Sump pump battery backup with inverter

Thread Starter

GSDH

Joined Jan 29, 2023
5
Hello,
Here’s my sump pump setup…
Main sump pump 1hp AC powered.
Backup sump pump 1/2hp AC powered by a large 12v DC marine battery connected to a 2000W inverter (4000W surge) with a manual on/off switch. I have a NOCO 5A trickle charger/maintainer connected to the battery at all times, but I don’t like keeping the inverter on and drawing power when it doesn’t need to be.

I’m looking for a way to turn on the battery to inverter via a separate float switch whether AC power is on or off (indicating the primary pump has failed or been overwhelmed), so the backup pump can do its thing.

The backup pump draw around 8-9 amps while running, but more during start up - not exactly sure how much though.

Is there a battery switch that can be triggered by a stand alone float connected to the battery (pre switch)? I am not an electrical engineer and am having trouble finding existing products that manage this. Do you have any recommendations what I should look for or where?
 

Externet

Joined Nov 29, 2005
2,217
A relay will solve it.
While 120VAC mains is present, energizes a relay to disable the inverter or DC from working.
When 120VAC is absent, the relay turns off and a contact in it enables the inverter or battery with the separate float switch in series to turn it on.

If you already have the inverter and 0.5HP implemented, fine. Alternative is a 12VDC pump running from the battery, no inverter needed.
 

Tonyr1084

Joined Sep 24, 2015
7,899
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When the float switch activates - the inverter will turn on. When the float switch deactivates - the inverter will turn off.
The charger is always on maintaining the battery at 13.8 volts float charge. If power goes out the charger will drop out. IF the water in the sump rises to a preset level the inverter turns on and empties the sump.

Since the backup pump is only used when power fails it should not run unless the primary pump has failed due to power failure or motor failure (or other failure). The battery will be maintained at all times and the backup pump will only run when either the main pump has failed or is overwhelmed.
 

geekoftheweek

Joined Oct 6, 2013
1,214
The only problem with this is finding a float switch capable of handling the power requirements on an inverter running a pump. At 100% efficiency a 2000 W load will pull 160 A at 12.5 V.

If the inverter has a remote on / off switch connection that would be where to wire the switch. Otherwise you'll need a starter relay or something similar between the battery and inverter.
 

crutschow

Joined Mar 14, 2008
34,412
a large 12v DC marine battery connected to a 2000W inverter (4000W surge) with a manual on/off switch.
You could connect a relay across those switch contacts.
I would think that it's a low current switch which electronically controls the inverter, so a high current relay would not be required.
 

geekoftheweek

Joined Oct 6, 2013
1,214
Standard starter relays are intermittent duty.
You would need one rated for continuous duty.
True. I know we have some high power relays at work that look exactly the same as what I've changed a few times in some old Fords back in the day. That was the only way I could think to describe it at the moment.
 

Ludens

Joined Nov 12, 2014
21
If the inverter has a remote on / off switch connection that would be where to wire the switch. Otherwise you'll need a starter relay or something similar between the battery and inverter.
If it doesn't have that remote connection, it's probably best to open the inverter and connect the float switch directly in parallel with the inverter's power switch.

Indeed that's usually a low current switch.

But it has to be made sure that the inverter is fine with starting up under the heavy load of a stopped motor! Some are, and some are not.

And powering an inverter up and down by connecting/disconnecting the battery is very risky, particularly with a 12V inverter! The survival of the inverter depends on the correct implementation of a fast battery fault protection circuit. Because they typically take the MOSFET driver supply directly from the battery. If the battery is disconnected with the inverter running, the MOSFET drive voltage drops, the MOSFETs go into partial conduction, while still under heavy load, and... kaboom! A good inverter contains protection against this, but not nearly all inverters out there are good...
 

Thread Starter

GSDH

Joined Jan 29, 2023
5
This has been really helpful. I appreciate all the thoughtfulness in the responses and the diagram!

It sounds like wiring a stand-alone float (directly in parallel) to the inverter on/off switch is the safest route. And then setting the backup pump float to trigger at a higher level of water (than the standalone float), giving the inverter time to start up before demanding it do anything would be my plan. And it could then drain as much as it wants (until the standalone float drops to the off position or not).

So my next newbie questions:
Are there any specific float switches I should look for and any specific wire? Or anything else I would need to implement this?
 

bassbindevil

Joined Jan 23, 2014
828
As Externet suggested, it might be better to get a 12 volt marine bilge pump. Much less to go wrong. A float switch could be combined with a 40 amp automotive relay or something.
 

boostbuck

Joined Oct 5, 2017
510
My experience is that an inverter on idle (no AC draw) will draw < 100mA, and many have an adjustable standby mode that is switched by load demand. It's draw should be a fraction of the 5A charge supplied.

