[To see the build without all the planning, click here to jump there.]Back in the day, 18650 lithium ion batteries were all the rage, and still are. So I had some experience with sourcing these and looking for battery holders.
Unfortunately, 18650 battery holders were not so easily available, especially in low quantities. I still have an unused aluminium pipe from my days of experimenting with diy 18650 holders. Mouser has a few holders but they require a minimum order total to qualify for free shipping.
Lead Acid batteries are clunky, heavy, and dangerous if you short it. If you were to short a small 9V alkaline battery on your tongue, you might get a small shock. But if you were to short, say a 6V, lead acid battery on your tongue, the shock would be not be so mild. This is because they can put out a lot of current. The upsides of a lead acid are high current, price, very easy charging, and stability/safety (you don’t really need to worry about heat issues, or about degrading battery life based on your usage or charging style).
Fortunately for me, when I made the decision to build this, DX, my favourite online shopping site, began to carry some. They weren’t anything like those from Mouser but they worked fine. (DX now carries holders that look very similar to those carried by Mouser).
The first thing you want to do when building a battery box is take out the calculator and ask yourself:
- How much power am I going to use? How much power do I need?
- How long do you want your battery to last between charges? Battery capacity?
- Do I need high current capability?
- How long do I need the battery to stay “good”? What kind of shelf life do I want in a battery? How many times can I recharge it before it starts getting “old”?
The following is a rough guide to the above points. Some liberty has been taken in assuming you have a general understanding of the terminology and concepts behind it. If I didn’t do that, this post would be much longer than it is now.
We will be considering secondary (rechargeable) 18650 sized 3.7V nominal lithium ion cells. We consider 18650 sized (18 mm diameter, 65mm height hence 18650) cells because they usually offer the most value since they are very very common. The battery for almost all old laptops and still a fair portion of the newer ones still use these 18650 cells.
Also note. A cell is the proper terminology for what most people will call a battery. A battery is actually many cells. So a single AA “battery” would actually be a cell, and 2 or more AA “batteries” would really be a battery. I may mix up the terminologies in this article because I’m just like everyone else. If I do, let me know and I’ll correct it.
How much power do I need?
To get a rough estimate of how much power you will need, you need to know how many watts your device will consume.
Power (in Watts) = Volts x Amperes (current).
Battery voltages sag under load and drop as they drain. This means, as you use your device, the voltage on your battery will drop. When you turn off your device, the voltage on the battery will increase slightly from wherever it was at. This usually happens because the battery can’t provide enough current to keep the voltage up. It also has a bit to do with the chemistry of the cells. The voltage also drops as the battery is drained but it doesn’t drop linearly. For example, generally, if you measure the voltage in a fully charged lithium ion battery (i.e. 100% State of Charge, or 100% SoC) the measurement will show 4.2V. Between 60 to 20%, it hovers around 3.7V, and below 20% it drops very quickly to 3V or below. This is known as a discharge curve.
This kind of ‘discharge curve’ (shown below) is very important to know for 3 reasons. Firstly, if the voltage drops too low, your devices may not work correctly or may not work at all. Secondly, if it drops too low, your device may try to compensate by drawing more current which could overheat your wiring, overheat your batteries (which could explode) or blow fuses. Thirdly, if you run down your batteries too much, you could destroy them.
From this curve, you can see that most of the “capacity” will be when the battery shows a voltage of about 3.7V. So this particular lithium ion cell has a “nominal voltage” of 3.7V. There are a few other types of lithium ion cells with different chemistries that have different nominal voltages.
So, on to the math.
As an example, on page 48 of the manual for my Lowrance Elite 5 HDI fishfinder, it says it typically consumes 1.1A (1.1 Amperes). This is not the maximum current that it can use, but how much it would normally use. What constitues typical/normal varies wildly between usage scenarios, environment, voltage supplied, and even between actual devices themselves, even if they are the same model.
