Guide to Batteries and Chargers for AEG's by Sarah Frazer - May 2002

There are currently two principle types of cells used for AEG's (excluding Lithium Iron and Fuel cells!), these are of Nickel Cadmium or Nickel Metal Hydride construction. Each type has its advantages and disadvantages and their differences will be explained below. The type of cell construction, its capacity and the number of cells will have an effect on how long the pack will last in a skirmish, the Rate Of Fire (ROF), how consistent the ROF will be and how we charge and discharge the pack. The more you understand the varies types of cells, the better choice can be made to get the best cells for the job and to keep them at there peak performance.

Cells and Pack sizes The cells which go into packs for AEG's fall into two main types, these being 'Sub-C' for the large battery packs (M16, G3 etc) and '2/3AF' for the small battery packs (P90, MP5A5 etc). There are slight variations of these two cell sizes and packs made from these may need slight modification to fit. Other size cells can be used to make custom packs, such as 'AF' size cells in a V-format for the M4-A1, or various lengths of 'AA' cells for other custom applications.

There are generally seven cells in a standard large or small pack, for a standard AEG. For modified AEG's with different springs, gearboxes etc, eight cells (or more) may be required. Note that using eight or more cells on a standard AEG will give a higher ROF, but will reduce the life of the gearbox.

Nickel Cadmium (Ni-Cad) cells Ni-Cad cells have been around for a long time, with the model car people having the most experience of getting the best, or worst!, out of them. They have found the best ways of charging, discharging and storing them to get the last little bit of energy out of them. Our use of Ni-Cad packs is not as demanding (the model car people can flatten a 2000mAHr pack in less than 5 minutes!), but we can certainly use some of their experience.

Ni-Cads, for a given physical cell size, do come in different capacities, though generally the higher the capacity, the bigger the drop off in voltage (and ROF) as the battery is used. The smaller capacity 'Sub-C' size (large packs) can vary in capacity from 1200 mAHr to in excess of 3000 mAHr. The larger capacity packs (SCE) will gradually drop off in voltage as the pack is used, this will also have an effect on the ROF as well. The smaller capacity packs (SCR) will have a lot flatter discharge voltage, right up to when they are approaching a fully discharged state when they will drop off very quickly indeed.

If your AEG takes a large battery pack you may be tempted to rush out, buy an expensive high capacity pack and be happy with that. Unfortunately Ni-Cads suffer from a 'memory effect', which means that to get the best consistent performance you will need to cycle your pack (from a fully charged state to a fully discharged state). If this is not done then the performance of the pack will deteriorate quite noticeably. If you have a 2000mAHr pack with a standard AEG, but only use 1200mAHr worth of its capacity on a skirmish you should discharge the rest (800mAHr) before charging again. If this is the normal use with your AEG then the extra capacity is wasted and you could have bought two packs with the money instead. If however you have a heavily modified gun then the higher capacity pack may be the better option than continually changing packs throughout the day.

Nickel Metal Hydride (Ni-MH) cells Ni-MH cells have a higher capacity and are lighter than Ni-Cad cells. The downsides are that they are more expensive, they have slightly higher internal impedance and are more sensitive to higher currents and overcharging.

Ni-MH cells have typically 2/3rds more capacity than the equivalent Ni-Cad cell, which (with a small pack) can mean the difference of lasting a day on one 1000 mAHr Ni-MH pack, or using two 600mAHr Ni-Cad packs (this is obviously dependant on the AEG and the user).

The Ni-MH cells do have slightly higher internal impedance, which limits the amount of instantaneous current that it can deliver. With a model car it will not accelerate as quickly as its competition, for an AEG the ROF will not be quite as high. The slightly lower ROF can be offset, as the physical size equivalent of Ni-MH will have a higher capacity that will allow a slightly higher ROF. So for a small pack the capacity of 1000mAHr will offset the lower impedance of a 600 mAHr Ni-Cad pack, giving approximately the same ROF but lasting a lot longer.

Ni-MH are more sensitive to damage from high currents in use and charging. The manufacturer of the cells is more important than with Ni-Cad cells. Poor cells are more likely to generate greater heat internally, which can damage the internal insulation leading to a short condition. Ni-MH are also more sensitive to heat damage when the packs are being assembled. If you make your own, use known branded tagged cells and a big soldering iron which will maintain its temperature whilst soldering the cells together (small soldering irons loose their heat too quickly when soldering larger surfaces).

