Inverter HIGH voltage shutdown

[brulaz]

This is a new one to me.

Simple plug-in 400W inverter my wife uses in the car to charge/run her computer. Has worked fine over the last 9 months. But just recently during the cold spell, it complained and refused to work.

I assumed it was a low voltage issue, what with the age of the car's battery and the low temperatures, and started looking for a new battery. But then decided to RTFM and, whaddya-know, the error indication was for "High Voltage Shutdown". Hunh?

Turns out the inverter doesn't like anything much over 15V and the chevy cruze's alternator can put out 15.5V when at 90% duty cycle (http://www.cruzetalk.com/forum/17-off-topic-discussion/1339-chevy-cruze-regulates-voltage-boost-fuel...).

Since it was very cold, we were running the heater fan and occasionally the electric defoggers so a 90% duty cycle on the alternator makes sense, although I haven't measured the V myself.

[brulaz]

GordonThree wrote:
Does the vehicle have an electric assist on the heater core or something? Seems odd the alternator would need that kind of voltage.

Not that I know of. It's a 2011 Chevy Cruze Eco.
The info in the link in my first post is all I know about it.

[DrewE]

GordonThree wrote:
Does the vehicle have an electric assist on the heater core or something? Seems odd the alternator would need that kind of voltage.

I suspect it's just temperature compensation for charging the battery in cold conditions. The charge controller/voltage regulator in many cars these days is surprisingly sophisticated. An electric heater or similar wouldn't in itself demand a higher voltage--just lots of current (and hence power).

[GordonThree]

Does the vehicle have an electric assist on the heater core or something? Seems odd the alternator would need that kind of voltage.

[brulaz]

theoldwizard1 wrote:

brulaz wrote:

Turns out the inverter doesn't like anything much over 15V and the chevy cruze's alternator can put out 15.5V when at 90% duty cycle

Get out there and measure the voltage yourself !

I have never heard of an auto alternator (with a properly operating voltage regulator) putting out over 15V.

Well, ScottG reported just that a few posts earlier. And if you look at the link in my first post, you will see :

From link in my first post wrote:
the system will set targeted generator output voltage to a charging voltage between 13.9–15.5?V, depending on the battery state of charge and estimated battery temperature.

But yes, I've been meaning to get out there with a voltmeter and today the temperatures rose to -7C (though with snow), so I shovelled out the car and driveway and drove into town.

Most of the time the V stayed between 14.8V and 15.15V and the inverter didn't complain. But for some reason, after stopping at a coffee shop, the V rose to 15.25 and the inverter shutdown.

So it's real. Perhaps the other day, when temperatures were -27C, the Vs were higher as my wife couldn't ever get the inverter to turn on.

[MEXICOWANDERER]

Yep the 2 rectifiers are put in SERIES. Back to back.

A 16 gauge wire with push on terminals does the bridge.

If you're fussy about exposed connections inside the cab spootch some black silicone onto the fast on terminals once the rectifier has been assembled and tested.

Remember, the base is insulated -- no power so using an extended length wire you can jam it under a seat out of the way.

[brulaz]

DrewE wrote:

...
With a bridge rectifier wired up in the usual (obvious) way, with the "DC Output" corners going to the inverter and the "AC Input" side to the power supply, at any given time two of the diodes are forward biased and two are reverse biased.
...
8A implies a bit under 15W of power dissipation. A heat sink of some sort would seem to me to be a good idea.

Thanks. I see it now, but it took a while. One of the two diodes in series is on the neg battery side and the other on the positive.

And that link I posted above shows a rather large extruded Al heat sink built into the rectifier.

DrewE wrote:
Parallel connections do not come into play here as they would for resistors; diodes are by design not ohmic. Two forward biased diodes in parallel still have the same 0.7V drop, but each one carries (ideally) half the current. It's the same idea as putting two batteries in parallel; the voltage stays the same.

Thanks again. Didn't realize that.