So I did a google search and found that some users had reported a maximum measured current of 1.5A. The current will vary as the voltage of the battery changes but we simplify things and just assume it can use a maximum of 1.5A @ 12V so it consumes 18W (watts). To digress a little, I’ve found my typical usage to be about 800mA with the brightness of the device at 8 out of 10 and with the ping speed set to normal. The backlight for the LCD screen draws the most current. Maybe they should talk to Samsung about AMOLEDs.
I also have an aerator for my live well (or as they call it here, an air pump). My aerator for my live well uses 250mA @ 1.5V so it consumes 0.375W, but let’s round it up to 0.5W.
We do all this rounding up and estimations because there are many other factors we have to consider but to make it easier, we just assume that we use more power than we actually might. High resistance wire, long cable runs, losses in voltage converters (e.g. to convert the 12V from the battery to the 1.5V needed for my aerator), these all contribute to inefficiencies and wasted power (most of it is wasted as heat).
So in an hour, at most, I will consume 18.5 Watts (18W + 0.5W). If I wanted my battery to last an hour, I would need an 18.5Wh (Watt Hours) battery, or a 12V battery with at least a 1.5Ah (Amp Hours) capacity.
So we’ve done our math and know what kind of capacity we need. Or do we?
The most obvious capacity factor we need to look at is of course how long we want our battery to last. In the results above, we found we need an 18.5WH battery to last an hour. If we wanted it to last 12 hours, we would want a 222Wh battery or a 12V battery with at least an 18.5Ah capacity.
The next factor we need to consider is the discharge curve. How much of the battery capacity is usable to us before it drops to a voltage too low for our devices to work? In that particular battery we saw above (the one shown in the discharge curve), we only get to use about 80% of its capacity. That discharge curve is mostly the same with most lithium ion cells. So let’s say we have a 2000mA (2Ah) battery, we’ll only get about 1600mAh or 1.6Ah of usable capacity. So in our case (12 hours battery life), we want a 22.5Ah 12V battery.
The next factor concerns voltages. Our lithium ion cells have a nominal voltage of only 3.7V but we need 12V. If we put 3 cells in series, we create a battery with a nominal voltage of 11.1V, and a max voltage of 12.6V. We could put 4 in series and make our nominal voltage become 14.8V and our max voltage 16.8. Some “12V” devices can tolerate voltages of 14.8V but even fewer can tolerate 16.8V.
Most “12V” devices will run reliably between 10.5V and 14.4V so in this example, we assume that we put 3 x 3.7V nominal cells in series, so our nominal battery (those 3 cells in series) voltage is 11.1V. This is a very common configuration anyway.
This voltage tolerance is due mostly to how 12V lead acid batteries are charged in cars and the actual voltages of those batteries. To digress a bit, a device that requires “5V” usually can tolerate voltages between 4.8V and 5.2V (like USB), a device that requires 3.3V can usually tolerate voltages between 3.2V and 3.4V (usually microcontrollers and Integrated Circuits), and a device that requires 1.5V (like those that use AA alkaline batteries) can tolerate voltages between 1.1V and 1.6V.
At full charge, our 3 cells in series will have a maximum voltage of 12.6V (4.2V * 3). It will have a nominal voltage of 11.1V. At full discharge, it might drop down to about 9V which is too low for our device but we’ve already compensated for this a few paragraphs above. So in our case, we find that the voltage of our battery throughout the discharge is within the range of acceptable limits for our devices. So we still stick to our 22.5Ah 12V battery.
Another factor we must consider is our cut-off voltage. Lithium cells will be irreversibly destroyed if you run them too low down. You are most likely to end up buying protected lithium ion cells anyway so we don’t need to worry about that. Instead, we must worry about when the inbuilt protection circuit will ‘turn the cell off’. Some turn them off at 2.5V or at least claim to do so, but for our purposes, let’s assume that it turns off at 3V. That means our battery would turn off at 9V. So no problem since it’s well below our estimates. The protection circuitry will also draw some current from the cell so you will lose charge over time but it’s small and can be covered by our previous rounding ups. Alternatively, you can use external protection circuitry with unprotected cells but we won’t cover it here. So, we are still happy with our 22.5Ah 12V battery.