Charging NiCad and Ni-MH cells Charging technologies for Ni-Cads and Ni-MH packs varies from slow trickle chargers, timers, voltage peak detectors and temperature peak detectors. Timers are not really worth considering as they will either undercharge, or overcharge and for a little more you can get a far superior voltage peak detecting charger. Thermal peak detecting chargers are only really used in controlled environments, though a cheap digital temperature gauge (from Maplin electronics) can be used if you want to manually confirm the peak charge of a pack. This leaves us with either a trickle charger (normally 1/10 of the pack capacity) or a voltage peak (also known as a Delta Peak) charger (which can generally charge up to 5 Amps).

The trickle chargers are generally cheap units, which will charge the packs in approximately 8 to 12 hours. They can be used for different capacities (within limits!) though the times will need to be adjusted (when the pack starts to get warm then it is charged). These are generally safe to use and can be left overcharging a pack, will do no real harm, though excessive charging will eventually reduce the pack capacity.

The best way to charge packs is with a voltage peak, or Delta Peak, charger (Note: Not all peak detecting chargers are suitable for Ni-MH cells). This type of charger will monitor the pack voltage whilst it is being charged and will be looking for the rate of change (Delta) of peak voltage which signifies a fully charged pack. In addition the charger will charge the packs a lot quicker, potentially in less than twenty minutes, though for most applications we would probably take one to two hours. Some of the more expensive peak chargers will also have a discharge function (some which will indicate the pack capacity as well) which is very usefull for cycling and conditioning cells.

Ni-MH cells do not have the same rate of change of peak voltage when they become fully charged as compared to Ni-Cads. So much so, that a peak charger which is only suitable for Ni-Cads may not detect when Ni-MH cells are fully charged. This will lead to an overcharge situation. With fast charging, if the charger continues pass the fully charged point then the excess charge, which the pack can not accept, will be dissipated as heat. With the higher currents the pack would get that hot, the cell internal insulation would start to break down and eventually the cells would start to vent electrolyte, if the cells are vented, or rupture (sort of explode!) if they were not of the vented type. This is why it is so important when fast charging to use a reliable peak-detecting charger for the right type of Ni-MH or Ni-Cad cells. A Ni-MH charger will generally charge Ni-Cads, but a Ni-Cad charger is not suitable for fast charging Ni-MH cells.

With new packs it may be necessary to cycle the cells a few times at lower currents before the packs will start to perform properly. If when during fast charging the cells become hot, way before becoming fully charged, reduce the charge current for a few cycles until they charge and are cool to the touch.

Connectors The connectors which are fitted standard with the large battery packs are of a type generally known as Tamiya connectors. The small battery packs have a relatively smaller connector, generally known as a Kyosho connector. These connectors are fine but over a period of time can get worn and the connector resistance can increase to a level (with a highly modified AEG) where the connector could get quite hot. (The model car people actually get the connectors to melt!). Keep an eye on the connector and if you have any doubts replace the connector (Most model shops or Maplin electronics stock the large Tamiya connectors).

Alternative connectors are available, again from the model car people, which are Gold plated. These have a lower resistance and do not loose their tension like the standard battery connectors. The only downside is that if you need to charge your pack from a site charger then you will need to make an adapter.

Summary Try and select packs to suit your AEG and the skirmish environment that you use it in. If your standard AEG takes large batteries, then a 3000 mAHr Ni-Cad pack may be too big, you could be wasting a lot of the packs capacity just discharging it at the end of a skirmish. A pack can only be charged a limited number of times, for the money it may be better to have two lower capacity packs than one huge capacity pack. My standard M4-A1 has a custom 1400 mAHr Ni-Cad that lasts me all day. If you are using a modified AEG then you will have to use a corresponding larger capacity battery.

If your AEG only takes small size packs then your choice is limited to either 600 mAHr Ni-Cad packs, or 1000 mAHr Ni-MH packs. My choice would be the 1000 mAHr Ni-MH pack, as this on my site can last me all day, whereas the 600 mAHr Ni-Cad pack will have to be changed.

If you have the space to use Ni-Cad cells (i.e. with large or custom battery packs), then do use them. They will give you a slightly higher ROF and can be abused (with charging) a little more. However if space is a premium (with small battery packs) the equivalent size of Ni-MH cell will give you the increased capacity (do use branded manufactures cells, such as Sanyo etc.).

by Sarah Frazer

This page last updated: Friday, December 17, 2004 6:52 AM