How to Size Your BATTERY: Beginner to Engineer

8,494

127

18

Battery cells are electrochemical accumulators, which means they allow you to store electrical energy in chemical form. The most common type of battery nowadays are Li-Ion, being used on almost any portable device, cordless power tool and electric vehicle such as skateboards, bikes and cars.

Step 1: Form Factors

Li-Ion cells come in various form factors, which means simply the shape of the cell. In the photos you can see the two most common ones:

  • Steel cilinder with the cell rollend insidie. Come in various sizes, the most common by far is 18650 -> 18mm diameter X 65mm high
  • Pouch, a soft wrapping with the cell folded inside. It's the type of cell used in cell phones

Step 2: Knowing Your Cell

Wether you already have a particular cell or are looking to buy one, you need to know some important parameters, which determine whether the cell fits your particular application or not.

All of the information you can possibly need for a DIY application is available on the datasheet of the cell. In the video I'm showing the Samsung INR18650-25R cell, for which you can find the datasheet here INR18650-25R

The most important parameters are the cell's nominal voltage, nominal capacity and maximum sustained discharge, which you can see highlighted in the photo.

Note that the nominal voltage reported on the datasheet (in this case 3.6V) is measured at 50% State Of Charge (SOC). The actual voltage will be higher when the cell is full and lower when empty

Step 3: C-rate, V, Ah, Wh, J

Units and their meaning:

  • Ah, is a charghe capacity -> Ah = (C/s)h = (C/s)3600s = 3600 Coulomb
  • Wh, energy -> Wh = (J/s)h = (J/s)3600s = 3600 Joule
  • C, current normalized on battery capacity. For a 2.5Ah battery, 1C means a current of 2.5A. If you discharge that battery at 1C, it's gonna last 1h. If you discharge it at 2C, it will be empty in 30 minutes.

In the photos you can see some examples relative to the same INR18650-25R cell.

Step 4: Scalin of Units in Battery Packs

The arrangement of the cells inside a battery pack is usually reported like 10s2p, for example, where 10 is the number of series cells (10s) and 2 the number of cells in parallel (2p). This means that the battery contains a total of 20 cells, as shown in the drawing above.

The C-rate, in this case, is calculated from the capacity of the whole pack. If for example I use the usual cell to make this 10s2p pack, I'll get a nominal voltage of 36V, a capacity of 5Ah and a maximum sustained discharge of 40A.
A discharge of 1C now means 5A but the battery is still gonna last 1h.

Step 5: Assembling Battery Pack

When assembling a battery pack you should use just one type of cell and balance them before assembling. Note that wiring in parallel cells which are not at the same voltage may make the cells blow up in your face. Not nice.

For the assembly itself you have essentially two options:

  • Soldering: Cheaper and easyer for sure, but also a bit dangerous and likely to ruin your cells. The problem is the heating, which you should avoid on any cell beyond 50/60°C. The exact value depends on the specific cell, but soldering is performed at 200+ °C, clearly too high.
  • Spot welding: This is the right way to do it, but a spot welding machine is way more expensive and you can use it essentially only for soldering cells. Also, making a nice spot weld takes some practice and patience, but the result is superior on many levels, including contact resistance.

Step 6: A Note on Safety

As stated a couple of times before in this Instructable, Lithium cells have the tendency to blow up. For real.
Underestimating that possibility is an error that you don't want to make. It's even shown on the datasheet! Check the photo.

The main causes for blowing cells are:

  • Overcharge and overdischarge. Not very likely to explode but defenitely possible
  • Short circuit, which overheats the cell, makes the electrolite boil and eventually a thermal runaway
  • External overheating, like putting the cells near a motor or under the summer sun.
  • Puncture, happens when the cells are not mechanically protected inside a rigid case. It's easyer to happen on puch cells which have a soft wrapping, but can happen on any cell.

It has happened on many cell phones (Samsung is maybe the most notorious) and some Tesla cars, which use the 18650 form factor for they batteries.

Take precautions! The flame is very high temperature!

Step 7: What's Your Project?

I'm working on an electric bike, for which I'm gonna build an 8s5p battery pack with those cells I showed you.


Tell me in the comments, on what project do you plan to use Li-Ion cells? In what configuration?

Did you ever have a cell blow up in your face? Hope your face is fine. What provoked the cell to explode? How can you avoid that? Let us know!

