August 08, 2018 18 min read 4 Comments

Our Best Advice for 18650 Battery Builders

The DIY battery community is bursting with energy. Many of our readers are interested in jumping in head-first and building their own solar batteries out of lithium 18650 cells. We have 42 pieces of advice to keep you safe during your build (keep on reading!), but the absolute best advice we have for new battery builders is this:


Don't Build a Duck Boat 

In July of 2018, a shocking and terrible accident struck Branson, Missouri. An amphibious bus-like vehicle called a duck boat sank in a storm on Table Rock Lake, drowning 17 of the 31 sightseers on board1. It was soon discovered that the entrepreneur who built the duck boat had no formal engineering training. If he had, he could have been exposed to safety best-practices and prevented the tragedy.

Why does this matter?

The Missouri Duck Boat being recovered from the lake in sank in.

Engineering school takes an uncompromising approach to teaching safety, but not everybody in the 18650 battery community has gone to engineering school – let alone for battery engineering. Charismatic YouTubers actively discredit basic safety measures grounded in reproduced scientific facts. As a result, well-known safety precautions are often disregarded.

We think that it's possible to build safe batteries without a degree, but only if people put in the time to educate themselves and implement battery safety best-practices. If DIY battery builders fail to do so, then the community is at risk of creating the next duck boat tragedy.



42 DIY Battery Building Tips

We made this list of tips to expose many of the ways a DIY battery build can be compromised, and we wrote it with beginner builders in mind. We've made sure to include highly actionable time saving tips and tricks that battery builders of any level will appreciate. If you're personally interested in building a battery with 18650 cells (or any other chemistry or form factor – even lead acid), this list was written for you.


1. Practice internet skepticism. Guard yourself against making a mistake by doing tons of research before starting your project. If this is your first time working with high power electronics, you should plan on staying in the research phase for at least a few months instead of trusting any single source outright.

Cover of Linden's Handbook of BatteriesA battery textbook written by some of the world's leading researchers.

2. Take safety seriously. "If it works, it isn't stupid" is a common piece of advice in 18650 battery forums. However, this is a precarious state of when building a large, flammable device. When working with high energy density batteries, disregarding years of battery safety research is like playing Russian Roulette with a bomb. When in doubt, take every measure to protect yourself, your home, and your family from the fire risks associated with thermal runaway.

A house on fire.Protect your loved ones and your possessions: practice safe battery building.

3. Do not underestimate the cost of the project. Wires. Connectors. Housing. BMSs. Chargers. Tools. And, of course, your time. The costs of building a battery from scratch add up quickly. But that doesn't mean you shouldn't do it! First, make sure you've thoroughly planned your project. Take note of every tool and component you need to acquire, and don't forget to estimate the time requirements. Second, look for ways to cut cost without reducing safety: build, borrow, or buy second-hand, but don't be tempted by deals on new items that look too good to be true. If you live in a large metro area, consider getting a monthly membership to a hacker space or any other community workshop with lots of tools.

Inside a maker space.
Inside a maker space.

4. Understand what types of misuse reduces cycle life as well at what types of misuses cause fires. What's worse for a lithium ion battery: letting it sit at 100% charge, or letting it sit at 50% charge? What's more dangerous: building a pack out of a hundred small cells, or four large ones? Charging, or discharging? Do you know all of the activities that will reduce the cycle life of different chemistries, including the ones that put them at risk of thermal runaway? Knowing these facts before you start your build is an essential part of being able to design around them. (Want to know the answers to these questions? Keep on reading!)

5. Research the nitty-gritty details of your chosen battery chemistry. Every battery chemistry is different, some more so than others. For example, a brand new lithium nickel manganese cobalt oxide (LiNMC) cell has a fully charged voltage of 4.2V, while a new lithium iron phosphate (LiFePO4) cell is fully charged at 3.65V. To make matters even more complex, not all cells of the same chemistry are created equal. Various manufacturer have patents and trade secrets which alter the characteristics of their cells. Furthermore, even cells from the same batch of a fully automated processes have slightly differing characteristics. Seek to understand the voltage, capacity, and thermal characteristics of your cells, at different states of charge and at different points in its cycle life.

