really know how to get you! Also, above Namche, there isn't a lot of reliable electricity available. Therefore, it would be very hard and/or expensive to bring a group of batteries and only record a LITTLE. By bringing my own charging equipment, I had no trouble recording 23 hours of video in 35 days on the trail. Of course, it took up a lot of my Christmas vacation to get all of the video listed and ready to edit down to a few one-hour tapes!)

Several people along the trek asked why we were carrying these solar panels, and we explained. Some of them said, "Hey, that sounds like a really good idea. I left my camcorder at home because I couldn't figure out how to charge its batteries. Let me know when you can tell me how to do it!" Well, that's what this column is about.

The flashlight that I hooked up to a battery can be seen in Figure 5. The LM334 and 2N3906 form a 100-mA current source. When you unplug the solar panel and plug in this flashlight, it's a pretty good little light. Normally, you wouldn't want any discharge path if you shorted the orange and violet terminals together. But because this is just a regulated 100 mA, the battery won't be abused. The components of the little current regulator are easy to mount inside of one of the A-frame members. You might switch out one or two of those 2-Ω resistors to adjust the brightness.

I arranged the LEDs (Digikey Part CMD333UWC-ND, about $3) in a fan array, to make it easy for reading. You can point them anywhere you want, though. The current source shown here isn't as efficient in voltage as the one I showed back in the September 5 issue. I did that on purpose, because I wanted this circuit to quit drawing current if the voltage supply gets down to 6 V to protect the battery. This circuit does have the advantage that it connects to the load and the battery with just two wires, rather than four. Therefore, the wiring is much easier and safer. Now my best flashlight is easy to recharge! A solar-powered night-light!

These are some of the circuits and procedures, the tricks, and the connectors that kept us running with plenty of charge for over a month. Did our batteries ever get low? Yeah, after three days of gray weather. That's why we like to keep our batteries charged up pretty full, almost all the time!

All for now. / Comments invited!
RAP / Robert A. Pease / Engineer
[email protected]—or:

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