Even a single laptop power supply can use 100 watts or more, especially with
the laptop on and charging the battery in same.
Yes you do have a battery problem but even with that corrected your use and
run time with engine off is limited.

Dennis

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Daniel Rotblatt
Sent: Saturday, July 30, 2011 12:16 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Inverter/Aux Battery question

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
>