6. Understand the role of the battery management system (BMS). Battery management systems (BMSs) are electronic devices that are attached to single cells or groups of cells in your pack. Their primary role is to manage charging and discharging your pack in order to prevent misuse and damage. Once a BMS is added to a battery, all charging and discharging is regulated by the BMS. However, BMSs are often an afterthought by DIY battery builders, which has lead to many BMS-related battery failures. We've included five major mistakes people make when choosing and installing BMSs in this article to make sure you get the most out of this important component.

The Electrodacus Solar Battery Management System.The Kickstarter-funded Electrodacus Solar BMS (SBMS).



7. Oversize your battery pack. Here's a great tip for pack builders working with lead acid or lithium ion cells that care more about cycle life than battery size. Oversize the capacity of the pack so that you only use 70% of its nominal capacity. For example, if you want 10kWh of usable capacity, create a 14.3kWh pack. For a 43% increase in upfront cost, you can avoid many of the dangers of overcharging and over-discharging your cells and double their cycle life2. In fact, Tesla uses this technique in their Powerwall to get many thousands of cycles out of their packs. However, if you're considering using LiFePO4, ignore this advice and design a pack that uses 100% of its nominal capacity. Why? Read on to find out!

8. Include cell-level fuses in large packs. Many people forego adding fuses between their parallel cell connections because they think that the chances of one cell becoming damaged and short circuiting their battery pack is low. This may be true for small packs made from closely matched cells, but fuses are a necessity on any large project, like a home battery made from salvaged cells. But how do you connect them? Within the past few years, pack builders have begun using a spot welder to attach fuse wires to cells. Although the technique takes some practice, it's well worth the added safety.

Spot welding fuse wire to lithium 18650 cells.
DIY battery vloggerDIY Tech & Repairs demonstrates fuse wire spot welding.

9. Ensure your series connections can handle the amps. Have you ever wondered why DIY battery builders have thick copper busbars between all of their cells? The reason is because series connections have to handle more amperage than parallel connections. Don't make the mistake of under-sizing your series connections. Double or triple up on whatever material your using, or spread the series connection over multiple cells.

26 batteries connected in an efficient 13S 2P configuration.
YouTube vloggerGreatScott! has connected 26 lithium ion 18650 cells in a 13S 2P configuration that makes efficient use of nickel strip. The nickel strip in the orange box makes on parallel connection, and the nickel strips in the two blue boxes together make one series connection. The series connection is spread out over two points, effectively doubling the amperage of the connection. This is the recommended way to make series connections using spot-welded nickel strips. 

10. Make sure your BMS can handle the amps. A common BMS mistake is to purchase a BMS that isn't rated to handle your battery's amperage. If you're making a 48V 50Ah pack with a 1C maximum discharge rate, make sure your BMS won't be fried by 50 amps. If you had trouble understanding that last sentence, check out our battery terminology guide here.

11. Use a balancing BMS. Cells in series or groups of parallel cells in series can develop charge imbalances through normal usage. This issue is worse for DIY packs built with salvaged cells. Human error and poor construction techniques can also produce uneven resistances between cells, causing them to become unbalanced over dozens of charge cycles. The solution? Use a BMS that will balance the charge of your cells. There are two main ways BMSs balance cells: either by draining the fastest charging cells (passive balancing), or by shuttling energy around from high charge to low charge cells (active balancing).

A longmon BMS.
Above: The LongMon, a passive balancing BMS by Batrium.
Below: The SBMS, an active balancing BMS by Electrodacus. In this photo, the SBMS is shuttling charge between the cell with the highest charge, labeled 4, and the cell with the lowest charge, labeled 5.

The SBMS balancing cells actively.

Use a BMS meant for your number of series connections. 
If your pack will  have six series connections, you need to use a 6S BMS. Using a BMS meant for any other number of connections will cause the BMS to malfunction.

13.Consider buying packs of cells with pre-wired BMSs. Another money and time saving tip that could even improve your battery's safety is to buy battery packs with prewired BMSs. Just make sure that the BMS that comes with the pack is a quality product by buying from a trusted retailer and doing your homework and buying a sample.

