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which battery?

jonnyboy

Jul 23, 2011
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Jul 23, 2011
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Hi, i have a TomTom satnav unit that i intend to use on my bicycle. What battery do i need if i wish the device to be powered for up-to 10 hours? - it is 5V/2AMP.

Regards John.
 

Resqueline

Jul 31, 2009
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5V*2A*10h = 5V*20Ah = 100Wh. You could use a 6V 20Ah NiMH battery pack and a linear low-dropout regulator.
Using a 12V battery and a switchmode regulator with 90% efficiency would require a 100Wh/0.9/12 = 9.26Ah capacity.
But I guess that it has an internal battery and that the 2A spec' includes the maximum recharge current for this, thus the gps may not draw 2A on a continous basis.
 

daddles

Jun 10, 2011
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I'd recommend getting a different gps unit. It sounds like that one is made for a car and doesn't really care about power consumption. Put a little generator on your bike and power the gps with it (you older farts probably remember the little generators with coin knurling on the small rotor that you put against your tire to power a bike's headlamp in the 50's and 60's). Use the generator to charge a battery so you've got backup power for when you're not pedaling.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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I'd measure the actual power consumption. It is probably much lower. If it has an internal battery, see how long that lasts.

On my bike (with a 24V LiPo pack for the power assist) I would build a switchmode buck regulator. -- but I'd probably just use my iPhone which will do the GPS thing for quite some time -- not 10 hours on a charge though.
 
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