I would have the inverter permanently on and the float switch operate the pump.
 

Thread Starter

GSDH

Joined Jan 29, 2023
5
Thank you (again)!

I measured at the inverter using the 200mV setting on the tester and got 92 with nothing plugged in, and a max of 110 with the sump & float plugged in (not on).
The NOCO will of course keep up with that, I was only hoping to reduce the standby waste. But I think it does make sense to just leave it as is and call it part of the cost of the preparation and backup.
I do also have a DC bilge pump, but I’m leery on how reliable they are in general. I’ve had a couple fail when needed, so it’s currently next to the sump pump ready to join the party if called upon (maybe).
 

boostbuck

Joined Oct 5, 2017
510
....the 200mV setting.....
I assume you mean 200mA setting and your reading is approximately 0.1 Amp.

In that case the standby waste is about 1.2 watts, so is negligible in my opinion - less than 30Wh per day.

Personally I think it is sensible to leave standby equipment powered on to improve reliability.
 

Ludens

Joined Nov 12, 2014
21
I do also have a DC bilge pump, but I’m leery on how reliable they are in general. I’ve had a couple fail when needed, so it’s currently next to the sump pump ready to join the party if called upon (maybe).
Those 12V bilge pumps are to be considered consumable items, rather than an investment for the life of the boat. They can last quite some time, but eventually they do fail - usually when needed most dearly. A guy I know who owns a small sailing yacht always keeps one or two spares aboard.

On an extended sailing trip we did years ago, we stayed afloat only thanks to those pumps... But that's matter for another forum!

Anyway those pumps are quite practical, often having built-in level sensing switches. Apparently not even float switches, but capacitive water level sensors or whatever.

On a sailboat, power consumption can be quite critical. Often there is nothing but a battered solar panel generating electricity. Let a row of bad weather days come, and you are happy NOT to need an inverter running all the time!!!
 

Thread Starter

GSDH

Joined Jan 29, 2023
5
I assume you mean 200mA setting and your reading is approximately 0.1 Amp.

In that case the standby waste is about 1.2 watts, so is negligible in my opinion - less than 30Wh per day.

Personally I think it is sensible to leave standby equipment powered on to improve reliability.
Yes, I meant 200mA - thank you!2E98227C-04FA-49EA-8102-92CD4F67C42C.jpeg
And yes, the <30Wh/day seems reasonable for now. I think the standby power will cost me about 20¢/month.
In the event we lose power, and I’m not home, it gives time to remain flood free and hopefully get power restored. Or I break out the generator if I’m home.
Ultimately I’ve had better luck with AC sump pumps than DC, and the AC pumps move a lot more water, which I need if it’s a really rough rainstorm.
On a different note, I’d like to relay that everyone here has been really helpful and generous with their knowledge, which I’m really thankful for.
 

Thread Starter

GSDH

Joined Jan 29, 2023
5
Those 12V bilge pumps are to be considered consumable items, rather than an investment for the life of the boat. They can last quite some time, but eventually they do fail - usually when needed most dearly. A guy I know who owns a small sailing yacht always keeps one or two spares aboard.

On an extended sailing trip we did years ago, we stayed afloat only thanks to those pumps... But that's matter for another forum!

Anyway those pumps are quite practical, often having built-in level sensing switches. Apparently not even float switches, but capacitive water level sensors or whatever.

On a sailboat, power consumption can be quite critical. Often there is nothing but a battered solar panel generating electricity. Let a row of bad weather days come, and you are happy NOT to need an inverter running all the time!!!
Yes, I can well imagine there is a different sense of power conservation on a sailboat. The battered solar panel reference certainly helps paint that picture! It’s clear that having multiple DC pumps would be preferable to an inverter that could break and leave your pumps useless in that situation.
 

MisterBill2

Joined Jan 23, 2018
18,464
For that high current float switch, consider that at least some of the old 6 volt cars had a directly operated starter switch. So look up the design of one of them. or let an antique car buff show you one of them. Then you will have more insight as to what is needed and what can be done..
 

Tonyr1084

Joined Sep 24, 2015
7,899
For sure a high amperage switching device can be built that can be controlled with low voltage low current. One does not need a one-million amp switch.

For starters, one could use the float switch to control an air conditioner contactor. The contactor can switch higher amperage. However, the problem with that is arcing when the switch opens. Here are some YouTube video's of switch arcing when working with DC voltages:
 
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