country1nphx
Phoenix
Full Member
Joined: 08/27/2006
View Profile
 Offline
|
I hope this isn't a totally stupid question...  I'm trying to figure out how long my batteries will last. I have 2-12V DieHard Platinum, D31, RC 205, Amp Hours@20 is 100 and I want to run, for example, a 120ac heater that shows it pulls 6.3 amps and 900 watts. I have a 1500 watt inverter. Knowing all the above figures, a)will it work and b)how do I calculate how long the batteries will last? Thank you in advance. (Side note - the heater is just an example so no need to tell me the best way to warm my rig, I live in Phoenix and don't use heat  )
|
smkettner
Southern California
Senior Member
Joined: 03/21/2005
View Profile
Good Sam RV Club
|
That 6.3 amps will become 63 amps into the inverter off the 12v battery. Battery capacity decreases the faster you discharge so it is an estimate that you would get 45 to 90 minutes to dead. And you do not really want to run them down 100% so maybe 30 to 60 minutes is more realistic. That is with both batteries connected.
Anf then I am not sure how they get 6.3 amps and 900w. It would seem 900w would need 7.5 amps at 120v. That shaves another 10 minutes off run time.
Take your amp hours divided by the amp draw gives the hours the battery will last. Example: 200 amp/hrs divided by 5 amp draw gives 40 hours.
2001 F150 SuperCrew
2006 Keystone Springdale 249FWBHLS
|
mike4947
N. Syracuse, NY
Moderator
Joined: 08/26/2002
View Profile
Good Sam RV Club
Offline
|
An old axium used by boaters/sailers is to divide your RC by 2.4 to get the actual amps usable till the battery is 50% discharged. So (205/2.4) gives apx 85 amps usable until you are at the recommended 50% discharge level.
And like smkettner said any 120 volt inverter draw is 10 times the 12 volt battery draw
blog.rv.net Your daily guide to the Open Road
Subscribe to the daily digest
They say you learn by your mistakes, in that case I must be a genius.
|
RJCorazza
Maryland
Senior Member
Joined: 12/17/2005
View Profile
Good Sam RV Club
Offline
|
P(power or watts)= I(current) X E(voltage)
900 = I X 12
I = 900 / 12
I = 75 (amps)
100 (50% ah capacity) / 75 = 1.3 hours, although discharging batteries at greater than the 20 hour rate can't be calculated strightforward like this. I forget the law (something beginning with a "P") that deals with higher discharge rates (?). 30-60 min seems like a good estimate as smkettner posted.
|
DeweyClawson
Enon Valley, PA
Senior Member
Joined: 09/22/2007
View Profile
Offline
|
Agree with all of the above, BUT: its all theory. The real world answer is don't do it. That large a draw off your batteries is a sure way to destroy them. If you don't pull them all the way down no problem. It doesn't take but a few more minutes and they are completely discharged. These batteries are not designed to be deep discharged, especially at that rate.
The fool learns from his own mistakes.
It is the wise man who learns from the mistakes of others.
|
|
|
tvman44
Southwest Louisiana
Senior Member
Joined: 09/25/2007
View Profile
Offline
|
You would be lucky to get more than an hour.
Papa Bob
1* DW "Granny"
1* 2008 Brookside by Sunnybrook 32'
1* 2002 F250 Super Duty 7.3L PSD
Husky 16K hitch, Tekonsha P3,
Firestone Ride Rite Air Springs, Trailair Equa-Flex, Champion C46540
"A bad day camping is better than a good day at work!"
|
hwybnb
Southern California
Senior Member
Joined: 05/02/2001
View Profile
|
Read this. There are two parts. A link to Part 2 is at the end of Part 1.
As others have said, running large loads like heaters on battery power alone is not practical.
Large joads or small ones, You should avoid discharging any battery more than 50%, which means that you should divide your rated battery capacity by 2.
* This post was
edited 11/03/09 07:29am by hwybnb *
|
pianotuna
Regina, SK, Canada
Senior Member
Joined: 12/18/2004
View Profile
Good Sam RV Club
Offline
|
Hi country1nphx,
The inverters are never 100% efficient. A rule of thumb is to divide the watts by ten--which will give you the draw on the battery in amps.
Battery banks have some peculiar properties--the faster they are discharged the shorter the time they will produce that much energy. This is called the Peukert effect.
Follow this link and click on Peukert calculator to down load your very own copy of a spreadsheet to figure things out:
Peukert look at menu bar to download
country1nphx wrote:I hope this isn't a totally stupid question... I'm trying to figure out how long my batteries will last. I have 2-12V DieHard Platinum, D31, RC 205, Amp Hours@20 is 100 and I want to run, for example, a 120ac heater that shows it pulls 6.3 amps and 900 watts. I have a 1500 watt inverter. Knowing all the above figures, a)will it work and b)how do I calculate how long the batteries will last? Thank you in advance. (Side note - the heater is just an example so no need to tell me the best way to warm my rig, I live in Phoenix and don't use heat )
Regards, Don
Kustom Koach Class C 28'5" 256 watts solar, 875 amp hours in two battery banks 12 volt batteries 2500 watt inverter.
|
wa8yxm
Wherever I happen to park
Senior Member
Joined: 07/04/2006
View Profile
Offline
|
This is a rule of thumb for using a converter...
As you know Amps = Watts/Volts.. So in theory a 120 watt lamp will suck 1 amp at 120 volts or 10 amps at 12 volts... but Inverters are not 100% efficent and....
It's way easier to divide by 10 than 12
So, just use this simple formula Watts/10 = 12 volt amps
You will be close, (NOT exact but close) and... Don't forget the 50% discharge limit
Suggestion re-that heater... Don't use it on battery.. Use propane for heat
Nothin adds excitment like something that is none of your business
Kenwood TS-2000 housed in a 2005 Damon Intruder 377
|
country1nphx
Phoenix
Full Member
Joined: 08/27/2006
View Profile
Offline
|
THANK YOU ALL for the very detailed information!!!! I knew I could find the answer here! Now I know how to calculate out items that I might want to use. Thanks again
|
|
|
