Output of the 50DN Alternator in the bus.

[Fred Mc]

I am looking to install an ampmeter in the bus and need some info so as to decide what size shunt I need. How many amps does the stock alt. put out and is this the no. of amps that flows to the batteries. I have 2x 6 volt golf cart batteries in the house system that I want to keep track of.

Thanks

Fred Mc.
Gm Pd 4106

[Sean]

Depending on specific model number, the 12-volt 50DN comes in ratings up to 350 amps.  If your shunt is between the alternator and the batteries, you need one at least that large.  500-amp shunts are common and that is the size I would suggest.

To keep track of current in both directions, the loads should also be connected on the opposite side of the shunt from the batteries.  In this case, your shunt also needs to be sized for the largest possible draw.  A 3kW inverter can easily draw 500 amps when operating in surge mode, such as starting compressors in refrigerators, air conditioners, and the like.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[Fred Mc]

I don't have an inverter but does the draw from the starter also come into play here.

Thanks

Fred.

[bevans6]

The current draw from the starter will probably be shown on the meter connected to the shunt, if the shunt is between the start batteries and the starter motor.  If you put the shunt between the house batteries and the alternator, and if there is a separate path between the start batteries and the starter, that draw won't show up.  Regardless, the alternator current is not flowing when the starter is operating so you'll never see alternator current flow when starting the bus.

There are kind of three places that you could put the shunt and therefore measure current.  In my bus, the wiring goes from the alternator to a main terminal in the engine bay that is connected to both the batteries and the starter motor.  If you put the shunt between the alternator and that main terminal, you will only measure alternator output, and never measure battery output (which is what you would want to see if you were starting the engine).  The second place you could put it in my bus is between that main terminal and the start batteries.  My house batteries are tied into the starts, so that shunt position would let you measure all current that flowed from the alternator into either/both sets of batteries, and measure current from the batteries into the starter motor, but you would not be able to measure current flow out of the batteries to any other load.  If you put the shunt before just the house batteries you'll be able to measure current from the alternator into the house batteries, from the house batteries back towards the alternator or  starter, but not be able to measure current out from the house batteries to downstream loads.  I guess a fourth place to put a  shunt is downstream of the house batteries to measure load current into the house, but that wouldn't let you measure alternator output at all.

The actual output from the alternator varies constantly depending on the load it is asked to supply.  There are two broad types of loads in the bus = battery charging loads, and accessory loads.  When you start the bus, you draw somewhere around 500 amps for the period that you are cranking, and as soon as the bus starts and the alternator starts charging, it replenishes that current back into the battery at a fairly high rate.  That level of current rapidly drops off as the battery resumes full charge state.  Anything else in the bus that uses electricity is the accessory load.  If you have your headlights and running lights on, you're pulling maybe 25 amps of current for that.  If you run an inverter and power your rooftop AC like I do, I'm pulling around 85 amps (at 27 volts).  So in your scenario, if your house batteries are fairly low in charge you may pull 40 amps for a few hours, tapering off as the batteries reach full charge.  Unless you still have your OTR stock air conditioner and it's high horsepower fans, you'll almost certainly never reach full capacity from the DN50.



Brian

[Fred Mc]

Thanks Brian.

[Fred Mc]

Brian
My main concern are  the two 6v house batteries. I want to measure the voltage to give me an idea of the % of charge left in the batteries. I don't really care about the start batteries as they are disconnected when boondocking. I think installing a voltmeter is pretty straightforward to do this.
I also want to measure the amps going out of the house batteries so I can get an idea of what lights/appliances/ etc draw for power so I can manage the power usage.. I would also like to measure the amps flowing into the house batteries from the alternator although this isn't as important to me. For this I need an ammeter and a shunt thus my wondering where to put the shunt.

In my bus(GM PD4106---12 volts) I moved the start batteries to the engine compartment and I have an isolation switch so when boondocking I can isolate the start batteries. I then installed two house batteries(6V)in series  in the old battery compartment and hooked them up to the electrical system that the start batteries were connected to. I also have an isolation switch to isolate them as well. When they are connected I can use them as supplemental start batteries as well.

Thanks for you herlp.

Fred

[bevans6]

Here is a neat gadget I just found:  http://www.rc-electronics-usa.com/ammeters/rv-battery-monitor.html

It will, if it is as advertised, give you some insight into battery usage.  They make a good point that if you can monitor Kilowatt Hour usage, you will get a truer indication of what you are using.  "Things" use watts, they don't use amps. Even though they draw amps that will vary depending on the battery voltage at the time, what they always draw is constant watts for a given useage.

Anyway, Sean has done extensive work on this and it's in the archives although probably impossible to find.

Brian

[Sean]

... I want to measure the voltage to give me an idea of the % of charge left in the batteries. ... I think installing a voltmeter is pretty straightforward to do this.

A voltmeter will tell you almost nothing about the amount of charge left in the batteries.  Voltage is only a good measure of state of charge when it is a "resting" measurement, which means the batteries are completely disconnected and have been sitting disconnected for at least an hour.  Since you will presumably have the batteries connected to loads at all times, the voltage will vary considerably.

I also want to measure the amps going out of the house batteries so I can get an idea of what lights/appliances/ etc draw for power so I can manage the power usage.. I would also like to measure the amps flowing into the house batteries from the alternator although this isn't as important to me. For this I need an ammeter and a shunt thus my wondering where to put the shunt.

Instead of separate voltmeter and ammeter, I would suggest instead you get a state-of-charge meter like the Xantrex Link-10 or the Bogart Tri-Metric.  These will give you amps, volts, watts, and, more importantly, once properly programmed, will tell you how much of the battery is left.  The device Brian linked does the same thing, but it is only rated to 50 amps and thus is not suitable for your application.  The Xantrex and Bogart units come with a shunt, and different sized shunts are available (standard is normally 500 amps).

...  "Things" use watts, they don't use amps. Even though they draw amps that will vary depending on the battery voltage at the time, what they always draw is constant watts for a given useage.

Careful... this is not true in general.  It is only true for constant-power loads such as induction motors.  It is not true for resistive loads such as lights, heaters, universal motors, and the like.  Outside of air conditioners, compressor-type refrigerators, electronics with switching power supplies, and possibly electronically dimmed LEDs, most of the loads on your coach are resistive.

For a resistive load, when the battery voltage drops, so does the current drawn by the device, and thus the power consumed drops as well.  The formula is P=V²/R, where P is power in Watts, V is volts, and R is resistance in Ohms.  Since it is a square relationship, power increase or decreases accordingly.  IOTW, if you halve the voltage, then only 1/4 the power will be consumed.  This is immediately noticeable with lights; when you drop the voltage by a mere 15% (say from the 14 volts when the alternator or charger is running, down to the 12 volts that represents a mostly discharged battery), your lights will dim by a whopping 28%.

-Sean
http://OurOdyssey.BlogSpot.com