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Old 29 June 2010, 16:39   #11
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Originally Posted by JABS View Post
So, how do you calculate it?

I have I have 120 watts of toys and a 60 amp hour battery.

In theory I can sit there for five hours after which, miraculously it will all go quiet.
120W, @12V = 10A. So would completely discharge a (full) 60 Ah battery in 6h. However its generally accepted that you shouldn't discharge normal lead acid batteries below 50% so thats only 3hours. And if the battery is "critical" then it would certainly make sense to add in a conservative safety margin.

If you google you'll find much more complicated approaches that account for temperature, infrequently cycled batteries etc - but the above approach seems to be the "normal" approach for sizing boat batteries.

Quote:
How do I know the battery is fully charged when I arrive?
How long can I discharge it and still get a start?
How has the capacity degraded due to life?
Sounds like the OP is just trying to work out if he has a sensible size of battery for the likely load though (and perhaps work out whether he should upgrade to say an 80 or 120 Ah battery if not). Indeed if I was running your 10Ah "load" constantly then I'd concerned about whether the alternator on a small engine was big enough too.

Quote:
My point is that a 'stiff' supply which maintains a high voltage at standstill gives a much better indication.
But you defined "stiff" as >12 V with all on in your first post and now its >12.5 V.

And if its not showing high enough - is that because the battery is too small? or a nackered battery? I'm not saying your method is bad/wrong - but its an additional rather than alternative way of looking at your capacity - and if it says the current set up is inadequate gives no indication of what size would be better.

Quote:
The moral of the story is get a dual battery system and make it stiff enough to hold on one isolated battery 12.5v even with everything on.
except he's got a 40HP engine, on a 4m boat so a large twin battery system is probably overkill - especially if he's got a pull start on the engine. Even with a twin battery presumably you still want to understand the size you need to avoid constantly depleting your battery which nackers it.
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Old 29 June 2010, 19:18   #12
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To answer the OP's question, yes that is how you would measure the current being drawn from the battery. Your meter needs to be set to measure DC amps, leads should be plugged into the appropriate ports on the meter and the meter should be rated to higher than what you'd expect to read. Most decent meters will measure upto 10A, but if you expect higher than that, it will blow the meters fuse (as will connecting the leads up wrongly.) Current can be measured between either the earth or live lead and the corresponding battery terminal, whichever is easier, and a -ve reading just means the current flow is in the opposite direction to conventional flow, red to black (you've got the leads the wrong way round but it's irrelevant.)

Converting power (watts) to current (amps) using Ohms law is all well and good in the classroom (or for sizing fuses) but it assumes that the stated power is accurate and that the voltage is fixed, we know both will not be true.

Nominal battery voltage is 12.6v, 12.4v would be fairly normal but a battery could quite easily sit at over 13v coming off of charge due to surface charge, try to crank over an engine and it might not even click. If measured voltage drops much below 10v then there's something wrong (could just be partially discharged) this is hugely temperature dependant, cold reducing chemical activity etc. Relays will stop operating at about 7.5v so if the battery drops this low during cranking, it won't start no matter how much you turn it.

Remember also that the amp hour rating of a battery is actally the 20hr rating, ie the notional rating that would discharge it to 10.5v (flat) over 20 hours. For example a 40Ah battery would sustain a 2A load for 20 hours, if you then placed a 40A load on the same battery, it would not sustain for 1 hour! As the load increases, the Ah rating decreases, you'd probably only be looking at a little over 30mins at 40A discharge and a lot of waste heat.

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Old 29 June 2010, 19:36   #13
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Originally Posted by free View Post
To answer the OP's question,
I think thats already been done (post 2!). This has the potential to develop into a ribnet how to drill a whole thread!
Quote:
Converting power (watts) to current (amps) using Ohms law is all well and good in the classroom (or for sizing fuses) but it assumes that the stated power is accurate
it is unlikely to be a significant under estimate (not least for speccing fuses / wiring)
Quote:
and that the voltage is fixed, we know both will not be true.
by diving by 12, rather than 12.5 or some other "full charged" voltage - ensures that if anything the "amps" are an over estimate (but we're probably talking about 5% error - not enought to worry about).
Quote:
Nominal battery voltage is 12.6v, 12.4v would be fairly normal but a battery could quite easily sit at over 13v coming off of charge due to surface charge, try to crank over an engine and it might not even click. If measured voltage drops much below 10v then there's something wrong (could just be partially discharged) this is hugely temperature dependant, cold reducing chemical activity etc. Relays will stop operating at about 7.5v so if the battery drops this low during cranking, it won't start no matter how much you turn it.

Remember also that the amp hour rating of a battery is actally the 20hr rating, ie the notional rating that would discharge it to 10.5v (flat) over 20 hours. For example a 40Ah battery would sustain a 2A load for 20 hours, if you then placed a 40A load on the same battery, it would not sustain for 1 hour! As the load increases, the Ah rating decreases, you'd probably only be looking at a little over 30mins at 40A discharge and a lot of waste heat.

free
almost certainly he isn't (or doesn't want to be) drawing 40A on his SR4 - unless he's got a fire extinguisher and a liferaft . This is all great at showing how much you know about battery tech - but doesn't help the OP work out if he's got the right size battery, and if not what size he needs.
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Old 29 June 2010, 20:06   #14
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Lol!

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Old 30 June 2010, 18:52   #15
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Thanks for all the informative and entertaining replies.

I Have measured and am drawing about eight amps. A little more with nav lights on. To be on the safe side to allow for transmitting on VHF I will round that up to twelve amps. As my maths was never a good thing and to ensure there is pleanty to start the motor with ( elec start ) we will call that fifteen amps. Not wanting to run battery into discharge so 50% use I'll need 30amps. As the battery never fully charges and on charge from engine under load will probably only get to about 75% we can add smother lump on for good meaure.

My guestimation was that I would be perfectly safe to venture out with my rig as standing on a 85amp hr battery with confidence that I could leave electrics on with engine off for a couple of hours with no issues when it came to restarting.

Only problem now us the battery isn't marked with it's amp hour rating..... Doh.

Is there a way of finding out what that is. I'll probably upgrade to new in any event to be safe but it would be interesting to know.

G
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Old 30 June 2010, 19:03   #16
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I Have measured and am drawing about eight amps.
What gizmo's are you running that are drawing a total of 8A with the nav lights off?
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Old 01 July 2010, 04:27   #17
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Personally I thought free's synopsis of batteries was very helpful and informative, and certainly concurs with my understanding of the subject. Best to give the OP the full story than some simplified test processes so that he can make his own mind up as to the shortcomings of any measurements.

To establish the size of your existing battery, if the cryptic numbers don't make it obvious, then I'd go to the manufacturers website, or do a google search with the numbers written on it. There are some standard dimensions and capacities for many car batts.
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Old 01 July 2010, 19:41   #18
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Hi Glenn, the current draw and Amp hour rating are not really that useful in this circumstance, what you really need is a battery with a good reserve capacity, that is, the length of time that the battery will support a 25A current draw and still start the motor. Saying that, I've noticed a lot of batteries no longer state the reserve capacity (RC) but if you find one with 125 mins, you know that you're easily going to get two hours out of it at your max current draw. I'd advise a marine battery (or deep cycle) as a standard Starting Charging Ignition (car) battery is not designed to be repeatedly discharged much more than 5%. As for make, Varta (also sold as Bosch) make the best batteries commonly available, if you've got money to burn, I've used Optima's it totally ridiculous situations and they just don't fail.

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