The View Up Here

Random scribblings about kites, photography, machining, and anything else

Care and Feeding of KAP Batteries

Posted by Tom Benedict on 23/06/2013

There are a lot of choices when it comes to the batteries used for kite aerial photography. Over the years I’ve used alkalines, nickel-metal hydrides, lithium ion, and now lithium polymer. Each has its own requirements for care and feeding. Here’s a look at some of these, and how to make sure you get the most out of them:

Alkaline

These are single-use batteries, and typically come in AAA, AA, C, and D cell sizes. For most KAP applications, AAA and AA are the only sizes that really apply. Alkaline chemistry provides 1.5V per cell, so a 2-cell battery pack will provide 3V, a 3-cell provides 4.5V, and a 4-cell provides 6V. Because alkalines aren’t rechargeable, I don’t like to use them for applications that will draw a lot of power. But for something like a small autoKAP rig, these can be an ideal choice. In case batteries go dead in the field, these can be found practically anywhere in the world.

NiMH

NiMH batteries can be built into packs, or can be used as individual cells. I like using the individual AAA and AA cells Sanyo sells under the Eneloop label. These will fit anywhere an alkaline AAA or AA battery will fit, so you don’t have to change your battery holders in order to make this switch. NiMH chemistry provides 1.2V per cell, so a 2-cell battery pack will provide 2.4V, a 3-cell provides 3.6V, and a 4-cell provides 4.8V. Depending on the manufacturer, NiMH batteries can sometimes have a high self-drain rate, so it’s worth checking. Eneloops tend to have a very low self-drain rate, so you can install them in a rig and leave them until they need charging. For transmitter batteries, I re-charge after each session. For KAP rig batteries, I’ll run on the same set for a month before pulling them out to charge.

NiMH batteries charge at a 1C rate or lower. The “C” in 1C refers to the cell’s current capacity. A 2000mAh cell can be charged at 2000mA, or 2A, but they can be charged at lower rates as well. Some chargers will charge until a particular voltage is reached. Others will charge until the current the battery is drawing during charge drops below a certain level. An inherent characteristic of NiMH chemistry is that the cell’s temperature will rise sharply when it reaches full charge, so some chargers have a temperature sensor that will tell the charger when to stop charging. In all of these cases, to charge a NiMH AAA or AA cell, the batteries are loaded into a charger, plugged in, and left until the charger indicates full charge.

Eneloops

Lithium Ion

When I started flying a DSLR, I could no longer use Eneloop AA batteries for my camera. As with most cameras these days, the Canon T2i uses a Li-Ion battery pack. Li-Ion batteries have a low self-discharge rate, so they’re great for applications like digital cameras, in which the camera may sit for long periods of time without use. Li-Ion chemistry provides 3.7V per cell, so a 2-cell battery provides 7.4V, and a 3-cell provides 11.1V. (I’m not going above 12V on cell voltage, because there isn’t much on a KAP rig that requires more than that.)

So far the only Li-Ion batteries I’ve used for KAP have been camera batteries, so these have all had dedicated chargers. Just like the Eneloops, I pop the battery into the charger and plug it in. When the charger says the battery is ready, I put it back in the camera.

Lithium Polymer

These are the real bugaboo when it comes to batteries. We’ve all heard the horror stories: unbalanced batteries gone bad, batteries burning or exploding, brimstone and hellfire. While all this is true to some degree, as long as you treat Lipo batteries kindly, they can provide excellent long-term service. I first started using these when I started flying RC airplanes, and have since introduced them into my KAP bag.

Lipo batteries provide 3.7V per cell, just like Li-Ion batteries. This makes things a little awkward for 5V systems like RC radios, but it’s perfect for 12V systems like video transmitters, receivers, and monitors. Some RC gear is designed for Lipo chemistry, and can handle the 7.4V of a 2S pack. But unless your radio and servos all claim this capability, though, don’t plug a raw 2S pack into your gear. It’ll fry gear that’s designed for 5V supplies. In order to use Lipo batteries on a 5V system, some sort of voltage regulator is required. I use a 5V 3A UBEC on my KAP rig. It provides regulated 5V power to the radio and servos, while still allowing me to use the 11.1V of the battery to power my video gear.

