5V SLA Portable Recharging Station

(12V 7Ah SLA Battery, 12V-to-5V voltage regulator, and micro B USB cable required)
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Hopefully it fits your 5v voltage regulator. Most 12V 7AH SLA Batteries should fit. (Mine is 150mm x 65mm)
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Battery- http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=12v+7AH+sla+battery&rh=i%3Aaps%2Ck%3A12v+7AH+sla+battery
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Voltage Regulator (although you'll need to resize the compartment for this one) - http://www.amazon.com/KEEDOX%C2%AE-Converter-Power-Supply-Module/dp