An eBay advertisement for a battery with a pre-wired BMS.
A eBay advertisement for a battery pack with a BMS pre-attached. The same battery pack is also offered by the vendor without the BMS. Bonus tip: eBay vendor alarmhookup is well-known in the DIY battery community for their wide selection of new and used lithium batteries.

14. Consider alternatives to lithium ion. Let's face it. If you want to build the absolute cheapest battery bank, nothing beats free batteries. What type of battery is easiest to find for free? Lead acid! There is nothing wrong with having a properly designed lead acid battery bank. There are other alternatives to lithium as well. If you're looking for a non-toxic, deep cycle battery that can handle extreme misuse, consider nickel iron batteries. Of course, lithium has its advantages over other chemistries, but its also more dangerous and expensive than many of the alternatives. If you're unsure which battery chemistry is best for you, we have a rundown of all the major solar battery chemistries here.

Ohmstead Deep Cycle Battery Guide comparison table
This table comparing the affordability of various chemistries is from our Deep Cycle Battery guide. Although lithium batteries have recently become affordable for many applications, there are many other affordable alternatives for stationary energy storage. Visit the guide to learn more.

15. Save time by using large cells. Want a ton of energy storage, but don't want to weld 500 cells together? Then consider using a dozen large cells instead. However, there's a catch: you should only take this approach if you're using a safe chemistry, like lithium iron phosphate or lead acid. Using larger cells is inherently riskier than using smaller cells. The larger the cell, the larger the damage cause by a single cell "event" (one of the battery world's many euphemisms for "explosion").

Large lithium ferrous phosphate cell.
This big boy is a 3.2V 300Ah LiFePO4 cell, manufactured by the Chinese company Thunder Sky Winston Battery.


Acquiring Components

16. Don't be fooled into buying cheap junk. The DIY community will often stop at nothing to find the best deal on a component. Some people see it as a matter of principle and pride. But this thinking constantly gets people into trouble. If you see a deal on lithium cells that look too good to be true, then it probably is. The secondhand battery market still has some of the best deals on lithium, but industry is catching up quickly and can offer compelling warranties and safety certifications.

Bad cells being advertised on eBay.

These cells test at only 1/3rd of the advertised capacity in spite being recommended by a prominent YouTube vlogger.

17. Buy a test batch. If you want to save money at the expense of time, here's one of our best money-saving tips. When purchasing second-hand cells that you plan on testing, buy small sample amounts to test for quality. This is a tried-and-true method of protecting your wallet, but it does mean waiting longer to buy and test all of your cells.

18. Spend money on a quality BMS. Perhaps the biggest reason that BMSs have a bad wrap in the DIY battery community is that the market is flooded with cheap BMSs that aren't made to last as long as the cells they're attached to. Attaching a bad BMS to high quality cells is an expensive mistake. Buy a high quality BMS from a trusted vendor fully loaded with all the features you need, including charging algorithms, balancing wires, state of charge reporting, and safety alerts.

19. Avoid mixing 18650 cells from different sources. Building a battery pack is a delicate balancing act. All batteries are different, and cells from the same factory batch can have slightly different characteristics. As cells age, their differences are exacerbated. When you start mixing cells of different ages, chemistries, and usage histories, these imbalances can become extreme. Weaker cells will become overstressed and will fail, possibly resulting in blown fuses, lost time, power outages, and grave injury. Using a BMS and fuses will go a long way in preventing a bad cell from causing damage, but they won't prevent the cell from dragging down the performance of the entire pack. Always build packs from closely matched cells. One way of finding similarly matched used cells from a single source is by buying modules from an electric vehicle.

Tesla battery modules contain hundreds of 18650 cells.
This Tesla battery module contains hundreds of 18650 cells.

