Battery Chargers Demystified

Chargers can seem a little confusing at first but a good charger will soon become a valued tool.

It seems that not long ago we had a very simple life when it came to batteries. Other than a few pioneering electric flyers, almost every R/C modeller had fairly standard equipment, usually comprising a model fitted with a two-stroke or four-stroke i.c. glow plug engine, 35MHz PPM or PCM radio gear with 600 mA hr nickel cadmium batteries fitted in the model and transmitter.

If you were really posh, you had an electric starter – commoners used a chicken stick (a length of broom handle if you really did things on the cheap!). Come to think of it, life was simpler all round really, computers were for big business' or geeks intrigued by Sir Clive Sinclair's hugely powerful ZX81 with a massive 4 k/byte of memory. Mobile phones were the size and weight of a house brick and only useful for making telephone calls, and there was no WWW , WAP, Bluetooth, MP3, or MPEG4! Of course, nothing stands still in this high tech world of ours – and our hobby is no exception. Take battery chargers for instance.

In those good ol' days we handed over a month's wages to the local model shop owner and took home our lovely new shiny four-channel proportional FM radio set. After we got over the excitement of all the features like servo reversing (hands up those who remember the black versus red label servos indicating the fixed servo direction?) we took out the supplied plug-in wall charger, shoved it into the mains socket, connected it to the transmitter and receiver pack, left it overnight, and went flying the next day. Then we would return home, plug it back in again, and the process continued. Once in a while, we whacked the car battery charger onto the long suffering flight-box 12V battery, and the glow plug clip usually just plugged into the flight-box panel. Simple eh? So, where did it all go wrong?

A typical charger package, some units supply charging leads and balance adaptors while others don't.

CHARGER PROLIFERATION

It's difficult to isolate a single reason for the plethora of assorted chargers we surround ourselves with today, but first off some bright spark invented the glow start stick – great idea for those frustrating engine cuts just as you arrive at the strip – trouble is, they need a charger. Then along came 6V receiver packs, high capacity NiCds (the correct abbreviation incidentally for generic nickel cadmium based cells – NiCad is actually a trademark of the SAFT corporation), portable engine starters, 2V PB accumulators for glow plugs – and this was just for i.c. models; throw all the developing electric flight gear into the mix and flyers started to find that they needed another charger for flight packs. Large capacity Nickel Cadmium cells were followed closely by the newer Nickel Metal Hydride, then came Lithium Polymer, Lithium Ion, and Lithium Ion Phosphate (a.k.a. LFP or LiFePO4).

Now we have more lithium ion based cells from A123 industries sometimes referred to as M1s, using something fancy called Nanophospate (c) technology. Yikes!

The manual should reflect the units power handling capabilities.

POWER IS WATT MATTERS

Many chargers claim to have the ability to charge batteries at very high charge rates, and if you're one of the growing number of modellers who are using high capacity cells in your electric flight packs, this may seem very worthwhile, and, of course, it is. However, all may not be as it seems, indeed it probably wouldnt do any harm to remember the following:

Charging a number of cells at a given rate of current, to a specified level of voltage, obviously requires power, and the charger and its associated PSU (Power Supply Unit) must be capable of supplying this power in order to get the job done. For example: a charger may claim to be capable of handling up to 10 lithium polymer cells, and have a charge rate of up to 8A, and if your latest power-hungry electric model is going to use a 10 cell 8000 mA hr battery, then you might think youve found your ideal charger. However, for this to be so, the charger will need to have a power capability of over 330 watts; for your battery to be fully charged up to its final voltage of 42V at the normal 1C rate of 8000 mA hr requires a hefty 336 watts of power.

Now, this may well be an extreme example, but the principle remains the same. A charger might indeed be capable of charging at up to 8 amps, and it could also be capable of handling the 10 lithium cells, but not necessarily together – this is limited by the total power capability of the unit (as shown above) and not all chargers readily admit this limitation.

So, first off, check the maximum wattage capability and decide if it is sufficient for your needs. Whilst were discussing PSUs, try to decide whether your charger will need to be used at home off the mains supply, or in the field off 12V. If you require it to be able to do both, then look for one that has an integral mains PSU as well as the more normal 12V input socket.

Separate mains powered PSUs are available, but for neatness, a combined unit is probably better. Of course, if the chosen unit has only the 12V inputs, and you dont want to bother with a separate PSU then a high capacity leisure battery makes a good alternative supply. Incidentally, remember that using a normal car battery at the field to power chargers can result in a flat battery and, of course, difficulty getting home! Ask me how I know…

Main or 12v input? Many units now come equipped for either.

FLEXIBILITY AND VERSATILITY

It's a fact that the best chargers will have several different user selectable programmes that offer alternative parameters for the varied types of cells and batteries you may encounter. Most of the units available these days are lithium compatible as this is definitely the power of choice for many people. There are at least three different lithium based cells available to modellers at present: lithium polymer, lithium ion, and LiFe. Each has its own charging requirement and so the better units have a setting that caters for each type.

Most units will also be capable of handling good old NiCds, and NiMHs, as well as PB accumulators (wet lead acid). Some smaller PB batteries are also known as SLA (sealed lead acid).

LOSING YOUR MEMORY

There is a belief (and as far as I know this has never actually been confirmed through hard scientific fact) that nickel based cells suffer from a memory effect (more correctly called Voltage Depression). Put simply, this means the battery, if not fully discharged to the correct level before being re-charged, remembers this shorter cycle and may not then give out all its potential next time around.

NiCds were more likely to suffer with this than NiMHs, but thankfully lithium based cells have no such problems at all. To cater for this, a good charger will have a discharge function, and may also feature a 'cycle' function (sometimes known as conditioning). This means the charger will be pre-set to gently discharge the battery down to around 1.0V per cell, and if a cycle mode is available, the user can normally specify how many cycles are required, during which the unit will automatically discharge, then re-charge the battery as many times as specified.

