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
>