20. Use pure nickel strips and avoid nickel coated steel. Use pure nickel strips for cell-to-cell connections in small projects that don't require cell level fuses, like an e-bike battery. When purchasing pure nickel strips, beware: nickel-coated steel is often falsely marketed as pure nickel! Steel is considerably less conductive than nickel, which means that you'd have to use more nickel-coated steel to make up for the difference. Avoid this mistake by purchasing your supplies from a reputable vendor and avoiding deals that are too good to be true. If you're wondering how to test the difference between nickel and nickel-coated steel, here's how: take a grinder to your material and visually inspect the sparks. This is called a spark test. If it shoots off long orange sparks that fork, it contains steel. Pure nickel gives off very short orange to deep-red sparks.

Metal spark test diagram.
Different metals produce different colored and shaped sparks.


Testing Cells

21. Get ready to spend weeks quality-testing salvaged cells. Testing salvaged cells is a tedious process that takes weeks to do properly, even for a small solar battery project. Why? Let's say you want to build a battery with 3kWh of usable storage and you're using lithium ion cells that claim to be 2000mAh on average each. That means you'll need 500 usable cells to safely stay between 90% and 10% charge. If you purchased a battery charger that can hold eight cells at once, then cycling enough cells to find those 500 will take weeks with only that one battery charger. Of course, you could purchase multiple battery chargers, but they aren't cheap. The time it takes to test cells will also depend on how many of your batteries test too low below their useful capacity. If you value your time, buy new cells or buy larger cells.

Four-cell 18650 battery charger.
A typical four-cell battery charger.

22. Perform self-discharge tests before capacity tests. In addition to cycling your used 18650 cells (or any used cells) to determine their capacity, you'll have to test your cells for self-discharge. To perform this test, charge all of your cells to their cutoff voltage, leave them alone for two weeks, then test their voltages individually. If a cell has maintained its voltage, then it isn't self-discharging too much and has passed the test. If it has decreased in voltage significantly, they are unusable and should be disposed of properly. Here's a time saving tip that will save you hours of testing: perform the self-discharge test before the capacity test so that you don't waste your time cycling leaky cells.

23. Test every salvaged cell you plan on using. Even if you purchase salvaged cells from a reputable vendor, you may be tempted to only test a handful. However, all it takes is one bad cell to destroy your entire pack, especially if you aren't using BMSs and fuses. But a reputable vendor shouldn't be sending you faulty cells, right? That's true, but even the largest cell-salvaging operations rely on manual labor, introducing plenty of room for human error.

Hand-testing cells at a used cell salvaging company.Thousands of cells are sorted by hand at a cell-salvaging operation.

24. Properly dispose of bad cells. There's no excuse for throwing your bad batteries in the trash. Plenty of electronics retailers will gladly recycle your batteries for you, so take advantage of their free services.

25. Replace damaged cell casings and missing gaskets.If you're breaking apart used electronic goods to harvest their batteries, you may accidentally damage your cells' protective casings and gaskets. Why is this a problem? Because in some cells, the entire outside of the battery is the anode! Leaving it exposed creates a short circuit hazard. Sometimes vendors will even send you cells with damaged casings or missing gaskets. Make it a point to buy new casings and gaskets in your favorite color and fix these problems as they arise. Cell casings are just small tubes of shrink-wrap, so you'll need a strong hairdryer or heat gun to shrink them onto your cell.

Cool custom 18650 cell casings.
Turn a chore into a tribute.


Constructing the Pack

26. Make a detailed step-by-step build plan. If this is your first build, think through the entire process beforehand. Write down every step of the build on paper to keep track of where you are and what to do next. Make sure you know which steps have the highest risk of short circuiting your battery or zapping yourself, and take precautions to avoid those risks.

27. Practice basic electronics safety. Before you start your build, take off all metal jewelry. This is a classic precaution for all electronics work, but it's even more important for working with batteries where the cathode and anode are on the same side. A ring could easily touch both terminals and create a short circuit. Also, clean your workspace and keep it clean. Never leave loose wire or nickel strip scraps around your workspace. Work in a methodical, step-by-step manner, and always keep your bench clean. Not only will your working environment be safer, you'll introduce less contaminants into your pack, such as dust and tiny pieces of debris, which will increase the quality of your build. There's a reason why sensitive electronics are made in a cleanroom!

A clean work environment is a safe environment.
Cluttered yet organized. A clean workspace is a safe workspace.

