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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Parallel Inverters?
Message-ID: <>
Date: Thu, 13 Mar 2003 01:18:37 -0500

On Sat, 08 Mar 2003 18:27:01 GMT, "Lobo-Cordero" <> wrote:

>Costco is selling  1000 watt Xantrex inverters for
>$70 or so, (forgot the exact price). Would it be feasible to parallel two of
>these units to simulate a 2000 watt unit (like the Freedom series, which
>costs $$$)?  The battery charger function would probably be lost with this

sure.  did it with the $29 400 watt vectors Sam's club sells.  Easy to do if
you understand circuits and how inverters work.  What I did on the Vectors was
designate one the master and the rest slaves.  On the slaves, find the 60 hz
drives to the FETs.  Cut the traces and bring the FET drive leads out.  On the
master find the same traces and solder wires to them.  Connect the master wire
to the corresponding slave wires.  Parallel the outputs of the inverters.
That's all there is.

What this does is designate one unit to generate the drive to all inverters'
output stages.  This is necessary to ensure they all stay in phase.

On the Vector, the circuit layout is logical so it was fairly easy to spot the
FET drivers.  Took maybe a half hour of noodling around with a scope to find
the right place and then probably no more than 10 minutes per inverter to make
the mods.

Can't see why such a paralleling scheme would have any effect on the DC
charging circuit.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Parallel Inverters?
Message-ID: <>
Date: Thu, 13 Mar 2003 14:10:40 -0500

On Thu, 13 Mar 2003 11:27:18 -0700, Alan Balmer <> wrote:

>On Thu, 13 Mar 2003 01:18:37 -0500, Neon John

>>What this does is designate one unit to generate the drive to all inverters'
>>output stages.  This is necessary to ensure they all stay in phase.
>Sounds like a good solution. I suppose that the FETs take next to
>nothing gate current, so the source impedance is not an issue.

Gate drive is actually a very important issue with power fets.  They draw no
static current but the gate has a lot of capacitance which means lots of
dynamic current draw.  If there isn't enough gate drive the FET will spend too
much time in its active region and generate heat.  OTOH, with sufficient drive
I can switch 30 amps of 12 volt current at 30 khz using a plastic pack FET
that doesn't even have a heatsink tab.

There is almost always a dedicated gate driver such as the MC34151 or discrete
equiv in commercially designed switching circuits.  The key is to find this
circuit and tap in on the logic side.  There the drive requirements are much
more relaxed.

>How's the waveform on those 400 watt Vectors?

Just fine.  It's a simple square wave switch some dead time at zero crossing
to give the proper peak/RMS ratio.  Price has very little to do with the
output waveform of sub $1000 pseudo-sine wave inverters.  They all work on the
same principle - step up the 12 volts to 145 vdc or so using high frequency
DC-DC switching, then switch this voltage to the output at the 60 hz rate.

The major difference is whether the neutral is ground referenced or not.  In
the cheap ones like the Vector, the neutral is at line/2 DC potential above
earth ground.  This allows the use of a monopolar high voltage supply.  the
more expensive ones have bipolar supplies with the neutral either at ground
potential or connected to it.

The first type works for most loads.  the major problem arises when the input
of the load contains heavy noise bypassing.  this bypassing can conduct enough
current between neutral and ground to trip the inverter's GF protection.  That
is the problem with this cheap inverter and Dell laptop power supplies not
working - at least some of them don't.  The 2 wire supplies work fine.  The 3
wire ones don't.

This little 70 watt cig lighter Vector inverter that I wrote about earlier
runs my Dell just fine while the 400 watt one won't.  The difference is there
is no connection to the ground pin inside the 70 watter while there is in the
400 watt one.  Simply snipping the green earth ground wire inside the 400 watt
inverter neatly solves that problem.

From the inverters I've looked at on the bench, the source of noise in things
like TVs is typically the INPUT.  Cheap ones either don't have input filter
caps or like the Vector 1kw unit, use aluminum electrolytics with high ESR.
All that high frequency crap gets conducted back into the 12 volt line where
it is in turn re-radiated to the surrounding area.  I put a couple of high
quality low ESR caps and a couple of ferrite torroids on the internal 12 volt
lines of my 400 watt units and removed all traces of buzz lines on my TV.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel,alt.rv
Subject: Re: Progressive Dynamics "Inteli-Chargers" are DUMB
Date: Wed, 31 Mar 2004 00:33:38 -0500
Message-ID: <>

On 30 Mar 2004 15:33:23 -0800, (dave martin) wrote:

>Progressive Dynamics tech support line says it is ok to parallel
>9100s.  They even have a wiring diagram on-line.  Nothing special,
>just put'm in parallel using TWO Charge Wizards. Like you, I'd
>presupposed that sharing a Charge Wizard would be required.
>It is clear from my data that the control points for the chargers
>differ. That apparently causes no problems for the PDs.

