Main + wire from battery

[TimC]

I am putting some plans together for my mostly LED 12v system. Battery will likely be a group 24 deep cycle. I have seen many drawings of typical wiring plans. I am wondering if I really need to go with 8ga. positive cable to a fuse/switch before continuing to my fuse block to distribute power. My total load assuming I would lighting up everything at once will be as follows;

Reading lamps (4) @ .3 a = 1.2 a
Porch lights (2) @ .16 a = .32 a
Fans (2) @ .15 amp = .30 a
200mm fan @ .42a = .42 a
Phone chrg @ 2.1a = 2.1 a @ 5v

12v TV/DVD @ <3a = 3.0 a
_________
Total amp draw 7.34 amp

I doubt I would ever use even half of this load at one time. I'm definitely in need of an education here, so... Can I put a 10a fuse/kill switch right on the + post of the battery and lead out of there with 16 ga to my fuse block? How about the - post?

Voltage drop in the 10' I would be running from the battery to my distribution block would be very low, right? I'm looking at the following chart for wire gauge size and wondering why folks would use 8 ga from their battery.

[url]http://www.sunforceproducts.com/Support%20Section/Solar%20Panel%20&%20Charge%20Controllers/WireGaugeSelectionTable.pdf
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[GuitarPhotog]

I use AWG 10 wire to my battery because I expect to recharge my battery faster than I discharge it. My use of LED lights and computer fan every evening for 2 or 3 days between recharges may add up to more than a 14 or 15 Ga wire can handle when a sunny day arrives and I can put out the PV panel to recharge.

That said, there is no point to over building by more than 15-20%. Assuming you will never want to expand your usage.

<Chas>

[MtnDon]

My calculator indicates that 12 gauge wire is good for an 8 amp load upto a distance of 12 feet while limiting voltage drop to 3% on a 12 VDC circuit.

GuitarPhotog raises a good point... if the battery charger is to feed back to the battery on the same wires that main wire should perhaps be bigger. depends on charger amps maximum.

And not just the + wire. The Negative needs to carry the same load, so should be the same size.

[TimC]

Thanks guys. I wasn't thinking recharging the battery, just the load side. I will probably recharge via solar panel most of the time and occasionally put a regular charger direct on the posts when I need a quick boost. I had a sailboat with similar 12v needs, with FM and VHF radios instead of the TV, and the 35 watt solar system kept the battery fully charged most of the time. Of course a sunny slip compared to shady campsites will make a big difference.

[Dale M.]

When I was in work world (communications industry) we had a saying, "when in doubt go larger"... Still applies, as there is never a danger of being to big, but being to small can bring you grief..... To restrict your self to minimal sized wire, also restricts you to what you may want to change/modify in future...

Dale

[TimC]

OK, I understand the go bigger idea. That is good advice.

I do want to go with smaller deep cycle batteries, like a 32 AHr U1. What is the calculation I would use to determine how long this will last at 7amp continuous draw to 1/2 capacity? Is it as simple as...

32 x .5 / 7 = 2.29 hours

I know there are other variables, just need a quick and dirty calc to see what I might be able to expect. I don't have a problem conserving power when needed, so, I'm not as concerned about available AHrs as I am about weight and space requirements. I doubt I will ever put a TV in this TD anyway. where I'm going to camp there are less than three stations available via rabbit ears and I'm more into reading...

Thanks again!

[MtnDon]

That is it in a nutshell. Variables will include things like actual capacity being reduced if the loads are large and a slight increase if the loads are light. As long as everything is DC, no inverter, you don't have that loss to deal with either. Along with the "discharge no more than 50%" rule keep in mind that the battery(s) life is shortened when the batteries sit in a partial state of charge too long. There are differing opinions on what constitutes too long. Many will say sitting at <80% for a week is too long.

AGM's may suffer partial state of charge storage better than an unsealed flooded battery. However they are more sensitive to being charged at too high a rate and can suffer permanent irreversible damage from that.