• JR-connector - 2X (5A each)
  • XT30-connector (30A)
  • Balance connector
Price: $24.99

    Item #: B2S850
    Availability: In stock
    Usually ships In the same business day

    This B2S850 is a dedicated pack consisting of 2 LiIon cells arranged in a series configuration (two cells arranged side-by-side). Outputting 7.4VDC (nominal), this pack is intended for direct connection to receivers.

    Representing a lot of juice in a compact package, this pack is equipped with two JST (JR-type) connectors rated at 5A each (combined they deliver 10A) along with an XT30 charge/discharge connector (rated at 30A). Plus, an HX-type balance connector for the charger (for monitoring SOC, or state of charge).

    • Type: 2S (two cells in series)
    • Capacity: 850mAH
    • Nominal voltage: 7.4V
    • Full charge voltage: 8.4V
    • Charge rate: 1C (full charge in 60-minutes)
    • Continuous discharge rate: 8.5A
    • Burst discharge rate (5-sec): 17A
    • Weight: 71g (2.5oz)
    • Dimensions: 52 x 31 x 16 mm (2-1/16 x 1-1/4 x 5/8 inches).
    • Discharge connectors: JR-type (two)
    • Discharge connector: Amass XT30
    • Balance connector: JST XH-type

    Q. At what rate should I charge my LiIon pack?

    A. In general, don't exceed 5C, and best practice is charge at 1C.

    Look, teaching you everything you need to know about battery charging is beyond the scope of this website. We'll share some examples to try and help guide you, but don't for a minute believe this is a comprehensive explanation. Point being, it's on you to learn enough to be safe and the charger manufacturer will also have information to use in safely charging your battery pack.

    To begin, you need to be aware of a bit of math. Like how the 'm' in mAh means mili, or 1/1000. Regarding the A, that means Amperes (or amps), and h=hours. So the capacity of the pack is measured amps delivered in one hour, or Ah (A*h or A x h), but since packs for models are often measured in fractions of an ampere, then they're also often expressed in terms of milliamps, or 1/1000th of amp. As for time, since the time frame is always based one hour, then . . .

    • 5000mAh = 5Ah - meaning it'll deliver 5A of current for one hour
    • 2000mAh = 2Ah
    • 850mAh = 0.85Ah

    . . . understand?

    Further to this, C (capacity) of a 5Ah pack is 5A for an hour, 2.5A for two hours. Similarly, 2A of consumption for an hour from a 2Ah pack, get it?

    So charging at 1C (meaning 1xC) it means a 5Ah pack can be charged at 5A, and if the same pack is being charged at 2C (meaning 2xC or 2x5=10) it means it can be charged at 10A and since we're dealing with full hours, this means it'll charge in 1/2 that much time, or 1/2 an hour. Confused? Continue reading.

    Let's switch to a 2Ah pack (same-same as a 2000mAh pack). Charging a fully discharged 2Ah pack at 1C means hitting it with the charger at 2A for one hour. Hit it at 2C (4A) means it takes half that much time (1h/2=1/2h or 30-minutes) Ditto, charging at 3C takes 1/3 of an hour or 20 minutes (60min/3=20minutes), or at 4C takes 1/4th of an hour, or 15 minutes. Get it? If not, review and consider learning about this material in other places until it clicks because if you screw up you're putting your life at risk.

    We're not kidding so please consider yourself warned!

    Note; all this is in theory because it's never a good idea to discharge below 20% capacity. Point being, a 5000mAh pack in practice should be considered 80% of 5ooomA (0.8 x 5000=4000), or 4000mAh instead of 5000mAh. This is important!

    There's more to learn, and it's not really rocket science, but it's on you to go learn about it before charging batteries. We're sharing some some rules of thumb that will for the most part keep you out of trouble - but - you can burn your house down by being stupid so don't go trying to blame us because a) we're telling you battery charging can be dangerous, and b) that what we're sharing isn't everything you need to know. The major point being, you should go learn how to do it safely before you begin!

