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Date:         Sat, 30 Jul 2011 09:15:36 -0700
Reply-To:     Daniel Rotblatt <d.rotblatt@VERIZON.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Daniel Rotblatt <d.rotblatt@VERIZON.NET>
Subject:      Re: Inverter/Aux Battery question
Comments: To: Dennis Haynes <d23haynes57@hotmail.com>
In-Reply-To:  <BAY152-ds9C6C0EEBA9F716B040554A0360@phx.gbl>
Content-Type: text/plain; charset=us-ascii

Dennis-

I didn't expect to have to use a full 400 watts - basically I just wanted to be able to charge some computers, etc. I don't imagine more then 150 watts as a sustained load - and even then it's a luxury - i.e. if the kids are bored and noisy and need to watch a video, or something like that. This issue is with no load on the inverter - just doesn't seem right.

Dan Los Angeles, CA '85 Westy Weekender

On Jul 30, 2011, at 4:30 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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