12V Power Consumption Chart

[RichardW]

For those of us who are often on the hook and running off of battery power I took a few mearsurements of current draw for various 12V loads on the CD. Current measurements were made at a nominal 12.6V with a fully charged battery in good condition.

I will add to this chart when time permits, but these are some typical loads during an overnight stay.


HERE is the LINK to the Chart:
http://www.c-brats.com/modules.php?set_albumName=C-Eagle&id=Power_Consumption_Chart&op=modload&name=gallery&file=index&include=view_photo.php

Included on the chart is a nominal Battery Charge vs. Voltage section which approximates the level of battery charge left. I conservatively try to leave 50%(~12.0V) of the charge remaining after an overnighter.
I use an inexpensive DVM that plugs into the cigarette lighter receptacle to get a quick measurement at a glance.
For example, 2 cabin fans, the FM radio and LED anchor light can be left on overnight with little draw down on the battery.
Assume a 600 AH battery, 2 cabin fans on low =.4Ax 10Hrs + LED anchor light =.18 A x 10 Hrs + Sony radio = .15A x 10 Hrs for a total of <8 AH. % Usage = 8/600 =~1.5% (negligible)

Richard

[Sea Wolf]

Richard-

Good Information.

Nice Table!

What battery combination would you be using to get 600 Amp-Hours?

I have four group 27's on my boat that combine to yield

4 x 105 AH/ea. = 420 AH.

Golf Cart Batteries? 8D's?

Cheers!

Joe.

[RichardW]

Sea Wolf,

Sorry to get your hopes up for exotic group 27 batteries that would fit into the lazarette and still produce 600 AH............
This was a hypothetical examle. My batteries are closer to 400 AH. Even 400 MA produces plenty of power for overnighters as long as they are charged daily from running. Running the Wallas heat all night is not even a problem.

Thanks for the feedback.

Richard

[thataway]

Richard,
Thank you for the current draw table.

I use 12.20 volts as 50% discharge. The 50% discharge value depends on the battery type (lead acid, AGM, Gel), the age of the battery, the usage of the battery and the temperature of the battery. Posted 50% discharge voltages run from 12.00 to 12.48 volts.
Trojan gives a value of 12.10 volts at 50% discharge (steady state).
http://www.trojan-battery.com/Tech-Support/BatteryMaintenance/Testing.aspx

The yaseau battery book gives 12.00 as 50% discharged as Richard has in his table.

In Wind/sun the following statement is made: "Voltage on a fully charged battery will read 2.12 to 2.15 volts per cell, or 12.7 volts for a 12 volt battery. At 50% the reading will be 2.03 VPC[12.18 volts] (Volts Per Cell), and at 0% will be 1.75 VPC or less." They also have a nice table (which is repeated in a number of net sites) which show 50% discharge at 12.06 volts, steady state.
http://www.solar-electric.com/deep_cycle_batteries/deep_cycle_battery_faq.htm

Most AGM battery sites give 12.20 volts as 50% discharge.


http://marine-electronics.net/techarticle/battery_faq/b_faq.htm#7 shows a 50% discharged marine deep cycle at 12.24 volts.

The voltage vs discharge is true for a steady state battery--that is no charging or discharging for 6 hours for a new battery, which is in good contion, well cared for. This is why I use a higher voltage value.

For us, the largest draw is refigeration--and this can be about 2.5 to 3 amps when running--often 50% of the time. So over 24 hours, we can use 30 to 40 amps for refigeration alone. I would suggest that engine start batteries by isolated from the house batteries. It just takes one time of depleated batteries to make a believer. Happened to me 45 years ago--never again.

[rjmcnabb]

Hi Richard W,
Can you explain your table a little more, please? What are the two columns labeled AMPS, and how do you come to your conclusions without factoring in how much time each day that you use each device?
I realize that time is factored into watts (they are a rate, just like knots are a rate), but still that rate is zero if the device is turned off. So, to determine battery draw-down, must you not tally (AMPS x HR), and compare that to the total AH available in your battery?
I have only a single battery, and carry a jump-start battery as a backup (belt and suspenders psychology from my sailing days in small boats.) So, I try to be very conservative in power use when the engine is not running. But still, I'm intimidated by the bundle of wires under the pilot's seat. I'm glad to hear (from someone's remarks) that the Walas's draw is low.
Thanks,
Rod

[RichardW]

Hello Rod,

I think that I can clarify the table and AH calculations for you. I threw in a couple of definitions ( AMPs and Watts) which you likely already know.

