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Boff

[WIP] Batteries and Motors Combinations

This is a work in progress article on how to pair up batteries and motors. It's designed to explain the theory behind motors and battery choice pairings. I started writing it as a stand alone thing but then realised that having a series of articles on wiring, motor choice and battery selection would be a good idea. This is the third in the series, I'll get onto the others as soon as I'm able. I'm looking for feedback, blind spots and other input. I want this to be a community effort to get information collated and eventually stuck in The Vault. I intend to leave this thread open for discussion, factor in all the correct information to the OP and then leave the discussion in tact as a reference and a film credits section if you will so those that contributed will be recognised. Smile

Please feel to rewrite whole sections if you need to. I'm very well aware that I'm a long distance away from the bewildered Boff that stumbled into the flywheel aspect of the hobby sometime in late 2012. More recent newcomers have a lot more insight into a market that has changed vastly since then and I'm sure the community would appreciate your efforts.

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You're looking at batteries, you're looking at motors and you have a bewildering array of options. Well, you've come to the right place. The BritNerf community has come together to give you an insight into how to select the right battery for your blaster. This article is the third in a series of three about flywheel blasters. Here we cover motor and battery pairing. If you're yet to choose your motors or work out why rewiring is important then we suggest you start at the beginning [link]. Smile

Now, if you're just wanting a battery for your blaster and aren't too interested in how stuff works then the link below is where you want to be. We've already done the leg work in finding a pack that will fit each blaster:

Motors and LiPo Pack Chart

The data above is based on buying batteries and testing if they fit in a particular blaster. The selections were made using the process described in this article. If you want to select a pack for a motor that's not listed or you're just curious then read on.

Choosing your motors:
Your choice of motors is the first thing you need to decide on. It informs everything else you do.

There are dozens of motors on the market and which one you choose is a matter for separate discussion (probably link a thread on choosing a motor here since it's a separate topic with its own considerations and stuff). If you've not chosen your motors then go read this article here and come back when you're done.

Rewiring:
Rewiring is compulsory on all motor upgrades for safety and efficiency purposes. If you're going to the trouble of getting more kick, there's no point throttling your build with sub-par wiring. [Link to another article here].

Finding out your motor's current requirements:
The most important characteristic in determining your battery is your motor's current draw. A motor will not draw more current than it needs but a battery needs to be able to answer that call when it comes. Current does not stack like voltage does so if you put lots of 8A cells together, they'll increase in voltage but not in current output. Asking too much of your batteries by putting motors that request more current than they can supply will result in things getting hot and doing significant harm to your build either in performance terms or safety when something melts or catches fire.

SSGT put together this handy spreadsheet that shows you the stall currents of various motors. The current at stall is the current asked for by each motor whenever you start them running or fire a shot. You need to be able to supply that current with your battery.

Look up the stall current at the voltage you plan to run your motors at. The voltage will be determined by your motors. If you're unsure what voltage our motors need then see our article on motor selection [link]. If you're running different motors on a build (for example, a Rhino flywheel pair and Honey Badger pusher) then take the reading for each motor at the voltage you're running things at. With the stall current for each motor in hand, you need to add them together.

For example, I have 2 Hellcats and a Honey Badger pusher in my Rapidstrike. I'm running them at 11.1V. The spreadsheet tells me the stall current for the Hellcats is 22A so I do 22A x 2 = 44A (because I have 2 Hellcats on the flywheels. Then I look at the current draw for the Honey Badger at 11.1V and see that it's 20A. I add the three together to give me 64A of stall current. That's my wiring system's current requirement. I need to find a battery that will happily supply 64A of current. I would recommend rounding these figures up to the nearest 5 or 10. A higher figure will mean you're more likely to pick a battery that can supply the current asked of it. Remember more current output from the battery is never a bad thing.

Another example (possibly use spoiler brackets or tags for these, thinking about doing exercises)
I'm running a pair of MTB Rhino 130s in my Stryfe, what's the current draw at 3S/11.1V?
Spoiler:

The datasheet tells me 8.42A at 11.1V so for 2 of them in a system would be 16.84A Round that up to 17A and you've got your figure for picking your battery.



Selecting your battery:
Now you have your current draw for your motors, you can start looking at your battery.

There are several characteristics of batteries that need to be considered.