    Q. My charger has a LiPo charge-cycle instead of LiIon. May I still use it?

    A. Yes.

    In general, chargers expressly made to charge LiIon packs are set to similar cell-voltages for LiPo-chemistry and thus, won't damage the pack. Basically, the difference internally is the LiIon cell has a liquid electrolyte while the LiPo, which is also a lithium-ion technology, uses a gel for the electrolyte. Anyway, always use a charger designed for the appropriate chemistry. Note; chargers are available to charge multiple chemistries.

    Q. My charger has a LiFe charge-cycle instead LiIon. May I still use it?

    A. In general, no because a LiFe-charge cycle is going to charge at a lower level than for LiIon.

    Q. Can I charge my 2S LiIon pack with a NiCd charger for 7-cells since that's designed to charge 7.2V packs?

    A. No. Hell no! Don't do it.

    The reason is the NiCd charger makes no provision to monitor the cell voltage of the individual cells the way a charger designed for LiIon packs will. This is the inherent advantage of the LiIon technology in that each cell is wired to the little white balance-connector so the charger can monitor the voltage of each cell as it charges. Be careful because this is a good way to start a fire and burn down your house! You've been warned!

    Q. I'm Canadian and fly year around, sometimes in sub-zero temperatures. Is it OK to charge my LiIon pack in these conditions?

    A. Yes, but be careful. Capacity is reduced maybe 20-30% at freezing. And at lower temperatures the data is inconsistent. Look, batteries basically like to 'live' at similar temperatures where 'we' like to live. Fortunately, in practice, what most folks do is charge their packs whilst in their car!

    That said, if you do charge in below freezing temperatures, reduce the rate of charge to 0.1C . . . e.g. 10% of the battery capacity.

    Q. I'm confused, isn't LiIon the same as LiPo? Also, why don't you recommend LiPo packs? I like that they're cheap so what's wrong with that?

    A. Yes, LiIon and LiPo are similar. But critical differences aren't so much in their chemistry (they're actually very similar) but in their methods of construction. This is the key to understanding our recommendation for LiIon versus LiPo.

    This is because the LiPo is built in a poylmer bag. This gives it the characteristic brick shape as the individual cells are flat-rectangles, which are overlaid upon one another. The shape is also the giveaway for the LiPo vs. LiIon where these packs are built within cylindrical metal shells (typically aluminum).

    Note; the Po in LiPo refers to the polymer in it's construction (aluminized polymer bags). Anyway, the individual cylindrical shells, because they're made of metal instead of thin polymer bags means they're more resistant to physical damage. By the way, this metal shell is the same technology used in old school NiCds and NiMH (and alkaline cells, for that matter). It's been around forever because it works!

    There are downsides to these metal shells. First, the metal is a bit heavier than the plastic bag use in LiPos. Second, simple geometry dictates two cylinders contain less volume than two flat cells (capacity). Third, they're more expensive to produce.

    Against these disadvantages are upsides. Like metal shell is FAR more sturdy. This turns out to be a crucial advantage because metal protects better against inadvertent damage (like a pack shifting during a maneuver and bumping up against the hard edge of a former). If this happens to polymer style packs, the dent may result in it puffing. Or in a fire. Need I mention our models are constructed of flammable materials like balsa, foam, and fiberglass?

    Bottom line? For an engineer, part of the remit is looking not at when everything is going right, but when things are going wrong. Look, nobody sets out to install their pack so it's dented due to shifting during a maneuver, but . . . shit happens, right? So it's when things go pear shape that a good engineer earns his pay.

    Our deciding against continuing to offer 'Po' style packs for control avionics is a direct result of data indicating it might sound good in theory, but in practice, leaves something to be desired. This reminds of the immortal words of a wise wag of baseball.

    Put another way, when the data changes, we change our mind! This is why our control avionic pack recommendation is to use durable LiIon instead of more fragile and less costly alternatives like LiPo brick style packs.

    Note; for propulsion, the weight and package volume (capacity) give an overwhelming advantage to polymer bag construction. This is why LiPo packs are used for powering RC models. But also know this, these packs are removed prior to charging (or should be), so the risk profile is somewhat different.