Why's my pack got two JR-connectors?

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So why do these packs have so many connectors?

As usual, another question, another white paper on the why behind something we do. This time in regards to why our receiver battery packs come with so many leads, and specifically, two of the little black Dupont connectors. So let's jump in and learn what's what and why!

Q. I bought a B2S2500 battery and am bewildered by the number of connectors. And do I use both JR connectors, and what do I do about switches?

A. Allow me to expand on the reasons for so many connectors on our batteries, and as heads up for others, we don't make batteries. Instead, I have them built to my specifications, e.g. to my heart's exact desires. So when it comes to batteries, we offer then in capacities ranging from 650mAh to 6000mAh and with several connectors to include an XT30, two Dupont-connectors, plus a balance connector - total 4 - because that's what I wanted.

What the heck?!?!? Let me explain.

This is what the fellow who wrote asking got from us, a B2S2500-15C pack, which looks like this;

- ProModeler 2500mAh 2-cell LiIon battery pack

So first, let's decode B2S2500-15C because as usual, ProModeler part numbers have meaning;

  • B = Battery
  • 2 = 2-cells
  • S = Series connections (means the voltage of each is added up for the total)
  • 2500 = Capacity (in mAh, or 2.5Ah, same thing because milli = 1000)
  • 15C = C-rating

To the latter, since it's a 15C rated pack, this is just a measure of how fast the pack will deliver it's capacity. So the math is simple and it's 2.5x15C=37.5A, which is the theoretical rate it will deliver current. Will it? Maybe.

But it doesn't matter. Why not? Because we (modelers) don't have applications for this kind of current flow within the control side of the avionics. So these are principally marketed as receiver packs and for our use, this realistically tops out in the 10A range, but we're getting a bit ahead of ourselves.


Next, let's discuss the connectors, of which there are four. A balance, an XT30, and two Dupont, or JR-connectors as they're also known.

- ProModeler battery packs are built with 4 connectors

So the small white one is the balance connector, there's a yellow XT30, and two black Dupont connectors (the ones many customers call JR-connectors). And in this order . . .

1. Balance connector is for the charger to know what voltage each cell is at. Why? Because the way hobby-grade chargers work is they charge full speed until the 1st cell reaches a set voltage while continuing to charge the other cells.

So the charger begins to discharge the 1st fully charged cell until the next cell reaches full state of charge (SOC). Then the charger also begins to discharge the 2nd cell as well, and so on until the last cell reaches full charge at which the programming for the charger dictates what's to be done, usually end the charge cycle because the entire pack has reached full. So because all cells don't reach full charge at the same time, it drains them off as they do until the last one reaches full charge. This is how it balances the full voltage level cell-by-cell for the entire pack. Hence the name, the balance connector. For a 2-cell pack, 3 wires.

Major point being, the charger doesn't slow down the rate at which it's charging, it simply begins to discharge individual cells as they reach full SOC until the last cell is at full SOC whereupon it stops.

So for all this to work, the charger needs to know the voltage of each cell, and if you count wires, the balance connector has one wire for each of the cells plus 1 more wire to complete the circuit. This is how it knows the voltage of each cell. In the case of 2-cell pack, three wires total. For a 6S pack, 7 wires.

Anyway, this has implications for how much discharge capacity the charger is capable of handling, the greater the ability for the charger to discharge, the more quickly it can complete a charge and the more quickly it can perform other functions, like discharge the charged pack down to storage more, for example . . . but this is a discussion for another day.

2. XT30 connector is for whatever purpose you make of it. It's rated at 30A and similarly, the XT60 is rated at 60A, an XT90 is rated at 90A, and on through 150A.

As it happens, 'I' (me, John, personally) use the XT30 connector for charging/discharging packs through a 1 meter extension, PDR331 (available on the site along with other charger accessories.

I use the XT30 instead of the Dupont connectors because I can move current around more quickly both in and out of the pack. If, for example I charge a 2500mAh (2.5Ah) pack at 10C, this means I am charging or discharging at 2.5x10=25 amps.

