High 120V current when running slides in/out

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BillB3857

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Jun 8, 2012
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St. Louis Area
Our Keystone Sprinter has a strange problem.  A couple of years ago, I added an AC Ammeter and voltmeter so we could monitor usage/supply.  Whenever we set up, we usually level, connect power, water, etc, and then slide the two slides out.  Ours are driven by a common 12V hydraulic pump.  We've noticed that when NOT connected to shore power, everything seems fine.  Slides go in/out well. Likewise, when connected to shore power, but I can see the AC current climb to about 12A when the slides are moving. Also, there is the unmistakable scent of something hot coming from the converter.  My first thought was that the battery couldn't carry the load, but the slides work well without shore power, so I figured that wasn't the problem.  Has anybody else noticed similar symptoms?  Thanks for any suggestions.
 
When you're connected to shore power, the converter supplies the load current first.  Then if the load needs more current than the converter can supply, the remainder is drawn from the batteries.

This happens because the output voltage of the converter is higher than the battery's resting voltage.  You won't draw any current from the batteries until the converter voltage drops below the battery voltage, and on a modern converter this doesn't happen until the converter is running flat out.

12 amps at 120 volts equals 120 amps at 12 volts.  That sounds high for a slide motor, so you may have a partially shorted motor or something mechanically binding the slide and making the motor work harder than it should.
 
There's nothing unusual about any of that.  Converter power factors are less than 1.0 as are converter efficiencies.  Hydraulic power packs draw enormous amounts of current, more than slide motors -- 100a under load would not be at all unusual.
 
I can see my 120vac current go way up when we operate slides or jacks too. Runs around 11-12 amps, just like yours.  As the others explained, the converter/charger is the primary source of power when shore power (or generator) is available.

I don't have a credible explanation of why the AC amps jump so high. My slide motors (not hydraulic) pull about 25-30 amps, according to the DC meter, but the AC current jumps to 12 amps while doing so. That doesn't really compute, since the inverter/charger is normally much better than 20-25% efficient. Normal charging suggests the efficiency is more like 85-90%. Maybe the efficiency is non-linear and drops severely as the DC amp rate goes up?

DC amps = AC amps x 10  x  Efficiency%
 
Thanks for the replies.  I really hadn't thought about the converter supplying power first, then the battery adding any needed support.  Had thought about it just in reverse.  Also, thanks for the confirmation that 10 to 12 amps on the AC is about normal when running slides.  Great site with great information!
 
I would measure the DC amperage to find out what's going on. So if the DC load is actually that high.

Because I was trying to figure it out too. If I'm looking at it right.

10A x 120VAC = 1,200W

1,200W / 12VDC = 100A Draw.
 
The converter output is determined by the battery voltage.  If the voltage is 13.6 volts very little output.  Most converters are multistage.  As the voltage drops the output rate will increase.  I suspect when you run the slides the battery voltage drops below the converters boost setting.  The converter now thinks it has a discharged battery connected goes into boost mode.  So only part of the 100 dc amps is going to the slide motor the rest is going into the battery.
 
That is certainly true, lavarock, but the mystery is where the 100+ amps are going if the battery  is only dispensing 25-30 amps.  The charger is going to be in a constant voltage mode, which allows the charging amperage to vary according to the batteries ability to accept it. 100 amps in, but only 30 out  - seems like it ought to burst!
 
Gary RV Roamer said:
That is certainly true, lavarock, but the mystery is where the 100+ amps are going if the battery  is only dispensing 25-30 amps.  The charger is going to be in a constant voltage mode, which allows the charging amperage to vary according to the batteries ability to accept it. 100 amps in, but only 30 out  - seems like it ought to burst!
30 amps to the slide and 70 amps to a battery bank with a 1200 amp cca rating.  I think the battery will be OK.  When a converter is in boost mode the output voltage is 14.4 volts.  But the 100 amp may be a little high as it was 12 amps AC which will probably only calculate to 80 amps due to the efficiency of the converter.  This would give us 30 amps to the slide and 50 amps to the battery.  We may have a bad battery which could be the reason the converter is going into boost mode.
To the OP.  You may have a weak or bad battery.  I would check the water levels first.
 
