Voltage sag during DC motor start?

What might be the voltage sag and duration during start-up with a 12Vdc water pump like this powered from a ~85Ah 12V deep-cycle flood-cell lead-acid battery? I have a travel trailer (RV) with a system like this where the **broken link removed**turns off momentarily whenever the water pump starts.

I connected the stereo to a lab power supply, and it will shut off as the dc input voltage is reduced below ~10V. It re-boots as the voltage comes back up beyond 10.2V. It draws somewhere between 0.5A and 0.75A depending on program loudness.

I'm thinking that the pump start-up transient momentarily pulls the battery/bus voltage below 10V. I am thinking of putting a Schottky diode into a large electrolytic storage capacitor (say 10,000 uF @15V) across the input of the stereo. The diode would make it so the capacitor discharges only into the stereo (not the pump), holding up the input voltage long enough for the pump to come up to speed and stop drawing its start-up current.

Questions: how long does it take the pump to come up to speed so that its own back emf reduces its current draw? Based on that duration, I can figure how big a capacitor I need...

How long it will take depends on the pumping load. That's something you will have to measure.
https://shurflo.com/images/files/pdf/Product-Data-Sheets/RV/Fresh-Water-Pumps/pds-4048-153-X75.pdf

Thanks for the data sheet. I suspect that the in-rush current is at least 3X the running current stated on the data sheet.

I will have to get the hose out, fill the fresh water tank, and run the pump long enough develop full system pressure, and then crack a faucet. The stereo doesn't reboot when the pump is running dry. I will also have to get my Laptop and USB Data Aquisition dongle to record the infrequent event. I was hoping to avoid all that...

MikeMl said:

Thanks for the data sheet. I suspect that the in-rush current is at least 3X the running current stated on the data sheet.

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I would imagine probably considerably more than 3X - I would also suspect the problem could be down to crappy wiring in the RV, in my experience they use long lengths of VERY thin cable, no where near good enough for the purpose.

We sold a portable CRTColour TV (complete with internal 12V inverter) to a customer, who had just bought a VERY, VERY expensive caravan - he rang up saying it wouldn't work, so I was sent out to find out why.

I used a car headlamp bulb to simulate the TV load, and measured the voltage drop across the caravan wiring - it was dropping more than 4V just feeding a headlamp bulb

So you might try that, stick a headlamp bulb in place of the pump, and see what voltage you get.

What you do is put a 1.2Ahr battery just next to the two wires coming from the main battery, near as possible to the pump. This will fix your problem. It fixed mine.
It costs $12.00 to fix the problem.

Do you have the pump directly connected to the battery, or does it have some common wiring with the stereo?

Nigel Goodwin said:

... I would also suspect the problem could be down to crappy wiring in the RV, in my experience they use long lengths of VERY thin cable, no where near good enough for the purpose.

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The wire between the battery positive pole and the fuse panel is #8awg (3.3mm); about 3m long, but goes through a self-resetting 40A thermal breaker (close to the battery). The negative lead is a combination of about 1m #8awg and the steel frame of the RV. The pump positive is fed from a 20A blade fuse in the panel. The pump ground is returned to the steel frame outside of the fuse panel. The pump wiring is not common to the stereo wiring; that is separate downsteam of the fuse panel.

The path between the battery and the fuse panel (and the esr of the battery) would be where the common-mode voltage drop is developed. I doubt I can improve the common wiring much...

Colin said:

What you do is put a 1.2Ahr battery just next to the two wires coming from the main battery, near as possible to the pump. This will fix your problem. It fixed mine.
It costs $12.00 to fix the problem.

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So you are "filtering" the 50Apeak pump by providing local energy storage. I was thinking of filtering the 0.6A stereo.
So what do you do when parking the RV (during which the main battery is disconnected)? Surely, the 1.2Ah SLA completely discharges into the un-switched parasitic loads in the RV (CO and LP gas detector, for example). You must be replacing the SLA every trip you go out...

MikeMl said:

Thanks for the data sheet. I suspect that the in-rush current is at least 3X the running current stated on the data sheet.

The stereo doesn't reboot when the pump is running dry.

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Did you try blocking the output line when starting, a impeller style pump is off loaded when blocked until up to run.
Max.