The next factor and arguably one of the most important ones is actual capacity. There are 2 parts to this factor. The first is, is the stated capacity real? Many of these China brand 18650 sized cells have claimed capacities of up to 4500mAh (4Ah) for a 18650 cell. At the time of this writing, the highest you are likely to ever get in a 18650 sized cell with a nominal voltage of 3.7V (we state the nominal voltage because different chemistries have different voltages and different capacities) is 3Ah but those are from premium brands and premium models like those from Panasonic.
Then there are imitations and there are fakes. For this article, consider an imitation a well made copy, or even an identical or almost identical copy, and consider a fake a very bad copy that just looks the same on the outside (then again, sometimes, it doesn’t even look the same). There are fakes of real brands like those from Panasonic or Sanyo or LG. Then there are imitations or former imitations such as Surefire. There are also imitations of the “original” Surefire such as Ultrafire, Uniquefire, Trustfire, Fandyfire. Those are mostly crap but occasionally there are “good ones” (but still nothing compared to the real Panasonics or Sanyos). The quality of these imitations vary wildly between models, build batches, age of cell (covered in point 4, regarding shelf life), etc. Then there are also fake copies of Surefire and fake copies of its imitations (fake Ultrafires, fake Trustfires, etc). In essence, trust nothing and do your research.
The next part of this factor is discharge current. The faster you discharge a battery (i.e. higher current), the less you get out of it. As an example that isn’t accurate, say I have a cell that can provide 2000mAh when I discharge it at 1A. If I discharge it at 0.1A, I might be able to get up to 2100mAh but if I discharge it at 2A, I might only get a usable capacity of 1500mAh.
An analogy would be like driving from point A to point B at full revs vs at economical revs. Travelling at full revs will lower your mileage and use more fuel for that same distance as compared to driving at economical speeds.
To extend that point further, will the protection circuitry prevent me from discharging at such high currents (Analogy: will the rpm limiter kick in and will it kill the engine)? Will the cell overheat (Analogy: will the engine overheat… and explode)? A safe discharge for a 3.7V nominal lithion ion cell is usually 1C, that is to say 1 x Capacity. i.e. a 2000mAh cell should be able to safely discharge at 2000mA or 2A.
Here are some photos of my cells with the tested capacities written on them. I tested each cell at a discharge rate of 1A and wrote down the measured capacity.
While I was taking these photos, I noticed that there seemed to be a burn mark on the box. I took a look at my board and discovered that there appeared to be a short on the circuit board. This would explain why on my last trip, I saw my battery indicator drop from 4 bars to 3 bars so suddenly. What probably happened was a drop of water had entered this case and shorted it out.
Fortunately, copper traces tend to burn off rather easily so after the water boiled and/or electrolysed away, the short quickly unshorted itself. There are also a lot of batteries with fairly thin wiring and those cells all have protection circuits in built so there are tons of “safety features” already integrated in there. Had this been a lead acid though, I’m quite confident there would have been quite a bit more damage.
Naturally, I have to fix this but for me, the main issue would be not discovering this and then wondering why my battery drained itself so quickly.
Yet another factor is matching, balancing and balanced charging. This is really a massive pain to plan for and to explain clearly. Say you have 3 cells in series and all are fully charged (balanced charging) and have the same capacities (matching). After using 50% of power, their voltages will have dropped to more or less the same level. If they had wildly different capacities (mismatched) or had different states of charge (unbalanced charging), one of them is going to drop to the cutoff voltage while the rest have a higher voltage.
Pretend you have 3 cups of water but the cup at the bottom is only half full. If you remove the water from all the cups at the same rate, about half way, the cup at the bottom is going to be empty. Pretend you have 3 ballons but one of the balloons is only half filled. When you let the air out of all the balloons, you get a certain amount of air coming out but the balloon that is only half filled is going to be empty before the other 2. When that happens, the amount of air coming out is now much lesser.
