Date: Sat, 30 Jul 2011 07:16:01 -0700
Reply-To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject: Re: Inverter/Aux Battery question
In-Reply-To: <BAY152-ds9C6C0EEBA9F716B040554A0360@phx.gbl>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Regarding inverters and the load they present to a battery, I have two
inverters. One is a 1,000-watt unit mounted behind the driver's seat.
The other, a 350-watt unit that plugs into a cigar lighter outlet, lives
under the driver's seat and is a backup in case the first fails. The big
one is a Harbor Freight unit, cheap. The little one is of unknown
provenance and it's also a cheapie. I think I got it at a truck stop or
in Walmart or something.
Neither is a fancy-pants pure sine wave unit, and I have yet to find an
appliance that needs it. I have run laptops, a DVD player, a coffee bean
grinder, an air compressor for watercraft, and an immersion blender off
the big inverter. None have griped.
I have an ammeter on the aux power system so I can monitor current going
into and out of the aux battery when camping.
Neither inverter consumes more than a couple hundred milliamperes when
idling.
When I connect up my laptop directly to the 12V system using a FleaBay
DC-DC adapter, I read about 2 amps on the ammeter. When I connect up my
laptop through the inverter, using the laptop's normal 120VAC supply, I
read about 2.1 amps on the ammeter.
That's with the big inverter, I have not tried these tests with the
little one. The results may be similar, or not. But the bottom line here
is that either way works just fine.
But regarding Dennis's first point about battery sizing and that it
doesn't take many watts at all to drain a battery, yup, you have to do
your homework. The math is simple, but for those that don't care for
mathy things, use the Battery Demand Calculator, found here:
<http://www.altavistaaudio.com/Westy/Vanagon/battcalc.html>
--
RJS
On 07/30/2011 04:30 AM, Dennis Haynes wrote:
> This is going to be difficult to explain but there are a number of things
> that are going against you.
> When sizing a battery for an extended load the battery should be sized for
> a minimum of a 5 to6 hour discharge rate. To get 400 watts out of a 120
> volt inverter you need 400/12 = 33.33 Amp/Hr. Consider some inefficiency and
> that fact that under load you will have less than 12 volts a full 40 A/Ah or
> more is required. The largest of the Yellow Top batteries is 55A/Hr. over
> the 6 hour discharge rate. So you do not have enough battery and this
> sustained load will damage it.
>
> Next problem is using a low cost inverter to charge batteries. Modified sine
> wave inverters basically switch and filter DC to make AC so most appliances
> can work. Motors due to their inductance do not care and Most DC power
> supplies can clean it up enough the only bad effects are usually some hum or
> buzz on audio equipment or fuzzy lines on old televisions. However, cheap
> power supplies for charging batteries use the batteries as part of the
> filter and the peaks of the waveform pass right through them basically
> making the batteries a heater they now have a load in addition to what they
> actually need to charge. As such my 2002 Toshiba Laptop and a 400 watt
> inverter can take down a 60 A/Hr. battery in about 2.5 hours.
>
> So for all those low voltage toys and cell phones get the 12 volt charger
> power cord for each device.
>
> Dennis
>
>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
> Daniel Rotblatt
> Sent: Friday, July 29, 2011 5:17 PM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Inverter/Aux Battery question
>
> Well, I just got back from a trip to Mammoth Lakes (separate post on that)
> and got a chance to try out the aux battery and inverter I got. The problem
> I had was that with the car off, the inverter didn't get enough power to
> work. When hooked up (and I tried two inverters and got the same results),
> the voltage would drop from 12.6 to about 8.x volts - not enough to run the
> inverter and it would automatically shut off. Thus, the only time I could
> charge the computers (keeping the kids happy), etc., was when the car was
> running. Is this how it works? What is the need for a aux battery, i could
> have just hooked up the inverter to the main battery and not used it when
> the car wasn't going an saved $175 on the aux battery.
>
> Hook-up:
> -brand new yellow top battery
> -Stock attachement to the starting battery (had to replace the relay)
> -Inverter hooked directly to the terminals of the yellow top - about 4' of
> wire, (I tried just 8 gauge with about 3' of wire, and finally with 3' of 8
> gauge and a few feet of 12 gauge hooked to that with a cigarette plug since
> that's what was on the inverter).
> -Inverter is a 375 watt Tripp-lite (good reviews on amazon).
> -Voltage at the battery (when new) was ~12.7 , with car running it's ~13.6,
> with car not running and inverter attached it's ~8.5 -All the inverters I
> saw shut off at a little over 10 Volts....
>
> So...is that what's supposed to happen or is something not right? I'm
> planning a trip to New Mexico next week, so it would be nice to have it
> working.
>
>
> Dan
> Los Angeles, CA
> '85 Westy Weekender
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