Voltage:
I've made a big thing about how having too much current capacity in your battery is not a bad thing. However, that is not true for voltage. You need to match your battery's voltage to the motors you've chosen. You have a little bit of leeway with voltage but it's optimum to use the recommended voltages laid out in SSGT's spreadsheet. Too much voltage and you risk damaging the brushes and the winding inside the motor. Remember that voltage will stack if you put two batteries in series so two 12V packs wired in series will form a 24V pack. Apply that to your Rhinos or Hellcats and they'll probably go pop.

So how much current can my battery put out?
Well, there are two main ways of working this out. The simplest is to see if a manufacturer states a current output on their listing or datasheet. This is usually stated in Amps (short for Amperes) or A. For example, this eBay listing lists these IMRs as 8A. That /can/ be taken to mean that these batteries can supply 8A directly to your blaster. However, take it with a pinch of salt and always build in a bit of head room.

Often you won't see a direct current rating stated in A on a battery listing. That's where C and mAh come in.

Current Discharge (C):
You will often encounter a number on a pack followed by a 'C'. This refers to the current discharge ability of a battery. If it's not stated then assume it's 1C. This will be important later.

You might see some listings with a 'burst' and a 'continuous' C rating listed. Burst is for short periods, usually up to 10 seconds that allow a pack to output more current than normal for short periods. This is often used by model aircraft hobbyists who need a bit of extra kit to get off the runway. For us, it offers a bit of head room. Continuous is the pack's rating for constantly discharging safely until it's flat. We recommend that you pick based on continuous ratings just in case. Burst would be fine but it's better to have a bit of head room built in, after all.


Battery Capacity (mAh or Ah):

Battery capacity is the amount of charge in a pack. It's important because it not only shows you how long a pack will last but it also gives you an idea of how much current it can supply. To start with, an Amp-Hour (Ah) is roughly a pack's ability to put out 1A worth of current for one hour so a milliAmp-Hour shows the same thing in milliamps or mAh. Remember there are 1000milliamp to an Amp. So if I had an LED that drew 1A and a 1000mAh pack then I'd, in theory, be able to power that LED for 1 hour before the pack went flat. Smile

So how do these two relate to how much current a pack can output?
Well, to give you your current output you take the Battery Capacity and convert it to Ah (divide the mAh figure by 1000) and multiply it by the C rating.

For example, I have a 1000mAh 35C (continuous) 3S LiPo. To work out its current output, I convert the 1000mAh to 1Ah and then multiply it by the current discharge figure. 35 x 1 = 35A. The pack is able to supply 35A constantly until it runs flat.


I've put together some questions below, with the information above do you think you can answer them?

Can you see why we don't recommend IMRs for anything other than stock motors?
Spoiler:

If you take 8A look for a combination of motors on the spreadsheet that work at their suggested voltages, you'll be hard pressed to see one that fits. As a rule, stock motors require around 5A and are therefore suited to this IMR. At the time of writing, I couldn't find any.



I have a TrustFire brand that states that its output is 1C. It says '800mAh' on the outside, can I use it for Nerf applications?
Spoiler:

No. At best guess, it as a 0.8A current output. Stock motors start at around 5A required current which is over 5 times the required current.



Battery Chemistry:
What battery type you decide on will vary on your application. Most performance modifications these days are done with Lithium Polymer batteries (aka LiPos or LiPoly). We'll look at Alkalines and NiMH here too. NiCd is not discussed as it is being phased out under a couple of EU environmental directives.

Alkalines:
These are your usual off the shelf packs. They tend to discharge anywhere between 0.5C and 1C with capacities in the range of 1500-2000mAh. They're great for the kids and bone stock blasters but without the current capacity of higher quality packs, that's where they belong.

Lithium:
Lithium chemistries are incredibly popular and come in a range of configurations. IMRs, LiPos, LiFe are all lithium at heart and need to be treated the same way. They mustn't be discharged past a certain safe point otherwise they will risk damage and become useless. They offer a lot of energy for a reasonably small package.

Understanding S & P:
Now with Lithium chemistries you will often see things like '2S' or '3S'. More unusually you might even see '2S2P'. These stand for Series and Parallel respectively. If you need an overview on series and parallel circuits then go see this useful Wikipedia article.

