Batteries are often sold branded with variations on the words Durable, Reliable, Heartbeat, or some critter like Viper. Some make claims regarding something special (the implication manufacturers do it just for them). We call bullshit on this because hobby vendors buy batteries in quantities measured in thousands from manufacturers making millions/year.
Meanwhile, nobody (not us, nor our competition) looks for poor quality cells because we're all aware of what's at risk, e.g. your RC model airplane. Especially considering we're modelers also. Anyway, our advice regarding battery packs is use common sense before ponying up for what amounts to little more than snake oil, understand?
This 2S2500mAh A123 (LiFePO4) battery pack is great for powering the avionics (receiver/servos) of a radio control model with 4-6 HV servos because it's reasonably light, offers incredible current delivery (50A continuous), and has enough capacity for 4-6 flights between charges. It's equipped with a balance connector and two JR-type discharge connectors (pin spacing of 0.1 in or 2.54 mm is the industry standard). What's the benefit of 2 vs 1 connector?
- Doubling the connectors cuts current flow through each by 1/2
- Two switches make for cheap crash protection in the event of failure
Reduced current flow: basically, splitting the current through two connectors means less resistance to current flow. This reduces failure due to heat build up. This is important because JR-type connectors are rated at 5A. Since this battery can deliver 50A-continuous, a single connector may melt if your system demanded that much current! Fortunately - in the real world - even a giant scale or jet model with 12 high torque servos doesn't see more than 5-7A of current so reducing current by half is plenty good enough.
Redundancy: while reducing current by 1/2 is a nice benefit, the real reason for using two connectors comes from using two on/off switches. This is because on/off switches are the single-most unreliable piece of equipment we install. At that, switch failure is rare, but the odds of two failing at the same time is literally astronomical. Think of it as cheap insurance!
Further thoughts on redundancy: of course, as with everything else, you can go overboard with redundancy. To the point we see folks equipping models with multiple receivers, dual batteries, plus expensive 'boxes' to manage two battery packs. And for what? While we'd love to sell you two batteries, the fact is we believe using two in a model is kind of strange because batteries (consisting of a couple of cells plus the leads - or just three solder joints) are practically the most reliable piece of equipment within the whole model.
Heck, why not have a back up pilot with a whole separate radio while you're at it? Put another way, when does redundancy become ridiculous? Of course, fear sells and nothing gets a rube to reach for his wallet more quickly than extolling the virtues of some pricey system that manages two batteries! So this is why the dual battery systems are big sellers, and while we also occasionally fly models costing a few thousand bucks, we sleep just fine using one battery and two switches. But it's your money and we won't say no to sales consisting of two packs for one model even though we know one is perfectly good enough.
Of course, maybe we think one is good enough because we're engineers and know what happens as you add components. After all there's nothing like a class in statistics to teach you what every good jackleg engineer knows without going to college; the more components, the higher the risk of failure! Ever eyeball inside one of those fancy 'box' system? if you do you'll realize they're more complex than the receiver 'and' servos. All we can say is there's a lot to be said for keeping it simple!
Failure mode: meanwhile, remember this; batteries don't just up and quit. They give ample warning they're getting tired because capacity goes down over time and once they get to about 75% of the original capacity, we recommend just tossing it and buy another. This is why our decidedly plain looking label has where to write the in-service date with a Sharpie.
By the way, these LifePO4 chemistry packs (A123 is the brand) are good for about 1000 cycles (and you probably won't see much degradation before you get maybe 200 flights out of them, which is a 'lot' of flying). The point is, you have to judge when you have enough redundancy to sleep. In our experience, one pack, two leads with two switches is plenty good enough. The point being; when someone's selling fear, a grain of salt is usually worth its weight in gold.
ProTip: pre-flight procedure with two switches. Switch one on (it doesn't matter which), verify control function, and switch it back off. Next, switch on the second one, and again verify control function. Now switch on the first (so they're both turned on), and go fly secure in the knowledge you're protected with the simplest and least complicated method possible because in the rare event of switch failure, everything isn't riding on just one!
Chemistry, construction, and charging; while LiFePO4 (lithium iron nano phosphate) packs shares a similar chemistry with a LiPo and LiIon (lithium-polymer and lithium ion) batteries, these cells are different in that they're constructed within a larger metal 26650M1B-cylinder (26mm diameter x 65mm long) vs a plastic bag no thicker than a Ziploc. And they're much more robust.
Similar to a really fat AA alkaline, these cells/packs are a great way to power your receiver and servos because they're very tolerant of abuse. However, your charger should have a LiFePO4 (or A123) setting - use it because it will charge to a voltage of 3.6 volts per cell with a tolerance of around ± 50 mV per cell. Discharge to 2.0V/cell is just fine with them. When not in use, they store best at around 60°F and a 40% charge level.
Thoughts regarding BEC use; if you fly electric models and like the convenience of powering the control electronics off the propulsion battery pack by using a BEC (Battery Eliminator Circuit), please don't! Isolating control electronics from propulsion is way smarter because the servos and receiver operate more reliably. Basically, a battery supplies perfectly clean DC voltage while voltage synthetically derived via FETs is garbage. Learn more here
Background: with the advent of surface mount technology, the reliability of servos and receiver has become exponentially greater. Basically, crashes that don't involve pilot error are usually due to a switch failure. This 6.6VDC 2-cell LiFePO4 battery pack aims to increase reliability by reducing cell count - and by adopting proven A123 lithium iron nano phosphate chemistry, plus two switches. The benefit of this chemistry is greatly increase current handling capability, which is important due to the demands imposed by digital servos. And using two switches is a no brainer!
Traditional receiver battery packs rely on NiCd/NiMH chemistry. The packs consist of 1.2V cells and depending on whether they are in a 4-cell or 5-cell configuration, output 4.8V or 6.0V. The beauty of the lithium chemistry battery (similar to what's used in mobile phones, laptops, and even a Tesla for powering the propulsion motor) is each cell outputs 3.3V. Using only 2-cells results in 6.6V, and with higher voltage, due to Ohms Law,
means current goes down. The practical effect of this is better
performance at the servo and reduced loses due to extensions and results in fewer interconnections for greater reliability, e.g. a win-win!
further advantage of the LiFePO4 chemistry is a greatly increased
ability to respond to the current demands. Basically, this 30C pack can
deliver significantly higher current than is needed by the avionics (75A continuous). This is important is with digital
servos because they may draw a lot more current than old-school analog
servos. Plus, if you're an aggressive pilot, e.g. flying 3D (or are a
competition driver) when the servos are moving more and deflecting
further, they're drawing more current. Bottom line? A higher C-rating is
a big deal.
Finally, if you'd like some Pro Tips on lithium chemistry packs, read this:
Best practices regarding the care and handling of LiPo battery packs.