Share

    Recommendations

    • Growing Beyond Earth Maker Contest

      Growing Beyond Earth Maker Contest
    • Sensors Contest

      Sensors Contest
    • Frozen Treats Challenge

      Frozen Treats Challenge

    18 Discussions

    0
    None
    graydog111

    1 day ago

    I bought 2 Lithium-ion batteries for a Makita 7.2v application on eBay. One worked fine but the second one was dead & would not charge, even after I zapped it. They sent me another battery to replace the bad on but it was bad also. The battery that was good out performs my other Nicad 7.2 Makita batteries. I am not positive, but I think it has 2 X 18650's in it. It is heavy on one end and feels empty on the other end. I do not want to tear into it just to find out if I am right.

    I would like to take the batteries out of some old dead Makita cases and replace them with 2 X 18650 batteries. I have Silver Solder & liquid flux I could use to solder the wires to the 18650 batteries or I could spot weld thin metal strips to the batteries with my MIG welder.

    I'm afraid if I ask you: "What should I do?", you would say: "Buy new Nicads" Seriously is it a worthwhile job or too dangerous?

    0
    None
    MaxM15

    2 days ago

    Did you ever have a cell blow up in your face?
    Yes, not one, not 10 but 960 of them (accident). I was shocked AF because I thought that's what hell looks like. Luckily no one was hurt and hangar sustained only minor damage. 10/10 would repair aircraft Li-Ion batteries again.

    0
    None
    FUSIMESTER

    2 days ago

    Only people who are smart enough, consistent, thoughtful should experiment with such batteries. If you have too much self-confidence, this technique is a danger for you!

    0
    None
    avantgardener

    4 days ago on Step 7

    If you drain one at 2C you will get less than half the amp hours. Higher current will reduce the energy you can retrieve.
    Do not forget that charging these frauds below ten degrees C will permanently damage them, and at that temp you will probably only get 10% of the rated energy out anyway.
    These are being foisted on us for their high cost and short lifespan. Go back to lead acid or NiCads if you want ten times the lifespan and no bother with dangerous products that need unreliable electronic monitoring to reduce the danger, and barely last ten percent of what's acceptable even then.
    Do you want to get less than 5% of your initial range on your ebike after six months? I built an electric microcar and two small lead acid junkyard car batteries give it 100 km range at 50kmph. Running lead calcium batteries in series brings them back to life.

    0
    None
    sylviahamilton

    4 days ago

    do you need an additional component for the battery pack to keep it balanced or is it built into each individual battery?

    2 replies
    0
    None
    RaymondR6sylviahamilton

    Reply 4 days ago

    You do need a protection circuit to monitor each cell in a group, either in series or parallel, for over voltage, over current, and under voltage conditions. Many battery packs or packages in laptops, notebooks and tablets have then built in the pack or attached on the cell.

    0
    None
    MatiasMamosylviahamilton

    Reply 4 days ago

    Yes, you defenitely need more components! Using unprotected cells is really dangerous. I'm working on another article explaining how balancing and protection works, will likely be out next week

    0
    None
    JimG50

    4 days ago

    When using an 18560 (only available battery at home) in low current applications, say, 10mAh, what is a good way to determine length of usable time? What is the recommended low voltage to begin recharging?

    1 reply
    0
    None
    RaymondR6JimG50

    Reply 4 days ago

    Some 18650 cells are rated at 2.5 Ah or 2500 mAh, so for a 10 mAh load, the usable time is about 250 hours or less. And for the low voltage for recharging, I recommend 2.8 V per cell or higher. Any lower can cause physical damage inside the cell, due to material migration, and if you try to recharge that level, the internal resistance increase will convert that charge to heat and continue to degrade the cell to its destruction. That is why battery manufacturers add a under voltage shut off protection circuit in packages to prevent that damage. Any raw cells that have dropped below 2.8 V should be carefully monitored during a recharge! Or just dispose of it with care at battery recycling centers.

    Use a good cell brand, such as Panasonic (that is what IBM, Ford, and Tesla uses), which can hold its charge for longer periods of unuse, but always remeasure unused cells after several months, and recharge to almost full (about 4.0 V) to keep the cell useful.

