What determines battery cable size?

[johnm1]

Hey all,

I'm looking into replacing the factory battery cables in the spring to hopefully get more efficient (as in shorter) generator charging time. My question is, what do I "need to do" vs. what would be "nice to do"?

1. I do not know the camper's wire size.
2. There are at least 14 crimp connectors between the 2 (6V) batteries and the "power center".
3. I have replaced ~8 of the very poorly crimped on ring connectors with real crimp battery lugs.
4. Out of the 8 replaced, I easily pulled 7 of them off with pliers with very little, to no effort.
5. I do not anticipate any heavy, or fast, battery draw downs. No inverter, no power hungry 12vdc appliance, etc.
6. I've replaced the WFCO with BestConverters Boondocker 75amp unit.
7. I have 2 Champion 2k (1.7K) invertor gennys with parallel kit to recharge as needed.
8. Right now the neg cable is screwed (self tapping screw) right to the frame near the battery and the same right near the power center.

What to do?
A. What wire size/gauge would be appropriate for my needs?
B. Should I run wire from the battery directly to the power center or replace the self tapping screws with something else more substantive?
C. The pos cable runs from the battery to some relay or circuit breaker thing (??) that's attached to the frame and exposed to the elements. Should I move it to more weather resistant area?
D. Should I install a fuse someplace to protect the batteries? If yes, what kind/size and where to put it.
E. I may, or may not, go solar. Depends upon many factors but in where we camp in NYS parks, there isn't that much sun (lots of trees) so it's not on my short list.
F. The distance from the batteries to the power center is around 20-25ft one way. This includes from the batteries down to frame and back up to the power center.

Hopefully I've listed all of the info that's needed to make proper recommendations. Let me know if I missed something.

Thanks,
John

[Shadow_Catcher]

This is one place where size genuinely does matter, bigger is better. However this is the way to check Cable Sizing
Eliminate joints/use proper splice connectors, I tend to solder mine.

[tvman44]

Whatever is required for the heaviest current draw you anticipate.

[BFL13]

About 4 ft of #4 each wire (pos and neg) from the 75 amper to the battery will get you the 75 amps. ( I have one of those converters)

I don't know how much longer of #4 will still get you the 75 amps, but a few less amps won't make much difference to your recharge time on 2 batteries if you have to go long. Stay with the frame for the neg--that counts as a fat short wire. Then you just need a long #4 for the pos.

Don't bother with anything less than #4 with the 75 amper.

You can use the 75 amper as a portable charger for generator charging, and leave the WFCO in as the converter for when on shore power. That way you get the short run from 75 amper to the batts. Using #4 AWG (cu, not cu-al) jumper cable ends to get the clamps is the easy way to do that.

[wolfe10]

Since you suggest no heavy loads, the heaviest load (i.e. most amps that will need to be carried) is your charger/converter (up to 75 amps) I agree, #4 fine strand wire direct from battery with fuse to converter output. Also upgrade the ground connection at the battery. Chassis is OK to use for ground, but make sure the metal is clean and shiny and you have a good connection there. If in doubt, use two grounds from chassis to battery.

[azrving]

Cable size depends on load. As mentioned for your charging situation 4 should be fine. If you want the most out of your charging system you need to check voltage at the converter output terminals vs at the battery. This will show you the voltage drop. It's common to need to upgrade the cable from converter to batteries. You are on the right track as far as removing as much resistance as possible. Inepensive hydraulic crimp tools can be bought on eBay for about $35.00. get the closed end style terminals and then heat shrink cover.

[enblethen]

A quality multi stranded #4 should do you good. Normal wire size is based on amperage. Your 75 amp converter should work very well with the #4.
Good clean connection ground for the battery to the frame of the rig and the converter end. Use electrical grease on all connections to prevent corrosion.
You say numerous splices, are these simple straight splices? Why so many? Every splice will or can cause voltage drop.

[time2roll]

I recommend pulling new #4 wire for the positive as that is about max to fit the connectors. Upgrade the ground connections as well. Though the frame is fine for the negative connection. Yes an 80 amp fuse or breaker should be near the battery. Any branch circuits should be fused according to wire size. That circuit breaker thing is probably a self resetting breaker and should be junked.

Solar is an independent direct connection to the battery.

Boondocker is well known to fast charge to about 75% then an extended trickle charge. If the voltage is manually controlled fine otherwise I would exchange it for Progressive Dynamics or IOTA.

[Chuck_thehammer]

Amp Draw and distance.

google 12 volt wire sizing... many calculators available.

here is just one
http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

[donn0128]

Cable size is really based on amp draw.