As NiCds become harder to find due to their unfashionable environmental chemistry, more and more batteries are now being made from NiMH cells, but nonetheless, this feature is a useful one for those people who already have several cadmium based packs. Lithium based batteries should not normally be manually discharged or cycled.

Balance charging usually entails using both battery and balance plug leads at the same time.

COOKED TO PERFECTION

Several of the new charger models now incorporate a temperature monitoring feature and can even connect to a PC for those who want to know exactly whats happening to their batteries during the charge process.

Temperature monitoring is actually a very good way of determining the state of charge for many types of cells and can, in some cases, be used to actually determine when the pack is full. Usually however on Nixx type packs something called Delta Peak is deployed, and the temperature monitoring probe (if supplied ) is more likely to be used to check that all is well and only shut the unit down if abnormal temperatures are detected.

Many chargers allow you, the user, to change certain default parameters, and providing you understand the implications, this can sometimes be useful. For instance, the delta peak system works when the charger detects a very small drop in voltage – milli-volts only, relative to time (delta V/delta T) which signals that the cells have peaked and are now full (zero delta V).

Now, cadmium based cells have quite a marked drop-off, in the order of around 30 mV or more, whereas the higher internal resistance and different chemistry of nickel metal hydride cells results in a much lower drop-off figure of around only 8 – 16 mV. This lower figure is much more difficult for the charger to detect accurately, as its looking at a very small change in voltage against time. If one charges slowly then this change can be extremely subtle and very hard to detect, but a couple of things can help improve this.

A screen is pretty essential so don't get a charger without.

CHARGING RATES

Charge NiMH cells at a higher rate than cadmium cells. The correct charge current for fast charging NiCd or NiMH batteries is between approximately 1/2C and 2C. This level of current is needed for the cell to exhibit the required change that occurs when it reaches full charge – although charging at 2C may cause an excessive battery temperature rise, especially with small, high capacity NiMH cells. Due to the chemical differences between the two battery chemistries, NiMH cells generate more heat. Typically 1C is okay; this emphasises the drop-off voltage a little, ensuring proper detection, after which the charger may enter trickle charge mode. Mind you, long term trickle charging is not good, so the battery should ideally be removed at this point.

THE PEAK OF FITNESS

Try altering the default value of peak detect used – often it is a little too high for many Hydride cells and the charger may fail to detect the change. However, setting it too low could cause the charger to stop charging prematurely, resulting in a partially charged pack. Several other parameters can also be user defined with some chargers, but obviously these vary across the range, and are outside the scope of this article; instructions on these settings will be in the instruction manual.

On this subject, its fair to say that some chargers have extremely good instructions, whilst others are, frankly, awful. If possible, try to get a look at the manual supplied with the unit before purchase, as it may sway your decision. Many manuals can be found on the WWW these days.

Although now rarely used, stand-alone balancers like these can be employed if the charger doesn't have a balancing function.

A BALANCING ACT

Now theres a good chance that many readers will have already discovered the benefits of lithium batteries, indeed you may be using them in your electric flight power packs, or even your radio gear. However, many other people have still to be persuaded, due in part to the many scare stories which abound on the internet or, in some cases, down at the average flying field on a Sunday morning. Often the die-hard old barnstormers will entertain the assembled throng with stories of complicated charging procedures, balancing taps, spontaneous combustion, indeed the end of the civilised world as we know it! All reasons to avoid these things like the proverbial plague. Of course, the vast majority of these stories are unfounded, but one aspect that must be seriously considered is the process of balancing.

Li-Po batteries are more sensitive to having unbalanced cells in a pack. One main reason for this is that these cells are far less tolerant of voltage discrepancies than nickel based cells, and mustnt be allowed to drop below a safe minimum value (normally around 2.8V per cell under load), or conversely be allowed to get higher than their specified terminal voltage (4.2V per cell ).

Early Li-Po batteries were not equipped with the necessary balance leads and multi-way plugs, but these days every lithium battery comes pre-fitted with such a plug although these plugs are unfortunately not, as yet, standardised across the various manufacturers. In this respect youll find it useful to read David Ashby's article here at Modelflying.co.uk for more on this particular issue. Balancing simply means ensuring that all cells in the battery are within a couple of milli-volts or better, of each other. Although stand-alone balancers were popular a year or two back, most charger manufacturers have recently wised up, and many chargers now come with a built in cell balancer unit, indeed this integrated approach further reduces the mess of wires and extra equipment required. Some of the smarter charge / balance combos will actually charge through these balance leads, where, (not withstanding the possible plug incompatibility mentioned earlier) one simply inserts the battery balancer plug into the appropriate balancing socket – usually located on the side of the unit.

This system ensures that each cell receives just the correct charge to reach its optimum level without allowing an overcharge, whilst simultaneously monitoring the pack as a whole, to ensure an even balance throughout.

Many chargers now come complete with balance board connectors such as these, so all battery balance plug types can be accommodated.

The majority of combo units however work on a more basic principle, wherein the actual charging is still done through the batterys main power leads. The leads supply the full current and voltage required for the complete pack, while the balancer function monitors the state of each cell as the pack is replenished. If, whilst monitoring the charge, the current to any cell is found to be creeping ahead of the rest, the balancer will briefly hold it back. This method, of course, is the system deployed by all stand-alone balancer units.

SUMMARY

So there we have it then. Ultimately, of course, its up to you to decide whether youre ready for a new charger, however one thing is for sure, with all the different models available these days, youll not be short of choice. Anyway, hopefully the decision about which one best suits your individual needs will now be a little easier to make.