Solar trickle charger

[pwl]

Our boat does not have a self draining stern deck so needs a bilge pump directly connected to the batteries via a float switch. To keep the battery bank topped up when not at the boat and to compensate for the pump operating in bad weather I was thinking about investing is a small solar panel that I can remove when on the boat. The one I am looking at is designed for battery top up and produces (in ideal conditions) 4.8W BUT its optimum working voltage is 17.5V at 260mA. Most alternators work at 14.2 V and alternator controlers raise this to 14.8V (I dont want to start a discusion on alternator controllers here). My concern is will 17.5V damage wet batteries or is the current so low it is not a problem?

 

P

[chris w]

Our boat does not have a self draining stern deck so needs a bilge pump directly connected to the batteries via a float switch. To keep the battery bank topped up when not at the boat and to compensate for the pump operating in bad weather I was thinking about investing is a small solar panel that I can remove when on the boat. The one I am looking at is designed for battery top up and produces (in ideal conditions) 4.8W BUT its optimum working voltage is 17.5V at 260mA. Most alternators work at 14.2 V and alternator controlers raise this to 14.8V (I dont want to start a discusion on alternator controllers here). My concern is will 17.5V damage wet batteries or is the current so low it is not a problem?

 

P

 

Don't worry. 17.5v is the "no-load" voltage of the solar panel. The moment you connect a battery, there will be voltage dropped across the solar panel's (relatively high) internal resistance and the voltage will drop to a safe value to charge the battery.

 

4.8W is only going to deliver around 300mA (0.3A) in maximum sunlight. While this will be sufficient to keep a battery topped up against self-discharge, when not in use, it will not compensate for a bilge pump that is operating regularly. Your bilge pump probably draws around say 3A. If it operates for say 1 minute every hour, that a total of around 1.5AH per day. To recharge the battery (due to battery physics) will need 50% more than this, so say around 2AH per day.

 

Your solar panel will need to be iluminated in bright sunshine for 7 hours a day to replenish this. Since, of course, the bilges will be filling with rain water on bad weather days, this situation is not going to obtain. You probably should look at a 60W panel to be confident that the batttery will not suffer.

 

Chris

[pwl]

Thanks. I will investigate higher output panels.

 

P