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Is There A Way To Adjust The Low-Voltage Setting In Power Inverters?

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rs14smith

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Basically I have an inverter (**broken link removed**) I hooked up to a 24v battery and I also have a motor controller hooked up to that battery too.

When the motors hooked up to the motor controller are activated, the battery voltage moves up and down 21v-24v very fast/slow depending on the speed and torque of the motors. I've seen this before nothing new...

Anyway now, this inverter has a Low-Voltage Cut Off voltage at 20v-21v. So when the battery voltage does hit 21v for just a split second, the inverter cuts off it's output power which I wish it wouldn't as the battery isn't really low...there's just a large load on it currently...

Is there a way I can modify the inverters low voltage setting internally, or does anyone know if they make 12v inverters that can have an input range from 12v-26v(battery fully charged)?
 
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You can do it but you have to have knowledge of the sense op amp circuit in the inverter.

Your real problem is your battery is not sufficient to run the load or your wiring is too small a gauge.

300 watts is not that large of an inverter. At 600 watts surge overload the 24 vdc battery current will be about 40 amps. A 300 watt load will draw about 16 amps. You should be using #6 wire or larger for battery lines assuming they are less then 6 feet long. Use clamp or bolted lugs not the alligator clips supplied.

Check you battery voltage at the battery terminal when load applied. Two 105 AH 12v batteries in series should be sufficient to run this inverter.
 
You can do it but you have to have knowledge of the sense op amp circuit in the inverter.

Your real problem is your battery is not sufficient to run the load or your wiring is too small a gauge.

300 watts is not that large of an inverter. At 600 watts surge overload the 24 vdc battery current will be about 40 amps. A 300 watt load will draw about 16 amps. You should be using #6 wire or larger for battery lines assuming they are less then 6 feet long. Use clamp or bolted lugs not the alligator clips supplied.

Check you battery voltage at the battery terminal when load applied. Two 105 AH 12v batteries in series should be sufficient to run this inverter.

If the battery was not sufficient, wouldn't the 10A rated motors stop running completely or run slower? If so, that's not what is happening for me.

The motors are DC which are hooked directly to the battery, and the inverter just has small devices (that use less than 50W, about 55W total) hooked to it.

So do you still think it's the wires?
 
If the battery was not sufficient, wouldn't the 10A rated motors stop running completely or run slower? If so, that's not what is happening for me.

The motors are DC which are hooked directly to the battery, and the inverter just has small devices (that use less than 50W, about 55W total) hooked to it.

So do you still think it's the wires?

If your motor is DC and the inverter is just sitting in parallel to battery with light load that says even more so that the battery is collapsing under the motor load. Measure the terminal voltage on the battery under motor load. If it drops more then half a voltage from no load voltage then the battery is either bad or too small for the application.
 
Well I was just hanging out with my friend Google for a little today, and I stumbled upon a device called a 24v Voltage Regulator. From my short time reading up on this little FET looking guy, from my understanding, it allows you to keep the voltage at a constant level.

So could we fix this issue by installing a 24v voltage regulator?
 
Measure the terminal voltage on the battery under motor load. If it drops more then half a voltage from no load voltage then the battery is either bad or too small for the application.

What do you mean by "no load voltage" as you also state to measure the terminal voltage on the battery "under motor load"...

I'm just not sure how to properly carryout this test. Are you saying to just let the motors run freely without any force being applied to them and then motor the voltage on the battery?

Thanks
 
So do you still think it's the wires?

Wires or battery - how long and thick are the wires?.

I once had to go out to a caravan, the customer had bought a 12V colour TV and it wouldn't work (on his brand new £20,000 odd caravan) - I measured voltage drop from the battery to the TV (using a fog light bulb as a load), it dropped 3V on the negative wire, and 4 volts on the positive. The wiring was pathetic, thin 5A cable (like a TV mains lead), and the wire to the battery even had a surplus 20 feet or so coiled up.
 
I believe this is what RC is getting at:

A 24 volt lead acid battery (or two 12 volt lead acid batteries in series) should show an open circuit (unloaded) voltage of 25.2 to 25.6 volts. That is between 2.10 and 2.13 volts per cell. This is normal for a fully charged battery.

When discharged that open circuit voltage will drop to between 23.6 and 24 volts. That tells you the battery is discharged.

Under a load when the voltage drops to 21 volts the battery is considered discharged and this is why the 24 volt inverter is designed to drop offline when the voltage gets that low. It is by design and should not be changed.

The symptoms you have seem to point to your battery being either bad or unable to meet the demands of your circuits. Simply put, it won't handle the load(s).

Another common problem that has been mentioned and Nigel covers is the wire running from the battery to the inverter (or loads in general). Wire has resistance and the smaller the wire gauge diameter the higher the resistance. Since the wire carries current to and from the load the current running through the wire produces a subsequent voltage drop across the wire lengths. Therefore the inverter in your case will see not the voltage at the battery terminals but the battery voltage less the voltage drop of the wires supplying it. If the inverter draws considerable current and the wire gauge is thin or light then the voltage drop can be substantial.

Discounting inverter inefficiencies generally speaking we can say a 300 watt inverter under full load will draw about 12.5 amps, a 600 watt unit will draw about 25 amps of 24 volt power. The wire gauge used to supply the inverter needs to be able to easily handle those currents and kept as short as possible.

Does all of this make sense to you?

Ron
 
I believe this is what RC is getting at:

A 24 volt lead acid battery (or two 12 volt lead acid batteries in series) should show an open circuit (unloaded) voltage of 25.2 to 25.6 volts. That is between 2.10 and 2.13 volts per cell. This is normal for a fully charged battery.

