Ready to make a UPS, just need some clarification

[pt95148]

I have read several threads here about making a UPS with the Prius 12V battery and I am ready to do it with just a few things to clarify for myself.

I can see one set up where marine quality connectors/switches/fuses were used but as I go through the regular car accessories shops, I can see automotive switches/fuses with ratings of 30A/40A and so on which means they are capable of handling the load. The only difference is that they are 12/14 awg wire size.

The inverter I bought has a couple of 6awg wires with terminals ready for direct connections with the battery, but like the designs I have seen here on this website, I want to build in a layer of fuses/switches and connectors.

That's where my question is. Can I use these components and wire size instead? What is the effect on heat/losses and so on? I think there is a difference and reason but not sure what is, can someone enlighten me please?

Reason for preference with automotive components - lower cost and availability.

TIA

[qbee42]

Marine components are designed to be more corrosion resistant. Since your setup will be inside of a car and not exposed to salt spray, that shouldn't be a problem for you.

Wire size is an issue because of voltage drop. All normal wire has electrical resistance, and this resistance causes part of the electrical power to be converted to heat. The problem with 12V systems is that you are not starting with much voltage in the first place, and for inverters, the current draw can be very high. The higher the current draw and the larger the resistance, the greater the voltage drop on the wire. This is expressed by Ohm's Law: V=I*R. The bigger the wire the lower the resistance.

You could calculate the voltage drop and determine the required wire gauge, but you probably don't need to do that. If you keep the length of the wire leads the same or shorter than those supplied by the manufacturer, you can use the same gauge wire. If you go longer, you need to use a heavier gauge. Bigger is better.

If keep all of this in mind, automotive grade products should be fine.

Tom

[problemchild]

I used marine connectors so I could remove the device for service. I dont want the dealer denying my warranty because of a ups device. Trust me they can void your warranty and YOU DONT want to be on the wrong end of that battle. Also the marine fuse go to 600 amp. I used 80 amp square fuses. You can order the parts online for a low price.

Originally Posted by pt95148

I have read several threads here about making a UPS with the Prius 12V battery and I am ready to do it with just a few things to clarify for myself.

I can see one set up where marine quality connectors/switches/fuses were used but as I go through the regular car accessories shops, I can see automotive switches/fuses with ratings of 30A/40A and so on which means they are capable of handling the load. The only difference is that they are 12/14 awg wire size.

The inverter I bought has a couple of 6awg wires with terminals ready for direct connections with the battery, but like the designs I have seen here on this website, I want to build in a layer of fuses/switches and connectors.

That's where my question is. Can I use these components and wire size instead? What is the effect on heat/losses and so on? I think there is a difference and reason but not sure what is, can someone enlighten me please?

Reason for preference with automotive components - lower cost and availability.

TIA

[pt95148]

Originally Posted by problemchild

I used marine connectors so I could remove the device for service. I dont want the dealer denying my warranty because of a ups device. Trust me they can void your warranty and YOU DONT want to be on the wrong end of that battle. Also the marine fuse go to 600 amp. I used 80 amp square fuses. You can order the parts online for a low price.

I agree with you. Was that your design I saw with the yellow Anderson switches and so on?

[pt95148]

Originally Posted by qbee42

Marine components are designed to be more corrosion resistant. Since your setup will be inside of a car and not exposed to salt spray, that shouldn't be a problem for you.

Wire size is an issue because of voltage drop. All normal wire has electrical resistance, and this resistance causes part of the electrical power to be converted to heat. The problem with 12V systems is that you are not starting with much voltage in the first place, and for inverters, the current draw can be very high. The higher the current draw and the larger the resistance, the greater the voltage drop on the wire. This is expressed by Ohm's Law: V=I*R. The bigger the wire the lower the resistance.

You could calculate the voltage drop and determine the required wire gauge, but you probably don't need to do that. If you keep the length of the wire leads the same or shorter than those supplied by the manufacturer, you can use the same gauge wire. If you go longer, you need to use a heavier gauge. Bigger is better.

If keep all of this in mind, automotive grade products should be fine.

Tom

Thank you. I understand the reason for wire size now.
One question - Not sure of the wire size from the standard battery but assuming it is either 4 or 6 awg, is it ok to mix wire sizes at this level?

[qbee42]

Originally Posted by pt95148

Thank you. I understand the reason for wire size now.
One question - Not sure of the wire size from the standard battery but assuming it is either 4 or 6 awg, is it ok to mix wire sizes at this level?