That’s the good. Now for the ugly: Lithium Polymer chemistry is inherently unbalanced. That is to say, if you build a 3-cell pack and charge it with a 12V charger, one cell will inevitably draw more than the other two, and will charge to a different base voltage. Or… it will over-charge, self-destruct, possibly catch fire, or explode. The solution is simple: keep your batteries balanced. More on this in a sec.

The other drawback of Lipo batteries – one that’s shared with Li-Ion and to some degree NiMH – is that once they discharge below a certain level, they cannot (or should not!) be recharged. So it becomes imperative to monitor the battery level to make sure they don’t drop below that threshold. The solution to this is simple as well: monitor your battery voltage and keep your batteries charged.

There are a couple of tools that will let you do both of these jobs.

Rig Lipo

In this photo I show one of my rig batteries – a 500mAh 3S pack – with a low voltage monitor on the left, and a battery balance monitor on the right. The low voltage monitor was about 2 USD off of Ebay, and the battery balance monitor was about 6 USD off of Amazon. Both will monitor the battery voltage and let out an ear-piercing shriek if the voltage drops below 3V per cell. The low voltage monitor also gives you visual feedback with a set of LEDs. Three green LEDs mean all three cells are good. If one goes red, that cell has dropped below the threshold voltage, and the battery should be balance charged. I use a low voltage monitor on my rig when it’s in the air.

Rig Lipo - Reads 11.4V

The battery balance monitor has some nice added features: It’ll cycle through the voltages on each of the cells in the pack, as well as a total for the entire pack. Here you can see my pack can provide 11.4V. A fully-charged Lipo battery will provide closer to 12.6V. I keep one of these in my KAP bag so I can check all the cells in each of my batteries prior to use. Since these also have a low-voltage alarm, you could just fly with one of these on your rig. But I opt to keep mine on the ground and use the lighter weight low voltage alarm.

Earlier I referred to “balance charging”. Remember that Lithium Polymer chemistry is inherently unbalanced. You can get chargers that will charge the entire pack, but they will do nothing to try to maintain balance between the cells. These are dangerous to use! They assume that the battery has balance circuitry built into it. Unless your battery explicitly says it has built-in balancing circuitry, don’t assume that it does.

Lipo Charger

Since I got into Lipo chemistry for flying RC airplanes, I splurged and picked up a nice charger. Most chargers require 12V power, assuming you’ll either be using your car to run the charger, or that you have a 12V bench supply. The one I got will take either 12VDC, or 110/220AC. This is a really nice feature to look for in a charger if you don’t already have a 12V bench supply.

This charger will charge NiCd (a battery chemistry I haven’t discussed), NiMH, Li-Ion, Lipo, and LiFe (another chemistry I haven’t discussed), as well as lead-acid batteries like gel cells or SLAs. For inherently unbalanced battery chemistries like Lipo and LiFe, it offers balance charging. All this means is that it monitors each cell individually, and makes sure that all of the cells in a battery pack finish charging at the same voltage. So far it’s been able to charge my Lipo batteries to within 0.01V per cell. Not bad.

If you take the plunge and jump into Lipo chemistry for your KAP gear, I highly highly recommend doing some homework and splurging on a good balance charger. Unfortunately there are hundreds of chargers out there, so there’s a lot of homework to be done. Most of these cater to the RC aircraft market, however, so some of their features may not be as useful for KAP. For example, some chargers can charge multiple batteries simultaneously. This is great if you have a power-hungry airplane or helicopter that can drain a battery in fifteen minutes. Put four of them on the charger, and in half an hour you have an hour’s worth of battery ready to go. Neat! But for KAP this feature doesn’t really help much. Our power requirements are so low, batteries last for hours. There’s really no need to charge multiple batteries at once.

