I have a Zylight Z90 that I power through an AC adapter or the D-Tap on my camcorder's Gold Mount. Looking for a way to use the AB battery as a portable power source to run just the Z90, used with a DSLR.

The lowest cost solution I've found is this:

Anton Bauer QR-GOLD Gold Mount Battery Plate QR-GOLD B&H Photo

It isn't cheap, and I wouldn't be mounting the battery to anything but the mount itself. The only alternative so far is Zylight's battery kit which costs $350.

Any other suggestions?

At a bit under a hundred dollars for something Anton Bauer it really isn't expensive but I know what you mean. When I used ABs I felt like I might have to mortgage one of my kids to be able to afford the stuff but it is great stuff! I don't think you'll find anything else out there that's reliable to do the job for you. At one time I thought I saw an article about putting a belt clip on a plate so you could carry it on your belt, throw a battery on it, plug into the power tap and be off and running. Shouldn't be too hard to do.

Only other thing I can think of is, forget the AB completely, and try to find a 3rd party battery and charger that meet the Z90's power specs. Might not be any cheaper than the AB plate, though.

Two ideas that are under $100. They're either clever or dumb.

1. This rechargeable battery pack. I don't think the jack is the right size, but there might be an adapter. However, I think it meets the electrical requirements.

Connectronics 12VDC 2.5AMP Hour Battery Pack With 2.1 Plug Rechargeable Batteries at Markertek.com (http://www.markertek.com/Expendables/Batteries-Chargers/Rechargeable-Batteries/Connectronics/CTX-12VPR.xhtml)

Of course, Zylight's documentation screams that you should only use their accessories.

2. A regular office-type UPS, charged and hauled around in a backpack. The power draw for an LED light would be minimal compared to a computer, and I already have a genuine Zylight A/C adapter. It would be heavy, but I don't expect to carry it very far from the car.

There are various Chinese knockoffs (of course) like this one (http://www.ebay.com/itm/Comer-Anton-Bauer-t-Sony-v-mount-Battery-Camera-Plate-/290598815598?pt=LH_DefaultDomain_0&hash=item43a90a076e) but as usual, caveat emptor.

I know there's some risk with a cheap battery. That's why I thought the UPS, though inelegant, doesn't require the emptor to caveat. It's a legit A/C supply fed into Zylight's own A/C adapter.

Anton Bauer QRP-EX1 Pouch-Style Gold-Mount for PMW-EX1 QRP-EX1

Or you can always use the Sony style BPU 14.4 volt series batteries that also have the barrel DC out connector like this one, much lighter than the AB battery. You just gave to get a charger for it.

ikan IBS-U65 BP-U Ultra-High Capacity Battery IBS-U65 B&H Photo

My camera is a Panasonic, and I already have a charger. I was thinking about getting the Universal Mounting Plate and just not mounting it to anything.

Is there any reason the plate MUST be mounted to something before it can be used for its Powertap?

Power Tap sometimes also referred to as P-Tap or D-Tap is a 14.4 volt DC connectivety plug/jack. Make absolutely sure 101% that any device or camera connected to it can handle the 14.4 V DC as input from that, as well as polarity. Any mistake is very costly.
The plate does not have to be mounted to anything but would be odd looking just dangling with battery on it. Most would mount it on the shoulder rest/braclet of the camera to also act as a good counterweight or have it attached to a belt. The belt option is very risky as when you rest the camera on a table and walk away from it, the camera goes down on the floor.

No reason to have to mount everything together to use it. I use my batteries similar to the one above to power the PIX-240 via a D-tap when I'm using the FS700 (the battery above won't work on the FS).

I usually hang a bag from the tripod to hold the battery or velcro to something convenient.

Sorry, I mangled my last response.

Until now, my only use for the AB battery has been on the preinstalled Gold Mount that came on my Panasonic HPX370. I also have a cable from Zylight that supplies power from the D-tap/Powertap to the Z90 light.

Now I want to use the battery for the Zylight to provide continuous light when I'm shooting stills with my digital camera. (The camera's internal battery would still power the camera.) The electrical configuration I've described for the light should be the same, I think, but with a new, unmounted "mounting" plate.

The battery does not have to be mounted to the camera to work. Just attach it to anything to secure it and you will be fine.