Yes, you can mix wire sizes. The size of the wire gets bigger as the awg number gets smaller, so 4 awg is larger than 6 awg. For your situation, it is always okay to use a larger wire (smaller awg number), but using a smaller wire will increase the voltage drop. How much voltage drop is acceptable depends on the device, the current, and the voltage of the battery. Try to keep the 12V wires short and use as large a wire as you can conveniently use. When you have an option, keep the 12V short and put the extension on the 110VAC side. Voltage drop isn't much of a problem on the AC side.

Tom

[ChapmanF]

Originally Posted by pt95148

Reason for preference with automotive components - lower cost and availability.

You might also look at industrial components, which you might find at your local big electrical supply house. I thought of them belatedly for my installation, mostly because I had the stuff lying around and hadn't found anything I liked much better at the auto supply places. One very nice thing is that the screw-clamp style industrial terminal blocks make very reliable connections with none of the hassle of putting crimp terminals on your wire ends. So I wound up crimping ring terminals only on the wire ends attaching to the battery, relay module studs, and inverter.

The positive-battery-terminal assembly includes a 120 A fuse (in my gen 1 anyway) that bolts in. The load-side fuse bolt works great as a place to attach a ring terminal for the + connection, so your wiring is protected by the 120A fuse without you needing to add any new fuse at that point. A little panel on the side of the terminal assembly pops open for access to the bolt.

Please post pics of your set-up!

-Chap

[Norm611]

Originally Posted by qbee42

Marine components are designed to be more corrosion resistant. Since your setup will be inside of a car and not exposed to salt spray, that shouldn't be a problem for you.

Wire size is an issue because of voltage drop. All normal wire has electrical resistance, and this resistance causes part of the electrical power to be converted to heat. The problem with 12V systems is that you are not starting with much voltage in the first place, and for inverters, the current draw can be very high. The higher the current draw and the larger the resistance, the greater the voltage drop on the wire. This is expressed by Ohm's Law: V=I*R. The bigger the wire the lower the resistance.

You could calculate the voltage drop and determine the required wire gauge, but you probably don't need to do that. If you keep the length of the wire leads the same or shorter than those supplied by the manufacturer, you can use the same gauge wire. If you go longer, you need to use a heavier gauge. Bigger is better.

If keep all of this in mind, automotive grade products should be fine.

Tom

Voltage drop due to wire resistance is an issue primarily for long runs. For the distances we are looking at here, overheating of the wires is a greater risk. For normal home wiring, 14 Gauge wire is rated for 15 Amps, 12 ga wire is rated for 20A. I believe there are insulations available which permit operation at higher temperatures, and thus slightly higher current (maybe 20 & 25A respectively).

Looking at Problemchild's pictures in another thread, it looks as though the leads from the battery to the inverter (12V) are probably about 6 feet long. 6 feet of 6 ga wire has a resistance of about 0.002 Ohms (0.004 Ohms for both leads). If the inverter INPUT is 1000 Watts, it will be drawing 83A, causing a voltage drop of ~ 0.33V. With the battery being charged at 13.6V, the inverter input would be 13.27V.

If the inverter input leads were replaced with 12 ga wires, the voltage drop would be ~ 1.6V. The inverter input would be 12V, until the insulation on the input wires melted or caught fire.

When determining wire size, two separate checks must be made. 1) the total resistance should be kept low enough to avoid an excessive voltage drop (remember a 50 foot extension cord has 100 feet of wire), and 2) the maximum current must not exceed the rated current for the wire (to prevent fire or physical damage).

Norm

[qbee42]

Originally Posted by Norm611

Voltage drop due to wire resistance is an issue primarily for long runs. For the distances we are looking at here, overheating of the wires is a greater risk. For normal home wiring, 14 Gauge wire is rated for 15 Amps, 12 ga wire is rated for 20A. I believe there are insulations available which permit operation at higher temperatures, and thus slightly higher current (maybe 20 & 25A respectively).
<snip>

Norm, in this sort of situation voltage drop becomes a deal breaker long before overheating can come into play. You can't have overheating without voltage drop, and a nominal 12V system starts with such a low voltage that voltage losses quickly reach an unacceptable level. Otherwise your comments are correct, and safety would be the main issue.

Tom

[pt95148]

Thanks everybody for the generous help here.
I have one more question that I forgot to ask, grounding.

The manual on the inverter says to ground it to the chassis, I am assuming here it is the car chassis, is this sufficient?

I think I saw in one design, maybe Problemchild, where he made an external ground plug into to house circuit.