

07022013, 14:42

#16

Registered User
Join Date: Aug 2012
Location: Live in N. VA but the boat, she's in MD
Boat: S2 11.0
Posts: 96

Can anyone recommend a good "boat electricity for dummies" resource? Not electronics, the basics about batteries, charging/discharge, inverters, wiring, power demand and draw, etc?
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07022013, 14:54

#17

Commercial Member
Join Date: Dec 2008
Location: St. Petersburg, FL
Boat: Gulfstar Long Range Trawler; 53'; BearBoat
Posts: 811

Re: Battery Draw
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Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."



07022013, 15:05

#18

Marine Service Provider
Join Date: Jan 2007
Location: Maine
Boat: CS36T  Cupecoy
Posts: 2,868

Re: Battery Draw
In addition to Calder, Charlie Wing's Boatowners Illustrated Electrical Handbook Second Edition, gets RAVE reviews from my customers. Most claim Calder is too indepth and Wing is right down the middle with easy explanations that all follow current safety standards.



08022013, 14:53

#19

Registered User
Join Date: Jul 2008
Location: Rockport, Texas
Boat: Tartan 41 #68
Posts: 304

Re: Battery Draw
Quote:
Originally Posted by bob_77903
I'm using 4/0 for the inverter now. I never use it to max output. Most I've seen on the LED readout is about 800W FWIW....

I need to correct myself. Not using 4/0 wire, geez that is .812" or 13/16. That would be a little over kill. I have 1 AWG for the negative that is longer than the 2 AWG positive lead....the positive lead is about 8', and the negative is about 12'.
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08022013, 15:08

#20

Registered User
Join Date: Jan 2011
Location: SFL& St Petersburg
Boat: Tartan 33
Posts: 1,576

Re: Battery Draw
Quote:
Originally Posted by bob_77903
I have 4 Gel Cell 6v 180ah batteries making up a 360ah 12v house bank. I have a 2000w inverter I use to power 120v ac appliances. If I pull 7080 amps from the microwave or toaster for 23 minutes straight will this damage the batteries in anyway?

While I understand the need to have a safety factor, I believe you may have overestimated. A 1000 watt microwave should draw less than 10 amps.
we know that volts X amps =watts therefore watts/volts= amps.
So 1000/120< 10 or 1000 Watts/ 120 Volts is less than 10 amps.
Or has something changed since I last used this many, many, many years ago?
Bill
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08022013, 15:36

#21

Registered User
Join Date: Jul 2008
Location: Rockport, Texas
Boat: Tartan 41 #68
Posts: 304

Re: Battery Draw
Quote:
Originally Posted by Snore
While I understand the need to have a safety factor, I believe you may have overestimated. A 1000 watt microwave should draw less than 10 amps.
we know that volts X amps =watts therefore watts/volts= amps.
So 1000/120< 10 or 1000 Watts/ 120 Volts is less than 10 amps.
Or has something changed since I last used this many, many, many years ago?
Bill

Guess that might apply if I were plugged into shore power (120 vac), but I'm powering the microwave from a 12 vdc source for this discussion.
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08022013, 16:36

#22

Senior Cruiser
Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 3,921

Re: Battery Draw
Quote:
Originally Posted by bob_77903
I need to correct myself. Not using 4/0 wire, geez that is .812" or 13/16. That would be a little over kill. I have 1 AWG for the negative that is longer than the 2 AWG positive lead....the positive lead is about 8', and the negative is about 12'.

Bob,
The diameter of the center conductor for AWG 4/0 is .46" or just under 1/2 inch. AWG 1/0 is about a third of an inch in diameter.
AWG 4/0 is certainly not overkill for a 2,000 watt inverter.
However, if you in fact limit your load to 800 watts maximum as you've indicated, that would be just about on the limit for AWG2 cable with a 12' oneway run.
Nevertheless, I'd certainly want to upgrade to the proper wire size when you are able.
Bill
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09022013, 01:48

#23

Registered User
Join Date: Sep 2011
Location: Patuxent River, MD
Boat: MorganByCatalina OutIslandClassic '41
Posts: 56

Quote:
Originally Posted by bob_77903
I have 4 Gel Cell 6v 180ah batteries making up a 360ah 12v house bank. I have a 2000w inverter I use to power 120v ac appliances. If I pull 7080 amps from the microwave or toaster for 23 minutes straight will this damage the batteries in anyway?

The OP is confusing. I'm going to assume that the appliances in question are AC appliances. Lets assume a 600 watt Microwave. Ignore power and inverter efficiency factors for a moment and rely on the notion that 600 watts is 600 watts. A 600watt load on a 12VDC source causes a 50 amp draw. 3 minutes is 0.05 hours. 0.05 times 50 is 2.5Ah at 12VDC.
My answer to the OP's question is, if you're using wire of sufficient ampacity and proper circuit protection, your batteries should not be harmed.
3 DISCLAIMERS: Microwave oven wattage ratings ignore power consumed by lights, fans, turntable motors, and displays. Inverters produce only about 80 watts for every 100 watts they consume. 100 volts of alternating current needs to be treated as about 75 volts for the purposes of Ohms Law. Conclusion  The actual Ah consumption will be a bit higher than these rough calculations indicate.
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09022013, 03:24

#24

Moderator
Join Date: Jul 2007
Location: Full time cruising. Currently in the Med.
Boat: Aluminium yacht
Posts: 10,005

Re: Battery Draw
Quote:
Originally Posted by CaptainDana
Conclusion  The actual Ah consumption will be a bit higher than these rough calculations indicate.