If I were to try that with the Dupont connector it would quickly melt as 25A greatly exceeds the 3.5A it's rated for (continuously). Are you beginning to understand the role of these various connectors?

- 1meter charge extension - 16AWG high strand silicone jacketed wire

Another example for the XT30; our battery packs are popular beyond receiver use with the turbine crowd. Why? Simple, it's because the XT30 connector is used for the electric start feature.

This photo of a small turboprop engine I purchased from the Spanish manufacturer Xicoy, shows where an XT30 is attached for the purpose of feeding the electric start circuit.

- Xicoy X45TP micro turbine equipped with XT30 for engine start

The reason the XT30 have become popular in the world of micro turbines is for the same reason they're useful for receiver use; they're a very versatile connector.

So a ProModeler pack (remember, available in capacities from a tiny 650mAh ranging through an enormous 6000mAh) isn't just suitable for receiver-use. It's also useful for the start duties of the incorporated electric starter of these engines. Or if you favor gas ignition engines like a DA120, then for this purpose, also!

3. This finally brings us to the two Duponts (or JR-connectors). Common as dirt in the hobby, what differentiates them is the housing (the black plastic bit). This is because various manufacturers will try to use any means fair and foul to rope you into their corral (where only they can sell you goods). Not good for modelers!

So, for example, one manufacturer molds their connector with a raised bit of blade so it won't physically go into another manufacturer's receiver, or switch assembly. Sure, you can just shave the bit of blade down and presto, it works - but - we don't feel you should have to.

ProModeler? As a matter of company policy, we don't believe in artificial incompatibility. Thus, we work as hard as we can to ensure our goods are compatible across a wide spectrum of users. We believe in open standards because its how all modelers benefit.

In other words, compete with us on the merits of your product instead of because you make it harder for folks to use another company's goods.

Barriers are bad business!

- Hobbyist call them JR-connectors, they're actually Berg, or Duponts

A bit of background, the 0.1" pitch connector (2.54mm) in the above photo originates within the computer industry. They were initially known as Berg connectors but when E.I. DuPont purchased the company they came to be known as Dupont connectors, instead. And FWIW, they're still widely used for connecting components like hard drives and such!

Anyway, they were adopted by Asian RF manufacturers (perhaps for convenience) but certainly as a cost saving factor. This, in order to more efficiently compete with American RF-manufacturers. The Asian companies came to dominate and we know them to this day as the American manufacturers were driven out of business by lower costs. These would be Futaba, JR, Airtronics (Sanwa), and eventually, Hitec, but there are many, many others.

Point being, back in the day, each RF manufacturer (these being American companies before customers put them out of business by buying cheaper imported units) had their own connectors.

For example, Kraft Systems had the Multicon connector. This nifty very well made connector was pricey, and it was slow and difficult to properly crimp, e.g. assemble - and - it was unique to them. By this meaning, Futaba couldn't use these connectors without license. So they (Futaba and others) turned to what had become ubiquitous in industry, the Dupont, instead.

Unfortunately, in my opinion, they learned what we felt was the wrong lesson, and instead of embracing a universal standard, raced to make it proprietary. How? Through the use of a custom housing thus duplicating what was wrong with the individual American company connectors. Argh!

- Back in the day, the Kraft Multicon was the connector of choice

. . . and as a side note, our first product - the genesis of ProModeler - was taking Kraft battery packs and adding a second connector, which Mr. Kraft kindly sold me, but I digress. Anyway, for the curious to learn more, review this article;

. . . because it delves into this subject a little bit further. And if you are curious regarding extensions and voltage loss, review this article;

So back to why a JR-connector is actually a Dupont, and the heart of the question, why are there two of them?

The reason has everything to do with current carrying ability, or capacity. While rated at 5A, the reality is these tiny pins will carry 3.5A continuously without overheating. Go 0.1A more than this and they begin to overheat.

