I see a lot of guys using dual batteries on their large airplanes. I don't - even with $3000 models. Why not?
- I never needed two battery packs with my 60-size models.
- Almost 50 years as a modeler means I know batteries don't fail like light bulbs.
. . . instead, they give lots of warning. For example, a 2500mAH pack; over time begins to deliver less capacity, maybe 2000, or 1800mA. That's when you replace it.
Switches? I throw them away once a year. Cheap insurance. And I don't use fancy electronic ones costing $40 that supposedly fail in the on-position because for that to actually happen you're depending on an electronic component - like how dumb is that? Instead, I like $10 heavy duty mechanical switches. Why? Simple, I can do math! Odds of two failing on the same flight are ASTRONOMICAL!
Two switches? Yup, one battery pack with two leads - and - I use two switches in parallel. Since all receivers uses a bus, this means you can plug in both leads (one connects to BAT and the other to any spare channel - it doesn't matter). Side benefit is current flow through each connector is reduced in half (total flow is the same, of course). Further benefit, this means less heat through the connector. This can be a big deal! Why? It's because the typical universal connector (pin spacing 0.10" or 2.54mm) is rated at 5A. Using two gives you 10A continuous, which is plenty.
Want proof? All you need is grade school math. A 3000mAH pack (3.0Ah) rated at 15C will deliver 3x15=45A continuous current. E.g. waaaay more than the model will ever in a million years demand! For example; our DS470BLHV will pull 3.5A while stalled (we list this in the specs tab). Meanwhile, the 630oz-in servo goes 4.1A. But in the real world, they don't stall because good modelers check for binding.
How? They use a cheapo VOM (volt-ohm-meter), cut the leads and solder them into a 3" servo extension (thus creating a custom harness for measuring current draw). Anyway, they connect it in-line with each servo, switch the VOM to DC current, and check each servo at full left, full right, and center. They also note current draw at idle. Easy to see if there's a problem. Inexperienced modelers? They depend on matching routines built into the 'box'. My advice? Trust but verify!
Anyway, in practice, a model with a full complement of high current servos typically draws 3-5A. In theory, 'if' they're all stalled, they could draw 40A . . . but in reality, 3-5A, maybe a maximum of 6-7A total - and this during an 'aggressive' flight. The average guy? Even with a 40% airplane equipped with 630 oz-in servos, he 'might' see 5A total with a few spikes of 7-8A. Note, this last, spikes, means momentary current, not continuous.
Anyway, using two leads through two switches (for redundancy) gives you;
- Reliability and,
- an inexpensive system
. . . and it's one capable of supplying 10A continuous, which is plenty!
Here's the rub . . . there's no money in two cheap mechanical switches. So marketing gets involved, and they sell fear of loss (honest to God, fear of loss is the technical term). Source? As well as taking a degree in engineering, I took a few business course to include marketing, accounting, statistics, etc). Heck, I even went back to college after selling my first business and took another degree thereby becoming a high school math and science teacher (no money in that, either but I digress).
The point? Nothing gets a rube off his wallet quicker than marketing something using the word 'safety'! Especially when he's got $3000 into a model airplane, understand? This is why it's so easy to persuade guys who should know better that it's suddenly smart for them to carry the weight of an extra battery pack, which they never needed before because they're so reliable. And the key to it boils down to a mind game, which they work hard to create - known as fear. And as we all know, fear is a powerful motivator! Me? I wryly shake my head in admiration of the marketing pros because they're good. Remember, the expression selling snow cones to Eskimos came about for a reason!
Think about it; marketing guys basically persuade folks that something nearly as reliable as an anvil (3-components total, the 2 cells and 1-lead joined by 3-solder joints) can be made 'safer' if you just buy two of them! Yet this neglect the simple fact you learn the first day in statistics that you've increased the odds of battery-failure by 2X. And it works because only a few modelers ever took a college course in statistics!
- AND -
They also persuade you to complicate things further. How? Once again by appealing to your fear of loss. Basically, they sell you either a more complicated receiver that accepts two batteries while adding features to prevent a battery failure from causing other unintended problems (but remember, batteries are reliable as anvils) and/or they sell you a pricey 'box-system' where all the servos connect, instead. All while ignoring your common sense; and by this I mean these 'boxes' have 30-50 components so they're a lot more complicated (and we all know about KISS - Keep It Simple Silly).
One last thing, the concept of using two switches mostly applies to large model, and especially nitro/gasser models. With respect to most electric powered models; there's a better solution than two switches.
Simply install a pair of short leads into the receiver. Typically one on the BAT port of the receiver and the other in any spare channel (same as with dual switches). Note; connecting at other than the port marked battery works because the receiver uses a bus system for power deliver. This just means all the +pins are interconnected and all the -pins are similarly interconnected. Since only the signal-pins are isolated from each other, you can connect DC-power anywhere! Taking advantage of this is how a BEC works, e.g. the power flows to the receiver (in effect backfeeding it) through the throttle lead and thus, obviating the need for an avionics pack.
Anyway, regarding the 'why' for using a pair of short leads? That's simple; it's just to make it easy for 'you' to become the switch. Basically, having two short leads is just for convenience so that you don't have to root around in the fuselage looking for where to make the connection each time at the receiver itself, which may be buried.
Preflight: Your new preflight procedure is; before connecting the propulsion pack, first make the two connections at the avionics pack using the two short leads and establish control.
Postflight: This procedure is the reverse; disconnect propulsion, then control. Since it's unlikely you forget to disconnect the propulsion pack, it's similarly unlikely you deplete the avionics pack by forgetting to disconnect it.
Note; the engineer in me rather likes using a pair of leads even more than a pair of switches. No data to back it up. Just feel better about the odds.
Regarding sources for extensions and switches? They're everywhere! Some are better than others so use your judgement. Yes, of course, we offer these bits as well - but - only in 20AWG. This; because weight savings between 20AWG extensions and 24 or 28AWG is negligible. Put another way, yes there's a weight difference but it doesn't make my radar despite knowing less weight is better for maneuvering because I judge the reduced voltage loss and higher current handling capability of thicker wire is of greater importance. Especially considering the 3" extensions we offer go a mere 3g and I don't really believe our scale will reliably measure the difference between those and lightweight 28AWG extensions.