Thanks Nefastor. I would be very interested in your follow up on this statement:
Naked cells can be combined in series and parallel like any batteries HOWEVER this is serious, potentially harmful business. If you're interested, I can post how to do it safely. If you make your own battery you'll also need to add battery supervision chips to it for safe operation (i.e. avoiding over- and under-charge). A bunch of these chips can be obtained as free engineering samples from companies like Texas Instruments. If you can make a high-current motor controller, you will have no trouble understanding how they work.
I thought you might
Okay, naked LiPo cells such as the ones sold for R/C modeling are the real deal, but without any form of protection whatsoever except a passive anti-explosion feature : a high tensile-strength internal net that will cause them to swell A LOT when mis-used, giving you enough time to disconnect them before they explode. I've seen it work, it's amazing. Problem is, this is not an elastic net, and a swollen cell is as good as dead. I've tried recharging one, and it wouldn't hold more than 10% its original capacity.
The main issue you face with interconnecting battery cells is balancing : when they are delivered to you, each cell will carry a different charge. This MUST ABSOLUTELY be taken into account ! Also, because of rather small voltage difference over the charge range of the cells, you CANNOT consider as equally charges two batteries that show the same voltage on the multimeter, unless your multimeter is a well calibrated unit with at least 20,000 points.
If you connect in parallel two cells at different charge levels, one will start charging the other. However, because there is no current limitation and the cells have a low internal resistance, you run the very real risks of exceeding the cells charge and discharge current (resulting in explosion) and turning your connections into molten metal.
If you connect in series two cells at different charge levels, it will be impossible for your charger to reliably measure the actual voltage on each cell or the battery, resulting in the most charged cell in the battery to be overcharged. That means KA-BOOM.
Therefore it is essential to manually balance the charge of cells (or batteries of cells) you wish to interconnect. There is NO easy, practical way to do that. The method I use, however, is the cheapest and least likely to kill you, because it's too simple you can't get it wrong.
The basic idea is to let the cells balance each other, in pairs, by letting the most charged one discharge into the least charged one. But what you want is to make sure you do not exceed the rated currents for your cells.
You do that by using a resistor :
- Connect the negative poles of two cells together
- Connect the resistor between the positive poles of the two cells
The resistor's value is the voltage difference between the two cells divided by the charge current for your cells. Keep in mind that, for large AND unbalanced cells, a regular quarter-watt resistor may not be strong enough. Also, since you'll be dealing with fairly low resistance values, you may need to use several resistors in parallel to get the right value.
You REALLY don't want a value smaller than what you really need (or else, KA-POW !) but also, a value too large will vastly increase the time it takes for the cells to balance each other.
In fact, as the cells balance each other, you may want to gradually reduce the resistance to make the process faster. In order to know when to do that and by how much, measure the voltage across the resistor : using Ohm's law, this will tell you the current flowing through it. To reduce the resistance, simply connect more resistances in parallel with the first one.
When you cannot detect any more current through your resistance, it means the cells are balanced and safe to connect in parallel or series directly.
I REALLY MUST STRESS, IF THERE'S EVEN THE SMALLEST DIFFERENCE IN VOLTAGE BETWEEN BOTH CELLS, YOU WILL DAMAGE THEM. YOU REALLY WANT TO HAVE A GOOD QUALITY VOLTMETER, ANYTHING BOUGHT AT WALLMART WILL GET YOU HURT.
On another note :
- for PARALLEL setups it is not necessary for the cells being balanced to be of identical capacity BUT it is essential that they come from the same manufacturer and use the same chemistry. You can get significant differences from two different manufacturers, regarding the actual EMF of the cell, its internal resistance and how much current it can take (or deliver).
- for SERIES setups IS IT VITAL for the cells TO BE IDENTICAL, especially in terms of capacity.
- to balance more than two cells, you MUST balance them pair by pair, then balance pairs together, and so on : while you can balance a pair, then balance that pair with another cell, and add cell after cell, if you do that you increase the currents you'll have to deal with, and it's never a good idea. If your number of cells is not a power of two, you'll have to do that eventually, so just be careful and remember to use high power resistors.
- it's a good idea to keep ANYTHING flammable WELL AWAY. A shorted battery will put out 60+ amps, usually in wires that aren't rated that high. I've seen people set fire to wooden benches that way.
- it is a GREAT idea to keep anything metallic WELL AWAY. The last thing you want is for a screw driver to roll against your naked cells and short them up : the current will fuse everything in place, in which case you'll have to run for your life.
Now, I know I seem a bit paranoid about naked LiPo cells, but I really can't stress enough : just like rare-earth magnets, if you're not careful these things will kill you, or at the very least maim you, blind you, burn you. You have to treat them with respect until they are finally assembled as a working battery.
Now, balancing the cells is actually only the first step in safely making a custom LiPo battery. What you need next is to ensure the cells will remain balanced during charge, and that they won't overcharge or undercharge. In the early days, this used to be a complex engineering challenge, luckily we've come a long way since then.
What you need now is to equip your battery with SUPERVISOR CHIPS. How many chips you'll need depends on how many cells your battery has. A great place to start is the BATTERY MANAGEMENT section of integrated circuit manufacturers. I personally use Texas Instrument chips because they are nice enough to send engineers free samples of almost anything we need. I can't post a link because the TI site is way too dynamic (so it would probably be broken after a few days or weeks) so :
- Go to TI's web site
- Look for a section such as "power management"
- There will be a subsection about batteries
- What you are looking for are any chips relating to lithium battery protection
Now, some bad news : LiPo batteries are intended for portable devices (mainly). That means the chips you'll be dealing with will always be small and hard to solder by hand. This cannot be avoided. Since this post is already getting gigantic I won't digress into this, but there are many ways to work with these small chips without breaking out the super-expensive professional tools. If you need pointers, you know who to ask.
And now the good news : LiPo batteries are intended for portable devices (mainly). That means they have many excellent features, some of which might go beyond what you need. The most important advanced feature is that these chips all have some sort of high-level management interface (typically an I²C bus) that will allow you to manage very large batteries using a microcontroller or any computer with a suitable interface.
You can use that interface to do any number of nifty things, such as checking the status of each cell in the battery independently and balancing the cells in your battery. You can decide yourself at which point the battery should be disconnected from its load to avoid undercharge, or disconnected from the charger to avoid overcharge, and of course the chip will provide you with an accurate reading of how much juice is left in the battery. In addition, supervisor chips often feature outputs to directly drive LED's that inform you on the status of the battery.
The most important feature of the chip is its ability to disconnect the battery in order to protect it. This is usually done through an external MOSFET transistor that acts like a switch. MOSFET selection is, in itself, worthy of its own discussion as the wrong MOSFET can reduce the battery's performance.
It's worth noting that you don't HAVE to use one of these chips but you MUST somehow implement their protection features. Many amateurs find it more comfortable to do so using PIC microcontrollers, but this will always be a suboptimal choice, typically shortening battery life AND reducing its available capacity.
Nefastor