DYI 18650 Battery Pack

Introduction

This article focuses on how to build a battery pack using 18650 cells, for use with a small digital amplifier; and is intended to support my post on how to build a Bullitt Boom Box.

Disclaimer: the ideas and information provided here are provided “as-is”, no warranty is provided or implied.  Building a system such as the one described here involves various risks, both during implementation and operation.  If you damage yourself, someone else or any property, through directly or indirectly being influenced by this content, that is entirely your responsibility.

The battery pack is one of the most complex parts of the system, because you need to find a battery system that:

  1. Matches the power requirements of your amplifier.
  2. Is mobile – can be mounted on your bike with relative ease, and cope with the demands of being on a bike (vibrations, weather, and so on).
  3. Can be managed (i.e. power level, when to charge).
  4. Can be charged.
  5. Is safe.

After a lot of research, the solution I settled on was to build a custom battery pack out of five lithium-ion 18650 cells, supported by a small voltage meter and a versatile hobbyist charger.

Let’s look at what’s involved.  First we’ll cover the 18650 cells, and then we’ll look at the whole battery pack solution including its design, implementation, and operation.

Images: (Left to Right) The battery pack next to the amplifier.  The SkyRC B6 Nano charger.  Small voltage alarm/meter showing the battery pack’s total voltage.  My second battery pack, with 18650 holders at allow cells to be changed.

 

Part I: 18650 Lithium-Ion Cells

Please note that the following information is based on my understanding and research, but I’m not an expert in this field, so you may want to verify with other sources.

18650’s are cylindrical lithium-ion rechargeable battery cells.  They are physically larger than AA batteries, and have a higher nominal voltage of 3.7V, compared to an AA’s 1.5V.  18650’s can be bought new, or – if you are careful and understand what you’re doing – recycled from old laptop battery packs.

In terms of recycling: there’s plenty of video’s explaining how to recycle 18650 cells, and if you plan to do this I’d suggest watching a few to get a better idea of what’s involved.  Treat the following info as an introduction,  If you plan to actually do it, do some further research so that you have a better understanding – or work with someone who has the proper experience.

The following content assumes you’re recycling 18650 cell’s from a laptop battery pack, but its still mostly applicable if you’re using new cells.

Safety

18650’s are somewhat dangerous – when treated badly they can swell and rupture, causing fire.  “Bad treatment” can be physical (knocks, puncturing) as well as electrical (incorrect charging, unsafe-discharge, use beyond safe lifespan, etc). The Samsung Galaxy Note 7 is a famous example of what happens when Lithium-ion batteries are mistreated through incorrect charging.  See: Washington Post’s: Why those Samsung batteries exploded.  You’ll also note airlines have travel restrictions for lithium-ion batteries, usually depending on how potent they are.  E.g. Air new Zealand: Travelling with lithium batteries.

That said, like anything electrical or mechanical, these cells are safe to use if you understand the relevant factors involved and don’t stray outside the safety constraints.

Technical Specifications: Voltage

Not all 18650’s are the same.  Check the manufacturer’s specifications to avoid fire and personal injury.  A reference that may help you identify the your recycled cells is Second Life Storage’s Cell Database.

A rough guide to a typical 18650 cell:

  1. Most will have a nominal voltage of 3.7V (but you may find some that are 3.6V).
  2. Maximum charge is typically rated at 4.2V.  In practical terms, my charger’s default setting is to charge lithium-ion 18650 cells to a maximum of 4.1V.
  3. Maximum discharge is typically 3V, and may be lower (say 2.5V – check the cell’s specifications).  Remember, over-discharging can be as dangerous as over charging.  In practical terms it may be advisable to discharge to a lesser extent, e.g. to 3.5V rather than 3V – if that is the cell’s stated maximum.

To put that in layman’s terms:

  • Cells can typically be charged up to ~4.1V, and discharged down to ~3.5V.
  • A cell’s “nominal” voltage is the voltage measurement used for specifications.

Note that the nominal value is not a simple average (e.g. 4.2V + 3.2V / 2 = 3.7V) because the voltage discharge is not uniformly flat – it curves:

Capacity-0.5AThe chart above shows the behavior of a number of different lithium-ion cells, in terms of how their capacity drops over time:

  1. Starting at the top left, voltage drops from the maximum fairly quickly, but then starts to plateau.
  2. The long plateau is where the voltage drops at a much slower rate, providing the majority of the cells useful power.
  3. Towards the right, the voltage starts to drop dramatically.

