Date: Sat, 30 Jul 2011 07:30:38 -0400
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: Inverter/Aux Battery question
In-Reply-To: <329EF91D-F64F-4727-B250-CBE38C633CCF@verizon.net>
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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