This is a simplistic example of the problem. In reality, many problems will occur. When a cell is empty, it isn’t really empty. There is actually more power to give. It’s just that by that point, your cells will be irreverisbly damaged by discharging them too low. So realistically, your usable capacity drops to whatever the lowest capacity cell has. The battery will likely continue to work but you will be damaging one of the cells. You will also get very unusual discharge curves and the batteries will wear unevenly as the lowest capacity cell has to work harder. You also get problems with current. In summary, don’t do this.
As you may see in the pictures near the end of the post, I tried out a lot of different cells, all with wildly varying capacities. The solution is matching then paralleling (balancing), and using a balancing charger. Let me explain. You want to keep the capacities in each 3 x series battery roughly the same. This is not always easily done.
Fortunately, there is a trick. You have to parallel everything and balance all cells both in series and in parallel.
Consider the following:
In the above, we see that our usable capacity is only 1500mAh. This is because when the 2 outer cells have about 500mAh left, the cell in the center is going to be empty. In reality, if you continue discharging this battery, the cell in the center will be discharged to such a low point that it will be irreversibly damaged. Alternatively, if your cells are protected, the cell in the center is going to shut itself off once it becomes empty which will render your battery (those 3 cells in series) inoperable.
So you either need to get bigger batteries or more batteries.
Let’s consider the following:
Here, you can see that the batteries (each 3 series can be called a battery) are mismatched. Because it’s mismatched, it only gives you a total of 3000mAh (1500mAh each).
The solution is to use paralleling:
By adding balancing leads, you see that the cells now appear to be matched. In the picture below, you can see a simplified version of what is happening above:
Of course, in this particular scenario, there are 3 cells that have the same capacity and the other 3 cells have their own same capacities. So we could have simply done this:
In reality, the chances of you getting well matched cells like these are unlikely, especially with low cost, non branded cells.
In the image below, you can see that we have 2 cells with 1000mAh, 2 cells with 1500mAh, and 2 cells with 2000mAh.
From the image above, we can see that we’ve made the best of a really bad scenario. We have optimised our battery to provide the most usable capacity. In this particular scenario, the capacities are perfect from a mathematical standpoint because the optimisation has given us the maximum capacity achievable. Again, in reality, the chances of you getting cells with capacities that magically add up to give you the best outcome is unlikely.
When I did my testing for my own battery pack, I ‘junked’ about 12 or 15 cells because they were simply too low to be usable. Even with paralleling, the math would not add up.
By paralleling your cells, you also lay down the ground work for balanced charging, so this is a bonus. Balanced charging requires connections to all terminals. You also get a much nicer discharge curve.
Often times, when your laptop battery has died, it usually is just one cell that has died. Some shops will buy your “dead laptop battery” or give you a discount on a replacement battery if you trade in your “dead battery”. They then open up the casing, find the bad cell, replace it, then resell the battery as “refurbished”.
This battery pack is not immune to this problem. This is why I use battery holders instead of spot welding cells together and wrapping them all up in tape (like those blue li-ion battery packs). This way, I can easily find and replace any bad cells. Using wildly varying capacities makes this problem also occur sooner rather than later, even with paralleling. The first to go will usually be the cells with lower capacities, as they have to work harder. (They also tend to be the crappier cells, which further hastens their death)
Another factor to consider is temperature. All batteries will appear to have less usable capacity when you use them in cold conditions. Unfortunately, in Singapore, we don’t have this lovely problem.
At this point, we’re in the clear. Not only will our usage require less current than the maximum safe of a single cell, but because we need such a big battery (for bigger capacity) that contains many cells, the amount of current going through the battery is spread out among many cells. On the upside, because we use so little current from each cell, and from what we learned above, our usable capacity has now increased.
But, let’s plan ahead, stick to our 22.5Ah 12V battery, and consider the capacity increase a hidden bonus.
Do I need high current capability?
So we’ve sort of covered this already but let’s go into a bit more detail.
What if you have a tiny battery, perhaps because of budget and/or space constraints and you’re fine with having your battery last perhaps only an hour.