This curve can be observed when charging: you may find that when charging a relatively “flat” battery, the initial charge up to about 80% is relatively fast, whereas the last 20% seems to take much longer.  That’s because you’re walking up that steep (top left) curve in reverse.

Discharging

Advice on how far you can / should discharge the cells depends on who you talk to.  general advice is to never discharge beyond 3V, and I’ve seen suggestions to not discharge beyond 3.5V.  As you can see from the chart above, the voltage curve varies between different manufacturers and models.

The idea is to utilize the plateau, and not let the voltage drop too far over the cliff on the right – as the discharge rate is faster it’s easier to over-discharge.

Ultimately you’ll need to do the research into the specific cells you have to determine what the safe discharge limit is, and then decide how far you want to discharge your cells so that they maintain a good useful life.

Be aware that some cells may have protection mechanism’s in-built to protect against operation outside of their safe voltage range – but many will not.

Side Note: Capacity vs Voltage

The capacity of cells will decrease as they age, but the voltage range they provide will remain constant.  Therefore, as your cells age, you’ll find that they still cover the same voltage range – but discharge faster than they used to.

Example Specifications

Let’s take an example: the Sanyo UR18650A, using information provided by the Cell Database: https://secondlifestorage.com/showthread.php?tid=6524, and then compare it to the official specifications.

Unofficial specifications:

Caution, this information is provided for reference only and is not guaranteed to be accurate.

  1. Capacity: 2100mAh.  mAh stands for Milliampere-Hour, a unit of electrical discharge.  As a rating, this is how long the battery will last (whilst remaining within it’s safe voltage range, and assuming a consistent discharge rate of 1 amp.  1 mAh = 1,000th of an hour, so a 2100mAh should last for just over 2 hours.
  2. Voltage: 3.6V nominal.  Slightly less than the more common 3.7V found in a lot of 18650’s.
  3. Charging: 4.2V Maximum; 1510mA standard.  Cells should never be charged above 4.2V.  1510mA standard represents the normal charging current.  Charging at a lesser current will be slower – which can be better for improving battery longevity.
  4. Discharging: 2.5V* cutoff; 420mA standard.  Should never be discharged below 2.5V and appears to have an in-built cutoff to protect the cell from discharge below 2.5V.  420mA standard refers to the expected discharge current.  Some cell specifications may also publish a maximum discharge current.  See the side note below.

* Note this appears incorrect when compared to the official specifications (see below) which state and end voltage of 3.0V.  If in doubt, use the more conservative figures or official specifications.

Side Note: Discharge Current

Measuring the current draw on my battery pack, using the sound system described in the other post, reveals that: when playing techno at maximum volume, “Doofs” in the music cause current spikes of up to 1 amp (1000mA), with the average draw being in the 500-700mA range.

Reviewing the thread we see that someone has found the official specifications…

Official Specifications:

  • End Voltage: 3.0V.  Would suggest not discharging beyond 3.0V, despite what the unofficial crowd-sourced info above suggested.
  • Capacity: 2100mAh nominal, typical capacity 2250mAh.  Here you can see the difference between nominal ratings and what you might find in the wild.
  • Discharging Current (Std) 2.15A.  suggests that the current drawn by my amp on maximum volume is well within the cells expected usage.
  • Discharging Current (Max) 4.30A.  presumably for intermittent spikes only.

References

Here’s some further reading:

 

Part II: Battery Pack Design, Implementation & Operation

 

Battery Pack Design: Power Ratings & Circuitry

The battery pack (i.e. collection of cells) needs to be designed around the power requirements of the device it’s expected to power, so you should start by understanding what those are.

For exmaple, my amp’s power requirements are an input range of 18.0V to 24.0V DC.  Five 18650’s (assuming a 3.7V nominal voltage) wired in series provide:

  1. A combined nominal voltage of 18.5V.
  2. A combined maximum voltage of 21.0V.
  3. Based on the amp’s minimum input range of 18.0V, we can run the cells down to an average of 3.6V, which is comfortably above the minimum voltage of most 18650 cells.

The thing about the 18650’s is that they have excellent capacity, and are well suited to an application of this kind (i.e. in terms of electrical discharge rate, and so on). They are also relatively small – so a pack of five of them are really easy to place on the bike. They are relatively more volatile than other battery types – you definitely will not want to pierce them, or give them any massive shocks. This is definitely relevant considering what can happen on moving bikes. That said, the batteries should be safe to use as long as you’re sensible.

