Battery size

[lenm]

Can anyone recommend a good size battery (Amp hours) suitable to run a sounder, VHF radio and GPS off for a full day fishing trip.
It will be used seperate to the outboard battery so preferably not too bulky would be good.
Thanks

[alimo88]

look in your manuals for the current draws of each item

add them together (say 1 amp)

Multiply by the number of hours you intend to use them (say 12 hours)

This is your Amp Hours (say 12 amp/H). 12 amps for 1 hour, or 1 amp for twelve hours.

For safety maybe go double

Just a thought

Alimo88

[steveg1100]

Batteries are rated in amp/hours a 1 amp/hour battery will deliver 1 amp for one hour. Just add up the current all your items draw (there will be a plate on the item or the manual should give you this information) and then calculate the size of the battery that way. With the radio it pulls a lot more current when transmitting so you have to factor this in also.

[steveg1100]

whoops just realised alimo answered already. Sorry.

[lenm]

For safety maybe go double

Thanks Guys, I've calculated a value but needed a feel for what margin to leave considering that most batteries dont like being drained to low.

[marlinqld]

One thing to consider with radios.......

If the battery is low or nearly flat, it will quite often receive fine, but wont have enough power to transmit. This can give you a false sense of security, so remember that.


Mike

[Fisherdan]

Depending on the battery type you should not discharge a battery more than 20% to give long life..
A traditional wet cell cap type battery (which you should never use in a saltwater boat anyway) should never be drained past 20%.
A good VRLA float battery will give you around 800-1000 cycles at 20% and as low as 200 cycles at 80%. this is an average figure.
An expensive DC Gel battery will give you 500 cycles at 100%. 1500cycles at 20% depth of discharge. remembering that a flat or 100% discharged battery is around the 10.8volts.. and not 0volts. anything past 10.5volts and you are damaging the balance of chemicals in your battery and doing permanent damage to the battery.

So if your total draw is 2amps an hour. for 8 hours thats 16 amps.
If you buy a cheap battery or standard wet cell dual pupose deep cycle that should only be drained 20% you need a rather larger battery.

So for the same price buy a VRLA DC.
If you buy an better and safer fully sealed good quality Gel VRLA DC you can get away with a battery as little as 40amps or less. This is much smaller and safer. but you will need a smart charger that can fully charge a VRLA DC properly. like a CTek. not just a cheap charger.
If your not sure ask you battery supplier to provide depth of dishcarge V Number of cycles chart.

Hope that helps.