Or consider this. Most devices (including echo sounders) don’t draw a constant current. However, well built devices often hide the problems that this can cause. For example, a transducer doesn’t constantly use the same amount of current. Instead, it comes in pulses. It uses a certain amount of current to emit a ping, then when it waits for the ping to return back, it uses a different amount. If you know about PWM (Pulse Width Modulation) in LED lighting or in anything else for that matter, then you should know about peak current. A device that uses 1Ah of power doesn’t neccesarily draw current at 1A for an hour. It could be using 2A every other second, or 4A for 1 second, every 4 seconds.
Lead Acid batteries have the same problem but it is barely noticeable because they can safely output a lot of current, way more than 1C. This is partly why you don’t see Lithium Ion batteries replacing Lead Acid batteries in cars as the starter motor requires a lot of current to run. The dangers of this became apparent in the lithium ion battery fire in the Boeing 787 Dreamliners. Lithium Ion cells should always be protected.
Or maybe you want to use your battery to power a very bright light that only flashes infrequently.
So anyway, let’s say you’re dead set on a lithium ion battery and can’t add more cells. What can you do?
Well, there are other alternative chemistries that can provide much higher current but first we need a bit of background (I’m writing this from memory instead of current research so please forgive me for and inform me of any errors).
Back in the day, there were lithium ion batteries. Then they started using polymers in the batteries. So it technically became a Lipo (Lithium Polymer) battery. There are many chemistries of lithium ion batteries out there.
– Lithium Cobalt Oxide (LiCoCO2) – This is really really old and is generally unsafe
– Lithium Nickel Manganese Cobalt Oxide (LiNixMnyCozO2) – Commonly just called Lithium Ion cells or NMC and is what we’ve been talking about here
– Lithium Iron Phosphate (LiFePO4) – For the same capacity, it’s heavier than the NMC chemistry above and is larger in size but it can put out a lot of current safely. Has a much longer shelf and cycle life than NMC. Can also be float charged much more safely than an NMC. Commonly, but technically wrongly, called LiPo cells.
So the best option for a high current operation would be to use a LiFePO4 battery. If you want to go down this route, you will have to redo the math as the nominal voltage for this chemistry is different. Do your own research as well regarding safe usage.
There is also a chemistry that is current being researched known as Lithium Air cells. These use Lithium, and.. air. And not air stored in the battery, but air from the environment. It promises much higher capacities at much lighter weights. It’s many years away though, but I can’t wait for it.
Be careful when putting in parallel, cells or batteries with wildly different capacities (either capacity or state of charge). Do your research on this. It’s a bit too long to explain here. Take note that small variances are acceptable and commonplace.
Also take note that cells in series have a capacity (and thus safe current rating) of the lowest capacity cell. If I have a 1000mAh cell in series with 2 x 2000mAh cell, the usable capacity is only 1000mAh (in practice it’s always less) and thus a safe current rating of only 1000mA. Having cells in series that are at different states of charge also have a similar effect. If I have 2 x 2000mAh cells in series, but one of them is only 50% charged, the capacity is only 1000mAh. In this particular case however, the safe current rating is still 2000mA. This is why balancing is important.
In our case, our current (Amperes) requirements fit the bill so we continue on with our 22.5Ah 12V battery.
Shelf Life/Cycle Life
Lithium Ion cells have no memory effect. You can recharge them when they’re empty, or when they’re half full. There will be no problems.
However, what they do have is Shelf Life and a Cycle Life.