I have two battery packs.  The first was made by my friend Pete, who is familiar with doing this – five 18650 cells wired in series (5S), with a balance lead also attached (giving two separate circuits – the balance lead is used for charging, the other circuit is straight output, which connects to the amp.  The battery cells have metal plate spot-welded to them, which serve as an anchor for the wires to be soldered to.  This leads to a very compact design, but fixed (not easily modifiable).

Images: various views of the first battery pack, with protective cover removed.

I also built myself a second battery using five 18650’s, but utilizing 18650 battery holders (shown below).  This means I can swap out cells with ease.  I’ve also added a switch so that I can fully power off the amp, and ensure the battery doesn’t get run-down.

Images: various views of my second battery pack. Left image shows the balance-tap lead on the left, main power lead on the right.

The top cover on my pack is a recycled shoe inner-sole.  This specific design is not overly water-proofed (it’s only seen summer operation so far), and I may yet modify it ahead of winter.

Even if you don’t want to build your own battery there will be other battery options out there, depending on what you’re after, and how much you’re prepared to spend – it’s just a matter of doing the research.

Circuitry

The actual battery packs have two circuits – the main power output circuit, and balance charging circuit – as illustrated in the following diagrams:

Images: (Left) The overall circuitry.  (Middle) The main power circuit.  (Right) The balance circuit for the 3rd cell, specifically.

Battery Charger

Hobby stores will have lots of electronics gear on offer, including battery chargers. I got this one (SkyRC B6 Nano) because it supported balance charging up to 8S (so can handle my 5S configuration), and a variety of chemistry types – including lithium ion (Li-Ion), so I assume I’ll be able to use it for various random charging needs in the future.

One thing to look out for is the power supply for the charger – check that it comes with one.  If the one you plan do get doesn’t have a power adaptor, just see what sort of power connector it has and what it’s power requirements are.  In my case I needed to fit an XT60 plug on to an old laptop power supply – which had the right power output range for the charger.

Images (left): SkyRC B6 Nano hobbyist charger. (Right) in action, main battery power lead top-right, battery balance-tap lead bottom-right.

Battery Management: Charge/Voltage

Assuming you use a battery without a BMS, you’ll need someway of reading the batteries voltage, so you know when to recharge it. For example: 18650’s can be run beneath their intended safe voltage range. If you do this you risk damaging the cells and/or causing a serious fire.

The amp I run draws a small amount of current – even when switched off. If the battery is left connected to the amp for a prolonged period, it will drain the battery to an unsafe level. At such levels your battery charger (like mine) will not attempt to charge them because the low voltage will fall outside the normal parameters of the 18650.

Fortunately there’s plenty of ways to manually measure battery voltage.  The cheapest way is to get a voltage meter like the one referenced below, which connects to the balance lead. It provides the overall voltage, and the voltage for each cell. Because it draws a small amount of power you may just want to periodically plug it in to check the voltage, then unplug it.

 

Images: (left) Lithium-Ion Lipo Battery Voltage Tester Alarm, 1S to 8S.  (Right) Reading the overall voltage.  This little meter will also read the voltage of each individual cell, cycling through each in sequence.

Battery Management: Supply Control

You might find that your amp draws power even when switched off.  Mine certainly does – around 0.025A, which seems to be due to Bluetooth receivers going into standby mode as soon as power is supplied.  This might not seem like much, but, it’ll be enough to fully discharge your cells if left long enough (trust me, I know from experience – more than once).

Accidently draining your cells is at best a pain, denying you from amplifying your vibes, but its also likely to retard your cells performance (reduced capacity) and may lead to other even less desirable damage.

To control the supply of power to the amp, I fully recommend building a simple switch that completely breaks the battery-amp circuit, such as the one shown below.

WP_20200319_08_27_50_Pro

This simple system is basically just a switch integrated into the battery-amp power cable.

Below is a prototype of a more advanced control system, one that includes a built-in voltmeter.  This specific unit uses a pair of XT60’s to fit in-line with any circuit that uses XT60’s between the power source and device.  This specific prototype does not have a power switch (I’ll be using this one with my on-board camera, so no switch required), but the next one, that I’ll use with the amplifier, will.

The idea of the button is to activate the voltmeter on demand, so it’s not a constant power drain.  It’s wired in parallel to the main power circuit.

References:

 

That’s pretty much it – good luck with any DIY projects you attempt.  Remember, using 18650 cells can be safe as long as you do the research and exercise some caution and common sense.

1 thought on “DYI 18650 Battery Pack

  1. Pingback: Bullitt Boom Box | Peruse Muse Infuse

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