MP7A1 battery & lipo' in AEG use

[Tankdude]

Wondering what type of space we have in the battery area of the MP7A1.
Two cell lipos 'might' be the answer, they are 7.4 volts and are damn small.

Example
LiPo 1250mAh 7.4V 20C Discharg

Length: 3.8 in
Width: 1.3 in
Height: .06 in

Link to Great hobbies 2 cell packs

Second thing.

FACT: 7.2 volt GP 3300 mah battery packs work in stock aegs. The rof is the same as far as I can tell. The only probly was the tight ass fit in the stock as the heatwrap was a bit thicker.....wondering if jackel ever got it out of his M16'.... Anyway, I'm 99% sure that 2 cell lipos at 3200 mah will run stock aegs. Why should you care? Look at the size of them (and the cost) link
There are a few risk in using them, but some basic saftey skills and you should be ok.

It might be a way to go when you have a small ass battery area and you want something more then minis. So far Madmax is the only person I have heard of that uses lipos.

[Mantelope]

Lipos are generally overkill. Nearly all guns have space for at LEAST an 8.4V 2/3A cell battery... for example, the MP5K is tiny and still has enough space for an 8.4V, or 9.6V if modified. Intellect 1400mAh batteries will last approximately 2000 rounds... unless you're a hose, that's enough for a day. They're cheap enough you can buy two. Purchasing a Lipo setup will eclipse the price of getting a good NiMH collection.

The only case for Lipo is guns like the MP7. It's freakin' microscopic.

[Gryphon]

Considering I can get a 4200 mAh 8.4V Intellect pack from eHobbyAsia for $45, I really see no point in endangering myself with an immature cell technology for double the cost and 2/3 the capacity. You also need a LiPo-specific charger and they don't come cheap. My Intellipeak ICE will do them but I still don't trust the technology. Regardless of whatever thermal protection circuitry the battery has, bear in mind what will be next to the battery should it fail and explode - your hand, or your face.

In an AEP they make perfect sense, and frankly it's typical of TM to use an archaic NiCd with only a 200-500 mAh capacity. The current draw on an AEP will be low(er) so that's good for LiPo safety. That's the one application I think they ought to be used in immediately.

[MadMax]

I'm really looking forward to carrying LiPo's in the nearish future. However there are a few technical hurdles to overcome before they could be really wide user friendly:

A circuit needs to be included to protect the user from overdischarging their LiPo as they get significantly damaged if you discharge them too deeply. The same goes for overcurrent draw. Protection circuits are available for the r/c plane market. I think they're light (plane flyers are weight conscious) but I don't know if they're small enough to shoehorn into AEGs.

Wow, those cells you linked to are good for 20C?! That's a whopping 66A for that 3300mAh pack. Hobby grade LiPo has progressed very fast. When I set up my P90, I think my packs were rated for only 15C. My packs are individually rated for only 22.5A which was a bit too low for an upgrade spring setup so I went with parallel packs for 45A current draw and high total capacity (3000mAh). In lieu of a proper protection cct I fully charge my batteries before a game. I've never come close to running out a 2400mAh NiCd 9.6v pack so I figured I'd never overdischarge my twin packs at 3000mAh. I play with std caps which also limits my lead trigger (you should see how fast I sneeze off a lowcap at 11.1v!).

As to catestrophic failure, I think LiPo has progressed pretty far from the days of burning laptops. Luckily for R/C, the usefullness of LiPo has made it also the preferred storage for a lot of consumer devices. Cellphones, laptops, Ipods all depend on LiPo so cell manufacturers have had to keep things safe.

I've tried to keep a bit of an eye on LiPo as I've been looking to include them in several products that I was designing. It appears that many of the early failures in LiPo were due to manufacturing defects. Thin spots in the battery windings are prone to overheating and developing breaches which allow an exothermic reaction. Also burrs or other sharp metal contamination from the outside during manufacture can later penetrate the separator in the battery piles and cause a similar failure. Failure of batteries with defects would occur if the charger overcharged or the battery was supplying a lot of current. Both cases would cause the pack to heat up and fail because the weakened separator would melt. Failures could also be caused by mechanical damage (say if you severly struck or crushed a cell) which could force a burr through the separator or even a piece of the casing if it sheared.

