Dual Battery accessory question

[joshh]

Hey all,

First off I will say that I am not the smartest person when it comes to electrical systems and wiring, I will have some friends helping me with that part of this. However I am looking for some opinions on the setup I am planning to run. I have a 1992 delica that I would like to convert into a part time camper van for some adventures in the near future. currently it has a single battery setup. I want add a second battery, with an isolator, and a 1000W pure sine inverter.

My main concern is for cold winter nights I want to run a small oil filled heater or convection heater (relatively low watts <500) on a thermostat to conserve power. Will I be able to power this sort of an accessory through a full night running off of an optima yellowtop battery? (my plan was dual yellowtops, they have been good to me so far!). I do not know how to calculate how much of the battery a 500W heater would drain over the course of several hours, and that is why I am asking the help of the forums electrical whiz's

I was thinking of possibly mounting a baseboard oil heater just behind the ridge behind the front seats.

Thanks for the help/input!

[rezdiver]

with those numbers you will be drawing about 42 amps with a 500 watt load.
I believe the standard yellowtop is a 55Ah battery.
this will give you approximately 1 hour and 20 minutes continuous runtime if you are not using power anywhere else. and this is best case senario. also as the battery voltage drops you amperage draw will increase.


the recharge time is around 1 hour with a 65amp alternator from a full discharge of 10.5 volts.

http://jci_media.s3.amazonaws.com/4713/4583/5068/YELLOWTOP_Full_Specs_Sheet.pdf

[tonydca]

IIRC, Jay and Christine of TardisDeli opt for electric blankets instead; they sandwich inside sleeping bags and run them at full blast while they are in transit to pre-heat up the bed while they can still draw from the alternator.

[joshh]

that would be an excellent idea.

[tonydca]

I was in a similar situation having installed a covered hammock outside on our balcony.

Rather than use a 1500W radiant heater to heat the great outdoors, I just bought a Sunbeam 50" x 60" heated throw from London Drugs for $70.

On High, it draws 120W (10 Amps @ 12 Volts DC thru an inverter) and gets extremely warm after only about 1/2 hour. Running it like this in your sleeping bag while in transit would get it most toasty.

On Low, it only draws about 40W (3.3 Amps) and it automatically shuts itself off after 3 hours. So if you went to sleep with it on Low, it would only draw a little under 20% of the capacity a 55Ah yellow top at full charge.

Plus you could get away with buying a much smaller (read: cheaper) inverter.

... also as the battery voltage drops you amperage draw will increase....

Actually, if the device is a simple resistive heater, its resistance will not change so as the supply (battery) voltage begins to drop, the amount of current it draws will actually drop (Voltage = Current * Resistance) so it will just put out less and less heat (Power = Voltage * Current).

But by this point, your battery will be getting so low on juice that you'd be worried about being able to start your engine, so you would never want to get to this stage anyways.

*****************

Edit: Also, the slower you draw power from a battery, the better the internal chemistry has a chance to keep up, effectively increasing your capacity.

For heater examples with my Odyssey PC1500 (see chart at the bottom of the page: http://www.odysseybatteries.com/battery ... series.htm ) -

I could draw 556W with a space heater for only 1 hour until the battery voltage drops to 10V (nearing full discharge I suspect) giving an effective capacity of 47.3 Ah at this load.

But I could draw 41W on an electric blanket for 20 hours, giving an effective capacity of 65.0 Ah - a 37% improvement.