MikeMl said:

The wire between the battery positive pole and the fuse panel is #8awg (3.3mm); about 3m long, but goes through a self-resetting 40A thermal breaker (close to the battery). The negative lead is a combination of about 1m #8awg and the steel frame of the RV. The pump positive is fed from a 20A blade fuse in the panel. The pump ground is returned to the steel frame outside of the fuse panel. The pump wiring is not common to the stereo wiring; that is separate downsteam of the fuse panel.

The path between the battery and the fuse panel (and the esr of the battery) would be where the common-mode voltage drop is developed. I doubt I can improve the common wiring much...

Click to expand...

Do the test I suggested with a headlamp bulb, and measure the drop from the actual battery terminals to the actual pins of the bulb - check both earth and positive, to see where you're losing most. Switches, fuses (and particularly thermal breakers) are places you make losses, as well as thin cables.

Inrush peaks are normally for Dc motors based on DCR coil resistance and usually 5x to 8x unless ICL limited.

I read 10A rated current thus I expect 50-80A pk
This is about 10% of a good battery CCA. (800A)

CCA is define by current @7.5V (when cold) drop from 12.5V full charge. or a 5V drop

But you don't want it to drop below 10.5V or 2Vdrop or 40% of the CCA drop spec.

This means the 80A/40% = required CCA rating =200A which seems low.

Adding a cap to a large lead acid battery just under a million Farads can be improved somewhat not by huge C but my Low ESR caps.

Using diode isolators won't help if the stereo uses a bridge diode.

Check State of Charge which increases ESR greatly when it drops.

All it takes is one bad cell with low s.g.

A diode pair (isolator) with a spare battery for electronic stuff is often the easiest fix.

Nigel Goodwin said:

Do the test I suggested with a headlamp bulb, and measure the drop from the actual battery terminals to the actual pins of the bulb - check both earth and positive, to see where you're losing most. Switches, fuses (and particularly thermal breakers) are places you make losses, as well as thin cables.

Click to expand...

I used the pump as the test load after adding water to the fresh water tank so I didn't have to run the pump dry.

Most of the 12V loads (water pump, lights, radio, gas frig, gas water heater, USB charging ports, 12V outlets) are all returned to a ground bus bar (ground mecca) that sits behind the fuse panel. Measuring between the mecca and the 12V input to the fuse panel, I noticed that this voltage dropped (0.1 to 0.2V) more than I expected even as low amperage loads (like LED lighting) were turned on. When I turned on the tap to start the pump, the voltage dropped from 12.7Vdc to ~11.9V.

As suggested by Nigel, I ran a test wire into the trailer from the negative pole of the battery (externally mounted on the trailer tongue) and measured between it and the ground mecca while running the pump. Viola, the difference accounted for most of the 0.8V drop . Next I measured (pump running) between battery - and the trailer frame; less than 5mV. Then I measured between trailer frame and the internal ground mecca; seeing the 0.8V again...

Carefully tracing the connections between the trailer frame and the internal ground mecca, I found the problem. The trailer builder had not connected the #8awg wire that runs through the floor of the trailer to the ground mecca. It was connected at the frame end, but the internal end was just bundled with some other wires, and had never been stripped and clamped under one of the screws in the ground mecca...

After connecting it, the voltage drop when the pump starts is now less than 0.1V. I substituted an old, partially discharged automotive starting battery for the new battery that came with the trailer, and now the radio does not reset even with the system voltage at ~11.5V.

This was a maker's oversight; a quality control issue at the trailer maker's factory. Its manifestation was quite insidious, because there must have been a secondary ground somewhere, likely through an appliance, which has small gauge wires leading to it, causing the large voltage drop... but yet most of the systems worked well enough that we had spend a few nights in the trailer before I found and fixed the underlying issue.

MikeMl said:

This was a maker's oversight; a quality control issue at the trailer maker's factory. Its manifestation was quite insidious, because there must have been a secondary ground somewhere, likely through an appliance, which has small gauge wires leading to it, causing the large voltage drop... but yet most of the systems worked well enough that we had spend a few nights in the trailer before I found and fixed the underlying issue.

Click to expand...

As I said (and you did), measure the voltage drop directly (and separately) from both battery terminals to the load - makes it easy to find the exact problem.

yes the predicted Low Battery CCA implied high ESR in the wiring. Good job.