AH battery drain

[johnad]

I'm not sure if I'm in the right place but hopefully someone can help.?
I have just purchased an electric outboard motor so I now need a battery to run it.
The motor is rated at 12v 35 AH although I believe this is on full power so the AH usage will be lower as it will be used on a lower setting so let's say 30AH average
So I would like a battery to run this for around 3 hours so is it as simple as getting a 90AH battery ?
But from what I have been reading on the Internet a battery shouldn't be completely discharged.
And also should a deep cycle leasure battery be used as some say leasure batteries are not really true deep cycle batteries .
Would I be better to get say two 60AH batteries and strap them together is there any benifit of doing this ?
And lastly my battery charger is rated at 4 amps so would this still carge a high amp hour battery or would I need a better charger or is it just a case that it would take longer to reach full carge?
Sorry for all the questions but it all getting a bit confusing hopefully I'm in the right place to find someone with the answers .
Many thanks

 

[sparkticus]

edit

 

[sparkticus]

The motor is rated at 12v 35 AH although I believe this is on full power so the AH usage will be lower as it will be used on a lower setting so let's say 30AH average

I don't know anything about outboard motors but I would assume that is with nominal load applied or no load. The more load applied to the prop the more work the motor will need to do and will draw more current.

So I would like a battery to run this for around 3 hours so is it as simple as getting a 90AH battery ?

More or less but again you have the load to consider.

Would I be better to get say two 60AH batteries and strap them together is there any benifit of doing this ?

Yes, this will provide longer duration and redundancy (still have one battery if one fails) The two batteries (in parallel) will discharge at slightly different rates due to minor manufacturing differences/differences in internal resistance etc.

And lastly my battery charger is rated at 4 amps so would this still carge a high amp hour battery or would I need a better charger or is it just a case that it would take longer to reach full carge?

It will still charge but take longer than a rapid charger though 4 amps is not too bad.

 

[Paul_C]

The motor is rated at 12v 35 AH although I believe this is on full power so the AH usage will be lower as it will be used on a lower setting so let's say 30AH average

You're using Ampere-hour figures there, but these aren't applicable to a motor. Do you mean that on full power the motor draws 35 amps?

So I would like a battery to run this for around 3 hours so is it as simple as getting a 90AH battery ?

Not quite. Ampere-hour ratings of batteries are normally specified over a certain discharge time period. For example, 90Ah at a 10-hour rate means that from fully charged the battery will supply 9A for 10 hours.

If you draw more than 9A, the battery will supply the required current for somewhat less than the pro-rata calculation by itself would suggest; if you draw less than 9A, it will generally supply that current for a longer period than the simple calculation would suggest. So 90Ah over 10 hours means 9A for 10 hours, but it would supply 18A for somewhat less than 5 hours, and 4.5A for somewhat more than 20 hours.

 

[sparkticus]

You're using Ampere-hour figures there, but these aren't applicable to a motor. Do you mean that on full power the motor draws 35 amps?

I had assumed so. Under load conditions which I assume would be highly variable on an outboard motor I would guess a 420 watt motor on a 90AH battery might discharge between 1.8 - 2 hours. A possible better option might be the two 60Ah batteries wired in parallel. Which might bring it close to 3 hours.

 

[ericmark]

You do seem to understand some of the problems but as already said a motor will not be rated in AH. It will be either amps or watts. As to running the motor at less than full output this would depend on the control it may not have the ability to run at different rates it may be designed as on and off only.

Batteries do come as many types for example see below:-
Lead-acid starter batteries
Gel batteries
Fleece batteries (A-G-M)
maintenance-free batteries

What you need is a traction battery. Likely the Fleece batteries (A-G-M) type. These are used in fork lifts, golf trolleys and mobility scooters. With the lead acid type they should be stored fully charged. With NiIr types these can be stored flat but very expensive so I will not go into these batteries.

Charging is normally done with a step battery charger. These will at first charge consent current until a pre-set voltage is reached this varies with venting type of battery but for a 12v open vented around the 14.8 volt mark. As the current drops to a pre-set level this voltage drops to around 13.2 volts.

Because of this charging cycle the batteries are matched to the charger. So for example 220AH battery will likely need a 35A charger and a 440AH battery will need a 50A charger.

Batteries are made of lead so are heavy as a result one can normally split the battery up into smaller manageable units. Either by the cell or by the battery. So for 440AH likely that would be 4 times 110AH batteries so you can physically move them.

So you start with maximum power wanted and add around 20% to allow for when batteries start to degrade then select the charger something like the Waeco MOBITRONIC 925-012TB according to battery size.

What you have to consider is the price of the Batteries. Traction batteries are not cheap and buying one 110 AH traction battery may cost the same as 2 times 90 AH leisure batteries. So it may be better to go for larger capacity leisure instead of just right size in traction.