When discharged that open circuit voltage will drop to between 23.6 and 24 volts. That tells you the battery is discharged.

Under a load when the voltage drops to 21 volts the battery is considered discharged and this is why the 24 volt inverter is designed to drop offline when the voltage gets that low. It is by design and should not be changed.

The symptoms you have seem to point to your battery being either bad or unable to meet the demands of your circuits. Simply put, it won't handle the load(s).

Another common problem that has been mentioned and Nigel covers is the wire running from the battery to the inverter (or loads in general). Wire has resistance and the smaller the wire gauge diameter the higher the resistance. Since the wire carries current to and from the load the current running through the wire produces a subsequent voltage drop across the wire lengths. Therefore the inverter in your case will see not the voltage at the battery terminals but the battery voltage less the voltage drop of the wires supplying it. If the inverter draws considerable current and the wire gauge is thin or light then the voltage drop can be substantial.

Discounting inverter inefficiencies generally speaking we can say a 300 watt inverter under full load will draw about 12.5 amps, a 600 watt unit will draw about 25 amps of 24 volt power. The wire gauge used to supply the inverter needs to be able to easily handle those currents and kept as short as possible.

Does all of this make sense to you?

Ron

Yep Ron, makes sense. Let me ask you this then, let's say we take the inverter out of the picture all together, and now all we have hooked up to the battery is the motors. Since the motors still run fine even if they are collapsing the battery depending on the load at the time, what harm effect will this have on the battery if any? I'm not sure if I stated it before, but after deactivating the motors, the voltage returns to it's normal level, and I've seen this happen with many personal/school projects in the past but thought it was normal.
 
Yep Ron, makes sense. Let me ask you this then, let's say we take the inverter out of the picture all together, and now all we have hooked up to the battery is the motors. Since the motors still run fine even if they are collapsing the battery depending on the load at the time, what harm effect will this have on the battery if any? I'm not sure if I stated it before, but after deactivating the motors, the voltage returns to it's normal level, and I've seen this happen with many personal/school projects in the past but thought it was normal.

Are you measuring the battery voltage directly on the battery?, or on the wires running to the motors - you need to measure directly on the battery, otherwise you're just measuring the drop down the wires as well.

Also, what condition is the battery in?, an old failing battery will exhibit a MUCH greater voltage drop under load.
 
Are you measuring the battery voltage directly on the battery?, or on the wires running to the motors - you need to measure directly on the battery, otherwise you're just measuring the drop down the wires as well.

Also, what condition is the battery in?, an old failing battery will exhibit a MUCH greater voltage drop under load.

All the components are connected to a busbar in parallel, even the permanent placed volt meter, so technically, no, it's not connected "directly" on the terminals of the battery, but logically, yes it is connected if that makes sense? Would it make any difference if I were touching the "actual terminals" even though the busbar is connected "directly" to the terminals?
 
Yep Ron, makes sense. Let me ask you this then, let's say we take the inverter out of the picture all together, and now all we have hooked up to the battery is the motors. Since the motors still run fine even if they are collapsing the battery depending on the load at the time, what harm effect will this have on the battery if any? I'm not sure if I stated it before, but after deactivating the motors, the voltage returns to it's normal level, and I've seen this happen with many personal/school projects in the past but thought it was normal.

OK, keep in mind what Nigel posted.

A battery, any battery is designed and built to deliver a specific amount of power over a given period of time. While delivering a current within its ratings its voltage should not drop below a certain level. While some drop is normal and to be expected a voltage drop exceeding set parameters is not acceptable. Under load for us to know what the battery is actually doing it becomes essential we measure the battery voltage at the battery terminals. Any point beyond those terminals the voltage we measure will be inclusive of the line drop. That is the point Nigel is trying to drive home.

Now I don't know your battery or batteries so we will use a rough example.

I have a couple of Werker 7.5 AH 12 volt batteries sitting here. They are part number WKA12-7.5F2. When I place these two batteries in series I basically have a 24 volt 7.5 amp hour battery.

I want to run my pump and my pump has a 24 volt DC motor and the name plate says 24 VDC 5 amps. OK, in theory my battery should run my pump. So I connect the pump and start it up. My pump should in theory run at least 1.5 hours. In real life it won't but right now matters not. I am measuring my battery voltage right at the battery terminals.

Before starting my pump I was reading 25.4 volts on my fully charged battery which is normal (see my last post) as between 25.2 and 25.6 is expected for my fully charged 24 volt SLA (Sealed Lead Acid) battery. Immediately after starting my pump my battery voltage drops to about 24.8 volts which is good and can be expected since my battery is under a load. Life is good and my pump is running. The Kingdom is happy as they all have fresh water to shower and wash dishes.

While my pump is running I watch my battery voltage slowly drop. Finally my battery voltage has dropped to 21 volts, my pump is still running but I shut it off. I shut it off because at 21 volts for all purposes my battery is dead and if I continue to run my pump I will destroy my battery. This may piss off the people of the Kingdom but screw them as my battery is important to me and I don't want to destroy it. Besides if they complain I'll raise their taxes!

Another possibility is when I start my pump I observe my battery voltage drops to below 21 volts. In this case my batteries have reached the end of their useful life and need replaced. I drag them to the Kingdom Recycle Center and use the tax increase to buy new batteries.

So some voltage drop is normal and to be expected. The trick is knowing when there is too much voltage drop. That tells us the batteries need replaced or they simply cannot supply the needed power because they are not rated for the load. In the latter case I need a larger battery. The voltage must be measured at the battery terminals. This precludes any I*R drop being measured if I measure voltage at the load. Measure at the source. :)

Ron
 
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