Another question needs to be asked here. What is the main fuse rated at? (30 amps, 50 amps, or what?) Because if there is a 30 Amp fuse and its not blowing then what is the converter doing?  ???
 
Don't forget, 30 amps at 120 volts is equal to 300 amps at 12 volts.  So it's highly unlikely the converter could produce enough DC current to trip the incoming AC breaker.
 
Lou Schneider said:
Don't forget, 30 amps at 120 volts is equal to 300 amps at 12 volts.  So it's highly unlikely the converter could produce enough DC current to trip the incoming AC breaker.

Actually I was referring to VDC fuse not VAC breaker.
 
30 amps to the slide and 70 amps to a battery bank with a 1200 amp cca rating. 

The dilemma is, assuming the batteries were charged to begin with, they won't be able to accept a 70A charge regardless of the charge voltage. Nor will they accept that much if low on water or have corroded connections. A shorted cell would probably allow it, though. The charger could be pumping high amperage right on through the battery if one or more cells are shorted internally.

It is also typical in a trailer like this to have a 50A or 60A fuse in the charging circuit and a charging system rated at only 40-50 amps. We don't know the details of this one - perhaps the OP upgraded the charger when the ammeter was added.
 
The OP needs to give us some more information.  Rating of the converter, what was the ac current draw (what else was using power before the slide operation) such as (water heater, refrigerator etc) before he operated the slides, how old and size of battery bank and condition (water level) , was the battery charged before he operated the slide, what is the ac voltage when operating the slide, dc voltage (at battery terminals) both when using only battery and then with ac when operating the slide.
The fact he said he had an unmistakable scent of something hot coming from the converter indicates to me the converter was at full capacity.
The dilemma is we do not have enough information to answer the question.  We can only make assumptions that would fit the data we have.  A discharged or defective battery would come close.
 
Sorry for the delay in responding.  Kids and Grandkids in town from out of state, so........  Original converter was boiling battery dry, so was replaced with a PD4655 about a 1-1/2 year ago.  Converter has two 30A fuses in parallel feeding the DC bus.  Battery was replaced at the same time.  Wal-Mart special with 675 CCA and 105 AH @ 1A rate.  I just checked battery water level and it is OK.  After disconnecting AC power, I used my Harbor Freight 100 amp load tester and by following the directions, it says battery is OK (voltage stays in green after test)  I deliberately did repeat tests to force the charger into full charge mode.  It works and voltage is right at 14.4 volts and the battery load tester shows charging system is good.  As for the AC loads, other than the possibility of the refrigerator, the converter is the only thing pulling power.  AC Ammeter only showed a load while slides were moving.  It starts out at a low current and as the slides progress, the current goes up. It is almost as though the battery starts out OK and then looses capacity.      I also placed a DC ammeter in the negative lead from the converter. With AC power applied, running slides results in 56A.  Of course, with AC off, couldn't read current from that point, but did measure battery voltage while slides were moving.  Voltage stayed about 12.5 volts.
 
Does all this mean that it is better to run the slides out prior to plugging into shore power? I have always plugged in first thinking I was being good to the system.
 
Everything sounds normal, Bill.  When the batteries are fully charged they provide part of the slide's operating current, then as they lose their surface charge the converter takes over.  That's why your converter only draws a little AC current to start, then picks up more as the slide motor continues drawing current.

Warsw, there's nothing wrong with plugging in before running the slides in or out.  And it insures you don't run out of battery capacity halfway through the slide's motion.

Most of this discussion has been academic, prompted by Bill's noticing the AC current rises while the slides are in motion.

 
Lou Schneider said:
Warsw, there's nothing wrong with plugging in before running the slides in or out.  And it insures you don't run out of battery capacity halfway through the slide's motion.

Most of this discussion has been academic, prompted by Bill's noticing the AC current rises while the slides are in motion.
Thanks.
 
Thanks to all for the responses.  Now I can sleep better knowing what "normal" is.  It's amazing how noticing something for the first time can create so much doubt.
 
It's amazing how noticing something for the first time can create so much doubt.

That's in itself is a rather common thing among RVers and in internet forums in general. Most any new observation elicits huge amounts of commentary coupled with FUD (Fear, Uncertainty and Doubt).  :eek:

You may find this message both relevant and humorous.
 

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