28. Use a spot welder. After years of debate, the DIY battery community has finally accepted that spot welding is the safer and superior technique for making cell connections. Do not solder directly to cells. But what if you've already created a battery pack by soldering directly to your cells. They didn't explode, so they weren't damaged, right? Don't be fooled by what people uploaded to YouTube five years ago. Soldering damages cells by heating them far beyond their healthy range. This causes charge imbalances to build up in your pack over time, reducing the cycle life and performance of your finished product.

Maletrics spot welder.The Maletrics spot welder. Quality spot welding at a budget price.

29. Test your spot welder on a bad battery to dial in the settings. Make sure you have an extra cell for testing your welds before moving onto your good cells. If you're spot welding with the probe extension, you will probably need to use a lot more power. To test your connection, use a pair of pliers and try pulling the fuse wire or nickel strip from your test cell. if it doesn't come of easily, it might be a good connection.

30. Always start with fully charged cells. Most beginners know that starting with imbalanced cells will create an imbalanced battery pack. But there's another problem. Connecting two cells with a large voltage disparity in parallel will cause the higher voltage cell to rapidly discharge into the lower voltage cell. This could blow the fuse wire connecting them and damage the cells involved.

31. Connect the B- and B+ wires of the BMS evenly to parallel cell groups. The B- and B+ wires of a BMS need to be connected in a way that allows electricity to flow evenly among every cell in the parallel group that they're meant for. The classic beginner mistake here is to connect the B- to one cell in the group instead.

A properly connected BMS.A properly connected BMS B- wire.

32. Know what to do if your BMS has an extra balance wire. When you buy a BMS meant for n series connections, it might come with n+1 balance wires. If you received a BMS like this, don't panic! The first balance wire is meant for the negative terminal of the first cell (or group of cells) in the series. The rest of the balance wires are attached to the positive terminals of the groups in series, as usual. For a 3S BMS that comes with 4 wires, the labels should be labeled as follows: B1-, B1+, B2+, B3+.

An 8S BMS with 9 balance wires.
An 8S BMS with 9 balance wires.


Charging & Discharging

33. Learn how to properly charge your chemistry. All battery chemistries have different charging requirements. You may have heard that all lithium cells need to be charged with the constant-current constant voltage (CC-CV) method, but there's even an exception to this rule – read on to find out what it is. Make it a point to find out how exactly your chemistry likes to be charged. However, a correctly matched BMS will handle the charging specifics of your chemistry even when your battery is plugged into a bulk charger.

Cutoff voltage vs cycle life of lithium ion batteries.

34. End charging lithium cells at 80% and discharging at 10%. If you've taken our advice and you plan on oversizing your pack to only use 70% of its nominal capacity, then this is how you'll want to charge your cells to help them last as long as possible. To do this, you'll need to make sure that your BMS or charger has the ability to set maximum charge and discharge levels. This advice does not apply to LiFePO4 cells. Check out this chart comparing maximum charge voltage cutoff to cycle life.

35. Don't confuse your chemistry's charge and discharge needs. Charging a lithium battery is a more dangerous and error-prone activity than discharging it. In fact, most battery fires occur during charging. Therefore, it's important to realize that the safe temperature range for charging your cells is more restrictive than the safe discharge temperature range. If a battery has a maximum discharge temperature of 50ºC, then charging it at that temperature could result in thermal runaway. But the differences don't stop with temperature: charge and discharge rates are also different. Most batteries can be discharged faster than they can be charged. Said differently, you can't charge a battery at the same amperage as you can discharge it. Make sure you know what your battery can and can't handle for both charging and discharging.

36. Charge and discharge rates for a DIY battery pack are constrained by the parallel group with the lowest cell count. Here's a tricky issue that's easy to overlook. Say you build a battery with two groups of parallel cells that are connected in series. Say you used salvaged cells of varying capacities to build the groups. If one of those groups has four higher-capacity cells in parallel, and the other has five lower-capacity cells in parallel, then the group of four cells limits the C rates of the pack. Why? Even though each group has the same capacity, the four cell group can't charge or discharge as many amps as the five cell group can at once.