However, as a practical matter, meaning for the duration of a typical flight, and for the way they're loaded by modelers (used), we (the industry) rate them as a 5A connector. And here's the rub. These days it ain't enough!

You see, back in the day (from the mid 1970s through the 2000s) when old-school analog servos consumed maybe 250mA while working, then having 6 servos in a model meant 1.5A of current (250ma x 6 - 1500mA, or 1.5A).

This left plenty of overhead when using a 3.5 ampere connector between battery pack and receiver. But these days? Nope, that's not enough. Why not?

Well, ProModeler, for example offer many servos that will consume as much as 3-8A each. Heck, some will suck down more because you can't make power out of thin air. So ponder this a moment . . . this is for each servo!

Moreover, we're not alone in this. Everybody making digital servos offers a product consuming more than in the old days of weaker slower servos. For example, our well regarded competitor Savox produce a servo (model number 2290SG) that will consume 8.3A when making rated torque! That's a lot.

As an aside, 'this' is why we give so much information within our specs tab. Unlike sites where it's a struggle to find details and for most you never do, ProModeler don't want to leave you blind to the realities of calculating current flow. We embrace the sharing of information!

Hence, we expressly tell you what each servo consumes and we share these in deep granular detail. This is why - for the various voltages common with our models - we tell you about the specs for a servo.

We tell you for 5V (4-cell NiCd or NiMH pack), for 6V also (5-cell NiCd or NiMH pack), at 6.6V (LiFe and LiFePO4), again at 7.4V (2S LiPo and LiIon), and for the 8.4V, which is typically what's delivered by synthetic means (from a voltage regulator or BEC circuit) instead of chemically (a battery pack). We do it because details matter!

So what's this mean in the real world?

Below are basic specs for a DS635BLHV servo. Note how it can suck down over 5A of current at full song!

Facts are ProModeler offer servos rated from 75oz-in through 2685oz-in - and for each - we create a table of specification. And we place it for your convenience in a specs tab so you don't have to hunt for it!

Last thing, remember, a servo's rated power happens when the servo is stalled (when it won't give you anything more). This is the line labeled Current (stalled). Importantly, this is the number you should use for your calculations of total because it represents the worst case . . . so no Polyanna pink lenses allowed!

- Every servo on the site has this info within Specifications tab

Now reality is, few modelers will operate all the servos when stalled. And even several powerful servos together may sum up to real world current flows ranging from 4-7A. But this, in the real world means one connector isn't enough. Not these days because after 3.5A, you begin to get heat. Heat is bad juju for any and all things electrical or electronic, trust me!

If all packs were offered with two leads, then we wouldn't have to bother with selling packs. Me? I look forward to the day all pack manufacturers are sufficiently enlightened such that we can eliminate all the SKUs associated with battery packs and simplify our business.

Making use of the current

Fortunately, RF manufacturers employ a bus structure for the juice flowing in and out of the receiver. As a practical matter, this means you can connect a battery anywhere, not just the port labeled BAT.

In fact, it's when this dawned on me that I realized (after crashing a model due to switch failure), that I realized I could use two switches in parallel to connect the battery to the receiver. And that idea became our original product. That it was a product based on another guy's product (Mr. Kraft's battery packs and Multicon connector and lead) didn't matter. This is fundamental physics, there's no ownership. In fact, I promised him we wouldn't try to patent it (and we never have). Anyway, the two-lead pack has been extended to today's 4-lead pack. Basically, that first product remains a mainstay of the ProModeler lineup!

These days, however, while being able to use two switches is still a fine reason all by itself to use a battery with two leads (after all, the odds of both switches failing on the same flight remain as astronomical as they were when we began), the other reason, and what's become the principal reason, is it's how it's become the only practical way for getting more current flowing in/out of our receiver.

And it's all in thanks to the foresight of receiver designers who employed a bus power structure. The same one that's built into all modeling receivers to this very day!

Wrapping up

Finally, to the specific question regarding switches, where possible . . . no switch at all.