[theoldwizard1]

brulaz wrote:

Turns out the inverter doesn't like anything much over 15V and the chevy cruze's alternator can put out 15.5V when at 90% duty cycle

Get out there and measure the voltage yourself !

I have never heard of an auto alternator (with a properly operating voltage regulator) putting out over 15V.

[DrewE]

brulaz wrote:
So a silicon diode gives a constant 0.7V drop *independent of current*. Somehow that little factoid never registered before.
So with no load and 2 diodes in series, the inverter would see 1.3V less and would not shut down. Nifty.

Think I need two bridge rectifiers to get 1.3V drop as each one would act like a single diode? Although each rectifier has 4 diodes, there would be 2 series strings in parallel = 1 diode?

With a bridge rectifier wired up in the usual (obvious) way, with the "DC Output" corners going to the inverter and the "AC Input" side to the power supply, at any given time two of the diodes are forward biased and two are reverse biased. The forward biased ones each have a ca. 0.7V drop across them, whatever the forward voltage of the particular diode is, and are in series. The two reverse biased ones have basically the full supply voltage across them but of course are not conducting since they're reverse biased and can basically be ignored.

Parallel connections do not come into play here as they would for resistors; diodes are by design not ohmic. Two forward biased diodes in parallel still have the same 0.7V drop, but each one carries (ideally) half the current. It's the same idea as putting two batteries in parallel; the voltage stays the same.

8A implies a bit under 15W of power dissipation. A heat sink of some sort would seem to me to be a good idea.

[brulaz]

So a silicon diode gives a constant 0.7V drop *independent of current*. Somehow that little factoid never registered before.
So with no load and 2 diodes in series, the inverter would see 1.3V less and would not shut down. Nifty.

Think I need two bridge rectifiers to get 1.3V drop as each one would act like a single diode? Although each rectifier has 4 diodes, there would be 2 series strings in parallel = 1 diode?

According to the "manual", the inverter should operate down to 11V before shutdown, or 12.3V at the alternator/battery with this device. That shouldn't be an issue as the alternator goes into "charge mode" at 13.9-15.5V as soon as it detects a battery V < 12.56.

I may try this with a smaller rectifier from another project first (if I can find it). The inverter's inline fuse is 8A for the cig. lighter plug&wire. (Inverter only goes up to 400W with the alligator clips and heavier wire)

And yes this is an el cheapo Canadian Tire (similar to Harbor Freight up here in the Frozen North) inverter. Have tested the inverter with a 13.8V power supply, and it does work fine as long as the input V is within range.

EDIT: just remembered that the other project diodes are Schottky because I wanted as little V drop as possible. So will prolly place an order for some of these (only 75A but much cheaper in Canada, slow boat from China though): ebay.ca

[MEXICOWANDERER]

Most of the Harbor Freak inverters do the same thing and yes they track. 15.4 volts input would yield close to 140 volts output.

My gizmo would work fine. I would not trust the rectifier heat build up at 400 watts inverter power but that isn´t part of the gig.

[DrewE]

Mex's idea is a pretty good one. A single silicon diode (or rectifier, same thing) has about a 0.7V drop when conducting, and that's mostly independent of the current flowing through it. With appropriate heat sinking for the power it would be dissipating, it should bring the voltage down sufficiently. Using the bridge rectifier gives you two diodes operating at a time, hence the 1.3 volts.

As an additional "advantage", you could hook up the inverter with it to the battery backwards and not have any problems.

I suspect the inverter may have an over voltage shutdown because the output voltage isn't really regulated, and so too big a swing of the input voltage would lead to too big a swing of the output voltage. I could also be all wrong about that.

[Kpackpackkelley]

Have you tried it in another vehicle?

[MEXICOWANDERER]

In and out then over to the second rectifier and in and out again.

• Push on terminals easy to assemble
• Finned body is insulated no short risk
• Automatically reduces voltage by at least 1.3 volts
• Fastest and easiest way to fix your issue
• Buy the critter and I will help you with the wiring diagram, a piece of cake

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[GordonThree]

If the inverter were built to SAE electrical specs, 15v wouldn't be a problem.