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TomC · 03-15-2009, 04:32 AM

if you want to run equipment for a specific time, you need a battery that can deliver the required energy in watt-hours. A 25 watt radio for 8 hours requires 200 watt-hours. Add in whatever stereo you are thinking about, as well - stereos can be power hungry if you like to crank it up. Add in a safety margin if you want to be sure. A solar panel to charge would be the only practical way to charge the battery but they get physically large for any reasonable current capacity. A 1-2 amp panel can approach 2 sq ft in size - and cost $200. At 25 watts your radio will draw approximately 2 amps at 12v so keep that in mind. To charge your battery, the solar panel (or whatever you use to charge it) wil have to supply current in excess of whatever you're drawing from the battery.

Finally, battery and battery life. You would need a deep cycle battery for this application. Capacity is proportional to size, so light weight is challenge. The energy-to-weight ratio for conventional (lead acid) batteries is around 30 watt-hours per kilogram. So assuming that you selected a battery with a rating of 600 wh to run your radio, stereo, and provide a safety margin, you are looking at a weight of 20 kg or 44lbs.

Like most engineering problems, this is one of selecting the best compromise for balance of features that makes the most sense to you.

03-15-2009, 04:32 AM	#2
TomC Senior Member Join Date: Apr 2004 Location: Lakes Region Posts: 818 Thanks: 42 Thanked 185 Times in 116 Posts	watt-hours if you want to run equipment for a specific time, you need a battery that can deliver the required energy in watt-hours. A 25 watt radio for 8 hours requires 200 watt-hours. Add in whatever stereo you are thinking about, as well - stereos can be power hungry if you like to crank it up. Add in a safety margin if you want to be sure. A solar panel to charge would be the only practical way to charge the battery but they get physically large for any reasonable current capacity. A 1-2 amp panel can approach 2 sq ft in size - and cost $200. At 25 watts your radio will draw approximately 2 amps at 12v so keep that in mind. To charge your battery, the solar panel (or whatever you use to charge it) wil have to supply current in excess of whatever you're drawing from the battery. Finally, battery and battery life. You would need a deep cycle battery for this application. Capacity is proportional to size, so light weight is challenge. The energy-to-weight ratio for conventional (lead acid) batteries is around 30 watt-hours per kilogram. So assuming that you selected a battery with a rating of 600 wh to run your radio, stereo, and provide a safety margin, you are looking at a weight of 20 kg or 44lbs. Like most engineering problems, this is one of selecting the best compromise for balance of features that makes the most sense to you. Last edited by TomC; 03-15-2009 at 08:06 AM.