Two safety warnings are important:
1. The cells contain HF (Hydrogen Flouride) which is a very bad substance. However the percentage is said by one manufacturer to be extremely small. Sensible precautions would be to avoid contact with the internal gel or fumes should the casing split. Standard HF treatment is to flush exposed areas for 15 minutes in running water and to consult a doctor. Encouragingly, some safety sheets now recommend piercing the plastic casings and soaking them in water before disposing them; this implies the risk is low if exposed to the gel.

2. The cells do ignite from time to time, usually when over-heated. It has been said that they suffer from thermal runaway once over 150'C. Charging is the highest risk because it is easy to set a charger to 3 cells to charge just 2. At best the cells may just puff up; at worst burst into flames. Drawing too much current can also generate too much heat, as can leaving them in a hot car without ventilation. Be careful! Have a foam extinguisher to hand or a bucket of sand. Don't charge unattended. Don't charge on top of flamable surfaces. Double-check settings.

It's probably also worth noting that laptop computer battery packs usually comprise 3 cells in series to achieve their required operating voltage (nominally 10.8v) with 2 to 4 cells in parallel to increase capacity (eg: 3 in series and 2 in parallel =6 cells in a pack which in modelling terms is often abbreviated '2S3P'). They always have built-in charging mechanisms and these monitor voltage of each set of parallel cells. There are also usually one or two temperature probes and current sensitive switches. These are the manufacturers normal requirements for safe operation.

Most modellers don't have these controls so we are taking greater risks. I would imagine that the result is that our cells get out of balance so it probably makes sense to check each cell from time to time. A good way of balancing cell voltages is to connect them all in parallel. Strong cells will charge weaker ones over several hours until they are all identical. Some suppliers have special 'balancing' plugs and add-on circuits to help keep cells voltages the same. I guess these are good but I've yet to find cells connected in series where the voltages have changed by anything significant so I don't use the balancing circuits yet.



This section added Spring 2006

When cells are connected in parallel (eg: to increase capacity), they balance each other and maintain a consistent voltage. When cells are in series there is nothing to do this and experience shows that they drift apart. Some cells may stay within 0.01v over a couple of years, but others can drift by 0.2v or more in a few months of usage. This may not sound much but the larger gaps are a problem because the gap is likely to widen every time you charge them (example below). This will impair the capacity and life of Lithiums so it is wise to address the problem.

The first and most important step is to KNOW whether your cells are drifting or not. Many packs now have extra plugs fitted to connect to 'charge guards'. It is easy to use these to measure the voltage of each cell. If there is a red lead it will be to the positive pole of one cell. The next lead is likely to be to the junction between the first cell's negative pole and the second's positive and so on... The last lead is likely to be black and should be the negative pole of the last cell. Each adjacent pair should measure one cell.

With a simple voltmeter you are able to assess whether your cells are drifting. If they are you can use the lead to charge each cell to full capacity one at a time. Effective but slow. A better approach is to buy or make a balancer... There are two primary designs:

Charge Guards - A charger cannot see the voltage of individual cells. So if the 3 cells in a pack are 4.1, 4.2 and 4.3v they will sum to 12.6v which is the normal cutoff for a 3 cell pack. However, the low cell will be nowhere near full and the high one will have been overcharged. A charge guard circuit monitors the voltage of each cell and bleeds off its share of the charge until all the others also get to 4.2v per cell. Although usually bundled into one package, there has to be a circuit for every cell and it has to be able to handle your highest charge current (which has cost and heat implications). Each unit also needs to be calibrated to be slightly higher than your charger's cutoff voltage so as not to impede its operation. Most chargers are set to 4.2v per cell, but some are higher 4.25V (eg: Schulze Lipo setting) or use pulsing techniques which are harder to match (eg: Astro). So, although they can be effective (and are the most common), these circuits have some challenges.

Balancers - A balancer is a device which simply tries to level the voltages between each of its inputs. It does this at its own rate independant of (and likely to be lower than) the charge current. It therefore may take longer to balance the cells than it takes to charge them so on cells that are significantly out of balance it should be used before charging. The comparator circuit can also be misled by cells with different internal resistances so it should not be used when packs are warm because the inner cells are likely to be warmer than outside ones (and resistance is temperature dependant).



This section added January 2007



This page describes the issues involved in balancing lithium cells. It also presents a simple and cheap circuits that work very well. I assume you understand the characteristics and risks of lithiums which I describe on my main lithium page.

Lithium cells are not a very robust technology so it is common for the voltages of individual cells to drift. This results in 'low' cells going below voltage in use and 'high' cells being overcharged. Both damage cells. Both means you throw them away! YOU HAVE TO KEEP LITHIUMS IN BALANCE!