37. Lithium iron phosphate batteries can get away with bulk charging. This is the exception to the lithium CC-CV charging rule. Lithium iron phosphate (LiFePO4) is one of the safest and most robust lithium chemistries. Its inherent safety and high cycle life makes it an ideal solar battery chemistry. What many people don't know is that LiFePO4 can be bulk charged safely up to 90%, making it a viable option for people who want to switch from lead acid to lithium without replacing their battery charger. Moreover, LiFePO4 can use its entire capacity without suffering damage – that's right, oversizing a LiFePO4 pack to operate at a reduced DoD will not increase its cycle life3. As a tradeoff, LiFePO4 has a lower energy density and a lower C rating than other lithium chemistries, making it less preferred in the e-bike community. This following chart shows the relationship between DoD and cycle life for LiFePO4 batteries: 

Cutoff voltage vs cycle life in lifepo4 batteries.


Maintenance & Final Tips

38. Store lithium cells at half capacity if they aren't being used. Lithium batteries are stressed when they're at full capacity. If you're going to be away from your battery for a long period of time, set it to maintain a range of charge in the middle of its capacity.

39. Avoid replacing single damaged cells with brand new cells. There may come a time when a cell in your finished pack needs to be replaced. A common misconception is that an old cell can be safely replaced with a brand-new, higher-capacity cell. This is not the case. The best replacement would keep the capacity of all the groups in series equal to each other. Replacing the damaged cell with a new, higher-capacity cell will upset the balance of capacities in your series connections, leading to damaging overcharging and over-discharging.

Damaged home made battery pack.
Damaged cells and connections in home made battery pack.

40. Do not mount your battery against a hot surface. When you're ready to mount your battery to its final location, make sure that one side isn't mounted against a hot surface or else the pack might fail prematurely. If one side of the pack is hotter than the other, this will cause changes in the internal resistances of your cells and may cause your BMS to fail. Instead, pick a place with good heat distribution and ventilation to mount your battery.

41. Use a battery monitor. The best way to keep tabs on your battery at all times is to stream data from sensors attached to your battery or battery charger directly to a convenient device. This way, no matter where you might be, you can know your battery's state of charge and even be alerted if any cells become dangerously hot or fail, or worse. This could be a lifesaver.

Batrium charts
Above: Batrium charts.
Below: Electrodacus SMBS charts.

42. For your first DIY 18650 battery project, choose something small. If you've never built a DIY battery before, start with a small project. There are endless opportunities to add rechargeable batteries and solar cells to everyday electronics to extend their usefulness and unlock new ways of doing things. Instead of choosing a large and potentially dangerous project as your first one, build a portable cell phone charger, a small emergency power supply, or solar landscape lighting, or a solar water fountain, or something completely different. Get creative!

DIY solar generator.
A DIY solar generator.


      You made it to the end – congratulations! We hope that more people in the DIY battery world will share safety information like this to help others build safer batteries.

      But will people listen? Or will DIY battery builders continue to make duck boats?


      DIYer Discount: Use the coupon code "18650" for $10 off the OffGrid 100 solar panel, bringing the price down to $79 per 100W monocrystalline solar panel.




      2:"The decrease in capacity fade at reduced DOD cycling translates for the end user in a four-fold improvement of cycle life when going from 100% DOD to 50% DOD cycling."

      3: "In this report, we have presented the cycling test results from an accelerated cycle life study on commercially available LiFePO4 batteries. The effects of test parameters (time, temperature, DOD, rate) were investigated and described. The results show that the capacity loss is strongly affected by time and temperature, while the effect of DOD is less important at a C/2 discharge rate."


      4 Responses


      February 08, 2019

      Great post! Lots of awesome tips!

      Andy Jones
      Andy Jones

      August 10, 2018

      Tip: Have a metal can or pie tin you can toss a smoking battery into to get it outside.

      Very timely otherwise. Covers a lot of key points.
      (Comment originally posted in Facebook)

      Rolando Sanchez
      Rolando Sanchez

      August 10, 2018

      Thanks for posting this.
      (Comment originally posted in Facebook)

      John Day
      John Day

      August 10, 2018

      This is great. Thanks!
      (Comment originally posted in Facebook)

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