save my batteries

I am planning my first trip away on my new (well new to me) Finnsport 650 cabin cruiser. There is 2 batteries so I am able to isolate the engine battery. What I want to do is bring a 12v cooler box for the milk etc. I am looking at a 48L cooler box http://www.minicoolers.co.uk/index.htm?url=products/index.htm&gclid=CICWpNKH15QCFQqNHgodtmm2QQ
My concern is the power consumption which will be approx 47watts.
Obviously the last thing I want is flat batteries. How long will the domestic battery be likely to last should I leave the cooler box on overnight. The only number on the battery is 100 which I assume relates to the power / strength of the battery. I have also considered buying a battery pack as a back up and any other advice would be appreciated.

Re: save my batteries

The amount of electricity is will use will depend on the temperature in the boat.
This comes with a cig lighter adaptor.Do you have one in your boat ?
Does the manufacterer recommend this cooler for a boat ? I would give them a call and ask them the questions you have.

Re: save my batteries

Or buy a solar trickle charger, I've seen them as low as $20 at an auto parts store.

Re: save my batteries

Bass Man Bruce said:

Or buy a solar trickle charger, I've seen them as low as $20 at an auto parts store.

Click to expand...

Those chargere are basically useless for anything but keeping a battery topped off since they are maybe 2W or so...most are even less.

The cooler that you are looking at is a peltier cooler and will pretty much be on all the time. Most of those types of coolers don't even have thermostats. As you can see from the specs, you can only get a 20 degree celsius drop from ambient temp.

Assuming a 12V battery @ 47W is about 4A/hour, your 100 amp/hour battery would last around 12 Hours if you only let the battery get down to a 50% discharge. Any more than that is bad for your battery.

You have to decide if this is going to be able to get cold enough for you. I use one similar to this but it is only used for things that don't need to be too cold like fruits and veggies. i wouldn't rely on it for meat, milk or other things that need to be very cold. I would only use this with temps below 25 celsious for anything that is likely to spoil. They are very slow to cool things down. If you start with everything cold, you may be ok but don't put anything warm in there and expect the cooler to cool it down right away.

Peltier coolers are very inefficient compared to regular compressor type refers. They are only 10% efficient compared to around 50% for a compressor type. You would be better off paying more money for a portable Norcold or just use ice.

http://www.thetford.com/HOME/Products/NorcoldRefrigeratorsHome/MRFT40/tabid/200/Default.aspx

Re: save my batteries

Thanks everyone for the help. I do intend to store milk and probably meat for a fry up in the mornings so maybe as suggested a larger investment is whats called for and get the compressor version. Like most things I guess you only get what you pay for.

John.