Lithium packs usually come in 'cells' of a nominal 3.7V each. Now that might be the single 14500 shaped 'AA' battery that you might put in when using IMRs or it might come wrapped as a bundle inside a LiPo pack. The key thing to remember is that the safe operating voltage for a Lithium cell is 4.2 to 3.6V. Outside of this range is bad. Your charger will know what's what and will charge accordingly. More on that in a bit. You can bundle cells together to get different effects. Your charged packs will read higher than the 3.7V per cell (2S LiPos usually read 8.4V when charged, 3S 12.4V etc), don't worry that's just an artefact of lithium's discharge curve. Trust in your charger. Very Happy

Series (S):
This denotes the number of cells wired in series. So for a '2S LiPo' that would mean there are 2 cells wired in series and since voltage stacks in series, you'd get a 7.4V nominal. A 3S is 11.1V, 4S 14.8V etc.

Parallel (P):
This refers to the number of cells wired in parallel. If it's not stated then assume it's 1P. Now, wiring cells in parallel can be a useful way of boosting the current output of a set up without affecting the voltage. If you have a 2S2P configuration of 8A IMRs then you have 7.4V from the 2S bit but 16A from the fact you're running 2 sets in parallel.

Self-Discharge
Lithium cells have a habit of self-discharging over time. Voltage will drop if left for a very long time (2+ months) and there's a risk of dropping below the critical safe threshold for a given Lithium chemistry. To prevent this being a problem, most chargers will have a storage mode which drops the cells to around 70% to slow the rate of self-discharge. If this isn't an option then simply charging the packs to full every few weeks. Provided packs are kept between the upper and lower bands of the safe voltage range for each particular chemistry.

Lithium Polymer or 'LiPo' or 'LiPoly'
This is probably the most common upgrade type of battery in use in the hobby and has an interesting reputation. It is critical to state that, on balance, LiPo is perfectly safe as a chemistry. All the stuff about houses burning down doesn't really apply to Nerf. Most often the sorts of catastrophic failure you see on YouTube comes from misuse or model aircraft accidents. If you manage to puncture or over-charge a pack in Nerf then you're doing something very wrong. LiPo safety has come a long way in the last 5 years and even trying to deliberately set one on fire has yielded disappointing results. It's far safer to run a Lithium polymer battery within its designed parameters than to stress IMRs or other batteries beyond their tolerances.

Lithium Iron Phosphate or 'LiFe'
A newer generation of Lithium chemistry. Unlike LiPo and IMR, the safe range for this chemistry is closer 3.0V and 3.6V with 3.3V being the 'nominal' as 3.7V would be for other Lithium chemistries. LiFe is considered more stable and generally safer than its LiPo counterparts. There also subtle variations in the chemistry that allow it to respond faster to current draw changes.

Lithium Manganese commonly sold as 'IMR'
Another Lithium chemistry that has been around for a while in the high powered torch community. Offering higher current output than other 14500 options, these are often opted for an entry level modification for flywheel blasters.

Lithium Cobalt, commonly sold as 'TrustFire' or 'Ultrafire'
LiCo is another Lithium chemistry that possesses the same characteristics of other Lithium chemistries. The main issue with this chemistry is that it is sold with terrible quality control. The number of low quality knock offs including recycled cells and the like mean you're in a mine field of potentially dangerous cells. They are often not rated to the currents we demand of them and stressing Lithium cells is bad at the best of times but when they're of questionable quality then you're in trouble.

Nickel Metal Hydride or NiMH
An old school rechargeable chemistry, this one has a reputation for stability and dependability. NiMH has a nominal voltage of 1.2V per cell. Low internal resistance means NiMH have a pretty solid discharge capacity making them ideal for higher current applications.

The problem with NiMH is the lower power density compared to LiPo and other Lithium chemistries. Packs tend to be a tad larger and difficult to fit inside Nerf blasters. Unlike Lithium Ion chemistries, the electrolyte isn't flammable and isn't kept under pressure. While the risk of Lithium chemistries is low, NiMH is pretty much unbreakable. Lithium is perfectly safe but for the super-cautious or lazy, NiMH is the way to go.

FAQs:
My battery has a current rating of several times what I need, will this damage the motors?
Nope. As I said above, your motors will only draw what they need.

How long will my pack last?
Well, that's a bit more tricky. It will vary greatly on the way you Nerf. A good rule of thumb is that 1mAh = 1 shot. From experience, I've had 1000mAh packs last all through a 3 day LARP event and only come back 40% full. If you're worried about longevity then you need to buy the biggest mAh you can find for the space you have.

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