If you try to charge two big of an amp hour battery with your charger then the current may not drop to the level required for the charger to drop into the final stage of charging called float charging. However there is another type of charger which uses pluses. Seen used with car alternators but not as plug into mains but sure they are available. These stop charging every so many minutes and measure the volt drop of battery when charging stops. And from this work out when to go into next stage.

You can float charge a battery but the curve flattens off so much as it nears fully charged it could well take a week to recharge batteries rather than around 8 hours with the step charger from flat.

 

[ban-all-sheds]

Disclaimer - I know SFA about boat engines.

Is a 0.5hp outboard going to be of any use?

 

[sparkticus]

Disclaimer - I know SFA about boat engines.

Is a 0.5hp outboard going to be of any use?

I was wondering the same thing

 

[ericmark]

Disclaimer - I know SFA about boat engines.

Is a 0.5hp outboard going to be of any use?

If you visit the slate mines of North Wales they use an outboard of about that size to take visitors the first 200 yards into the tunnels about 20 at a time.

However I don't think you can ski with them!

 

[johnad]

Thanks for all your replies I just thought I would paste the information from the place I got the outboard from


To power this motor you need a 12 v car type battery, there is NO battery in the motor or offered with it here.  The length of time it will run depends on the size of the battery, a Typical 60 AH Car battery will give 2-3 + hours running and a 110 AH Deep Cycle Leisure Battery will give 4-5 + hours.  This motor has a LED indicator that shows 10 stages of battery condition, this indicator is on the top of the motor,  The twist grip throttle gives 5 forward and 3 reverse gears.

Sun sport have 4 motors in the range 32 - 40 - 46 - 55 Lbs Thrust, these are all listed in the Electric Outboard section of our shop

Specifications of Sunsport SS 40 Electric Engine

 

Static thrust 40 Lbs
Amp draw 35
Voltage 12
Speed Control 5 Forward / 3 Reverse
Shift Twist
Shaft 28-30" (Suits 15 - 20" Transom)
Weight 7.2 kg
Bracket twist Yes
Composite shaft

I had been looking on the web for information and although it has 5 forward speeds top speed is a drain on the battery and not needed (so I'm told).
I assume the information on battery drain above is a tad optimistic ?
I'm now thinking two 60AH batteries would be the best cost effective option

Any more ideas would be appreciated
I'm now starting to think at £130 for the motor possibly £100 + for two batteries I may as well have gone for a small petrol outboard ?
But it is only to power a canoe when I don't fancy paddling
Thanks again to everyone

 

[bernardgreen]

Connecting two lead acid or similar batteries in parallel should be avoided.

Unless they are a perfect match one of them will take some current from the other and this will be wasted. If a cell in one them fails then a lot of power will be lost as the good battery tries to charge the defective one

Either use a change over switch or a pair of Schottky diodes ( electrical one way "valves" ) that prevent one battery charging the other battery. And in the event os a catastrophic failure of one battery enables the other to survive and continue providing power the motor. Schottty diodes have very low voltage drop but are not cheap at 35 amp rating

 

[ericmark]

Although what "bernardgreen" says is to an extent correct most narrow boat owners will buy a set of batteries together and connect in parallel without any real problem. That is assuming all bought at same time and have the same use throughout their life. What "bernardgreen" says is correct and using diodes would extend the life but as to if really worth the expense is another question. Isolation diodes in a package are sold for galvanic protection and likely would do the job but I would not bother.

Car batteries are no longer rated in AH as that's not what they are designed for they are rated in start amps and are useless for what you want.

The leisure battery seems to come in 60 AH and 90 AH and you have to consider you have to put this into the canoe. I can't remember a canoe having a Transom and not sure how you are going to mount it?

The pitch of the prop is very important and if one looks at Turbinia

this has been a problem since the first use of props in boats. And likely with such a slim craft some changes will be required to get the most out of the unit.

I am not suggesting you put three props on the shaft but there may be a selection of props to suit the boat.

If one 60AH battery will last 2 - 3 hours then two should last 4 plus to 6 plus hours as the less one draws the more efficient the battery is. The AH is set at a 10 hour or 20 hour rate. So a 60 AH at 10 hours will likely only give 40 AH over 4 hours.

Assuming you already have the motor I would try at first with one 60 AH battery of leisure type. As they are not light and although the punt used in the slate mines may have 4 times 110 AH batteries likely these are charged in the boat, and so there is no problem carrying them.

I would also look at fully sealed batteries. Although they may still fail if submerged, they will stand more of a chance than open vented type.

So next is cooking stove and sink to wash dishes it it?

 

[empip]

Trolling ( ) motors ?

http://www.islandhopperoutboards.com/hobietrollingdrivesystem.html

Outrigged...

-0-

 

[ericmark]

Is that real or just a "Mirage"?

 

[sparkticus]

Bet its got a lot of YAW