Ready to make a UPS, just need some clarification

[qbee42]

Originally Posted by andyprius

Hello tochatihu: So the only time a larger fuse would be needed is with a much larger 12VDC battery? If the Prius is rated at (about) 30AHr then what is the criteria for determining the 80Ampere fuse? Andy.

A larger fuse is needed to draw more power. A 1250W inverter will draw over 100A when it is fully loaded. The exact amount depends on efficiency and input voltage, but we don't need to be exact for this discussion.

Note that I said fully loaded. Just idling, that same 1250W inverter might draw only a couple of amps. The amount of current goes up more or less proportionally with the load.

When you use a smaller fuse, the inverter will work fine up to the point that the fuse blows. In other words, by using a smaller fuse, you effectively have a smaller inverter. Instead of 1250W, your inverter might be good only to 800W (just as an example).

As one of the earlier poster said, there is a safe limit to how much current you can draw from the Prius 12V system. The amount varies depending on whether you are taking it from the Prius 12V bus, or just drawing it from the little 12V battery. I wouldn't want to run an inverter from the Prius 12V battery alone, as it is a very small battery, and it is not designed for deep cycles. The smaller fuse size is suggested to protect the Prius 12V system and battery from large current loads.

Tom

[andyprius]

Hello Tom. OK that all makes sense, especially the last sentence. I was thinking about having a inverter at hand and the car wired up for the adaptation But now I wonder about the total logic of it and would such a emergency (likely) occur? Thanks for the reply. Andy.

[pt95148]

Originally Posted by andyprius

Forget all the other hyperbole, Tom's post #28 answers your question. If your connecting to the house, the green wire (safety ground) must always be there. Yes it is usually the round, third post. If your using the output of the inverter for camping then the safety ground (green wire) has no effect. For the 12VDC Aux battery, chassis, ground, negative are all one and the same. As you have no doubt observed, the Neg lead on the 12VDC battery connects to the chassis. The really important point in all this IS: Watch out for wires getting hot, if they are, disconnect immediately and figure out why.

OK Thanks for the clarification.
One more question - if the negative wire of the Prius battery is already grounded to the chassis and the negative wire of the inverter is connected to the same terminal, is there a need to ground the inverter again or will the same connection suffice?

[bwilson4web]

I'm going to offer one suggestion: single ground point.

It doesn't matter if the ground is vehicle-to-ground or ground-lead-to-ground but having both the vehicle and ground-lead tied to SEPARATE grounds is a bad thing!

The problem is multiple grounds can sometimes lead to a ground loop. They are pernicious and though seldom fatal, hell to pay for the grief they can cause.

IMHO, the best answer is to ground the vehicle to earth and the problem is likely to be solved.

Bob Wilson

[tochatihu]

It has previously been suggested that a good way to get max power (including some big current peaks) from the Prius 12 v system is by adding a larger 12 v battery near the outboard inverter. Using short, heavy gauge cables. I don't remember the exact size now, but the Prius' cabling from the front is about 4 gauge, and it's several meters long. Thus it would appear to make sense that the demand peaks of the outboard inverter would largely be met 'more locally' from the added battery and its big pipe. I would still like to see conservative current protection/limiting for the Prius main fuse, but the other positive connections could be 'wide open'. Then a 1250 w (or even larger) outboard inverter could deliver power peaks without bring the whole system down.

[andyprius]

Originally Posted by pt95148

OK Thanks for the clarification.
One more question - if the negative wire of the Prius battery is already grounded to the chassis and the negative wire of the inverter is connected to the same terminal, is there a need to ground the inverter again or will the same connection suffice?

Correct me if I'm wrong, the input to the inverter is 12VDC, one plus and one minus with the minus at Vehicle chassis ground. The output(s) are 3 or 4 ordinary 3 prong AC outlets, just like your house right??? Should you make a nice heavy cable, the type an RV has, with a male on each end , wiring up the plug-ins, with the safety ground, then......you could plug it straight into the house , at a good secure female outlet. The green safety is automatically protecting you, the house and the chassis of the inverter. In fact the green safety is actually attached to the inverter chassis. Should you actually be using the inverter in this manner, Make sure you pull the main circuit breaker on your house FIRST! And if you run an extension from the inverter, make sure it's heavy duty and as short as possible. If you do not have a multimeter, by all means buy one and learn to use it, it's an education. Never run inverter current and house current together within the house circuit. There are many reasons for this, but I will not go into it. One more point whenever we speak of chassis ground on the vehicle (any vehicle) any metal lic point thruout the car is electrically one and the same and they are the same as the negative lead at the battery, HOWEVER there are preferred points of connection as some random chassis point could be floating or of a slightly higher resistance thereby lowering current flow. Paint, rust etc could affect the ground connection.