Low inverter volts

[magaliasram]

Here is my problem when I start my 04 Fleetwood Discovery my inverter voltage at my fridge drops to 108 volts and the heater on the fridge will not operate. Turn the engine off and the outlet comes back to 120 volts. Any suggestions?

[twinboat]

Is your inverter indicating high or low voltage alarms ?

What brand inverter ?

[Soppy]

I think it's time to buy a new inverter. The inverter should maintain 120 volts from the battery voltage range of 10.5 to 15.5 volts. Could also be a problem with the alternator on the coach . Measure the voltage on the battery with the coach off and running. With the coach running measure the AC voltage on the battery. It should be near zero. If there is AC voltage on the battery , could be an alternator problem. Could also be bad batteries . Check and clean all the connections. Bad connections can cause wied problems.

[Kiawah]

Not quite enough diagnostic information to know exactly from the symptoms you indicate, however here's my thoughts.

First, there are two battery banks (house and chassis), which are normally separate. However when the engine runs, then there is typically a charge solenoid or BIRD device (don't know which your mfg put in), which ties the two battery banks together. This is so the engine alternator which normally charges the chassis battery, can also recharge the house batteries.

Secondly, when you start a diesel and it's cold, there is typically an intake heater which draws a LOT of current for a period of time. This is so the engine will run when it's cold out. This current is drawn from the chassis batteries.

What I speculate is happening, is when you go to start the cold engine, the intake manifold heaters come on drawing a lot of power from the chassis batteries, then you crank the starter which draws even more power from the chassis battery, then the engine starts to idle and the little alternator tries to put current back to the discharged chassis batteries (which by this time has lower voltage). Then the charge solenoid engages, and the chassis batteries draw down the house batteries, which then effects the inverter and refrig.

So before you go replacing anything, check the condition of both your chassis and house battery banks, and put your voltmeter on the battery banks and monitor their voltages while you go thru the start process. This will verify what is happening.

You may need to replace one or both sets of batteries.

[will stevens]

I have noticed a modified sine wave inverter will not give a true reading on a volt meter. Do not know what inverter you have, but the voltage reading could be false.

[twinboat]

Originally Posted by Kiawah

Not quite enough diagnostic information to know exactly from the symptoms you indicate, however here's my thoughts.

First, there are two battery banks (house and chassis), which are normally separate. However when the engine runs, then there is typically a charge solenoid or BIRD device (don't know which your mfg put in), which ties the two battery banks together. This is so the engine alternator which normally charges the chassis battery, can also recharge the house batteries.

Secondly, when you start a diesel and it's cold, there is typically an intake heater which draws a LOT of current for a period of time. This is so the engine will run when it's cold out. This current is drawn from the chassis batteries.

What I speculate is happening, is when you go to start the cold engine, the intake manifold heaters come on drawing a lot of power from the chassis batteries, then you crank the starter which draws even more power from the chassis battery, then the engine starts to idle and the little alternator tries to put current back to the discharged chassis batteries (which by this time has lower voltage). Then the charge solenoid engages, and the chassis batteries draw down the house batteries, which then effects the inverter and refrig.

So before you go replacing anything, check the condition of both your chassis and house battery banks, and put your voltmeter on the battery banks and monitor their voltages while you go thru the start process. This will verify what is happening.

You may need to replace one or both sets of batteries.

Most modern Class A charge ( Isolation ) relays will not close until the chassis batteries are charging above 13 volts. If they close and the volts drop below 12.6 they will open again.

The second point is that inverters output a steady 120 volts, until the input voltage falls below or goes above the cutout settings.

[Sbrownstein]

Originally Posted by will stevens

I have noticed a modified sine wave inverter will not give a true reading on a volt meter. Do not know what inverter you have, but the voltage reading could be false.

Voltmeters are not calibrated for MSW waveforms, they are designed for sinusoidal waveforms. That aside, the change in voltage might concern me. However, when the engine is running, chances are the battery voltage is close to 14.3 or so. The inverter may be changing its MSW profile with the higher input voltage. In other words, this might be a normal occurrence...if it is a MSW inverter.

[MSHappyCampers]

Hi ! Welcome to IRV2! We're sure glad you joined the gang!

Can't help with the problem but wanted to say hello! Keep her between the ditches!

Good luck, happy trails, and God bless!

Merry Christmas!

[96 Wideglide]

Originally Posted by will stevens

I have noticed a modified sine wave inverter will not give a true reading on a volt meter. Do not know what inverter you have, but the voltage reading could be false.

Makes sense. I've seen lower voltage readings on my plug-in volt meter while on inverter power, even with full charged batteries.

[Gary RVRoamer]

I'm struggling with this because 108vac is plenty for the fridge to operate. I don't think it's an inverter issue, even though the inverter should be delivering 120v or very close to it. However, the inverter cannot do that is the DC voltage is low. It needs 12.0-12.6v to do that.

The heater on the fridge is a pure resistance device and works at any voltage. The heat output will be a bit less at 108 vs 120, but not a huge amount. The fridge should work fine. Therefore I think there is some problem in the fridge and the voltage concern is a red herring.

[twinboat]

True that 108 volts should run the fridge, but if the inverter is MSW and the meter doesn't accurately measure MSW voltage, the reading is useless.

[Outbound]

I have always heard 10% of desired voltage is acceptable.

[Gary RVRoamer]

I have always heard 10% of desired voltage is acceptable.

Not a bad rule of thumb, but what voltage is "desired"? That is determined by the engineers who designed the appliance or device. Further, some types of devices have inherent differences. Without getting into the messy electronic details, resistance devices like light bulbs and heaters, are very flexible on voltage and wave form, but inductive devices like motors are not. And solid state electronics can be made very fussy or very accommodating to either voltage or wave form/timing.

[Gary RVRoamer]

True that 108 volts should run the fridge, but if the inverter is MSW and the meter doesn't accurately measure MSW voltage, the reading is useless.

True enough, but most any digital meter will be an RMS type and read accurately enough. An older, inexpensive analog meter, maybe not.