Can't be wrong 'cause all I said is I haven't seen them.

Take my word for it.

Trust me.

Although the batteries described at the Wiki article are loosely called 9V by some, most of the manufacturers who supply my retailers have started labeling them correctly at their actual 7.5V (nominal) output, and that's how they're sold now at most retail outlets here - where I shop.

Note also that the "cell voltages" for other kinds of cells can't really be quoted with great authority, because commercial products are doctored with mysterious potions and additives, usually with trace elements in the electrodes and "concoctions" in the electrolyte pastes to "adjust" the elemental voltages to what's wanted in the product device.

In the US, most of the battery powered pickups use the 7.5V battery, or less often sets of AA 1.5V; but once again, that's "of the ones I've seen," which is by no means a wide sampling.

The common cells sold in the US are the cylindrical AAA, AA, C, and D, all of which are nominally 1.5 volts. The 7.5V battery is a rectangular assembly of cells with "snap" terminals at one end, and they're also widely available.

A few places carry 6V (rare) and 12V "lantern batteries" with either spring terminals or threaded terminals with thumb nuts, but at about 1 LB_w each they'd be a bit heavy for inside a guitar.

Rechargeables (at retail shops) are pretty much limited to the 1.5V cells, in AA, C, and D sizes. NiCds were widely available not too long ago, but have almost disappeared from the consumer shops. A "mercury" rechargeable was available some years ago but is now prohibited in retail markets in most places.

Most "rechargeables" here now are NiMH (Nickel metal-hydride) which when doctored by the manufactureres have the advantage of more closely replicating the 1.54006 Volts of a fresh carbon zinc non-rechargeable, although I can't say how close they come without looking it up. With NiCd batteries, there often were problems of voltage mismatch in devices designed for throwaway carbon zincs (which for many years haven't really been carbon zinc), but they seem to be much less common with NiMH ones. The NiMH versions are also considered "less toxic" in the landfill, although how much less toxic can be debated.

Lithium rechargeables in standard sizes are only "theoretically available" in my area, but may be more common elsewhere.

Battery suppliers list some 300+ different "standard packages" that one can order, but the common retail market here is pretty limited.

As noted, the "special batteries" used in many rechargeable devices - again the ones I've taken apart - are commonly just stacks of "disk cells" resembling hearing aid and/or watch batteries, and appear to be mostly NiCd. If there's difficulty getting a replacement, a battery shop (or clever user) could quite easily assemble a replacement for most of them. As I generally only buy the cheap stuff, things popular with the idiots mass consumers may differ.

The devices that use Lithium batteries are an exception to DIY assembly, as I don't consider them truly "safe" even when purchased from a reputable manufacturer - the good ones are just "acceptable risk." Lithium rechargeables are used in virtually all portable computers, and in quite a few cell phones, GPS units, etc. If that's what you've got, I'd say get ONLY the manufacturer's exact replacement battery, and use ONLY the recommended charger.

Few places here stock rechargeable disk cells that would be useful for constructing your own battery, but the kind needed are used for the "clock battery" in many computers, so they shouldn't be too hard to find at your local computer shop. The last time I looked for one, they were mostly NiCd, so some compromises on voltage may be needed to replace a throwaway Carbon Zinc and get the voltage "just right." (Computer batteries are so reliable, usually, that they're seldom discussed, so manufacturers may be using another kind of battery now, as well as the NiCd ones.) It depends a lot on the voltage tolerance of the pickup, whether you'd have a good chance of being successful with a "homemade" battery.

It should also be noted that the chargers used with most rechargeable devices are intended for use only when the device isn't in use. Most of them are pretty noisy (there may be exceptions), and you probably wouldn't want them connected during play. That probably means adding a jack for easy connect/disconnect of the charger, and of course means carrying the charger along if you'll be out for a while.

The original problem statement sounds like the optimal solution might be just putting a bit longer leads on the connector(?). Or maybe getting a "roadie" to change the batteries.

John