Not going by the usual conventions of crane stock batteries, Tokyo Marui have redesigned their NiMH battery to fit such stock battery compartment, and it's a cinch changing batteries, especially in the middle of a firefight. Many reviewers have complimented Tokyo Marui for such a radical departure, eschewing cheaper airsoft battery making processes in the name of convenience. That's how M4 batteries should be made.
However, as typical in the requirements of Tokyo Marui AEGs, they always require mini-batteries. While these maybe enough in the old TM AEGs (and their clones which you know which these are), in the new generation TM AEGs, sporting blowback and recoil, they fall short in performance. I have to use up to 6 8.4v mini batteries in a single skirmish day with these for the TM AKS74U , where before 2 to 3 would do for the same number of rounds used up. The power requirements of the new generation are greater in order to give more feedback through the recoil and blowback for the user, and a new level of realism.
Another thing, which is specific to the TM M4 SOPMOD battery, is that the cells in the batteries seem to be fragile, that carrying them in your pockets would snap them --- they need sturdier cell casing or wraps.
And these are the issues that Eagle 6 Airsoft have encountered as they've gotten a lot of requests to fix the TM M4 SOPMOD batteries and heard the complaints of low capacity that they've worked on prototyping a replacement. What they have done is to build a high quality, sturdy SOPMOD Battery with the upper part CNC'd to keep the cells together. Furthermore, they are releasing two versions: the Lipoly Battery version, which is 7.4v and 2800mah, and an NIMH version which is 9.6v and 2000mah. These come with Eagle6 trades engraved on them.
You can already order the Lipoly battery pack, but since they are still not mass producing, and looking for sources that can help lower the costs, it will still be a hefty £75 battery, a price that can get you a good-priced gas blowback pistol. But if it's convenience that you are looking for, and cannot stand having to replace the original battery more often, then it's a compromise.
The NiMH version, while not yet released, should cost lower than the Lipoly pack as NiMH battery cells are cheaper nowadays.
I have been wondering why other airsoft manufacturers have not copied such innovation by Tokyo Marui, especially for their rear-wired AEGs using the version 2 gearbox. It is such a great convenience and this is one of the reasons that I am sticking to my TM M4 SOPMOD. I've been hating installing batteries in the handguard or using an external battery box when I don't need another attachment to my AEG to lessen the load. And it sucks having a battery box in the form of AN/PEQ ones, which is a not something I would want to do with an AN/PEQ box, I'd rather use it for illumination than as a battery box. Such approach by Tokyo Marui also lessens the wires within and outside the AEG.
Yes, the caveat of Eagle6 Airsoft's SOPMOD Battery is the price. But then, it's hell a lot better than the original SOPMOD battery in terms of durability and capacity. If only other airsoft manufacturers would use such battery for their AEGs, then Eagle6 airsoft would be able to lower the costs due to volume sales.
If you're a TM M4 SOPMOD fan, then you now know where to get replacement batteries.