Newb battery question

[etd]

I'm trying to upgrade batteries on my gun for two guns.

One is a broxa evolution and the other is a TM Compatible Sig 552. My question is what will fit in these two guns and how do I know how powerful the battery is?

[Muffin]

Standard batteries are 8.4v, of which is all that I would suggest with the guns you have. However it wouldn't hurt getting a higher quality battery.
I'd reccomend a Intellect Large 8.4 for your MP5(if it has the full stock) and an Intellect mini 8.4 for the SIG.
You can find what you need at cheapbatterypacks.com

[Jimski]

intensity (mAh) gives you an idea of the discharge rate and the capacity.
(usually, bigger capacity=bigger discharge rate=bigger push for bigger springs)

tension (V) gives you the ROF you'll have: 8.4 standard, 9.6 or 10.8V for high rate.
PDWs use 12V but they have a circuit that's matched to those specs.

[namloot]

Originally Posted by Jimski

intensity (mAh) gives you an idea of the discharge rate and the capacity.
(usually, bigger capacity=bigger discharge rate=bigger push for bigger springs)

This is not correct.

The mah rating of a battery is it's capacity, or how many miliamp hours of current the battery will provide into a load (an AEG), on a single charge. It has nothing to do with the discharge rate of a battery.

The peak discharge rate of a battery is actually totally independent of the capacity of a battery, and is often not given by the battery retailer. As a matter of fact, with many batteries, the peak discharge rate goes down as the capacity goes up.

Discharge rate is directly related to the internal resistance of the cells that make up a battery pack. The lower the internal resistance of the cells, the higher the peak discharge rate of the battery pack. Lower internal resistance cells are also more efficient, being able to provide more of their power to the load and losing less to internal heating.

This is typically seen with NiCad packs vs NiMh packs. NiCad batteries have a lower internal resistance then NiMh batteries, thus they typically have a higher discharge rate. A lower capacity NiCad pack will typically outlast a higher capacity NiMh pack. NiCad packs, being able to deliver more power to a gun, typically give a gun a higher rate of fire then the equivalent voltage NiMh pack (see below).

LiPo batteries have even a lower internal resistance then NiCad batteries do and are capable of delivering the most power to an AEG of any commonly available battery. This is why a 7.4v LiPo battery will typically outperform an 8.4v NiCad or NiMh battery; It's all about the discharge rate (see below).

Originally Posted by Jimski

tension (V) gives you the ROF you'll have: 8.4 standard, 9.6 or 10.8V for high rate.
PDWs use 12V but they have a circuit that's matched to those specs.

It's not just the voltage that is responsible for higher rates of fire, it's a combination of the voltage and the internal resistance of the battery (peak discharge rate). These two factors determine the battery's ability to deliver power to a load (an AEG).

The power, in watts, used by any load (an AEG), is defined by Ohm's law to be the voltage ^2 / the resistance of a circuit.

For an Ohm's law chart:
http://www.the12volt.com/ohm/ohmslaw.asp

The load resistance in an AEG is the sum of the motor resistance plus all the electrical system components (switch contacts, battery connectors, etc) plus the internal resistance of the battery.

Therefore the power used in any AEG is:
voltage squared divided by the AEG resistance + the battery resistance
P = E ^2 / Ra + Rb

From the above formula, and knowing that AEGs typically draw high peak currents during firing, a battery with a low internal resistance (high peak current) would be significant. A higher voltage battery also makes a big difference.

This is why a higher voltage battery or a higher peak rated battery will generally provide a higher rate of fire in any given AEG. The lower internal resistance of the battery will cause the AEG to draw more current. If you combine a higher voltage battery that also has a high peak discharge rate, you will cause your AEG to use the most power.

Keep in mind that if your AEG does not have a high quality electrical system and internals, a high power delivering battery can damage it.