I realise you are trying to simplify the calculations, but you really need to consider Peurkets equation with these sort of power draws.
Strictly speaking this does not alter the power consumed, but the battery capacity.
Nevertheless the battery capacity will be diminished considerably more than the
calculation without taking the effect into account would indicate.
For high discharges from LA batteries even rough calculations need to take this factor into account.
It can easily add 50% to the effective consumption.
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09022013, 04:38

#25

Registered User
Join Date: Jul 2008
Location: Rockport, Texas
Boat: Tartan 41 #68
Posts: 304

Re: Battery Draw
Quote:
Originally Posted by btrayfors
Bob,
The diameter of the center conductor for AWG 4/0 is .46" or just under 1/2 inch. AWG 1/0 is about a third of an inch in diameter.
AWG 4/0 is certainly not overkill for a 2,000 watt inverter.
However, if you in fact limit your load to 800 watts maximum as you've indicated, that would be just about on the limit for AWG2 cable with a 12' oneway run.
Nevertheless, I'd certainly want to upgrade to the proper wire size when you are able.
Bill

Thanks Bill,
Yes I see where 4/0 is .46 in another chart I looked at. My 12' run is the negative cable, and it is AWG1. I will go ahead and upgrade to 4/0 first chance I get...Thanks to all for your responses...
Bob
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09022013, 04:57

#26

Registered User
Join Date: Sep 2011
Location: Patuxent River, MD
Boat: MorganByCatalina OutIslandClassic '41
Posts: 56

Quote:
Originally Posted by noelex 77
I realise you are trying to simplify the calculations, but you really need to consider Peurkets equation with these sort of power draws.
Strictly speaking this does not alter the power consumed, but the battery capacity.
Nevertheless the battery capacity will be diminished considerably more than the
calculation without taking the effect into account would indicate.
For high discharges from LA batteries even rough calculations need to take this factor into account.
It can easily add 50% to the effective consumption.

With great respect to Herr Puekert, I think it's a fair characterization to refer to 1.25Ah (your suggested 50%) in this particular 3minute microwave case as 'a bit'.
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09022013, 05:56

#27

Marine Service Provider
Join Date: Jan 2007
Location: Maine
Boat: CS36T  Cupecoy
Posts: 2,868

Re: Battery Draw
Quote:
Originally Posted by noelex 77
I realise you are trying to simplify the calculations, but you really need to consider Peurkets equation with these sort of power draws.
Strictly speaking this does not alter the power consumed, but the battery capacity.
Nevertheless the battery capacity will be diminished considerably more than the
calculation without taking the effect into account would indicate.
For high discharges from LA batteries even rough calculations need to take this factor into account.
It can easily add 50% to the effective consumption.

AMEN! Thanks for finally mentioning that. I have not had the time to chime in....
On a 360Ah bank, at the 20 hour rate, with a Peukert of 1.25 the 80A load turns into a 116A load to the battery, after Peukert corrections.
With a three minute load this only makes 2 Ah's worth of difference but when using loads, at higher than the banks 20 hour rate, you can really chew into capacity. I have seen this numerous times with DC water makers where the bank is over drawn because the owner did not correct for Peukert.
Take that same 80A and run the load for two hours. One would assume you're only down 160Ah's on a 360Ah bank or at 55% SOC, so you're ready to recharge, if you started with a full bank. In reality you are now down nearly 240Ah's and at close to 70% depth of discharge and well beyond the 50% DOD threshold......
While not a huge issue to the OP's question, the batteries will still survive it, the load on your batteries is directly related to where the load lies in relation to the 20 hour Ah rating and its Peukert number..
A 360Ah bank can support an 18A load for 20 hours before hitting 10.5V. This is the 20 hour rating "load". It is calculated by dividing the Ah capacity by 20 so 360Ah/20=18A.....
Do that same thing with an 80A load and the time you have at 80A drops to just a little over 3 hours..
Conversely apply an average load of just 6A and you'd have nearly 79 hours of use before the battery was flat/10.5V....
Basically you can't just take the amps and multiply by time to get the Ah's used unless you also calculate for Peukert. This is more true when applying high loads because the high loads drain the battery faster. Loads at less than the 20 hour rate give you MORE capacity so doing these calculations is not really as necessary.. Doing these calculations gets you much closer to the reality of the actual consumption from the battery at a given load.
This is why battery monitors, that correct for Peukert, can be valuable and why the Ah's consumed screen hardly ever agrees with the % charged screen.... % charged is usually (with many good monitors) Peukert compensated and Ah consumption screen is not...



09022013, 11:49

#28

Registered User
Join Date: Jul 2008
Location: Rockport, Texas
Boat: Tartan 41 #68
Posts: 304

Re: Battery Draw
I really appreciate the knowledgeable explanations from those that understand these dynamics. Not only do I learn things, but I'm sure others do as well.
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09022013, 12:50

#29

Registered User
Join Date: May 2011
Location: Toronto
Boat: Sandpiper 565
Posts: 2,597

Re: Battery Draw
Toaster
current draw: 0.0A (averaged)
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09022013, 12:55

#30

Senior Cruiser
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 17,146

Re: Battery Draw
Quote:
Originally Posted by bob_77903
I have 4 Gel Cell 6v 180ah batteries making up a 360ah 12v house bank. I have a 2000w inverter I use to power 120v ac appliances. If I pull 7080 amps from the microwave or toaster for 23 minutes straight will this damage the batteries in anyway?

a 1200 watt microwave at 120volts should be drawing 10 amps AC .... that would be 100 amps DC 12v right?
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