Interesting.  I'll anxiously await your findings.  That would greatly simplify
things for me and you both.

>You are the electronics wizard here; tell me how to sync two
>inverters. It should be easy. Then a BIG system could be assembled
>pretty cheaply.

It's actually not all that easy.  I've done it.  The hard part is figuring out
the sync points on the PCB without schematics.  Basically, the outputs of the
DC/DC converters have to be paralleled, the HV voltage sense circuits have to
be paralleled, the 60 hz drivers phase-locked and the outputs paralleled.

The DC/DC converter paralleling is easy - just tap on at the HV filter caps.
Finding the sense circuit requires probing around the PCB and tracing circuit
paths.  Phase locking the 60 hz drivers consists of designating one oscillator
as master, disabling the oscillators on the others and slaving the others to
the master.  Paralleling the outputs is easy.

On the ones I did, I didn't have to mess with the shutdown circuits, as when
one unit shut down it tripped the others.  That might not be the case with
other designs.

I have 3 identical 1kw inverters that I'm going to look at when I get a round
tuit.  I'd like to have them paralleled in my rig so I could run my roof AC
from the engine electrical system when underway rather than running the aux

>Maybe if I started over I'd directly control my alternators, but I've
>zero experience in that area. Perhaps you can tell me how practical it
>is to use something like a Charge Wizard to control an off-the-shelf

Controlling an alternator is very easy.  Most are low side regulated these
days so all that is necessary is a transistor to switch the field to ground
and a few bits to bistable across the setpoint.  You might google around a
bit.  I've seen several schematics online that look like they'd work. I think
the Charge Wizard, if it works the way I think it does, could control one very

>> If I can figure out where to put two more batteries on my rig I'm thinking
>> very seriously about going to this two bank architecture.  I'd like to have 2
>> sets of 2 Group 29 batteries in parallel.  I know that I can go 2 days at a
>> time on two Group 29s while being my usual electrically wastrel self :-)  In
>> that setup I could charge for about an hour a day, alternating banks so that
>> the charging voltage does not reach the house loads.
>I'm not convinced about the need for the switch. It allows some
>redundancy at the cost of doubling the total time my engine's got to
>run to get a particular number of amp-hrs.

The reason for using two banks is to separate the house load from the charging
circuitry.  Once the house voltage constraints are removed, the charging
compliance voltage can be allowed to rise, resulting in a longer bulk charging
phase and thus an overall faster charge cycle.  For example, if the charge
voltage can be allowed to rise to 15 volts, temperature permitting, the bulk
cycle can be extended, resulting in a much shorter overall charge interval.

You might have noticed that several of the Ample Power regulators has a "light
saving" mode that limits the voltage to about 13.8 volts.  This is designed to
still permit some charging when the house batteries have to be used at the
same time they're being charged.

>> I have about decided that for my style of dry camping, mounting a second large
>> alternator on the main engine, along with one of the aforementioned
>> intelligent charge controllers will provide the most satisfactory charge
>> regime.  Though idling the engine for an hour a day might use a little more
>> gas, it will be much quieter and cleaner than running the main generator.
>> Plus the battery bank will receive the same quick charge while driving.
>About parallel devices; I'm operating my systems far from the control
>voltages and all each device knows about is the voltage at its output
>terminal. So if one alternator's control voltage is 13.8V and the
>other's is 13.9V it makes no difference if they share a common output
>terminal that's at 12.5 volts; they'll both function as if the other
>doesn't exist.
>The same logic holds for chargers whose outputs are tied together. I
>don't care if the last 10% of the capacity is supplied by only one of
>the chargers. By the time that happens, I'll have recovered the charge
>I need.

That's only because the wiring is serving as ballast resistors.  If the two
alternators or chargers were connected in parallel using wiring heavy enough
that the resistance is insignificant, the situation would be that one unit
would hog the load.

>One problem I've not yet thought about is what happens to my
>lightbulbs etc. as the end of the bulk charging phase is approached
>and the coach voltage approaches 14.4 V.  Bye Bye bulbs I guess.

Yup.  That's the reason for splitting the set into two banks.


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