For me, I wanted something that would take AC or DC power, charge a wide range of battery chemistries, and could balance charge Lipo batteries up to 24V. And that’s exactly what I got.

Why Lipo?

Fair question! For years I didn’t have a compelling reason to switch to Lipo chemistry for KAP. Think about it: The voltages are weird, they require special monitoring and charging equipment, and there’s the risk of fire or explosion. Why go there if you don’t have to? The only reason I started using Lipo batteries for KAP was because I finally had a reason that outweighed all the negatives: I needed 12V power for video.

Even using AAA batteries, sticking with alkaline chemistry would’ve required eight batteries to get the 12V I needed to power a video transmitter. I tried it, and the resulting battery pack was ungainly, heavy, and an incredible pain to use. I even tried a battery pack that had the batteries separated into two banks of four each so my video gear would get 12V while the radio gear got 6V off the same pack by only using half the batteries. I almost fried my GentLED cable doing this, and wound up with all sorts of other problems. My last ditch effort to avoid Lipo chemistry was a 4xAAA NiMH pack powering a boost-buck regulator that output 12V for the video system. But the resulting power was so glitchy, I never got a decent video signal.

Using a 3S Lipo battery on the rig solved that so cleanly, I couldn’t go back. Eventually I ditched the 4xAAA NiMH pack I’d been using to power the radio and servos, and replaced it with a 5V UBEC, fed by the same Lipo battery that was powering the video transmitter. The weight of my rig went down, the overall complexity of the power system went down, and the video no longer had power glitches.

Once you take the leap to using Lipo batteries, most of the real heartache is behind you. One charger will charge all your Lipo batteries. One battery balance monitor will monitor all your Lipo batteries. Adding one more battery to the mix doesn’t make life any harder than you’ve already made it. So I started looking for other places I could use them.

Monitor with Lipo

My video receiver and monitor was a good application. Both required about 12V and had fairly small current requirements. One of the real marvels of Lipo batteries is that they come in all sorts of sizes and shapes, provided those shapes are all rectilinear. (Lipo, Li-Ion, and Li-Fe cells are all flat rectangles. NiMH, NiCd, and alkaline cells are all cylinders.) The monitor had a rectangular cavity on the rear right side of the case. All I had to do was measure it and find a 3S Lipo battery of that size that could provide at least 2500mAh of current. It took less than ten minutes of cruising the web to find just the battery I needed. It fit perfectly, and provided plenty of capacity to power both the receiver and the monitor. Powering my ground-side video system went from being a logistical nightmare to being a simple click-n-ship web purchase.

Why use batteries at all?

All this talk of batteries and KAP assumes that batteries are required at all. Honestly, they’re not. If you’re willing to use film cameras, the requirement for batteries goes away. A wound timer can trigger a shutter as well as any battery mechanism. One of my KAP rigs holds the camera lens-down – no pan, no tilt, no motion at all. Even rigs that move don’t strictly require batteries. Most early KAP rigs used spring-wound or rubberband-wound mechanisms to pan and trigger the camera. One of my favorites – a rig built by Timonoko – uses a propeller to turn a worm drive that pans the rig slowly using wind power alone.

If you don’t need batteries, don’t use them. If you need batteries but don’t need the benefits that bizarre chemistries like Lithium Polymer provide, skip the headaches that come with them. But if you find yourself in a corner, trying to design around something that a Lipo battery would solve, don’t worry. They’re not that nasty to work with.

– Tom

P.S. Bonus points to you if you saw that I had the charge leads plugged into my balance charger backward! When I plugged it in after taking that picture, it came up with a warning, “Polarity Reversed!” and refused to let me charge the battery until I’d plugged the cables in correctly. See what splurging on a charger does? It pampers you!

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