In any case, manufacturing techniques have improved significantly. Now exothermic cell failures are down to about one in a million in about 5 months of pack life. The severety of cell failures has become significantly less because of design changes. I think manufactures are aware of the increase in field failures from the introduction of sharp contaminants (burrs) or defective separators so they've tightened QC. I don't know how they've reduced the exciting factor in failures. Perhaps a change in composition like a fire retardant paste or a suspension of inert particles which retard oxidation if they're heated significantly.

About two years ago I was pursuing a product which was to use a LiPo for a head mounted device. I was concerned about the safety issues of a head mounted battery so I did do some crude testing on some LiPo packs. I could not get them to fail in an exciting manner with significant abuse.

First try was to short cct a fully charged pack. One cell failed first but not in an firery exothermic manner. On close inspection it looked like a single shot fusing feature failed before the cell could overheat. The connection to the outer terminals is not a huge heavy chunk of metal. It was a smallish conductor apparently designed to fry so one could not subject the cell to a prolonged short cct.

I tried a crushing test on an undamaged cell by mashing it with a mini sledge hammer on a concrete floor. Crude, but I figured that if the battery was mounted to someone's head, a sledge hammer blow would cause more damage to the user than the battery getting mashed so it was a pretty severe worst case scenario. Anyways the cell got a bit on the hot side (I'm guessing around 70C which is hot to the touch, about as hot as household hot water), but it did not burst into flames. I guess what would have happened would the the user would be knocked unconscious, suffer some brain damage from the steel hammer blow and have a stingy burn on a massive bruise if they woke up.

Last test (only one cell left at that point) penetration test. I held a nail with a pair of pliers and hammered it through the side of the cell. I had to saddle the cell into a groove in the floor so it wouldn't roll around and I had to ding the side to set the nail. Samish kind of exotherimic failure as the crush test. Got hot, but still no fire to my disappointment. If the nail didn't go clearly thru the battery, you'd have time to remove my head accessory and kick the shit out of the guy with the nailgun before getting a burn on your head from the cell heating up. Heck, hold him down and burn him with the cell.

The cells leaked some pretty caustic stuff (stings the hands), but nothing worse than what I've found in NiCds which is also pretty alkaline. Cadmium is also toxic and a cumulative poison so I didn't consider the leaking issues with LiPo significantly more dangerous than NiCd.

I'm not sure if LiPo has ever had a history of exploding. It's probably an exaggeration of the spectre of a laptop causing a fire in an airplane.

I do have a bit of concern leaving a LiPo unsupervised while charging it. I slow charge them overnight so I sleep better if I put the battery into a steel ammo box while it charges. Pictures of burned LiPos don't show the thin casings melted down so I'm pretty sure they wouldn't melt through my steel ammo box if they burned up.

[Gryphon]

It's stories like this that make me wary of lithium chemistries.

http://www.cdc.gov/niosh/fire/pdfs/face9939.pdf

[MadMax]

Sounds very bad, but it only references similar failures from the same manufacturer of packs manufactured about 10 years ago now. Secondary (rechargable) LiPo was a pretty new technology back then. Even primary (non rechargable) lithium cell have sometimes combusted. I think primary cells are more prone to this because they contain elemental lithium. Secondary packs use a lithium oxide which is less reactive. Primary cells are still popular with lifesaving devices because they have a very low self discharge rate. Before lithium ion, rechargable batteries have typically had over 20% self discharge vs 1-2% for secondary Lion over one month.

The accident report does not indicate if primary or secondary cells were used which really does matter if you suspect the viability of secondary lithium cells let alone modern secondary cells.

Fires have been started by batteries of many types through short circuit overheating in unsupervised conditions. During recharge, lead acid cells evolve hydrogen which sometimes collects and causes a small explosion or fire every now and then.

Heh heh, I've heard some pretty interesting stories from electric car design projects running a few decades ago. At the time molten sodium batteries had shown to provide the winning combination of highest volumetric and gravimetric energy density (joules per cubic metre or kg). The violent fires were not controllable through conventional means because sodium generates hydrogen when it contacts water. I think the only way to fight a sodium fire is with halon or argon blanket gas. Chemical retardant is in a water suspension so it's no good against a sodium fire. To think that liquid sodium electric cars were the "way of the future" considered by major car companies like Ford and GM.

[Tankdude]

Friend at school designd a quick little circuit to tell us when the battery is low. It involves an opt amp and a few parts.

When the battery drops to 3 volts a cell a light will come on.

I'm glad you guys agree in the case of the MP7, I figured I couldn't be too wrong with that.

I had a dream last night where the battery compartment was huge. It held the battery, a repair kit, a can of pop and some yogurt and other misc stuff. So ya....