Always buy lithium packs with balancer leads. These 'tap' each cell and allow you to measure each one and balance them if necessary. What does 'tap' mean? I show a typical layout of cells and balancer lead in the veroboard layout below.

Even if you do not have a balancer, check each cell regularly with a voltmeter. Don't use them if more than a few hundredths of a volt out. Please note that your cells may not drift but if you are not checking, you will not know.
Parallel cells

When cells are placed in parallel, they balance each other automatically. A 3S2P pack comprises six cells. For balancing purposes, you only need to worry about keeping each of the three pairs in balance (ie: this is a three cell balancer requirement).

If for some reason you had a 3 cell pack and the cells had got badly out of balance, one option would be to unsolder all the connections, and to reconnect all three cells in parallel. If left for some hours, this would balance all three perfectly. Clearly a balancer is a much more practical approach.

In a similar vein, if you had two 3 cell packs that you use in series to form a 6 cell pack, you could connect the two packs in parallel to level them off. This would make both packs identical but would not balance individual cells within each pack. Note that you can only parallel packs with the same number of cells, and preferably of the same type and vintage.

High cell counts

Your ESC should (must) have a low voltage cutoff. While effective with two or three cells, these become less effective at protecting individual cells as the number of cells in series goes up. One 'low' cell will easily be masked by higher cells. As a result, until we get ESCs that monitor every cell (as happens in laptop battery packs), your only defences are to set a higher cutoff voltage or avoid using the full capacity (to keep the volts up). For high cell counts, I therefore recommend buying packs with a slightly higher capacity than you really need (eg: 5000's instead of 4000's).

There is a similar issue with charging. Again, the charger only sees the total voltage so it does not know if one cell is 'high'. As the number of cells increase (>3), the risk and magnitude of an overcharge goes up. Where possible you should spilt large packs into smaller ones for charging. For instance, I have a four 3 cell packs which I use as a 12 cell pack. I charge these as a 12 cell pack when in a rush but prefer to connect the four in parallel and charge them as one 3 cell packs when I have more time. A balancer or charge guard can help but people who use high cell numbers probably also have high cell capacities which can take longer than a charge cycle to balance.

Two types of balancing circuits

Charge Guards - A charger cannot see the voltage of individual cells. So if there were 3 cells in a pack with 4.1, 4.2 and 4.3v respectively, they will sum to 12.6v which is the normal cutoff for a 3 cell pack. However, the low cell will be nowhere near full and the high one will get overcharged. A charge guard circuit monitors the voltage of each cell and bleeds off its share of the charge until all the others also get to 4.2v per cell. There has to be a circuit for every cell and it has to be able to handle your highest charge current (which has cost and heat implications). Each unit also needs to be calibrated, and needs to be slightly higher than your charger's cutoff voltage so as not to impede its operation. Most chargers are set to 4.2v per cell, but some are higher 4.25V (eg: Schulze Lipo setting) or use pulsing techniques which are harder to match (eg: Astro). So, although they can be effective (and are the most common), these circuits have some challenges.

Balancers - I use this term to describe the device which simply levels the voltages of all the cells (irrespective of the state of charge). It does this at its own rate independant of (and likely to be lower than) the charge current. It therefore may take longer to balance the cells than to charge them so on cells that are significantly out of balance they should be balanced before charging. Balancers have a comparator circuit which can be misled by cells with different internal resistances so it should not be used when packs are warm because the inner cells are likely to be warmer than outside ones (resistance is temperature dependant). Balancers tend to be cheaper than charge guards and usually require no calibration.

Please note that all circuits draw current even when their balancing or 'charge guarding' duties are done. So all designs will flatten and potentially kill your pack if left connected for long periods. So please remember to remove your balancers when the job is done.






This section has been truncated due to its tecnichal nature but the links work.

Go to the website FLY ELECTRIC for the full version.


I prefer the 'balancer' approach for small packs. So I describe my experiences with one design below. We have many in our club and they work well. This particular circuit was started by Sebastian Zettl and I think first appeared in this thread. Richard Szym added components to allow it to balance at higher currents and extended the design to handle any number of cells (with an appropriate number of 'units'). These designs can be found on Richard's site. I have merged a couple of his suggestions and made some minor changes based on suggestion by James Hopper and our experiences with the design. I have 'our' design as a veroboard layout, in Eagle for those than want that format, and an image file for PCB creation (best option if you can etch). The layout of the PCB version is broadly the same as the veroboard one, but much simpler to construct with only one link on top. A 7 cell version is shown at the bottom of the page.



Reprinted from the website FLY ELECTRIC
By kind permission of David Theunissen ©
This site is well worth a visit with loads of interesting electric flight information.