In general, any lithium ion cell will start to lose capacity the moment it leaves the factory, even if you don’t use it. If they started with the same capacity, after a year, those NMCs we talked about above will generally have less capacity than the LiFePO4 cells. NMCs will generally become completely useless about 4 years after the date of manufacture. LiFePO4 cells will lose capacity as well but over a much longer period of time (I don’t have the exact number). This is called Shelf Life. Shelf Life is also affected by storage conditions, and State of Charge. If you store your battery at 100% without using it for long periods, it wears the battery down. Think of it as revving your engine at full but with the clutch in. Your engine is ready to go but with nothing to work on. Storing your battery in hot and/or humid conditions also affects shelf life. Storing it when the State of Charge is too low is also a problem as all batteries will deplete themselves over time, even when not connected to anything (Sanyo Eneloop has Low Self Discharge NiMH batteries that don’t self deplete as quickly). When stored at low charge with protection circuitry built in, it makes the problem even worse as the protection circuitry further depletes the cell. Some chargers and some laptops (Lenovo Yoga 3 Pro) have “Storage Charge”. This charges the battery to betweeen 40% and 80% (it depends on the charger/laptop) so that shelf life is increased, or rather, not decreased.
So the next time someone chides you about leaving your lithium ion battery powered device in the charger, the only damage is in keeping the battery fully charged (which is what happens when you leave it charging). So removing your fully charged battery to ‘save the battery’ is pointless. (Note: leaving it ‘charging’ all the time will also contribute to cycle life – which is explained in the next paragraph).
Then you have cycle life. Like almost all (if not all) types of batteries (Lead Acid, Li Ion, NiMH etc), you can recharge a cell only a certain number of times before the the capacity starts to drop to unusable levels. Call it wear and tear if you like. A lithium ion cell can usually be recharged about 500 times or more. A LiFePO4 cell can be recharged about 2000 times or more.
Luckily, anytime you discharge then recharge a battery, it usually isn’t a complete charge cycle. If I ran a cell from 100% down to 0%, then recharged it back to 100%, that would be 1 charge cycle. However, if I ran it down to only 50%, then recharged it back up to 100%, it would only be considered half a cycle.
Recent developments in lithium ion technology show promise in reversing the effects of shelf life and cycle life.
In this instance, we don’t really care about that and just plan to replace the whole thing once it dies. So we stick with our 22.5Ah 12V battery.
So we want a 12V 22.5Ah battery. Let’s assume that each cell has an actual capacity of 1500mAh. Putting cells in series increases voltage and putting them in parallel increases capacity (and also maximum safe current).
We need 3 cells in series to get our 11.1V nominal voltage which is within “12V” device specifications. With that, we have a battery with a 11.1V nominal voltage and a 1500mAh/1.5Ah capacity but we want a 22.5Ah battery.
So we put 15 of those batteries in parallel and get a battery with a 11.1V nominal voltage and 22.5Ah capacity. This would mean we would need to get 45 cells. This is where a lot of the work comes in. You have to research which cells offer the best value while being pratical. If you had to choose between a 1000mAh cell that costs just $1, and a 2000mAh cell that costs $6, which would suit you best? If you need a very small capacity battery, you could probably get away with the 1000mAh one. If you need a large capacity battery, you will probably have to take the 2000mAh one, simply to keep your battery to a physical size that doesn’t require you to transport it with a trolley. Keep in mind, low capacity cells usually have low capacities because they’re crap, not because they’re “empty” inside. So they will mostly weigh the same.
Say we bought “good” Ultrafire cells with 1500mAh at $8 a pair, just the cells alone would cost us US$180. But remember, we calculated for max usage and rounded up.
Assuming that everything is the same, let’s pretend that in the real world, my fish finder typically uses 700mA on average (pretend I use it from noon to midnight – once evening comes, I lower down my brightness setting, drastically reducing power consumption). Now I only need a 14.4Ah battery. That would require 9.6 batteries of 3 cells in series, so let’s round it up to 10. I now only require 30 cells, which brings the price down to US$120. Significant savings for being realistic! How nice.
We’ve glossed over the fact that batteries lose capacity over time but oh well. We’ll just squint at our fish finders to save battery consumption.
I can’t recall where I bought the waterproof connector shown in the pictures from. I can’t find it on DX either.
Products on DX:
The balance leads. (Note: fairly low current capability)
A simple balance charger (note, there is a cheaper one that goes by the brand name mystery. it is unreliable and will spoil quickly)