Drawing more than 12 volts off auxiliary battery

[PatrickFisherPr]

I have a 1984 Toyota Sunrader. Under the hood is an auxiliary battery, which charges off the alternator. The auxiliary battery operates my water pump and the furnace.

The furnace requires approximately 13 volts to open the gas valve and spark the ignition. It appears I only get the proper voltage to start the furnace when I turn over the engine.

Any thoughts on how I can draw more power from the auxiliary engine without having the vehicle's engine on? The furnace kicks off if the RV reaches the temperature to which my thermostat is set. If the RV reaches that temperature and the engine isn't running then it doesn't have enough power to reignite the ignition if temperatures drop.

Any insight would be very appreciated.

[CavemanCharlie]

12thgenusa wrote:
Sounds like a bad aux battery.

My first guess too. Have the battery load tested.

[prichardson]

Were you measuring the voltage at the battery or the furnace? Check the voltage at the furnace both with and without it calling for heat. If there is a big difference, I suspect that you are getting a voltage drop due to poor wiring connections somewhere.

[DrewE]

naturist wrote:
As others have said, a fully charged battery is only going to read 12.7 or so, and that's only with an open circuit, i.e., nothing connected to it. Either your battery is bad and not able to deliver power in spite of reading fully charged (I had one do that once), or you have a bad/high resistance connection or broken wire somewhere.

Well said. I would generally tend to suspect the connection first, as even a not very good battery often (in my limited experience, at least) has enough of a surface charge to maintain voltage for a short while, but either one is entirely possible.

Do not neglect to verify that the ground connections are tight and sound, both at the battery and at the furnace (however it connects to ground). Also check that the wires are not corroded within the connectors, particularly at the battery connections; I've seen a few terminals that appear nicely crimped or clamped on the wire, but with the wire turning to dust at the other side within its insulation.

[naturist]

As others have said, a fully charged battery is only going to read 12.7 or so, and that's only with an open circuit, i.e., nothing connected to it. Either your battery is bad and not able to deliver power in spite of reading fully charged (I had one do that once), or you have a bad/high resistance connection or broken wire somewhere.

[Golden_HVAC]

PatrickFisherProject wrote:
The furnace is a Suburban NT-12SE.

I appreciate all the suggestions provided.

The furnace should be running just fine on 11.8 volts. I would be checking the voltage of the battery running it. Probably not charged, or it is worn out and not holding a charge.

Good luck,

Fred.

[PatrickFisherPr]

The furnace is a Suburban NT-12SE.

I appreciate all the suggestions provided.

[Chuck_thehammer]

bad wiring under load.. bad aux battery..

or control valve is not for 12 volt operation... older aircraft uses 24 volts with a requirement of operating at 14 volts minimum.

[12thgenusa]

Sounds like a bad aux battery.

[Golden_HVAC]

Have you measured the voltage at the battery? At the battery while under load?

Most furnaces will ignite at 11.6 volts, and that is a battery that is about 20% full. Your engine alternator is probably putting out about 14.5 volts and will allow the furnace to run while trying to charge the battery. Once the engine is off, typically a 'full' battery will have between 12.7 and 12.8 volts, while one just off the charger might have as much as 13.3 volts for a 1/2 hour, then will settle to 12.8 volts.

Your RV will use 1.2 amps per hour just to run the CO detector, propane monitor and refrigerator. This is enough to discharge a full battery in about 2 days, even with no lights running. Add a 6 amp per hour furnace, and the battery can go dead in a few hours.

Are you plugged into shore power? If you are, then the converter / charger should be supplying the power to keep the battery full. Check for a tripped GFI someplace that might be supplying power to the converter. If there is no power to the receptacle it is plugged into, try running another extension cord from a good power source to the charger. Even if you do not have a digital volt meter, you can look at your gauges and see if battery reads 1/3 or full. IF it reads empty, and you are on shore power, suspect a problem with your converter.

You have a 1984, probably is a converter, not a modern battery charger. You would be much better with a modern charger, but they are around $300 - $400. They can put out up to about 45 amps, and accurately charge the battery. However the prior owners might have already upgraded to a charger. You will need to look for it, and report it's model number if you have low battery voltage and problems.

Good luck,
Fred.

[Chris_Bryant]

On your era furnace, all of the power to the circuit board goes through the thermostat, limit switch, sail switch as well as the thermostat wiring. Each of these items can easily have a voltage drop acros them.
Easiest is to test, clean or replace the thermostat. You can test the thermostat by turning it on and measuring the voltage between in and out- you should get zero, but I would bet you will find a good half volt, at least.
FWIW, the furnace should run down to around 11 .5 volts, though 12 should be the limit.

[Old-Biscuit]

What furnace brand/model do you have?

Most RV furnaces have a DC operating 'range'

Transformer increases DC voltage to 10K for spark ignitor

[RoyB]

A fully charged battery only produces 12.6-7 VDC

Never hear of deal where you had something in a 12VDC vehicle setup that had to have 13VDC to work...

Only way you can get more DC voltage is using your alternator which will be producing 13.2 to 13.6 in normal mode and may jump up to 14.4-4VDC when in charge mode...

If you valve doesn't draw much DC current then maybe install another 6VDC battery in series with the 12V dc outputs and set it back to 13VDC with a resistor...

Maybe you need a newer toy haha...

Roy Ken