That's right, I try hard to avoid using a switch at all. Instead, I use a pair of extensions from the receiver to where I can conveniently make the connection to the pack, manually. Yes, I do this twice, two extensions between receiver and pack.

One is connected at the BAT port of the receiver, the other connects at any free channel, (by preference the one physically furthest from the BAT port). The current balancing benefit is automatic and happens because of physics. Thus, each lead automatically carries 1/2 the load out of the battery and into the receiver. Sweet!

And when forced to use switches instead of extensions (sometimes making/breaking the power isn't convenient, think scale models, especially) then I always use two switches. And this delivers the 2nd benefit of dual leads, which is safety against model loss due to switch failure. This protection is due entirely to switch failure mitigated via redundancy.

Oh, and by the way, sometimes I need more juice out of a pack, like for driving a lighting circuit or maybe to power electric retracts. So what do you do when you need more current carrying ability?

This is why we offer this nifty little adapter, the XT30 to dual-JR. End result is four Dupont connectors from which you can draw current!

- PXED adapter, or an XT30 to Dual Dupont

So when used with a PXED adapter, the pack through the XT30 will now deliver current through a grant total of four Dupont connectors for a total of 14A with zero heat, or as a practical matter, about 20A. Note, this is still less than the single XT30 will carry.

Follow on question

Q. Thank you for taking the time to explain. Just one more question. I have been using the XT30, or similar connector from the 2-cell batteries to power the receivers, leaving the JR type unconnected. Does that deliver the same "Power" for want of a better word, than using the two JR type connectors?. In other words which provides the greater power to the servos? I understand that the outputs are all rated differently, but is that the max amount they can handle, or is it how much is actually outputted?

A. Something I hear 'outputted' expressed as you have. It shows a curiosity regarding flow, and maximum flow, but also a fundamental misunderstanding (I am assuming, but this is why I don't use names in these to avoid embarrassment for either of us).

First, the battery doesn't know from whence the draw is happening. So it doesn't know (or care) whether the load is applied at the XT30 or at one or both Dupont connectors. Reason is because all three discharge connectors (XT30 and both Dupont) attach to the same place on the battery.

Yup, slit the heat shrink and peel it back reveals the discharge connectors are all soldered to the same place - logical since this is where current flows in at out of the pack!

Point being, loads will draw the same amount of current via either the XT30 or the dual Duponts (as long as the consumption doesn't exceed 10A). And note; if you don't need more than 10A, you're equally fine using the dual Duponts or the XT30. Heck, as long as you don't draw more than 5A, you're fine with a single Dupont!

It follows that only if the load becomes greater than 10A will using the XT30 then become beneficial, understand? Otherwise, same-same!

Second, allow me to clear up one more thing, the battery doesn't 'give' or push the current, e.g. force it on the servos. Instead, the battery is a reservoir, like a glass with a milkshake is a reservoir of goodness on a hot summer afternoon.

- John's favorite, a Steak 'n Shake strawberry milkshake

Continuing with the milkshake analogy, think of the servos drawing the current through one Dupont like you sucking on that milkshake through a straw. Now think of putting a second straw in the glass, same size, and now your sweetie can join you in slurping down the milkshake - or - you can slurp it down faster. That's the dual Duponts, OK?

The XT30? That's me handing you a short piece of garden hose to replace both straws in the glass. Now you can suck the milkshake down 3X faster than with 'both' original straws, understand?

But in no wise was the glass with the milkshake ever trying to force the milkshake upon you (no matter which size straw you used). My point? The milkshake is passive and will just sit in the glass unless and until you use a straw and suck the contents out . . . same holds for a battery pack and whatever device needs current (servos in this instance but it could be an automobile headlight).

Basically, 'something' needs current the same way you thirsted for the milkshake. The wires and connectors? They are what facilitate this movement of current. And just like with a larger diameter straw, the thicker the wire and larger connectors do for the battery the very same thing, they allow you to more greedily drain the milkshake out of the battery pack!

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