Charging tutorial

All chargers do the same basic job. They move electricity from a source into your packs. For our purposes the source is normally a 12V car/leisure type battery (or sometimes the mains A/C supply).

However chargers cost anywhere from about 20ukp to several hundred ukp. The main specifications which determine the price are :

How many cells can be charged at once ?
What is the maximum current ?
How does it detect that the charge is complete ?
What other facilities does it have (cycling, discharging etc)

How many cells ?

You can start off with a simple charger for 7 or 8 cells. Even then look for one that has proper charge termination (see below) not just a timed charge. You may think that you will never need more than 7 or 8 cells in a pack so this won't be a problem. Possibly, but I doubt it. I already have 2 models with 16 cells and I can see even more cells on the horizon.

Any way most of the chargers with the better facilities also tend to charge larger packs. Somewhere around 30 cells is probably enough for most people. After all these cells all cost money. I believe that chargers are certainly one of the areas where it is worth buying a good one if you can afford it.

However if you are on a budget you will notice that the prices go up drastically as soon as the charger can handle over 7 or 8 cells. The reason for this is that 8 is the maximum that can be charged from a 12V battery without including some clever and relatively expensive circuitry to step-up the input to a higher voltage internally in the charger. It is probably better to get a good quality low cell-count charger than a cheap and nasty one with a higher cell-count.

Maximum current

Many of the cells we use can be charged at up to 4C (4 times rated capacity). FOr a 2000mAH cell that's 8 Amps. If your charger will not handle high currents then you will sit around longer waiting for your batteries to charge. I would suggest that you get a charger which will do at least 5A. It is also worth checking if the charger will maintain this rate for higher cell counts. There are some chargers about which advertise quite high charging currents but will only deliver those on lower cell counts. When you get up to the maximum number of cells the current may be under half the advertised maximum. Just something to look out for.

Charge termination

Ni-Cd and even more NiMH cells do like being overcharged at high currents. In extremes they can burst into flames or explode. Therefore most good chargers have a way of automatically stopping the charge when the cells are full.

There are still some chargers around which have no charge termination or which simply have a timer so they charge for a fixed time. I would not recommend one of these. The better chargers are able to "read" the voltage and current in the battery being charged and tell from those readings that the battery will not take any more charge. The term most commonly used from the mechanism they use is "delta-peak" charging. If you are not interested in how this works just look for that phrase when buying a charger and skip the next section.

Delta peak charge termination

Lets' be clear about this. "Delta peak" is short hand for "The peak charge is detected by measuring the Delta Voltage". "Delta" is a mathematical term which means "The change in". There is no such thing as an actual "delta peak". Many people obviously do not understand this, I have recently read an article in a model magazine which claims that trickle charging is used to keep batteries "at their delta peak". Of course it just means "at their peak charge". SO what is delta peak charging ? As a Ni-Cd battery is charged its voltage increases. This increase in voltage is technically called a positive delta V (or voltage). Positive because the voltage is rising. It is a basic characteristic of these cells that when they are fully charged the voltage levels off and even goes down very slightly. These conditions are known as Zero delta V and negative delta V. The so-called Delta Peak chargers look for this change and terminate the charge when they see it (or sometimes drop into a maintenance low trickle mode). The best chargers have different programs which allow them to terminate the charge in various ways. E.g. for Ni-Cds it is fine to watch for -ve delta V but the vlotage dip is not so pronounced in NiMH cells so it is better to charge them using Zero delta V i.e. don't wait for the voltage to drop but cut off when the voltage levels off.

To be added when I've drawn the pictures.

Additional features

Some of the modern chargers have long lists of additional features over and above charging batteries. Here are a few which are, in my opinion, fairly worthwhile :

Multiple outputs - these are usually one main output and other lower rated ones for specific uses e.g. charging Tx and Rx batteries.
Output information - while not vital to have a screen which shows things like charging current, time, total power etc. it is quite useful. To me the main value is in checking how much capacity has been put into the battery (and also how much taken out if the battery have a discharger).
Discharger - the ability to discharge batteries, usually at a relatively low current, perhpas 2 or 3A only. On its own this allows you to manually cycle your batteries.
Cycling - cycling is charging and discharging the battery, often several times. The main objective is to measure the battery's capacity as batteries do deteriorate over time and the first symptom of a failing battery is often a reduction in capacity.
Cycling batteries is a controversial subject. On the plus side it is the only way to get an accurate feel for the capacity of your batteries. On the negative side Ni-Cds only have a limited life in terms of charge cycles so it actually uses up some of your batteries life. I'll leave it to you to decide if you want to cycle your batteries and, if so, how often.

Recommendations

There are plenty of good chargers around. Some of the ones which are most often recommended on the Electric Flight mailing list are :

Any AstroFlight charger particularly the 110D and 112D
Hitec 330 and particularly 335
Robbe Infinity II
Any Schulze charger, isl6-430 is the base model and they get better (and more expensive) from there.