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Date:         Wed, 20 Nov 2002 05:27:39 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Electrical Consumption &  Battery Recharge Calculations
Comments: To: Larry Chase <lchase@ATTGLOBAL.NET>
In-Reply-To:  <001801c28ffc$686bc8e0$94106620@laptop>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 01:49 PM 11/19/2002, Larry Chase wrote: >My Usage Assumption One: > >Power Consumption ... (running off batteries only) > >Per night = 20 amp hours * 12V = 240 watts per night total

Units problem here -- answer is in watt-hours (amps * volts * hours = watts * hours)

>Details, > >18 LED Strip Light = .180 amps per hr * 3 hrs = .72 amps

Or rather, .18 amps * 3 hrs = .72 amp hrs.

>18 LED Reading Light = .180 amps per hr * 3 hrs = .72 amps >LCD TV = 3amps per hr * 4 hrs = 12amps >Sat Receiver = 1amp per hr * 4 hrs = 4 amps >Misc Ghost Usage = .500 amp per hr * 4 hrs = 2 amps > >Total per night = 19.44 amps or 233.28 watts

Amp hours / watt hours. Note that actual battery voltage will typically be below 12v under load, so watt-hours would be a bit less. Also suggest you knock at least one decimal place off these numbers.

>Recharging: > >Solar Panels ... Two Uni-Solar 32 Watt Flexible Panels @ >64 watts * 4 hrs per day = 256 watts

I'd be interested to see the assumptions here. For example, in Providence RI the direct solar radiation averages between 2500 and 5,000 watt-hours per square meter per day according to statistics from Sandia <http://rredc.nrel.gov/solar/old_data/nsrdb/dsf/> -- how does this fit in with your calculations? Are you planning a tracking system (seems unlikely)? I have a suspicion that there is a lot of specsmanship in solar panels...

>My Usage Assumption Two: > >Running the Vehicle Stereo instead of the LCD TV & Sat Receiver > (not included in the above assumptions). > >Average Power Consumptions for a Vehicle Stereo System (actual system >not purchased yet) > >CD/Receiver = 6-10 amps per hr >Satellite Radio Receiver = .500 amp per hr >Amplifier = 10-20 amps per hr

15-30 amps, or roughly 150-300 *average* watts input for your stereo? Aren't you afraid of being mistaken for a drug dealer? Seriously, to me this implies sound levels that would be un-neighborly to say the least. You're talking about a micro-house here - -why not extend the concept to micro-sound, i.e. headphones for the loud stuff, and knock it back to say 75 watts max, average more like ?30? for regular listening?

>The mean average is approx 23 amps per hr.

Amp-hours per hour... :)

>The extra battery capacity of 84 Amp Hrs would enable the stereo to be >played for approx 3.65 hrs.

I'm reminded of the ensign who worked his fix and placed a neat dot where the lines of position crossed. The mate saw it and drew a good-sized circle around it, and then the captain came and smeared it with his thumb. Can we say "more than three hours?"

>If I assume a 75% efficiency on the VW 90 amp alternator then I would >need to run the engine approx 1 1/4 hrs to supplement the Solar.

Here's where you run into trouble. The alternator may well be capable of putting out 68 amps continuous (which is how I interpret the above statement) but first of all there's the parasitic drain of the engine to consider -- fuel pump, ignition, injectors, radiator fan, and second, no way is the stock regulator going to dump anything like that level of charge for more than ?a few minutes? -- the charge will all too quickly taper off to 10-15 amps and then asymptotically approach zero -- and the battery still won't be fully charged even if you run all day.

>Disclaimer .... none of this assumes three staging Charge rates/times >required by Deep cell batteries. But have built in a large "Fudge >Factor" in excess of 15% to compensate for this.

IF you want to FULLY charge your batteries, you're going to have to bring them up to a higher voltage than the stock regulator will supply. And if you don't want to boil them dry afterwards, you'll have to reduce the voltage after they're charged. There is simply no way around this. You can satisfy the requirement by manually controlling the alternator field, or by getting a smart regulator. You're going to need a regulator of some sort for the solar -- maybe possible to combine the two functions? Also, it hasn't been demonstrated that the Bosch alternator can handle continuous high-current charging without additional cooling. Since you'll need to modify it (slightly) anyway to bring out the leads for the external regulator, maybe it would be better to get a hot-rated marine-type unit.

>In conclusion. > >Solar will recharge "Usage Assumption One UNASSISTED. > >Solar plus 1 1/4 hrs of the VW Alternator will recharge "Usage >Assumption Two".

Getting the last 15% of charge into the batteries will take several hours -- if you run the engine in the morning and take care of the bulk phase, the solars can take the absorption/float phases.

>The actual system will also be supplemented by a small 1000kw Honda Gas >Generator.

One thousand kilowatts? I know where you can get a 1250 KW Caterpillar complete with 4,500 gallon fuel tank for only $290,000. But I'm not sure the Vanagon will pull it. <grinning, ducking, running like heck> Seriously, Honda makes a lovely little tiny generator that only puts out 350 watts or so -- but you can hold it in one hand, or nearly.

In conclusion -- If you're really going to go the high-power route, you need to bite the bullet and spend money on proper mechanical charging equipment (regulator and maybe alternator), and ought to strongly consider serious consumption-monitoring instruments as well. And I would suggest to increase by 1.5-2 the battery capacities you're talking about, under the rule of thumb of carrying a capacity of four times your daily consumption (which if used conservatively will get you 1-1/2 to 2 days of operation, or three if you push it a little as I would).

Alternatively, scale down your consumption a lot.

cheers, david

-- David Beierl -- dbeierl@attglobal.net


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