Date: Tue, 20 Jun 2006 16:09:46 -0400
Reply-To: dhaynes@OPTONLINE.NET
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: Understanding Amp Hours
In-Reply-To: <75567D6D-F104-479E-B141-F39A5068CFF5@knology.net>
Content-type: text/plain; charset=us-ascii
Battery discharging and its ability to sustain a load is not a linear affair and there are many variables. You can do an experiment today at one temparature and get different results tomorrow. Theoretically, a 65 A/H battery should support a 1 A/H load for 65 hours before dropping below 10.5 volts. It should also be able to support 65 A/H load for 1 hour. However, this will not happen. Most likely, the battery will be done in 15 minutes due to internal heating. That is why real deep cycle batteries have different capacities based on discharge rates.
Most likely, that inverter is cutting out at ~10.5 to 11 volts. Remember that in a normal operation with the engine running the voltage will be somewhere between 13-14 volts and low cost inverters need this help. Another issue is the size and wiring to the inverter. A .5 volt drop will cause the inverter to shut down earlier. Anyway, when the inverter cuts out, is the time to check that open circuit voltage on the battery. if it is 11.5 or more, things are ok and the battery will need to be charged. If the load was low when the inverter cuts out, most likely, the batteries open circuit voltage will still be near the 10.5. This is not so good. At this point some active material on the plates has moved into the solution and the battery will not be the same. Fast charging will make things worse as the material will not travel back to the plates evenly.
Since you have the inverter and the fan, do some experimenting. Charge a battery, and then test the battery, inverter, fan combination see how long it will last. Personally, due to space limitations and the like, I don't think I would be traveling around with a 20" box fan to get a breeze. Those little truck type 12 volt fans can do wonders.
Dennis
----- Original Message -----
From: Jim Felder <felder@KNOLOGY.NET>
Date: Tuesday, June 20, 2006 2:01 pm
Subject: Re: Understanding Amp Hours
> This information is getting towards my real question.
>
> My inverter shuts off whatever it is powering and starts buzzing to
> warn me whenever the battery reaches some pre-set point. Using the
> inverter, though it throws a little(?) overhead on top of what the
> fan draws, doesn't let the battery deplete the way a direct hookup
> does--say a fan or the fridge--by running it to zero volts or near
> zero volts.
>
> What that point is where it shuts down, I don't know, but I guess
> it's around 10.5 or 11 volts. So the question is not just how many
> amp hours are available from the battery, but how many amp hours are
> sustainable above say a 10.5 volt window.
>
> Using Dave's example, and assuming that I had the same Sears Diehard
> 650 CCA battery, could I divide the amps (since we're talking amps
> here) of my fan and inverter combo into the eight amps the fridge
> draws and get a multiplier for the two hours. For example, Dave
> assumes (rightly or wrongly) that 50% depletion from 8 amps happens
> in two hours. Does that mean that 50% depletion at one amp will occur
> at 16 hours?
>
> jim
>
>
> On Jun 20, 2006, at 11:58 AM, Dave Vickery wrote:
>
> > A real world example. A new Sears diehard 650 CCA aux battery will
> > last 4
> > hours until completely dead running the 8 AMP fridge. On 12 V the
> > fridge
> > runs continously pulling 8 amps. So assume 50% depletion in 2
> > hours = 16
> > Amps available.
> >
> >
> >> From: Loren Busch <starwagen@GMAIL.COM>
> >> Reply-To: Loren Busch <starwagen@GMAIL.COM>
> >> To: vanagon@GERRY.VANAGON.COM
> >> Subject: Understanding Amp Hours
> >> Date: Tue, 20 Jun 2006 08:43:35 -0700
> >>
> >> The answer to another question here on the list raised this
> question.>> Given
> >> that we have the amp hour rating for a battery and also given that
> >> we don't
> >> want to draw it down below about 10.5 volts, about 80%. Does that
> >> amp hour
> >> rating assume draw down to the 80% level, or draw down to 'dead',
> >> zero?
> >> I've always assumed that if I have a 50 amp hour battery that I
> >> can draw 1
> >> amp for 50 hours before reaching the point that will require
> >> charging, or
> >> will I only get 20% of that, 1 amp for 10 hours, before reaching
> >> the point
> >> where I need to recharge?
> >
>
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