Charging CCTV li-ions via 5v USB

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Quick (possibly stupid) question regarding those cctv li-ions you can get on ebay.

Some have a 5v USB out as well as the 12v normal out. Can you charge via the 5v port?

I know it's a long shot but I'm wondering if anyone knows.
 
USB power is +5V and 900mA (for version 2 or newer, and 500mA for version 1.1 or older), that's 4.5W output max. Even if we assume 90% charger efficiency that's only 4W. With a 12V 7Ah battery, that means a full recharge would take over 60 hours.

In other words, it's possible but it's not realistically something you'd wanna do.
 
Ok, on a related note, if I

1) mount a 2.5 power jack on the outside of my boombox
2) link this to a lead acid
3) link the lead acid to the amp
4) plugged a 13.5v 1A power supply into the external power jack

Then everything should work, shouldn't it?

The battery will charge (although slowly towards the end of the cycle) and won't be damaged (13.5v is float voltage). Amp will draw power from the supply if present, battery if not (well, sort of). I'll be able to plug it in and unplug it at will. Or am I missing something here?

I was thinking something like this:
AC Power Adapter For Sony Camera EVI-D30 13.5V 1A New | eBay

or this:
http://www.cableuniverse.co.uk/switch-mode-bench-top-power-supply-3a-13.8v.html

(cheers for your continued help btw)
 
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