The calculation of Amp Hours was done in the text of my orignial post (above) by multiplying the amps used by each device by the time used (assumed to be 10 hours) and totaling them. The table is intended to list typical current (AMPS) drawn by common devices in a boat. As stated in the post, I assumed a nominal fully charged value of 12.6V. I compute the Watts used for each device as a reference value of power consumed. This is useful for knowing the power hogs and for managing power consumption. For example, for a cabin fan on low speed, P (2.6W)= V (12.6V) x A (.21) rounded off to one decimal. At a glance, you can see that an incandescent cabin light consumes (18.3/2.6= 7) 7 times the power of a cabin fan on low. The table also shows that an incandescent cabin light uses (18.3/8.2) 2.2 x the power of the fluorescent cabin light, which is why I am using fluorescents.

To estimate the battery state of charge after running specific devices for a specific time, you must tally the total AMP-HOURS as you suggested and as I did in my example calculation. I use the plug-in DVM (digital volt meter) to get a quick visual indication of the actual state of charge of the battery. If I used 50 AH from a 100 AH battery in good condition that was fully charged to an initial reading of 12.6v, I would expect to read a battery voltage of around 12.0V (~50%) after this rate of discharge. I try not to allow the house batteries to go below 50% without recharging. These numbers are approximations for a typical lead acid wet cell battery and are affected by several variables (cell specific gravity, temperature, rate of discharge, how the battery is charged / maintained, manufacturing processes, etc. , just to name a few).

AMPS
AMPS (amperes) are a measure of the rate of electrical current flow through a given load (device drawing current, such as a fan or other appliance). The only reason that I used two columns is that some devices, such as fans, have two speeds and the current differs for each speed. For example, the Hella Cabin fans in my table draw .21A on low and .41A on high. In my calculations, I used the low speed value because that is where I operate the fans at night.

Watts
Power ( measured in Watts) = Voltage (volts) X Current (measured in Amps) in a simple DC electrical circuit. A Watt is a measure of energy used over a unit of time. The table simply converts measured current for each device to Watts by multiplying the current by 12.6 V. As I explained in the post, I assumed a voltage of 12.6 for a fully charged lead acid battery in good condition.

I hope that I answered your questions without telling you much more than you ever wanted to know.

Richard

[JamesTXSD]

Here's a good link from BoatUS...

http://www.boatus.com/boattech/12volt.htm

As a side note, the house batteries in our boat seemed to go flat very quickly. We found out that we had a dead cell in one (apparently came that way new), and wound up replacing both. No problem since.

Best wishes,
Jim B.

[marco422]

Honda Canada has posted this useful guideline... geared to household use, though.

http://www.honda.ca/HPower/_Global/Files/Honda_Generator_Usage_Chart_En.pdf

[NewMoon]

Hi C-Brats!

I just joined the web site. My wife Cindy and I had a 1991 CD22 cruiser "Cindy Sea" until we sadly sold it in 1998. Cindy Sea was family! We're now cruising in a 26' diesel boat, but we did much of our learning in the C-Dory, starting at Lake Powell and working on up to BC and SE Alaska.

Anyway, the reason I'm posting is to offer anyone who wants it an Excel spreadsheet that helped us calculate current usage for our cruiser, and thus the battery capacity we would need, how long it would take to recharge, etc. It takes into account the amp usage of your devices, how many hours they'll be running, etc. Does a very good job of helping you plan - doing the arithmetic for you, and letting you fiddle with it as many times as you want easily until you get it to where you think it's about right.

Holler if you'd like a copy.

[marvin4239]

Ricard thanks for contributing your chart. I'm curious do you have 2 group 27 house batteries in your port Lazarrete? I have 2 group 24's in mine because I didn't think I could get two group 27's in there.

[NewMoon]

In our C-Dory we had two G24 batteries - all that would fit as I recall.

In our current boat we have a pair of G31 AGM's for house bank, and a pair of G34 AGM's for starter bank (for the big diesel).