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[regiment]

charged my 6 volt 4 amp battery  left it for a couple of hours  put the charger back on and it took 20 minutes to come up to full charge again running a 540 motor  lasted about 1 and 1/2 hours then went dead is the battery on it way out it is 3 years old also best place to buy a new one  thanks regiment

[Raen]

I am no expert in this area however I remember when bluebird built his cervia he used a 550 motor and mentioned this:

Now the motor I have installed into this model is a 550 motor.  When we tested it ‘free running’ coupled to nothing, the motor was sitting on the bench and we ran it at 6 volt.  According to the book we have and the information sheet, for this particular motor, at 6 volt; she should draw between 0.4 and 0.5 of an amp and she should rotate in the region of between 15000 and 16000 rpm.

Assuming that there is no load on your 540 and assuming a similar unloaded current draw I would expect to see something similar to the following:
0.5 amp draw
4 amp/hour battery

4/0.5=8hours run time

Now nothing is perfect so I would assume less run time than this maybe 6 hours
http://www.jaycar.com.au/products_uploaded/SB2496.pdf
are some specs for a similar battery

Given this I would assume either the battery is on the way out or it was not actually fully charged.

Given the batteries age its also important that they are stored correctly. I have assumed the battery is a sealed lead acid and I believe these should be regularly "cycled" to keep them in top nick

By cycled I mean discharged and charged every so often.

Hope that helps,

Tim

[Edit] Not wanting to hijack this thread but I'm curious as to how low people discharge a 6v battery too, I'm thinking you probably don't want to go below about 5.25V but im not sure.

[malcolmfrary]

Doesn't say whether the 540 was loaded or free running, no indication of the efficiency of the 540 (some pull a couple of amps all by themselves), so no real clues there.  Expecting a 4AH battery to charge in 2 hours and survive is asking a bit much - they were designed around 10 and 20 hour charge and discharge rates - that they can intermittently deliver an enormous amount of power is both a boon and a curse.  Its a boon for starter motors.  Its a curse because it becomes expected that they can deliver a medium high current for a long time without internal damage.
A voltmeter can tell a lot, and is cheaper that buying replacement parts on the basis of guesswork.

[nick_75au]

Sounds like the battery is fine, depends on the prop and RPM but a 40 mm prop at 6000 RPM draws about 4-6 amps in the water, 540's are usually higher RPM. I didn't think SLA's need cycling, only a top up charge every month or so not in use.

Nick

[Raen]

Well as I did make a fair few assumptions, just how he worded the original question made me think it was an unloaded 540 motor. 

I didn't think SLA's need cycling, only a top up charge every month or so not in use.

I see, but when to apply the top up charge? at 5.5V, 5.25V 5V or some other arbitrary figure?

Thanks,

Tim

[regiment]

thanks to all who answerd bit more information  boat was on the water converted from a tug rtr useing   the prop shaft and prop  howes sell them .. start of charging 0.6a then decreases as battery is being charged  when charged ? //light gose out it is a powertec  automatic charger from howes  thanks again