    0
    None
    RaymondR6

    Tip 4 days ago

    This is a very good instructable, even if it has a few spelling errors such as "puch" for "pouch". There are other points that you missed, such that there are two other "construction" types. One is just a raw cell such as the 18650 or pouch cells, and the other is the packaged cell that has a protection circuit in the package usually shaped like a square with contacts on one edge. These second types are commonly used in cell phones and other devices with removable cells, and that the vast majority of failed cells that will not charge is that the protection circuit (over voltage, over current, and under voltage) will either shut down or blow an internal fuse, and render the cell useless. I had a HP Chromebook with a bad USB changing post and after the port was repaired, the battery would not charge again. The protection circuit did a under voltage shut down (the cells dropped below 2.6 V), and I had to buy a $30 replacement to use the Chromebook again. If some experimenters are very careful, those packages can be opened, the circuits can be removed, and the raw cells can be recharged and used differently.

    I am presently working with four large Panasonic rectangular cells from a Ford Fusion Energi (a plug-in rechargeable hybrid) rated at 24 Ah each, and I plan to set them up in series (for a total of 355 Wh) to replace the lead-acid battery of my car. I have monitoring circuits that can check each cell and report if they are unbalanced.

    One last suggestion that I see many time: use correct engineering suffixes for small or large values. I see many sites promote packs with a rate such as 20,000 mAh. That is wrong! It should be just plain amp-hours or Ah, such as 20 Ah. And for your large numbers, 3600 Joules is 3.6 kJ (kiloJoule). Just remember 1000 = 1 k and 1,000,000 = 1 M. Apply the same for small values.

    1
    None
    SteveS101

    4 days ago on Step 7

    I have worked with lithium batteries off and on since they were first introduced. They are not a technology to be handled carelessly. As the author suggests, they can and will explode. Drill a hole in one, drop it in a bucket of water, and run for it... Once certain types are compromised (especially, LiOH), they can explode if dropped or subjected to shock. Soldering them can compromise them; so, minimizing the heat of the soldering process and knowing what you are doing is critical. Otherwise, you could lose a finger or an eye quite easily. Be careful...

    1 reply
    0
    None
    hugof224SteveS101

    Reply 4 days ago

    Steve... Rather than soldering; why not use a 'modern-day' super glue? Solves the potential for over-heating, no? Of course, care must be taken to thoroughly clean all contacting surfaces and to make sure of a good contact, as well.

    0
    None
    gizmologist

    4 days ago

    The easiest way to tell when a phone battery (or any flat battery) is getting dangerous is when it starts to swell. Unfortunately, with the modern trend of non-replaceable batteries, you might not know it until the swelling battery cracks your $800 phone - or catches fire.

    0
    None
    keesj

    4 days ago

    I use a bmc pcb board to have an equal discharge over my lion cells, it also protects charging batteries in serie and undercharge. When I measure the voltage from time to time the batteries stay on an equal voltage. I also have a voltage meter over the batteries, which will tell if one of the cells is not connected or getting old. While using a bmc the cells are getting old in the same rate. I use standard containers and standard bmc boards, and solder the threads for the bmc board on the contacts of the containers. These bmc boards come for a certain number of cells, so it will be limited. Sometimes on the cells, it says it is being protected against overscharge and decharge. A warning is that the cells must be charged, before being put in the container and bmc pcb boards.

    0
    None
    Alex in NZ

    14 days ago

    Thanks for a great tutorial. I might be tempted to start using lithium batteries now.

    2 replies
    0
    None
    nic.bryan.73Alex in NZ

    Reply 4 days ago

    I recommend the flat packs. Higher discharge rates, better heat dispersal, better capacity-by-volume, and most come with solder-tabs. Also a lot lighter than the 18650s. If you have a specific voltage you're looking for, divide it by 3.7V to get the number of cells you need in series, and head to the local RC shop. They've got voltages up to 22.2V (6S). And parallelling premade RC batteries is super easy, you just add a second (third, fourth, etc) connector. And then the batteries can be disconnected easily. They also come with balancing plugs, which makes protecting the cells much easier.

    0
    None
    MatiasMamoAlex in NZ

    Reply 13 days ago

    Well, thank you for the feedback! I'm aware it's not the most popular topic, so some feedback is always appreciated

    0
    None
    bobstuart

    4 days ago

    This looks like an interesting article. I hope to see it again with enough contrast for reasonable legibility.