Cable sizing for charging from the Alternator

[MikeRuth]

Hey guys, I hate starting new threads but I couldn't find an answer to this...


In my case my House batteries will be a sizable run from the Alternator.
Of course there will be a high amp continuous duty solenoid in between for a disconnect.

What I'm trying to figure is size of the wire from the Alternator through the solenoid to the House bank. Worst case setup is two Trojan T-125's in series and they will be I'm guessing right now about 10 Feet in wire length from the alternator.

so two questions, use 1/0 cable or 2/0? or?
Second, are we good with a short ground to the frame rail or should I also run a ground up closer to the alternator/engine?

Thanks

 

[Spaceman Spiff]

Assuming a 12 volt, 30 amp fused load, 3% voltage drop and 20 foot round trip: AWG 6 (AWG 3 for 1% drop).

Calculated using http://circuitwizard.bluesea.com/#

It is always better to run a dedicated ground. Steel is a poor conductor and tends to increase voltage drop over time (rust at the connections). I ground directly to the alternator case.

-- Spiff

 

[MikeRuth]

Thanks and I just added that calculator to my favorites.

 

[ccbreder]

cable size depends on distance and max amp flow. 10+ feet distance. What size your alternator? Will you ever use the house bank for starting? There are wire size calculators on the web, here is a simple one; http://boatstuff.awardspace.com/awgcalc.html
Yes run two wires, the slight extra expense is worth doing.

 

[MikeRuth]

Alternator is a 135 Amp. And I prefer the first calculator listed as it takes in all the factors.
My gut selection of 1/0 or 2/0 would be fine, although the Calculator let me off with 3/0
Having a second ground is no big deal and cost is not an issue when it comes to doing this work right!
I decided to bump it up to a 40 Amp load since these deep cycles cam probably take it if down to 50%. Hope not to do that but ya never know.

Thanks Again folks.

 

[ccbreder]

Of course, the more complicated something must be better. The one I posted gives you the information you need. But, it is too simple.

---

A question. If your calculator gives you 3/0 why would you use a smaller gauge such as 1/0? Using 150 amp charging, 12 feet and 2% voltage drop, 1agm will be fine, 1/0 is a bit better and easier to find.

 

[SternWake]

Batteries themselves decide how much amperage they can suck from a charging source. At the voltage allowed by the voltage regulator.

the limiting factors are the wiring. And the voltage regulator. And how well the alternator can cope with the tremendous amount of heat it will generate when asked to perform near its maximum output.

A big issue is the voltage regulator. It might 'see' the more fully charged engine battery and decide 13.7 volts is plenty. And then amps making into depleted house battery is a third or less of what it would be if the regulator was allowing 14.8v.

They bigger the cabling the more visible the depleted battery is and charging can be orders of magnitude faster.

Also. Many take power for isolation device from engine to feed house battery. This leaves the too thin original charging circuit as the weak link. One can take power for isolator directly from alternator (+). This usually makes for a shorter circuit path and eliminates the starter battery fooling the voltage regulator.

Those who do not want to take power directly from alt positive stud can run a fused parallel circuit from alt positive and o starter battery positive or engine battery side of isolation device.

Huge amperage increases can be obtained on a depleted battery with thick cabling in this circuit. Wire ampacity charts are not really valid if the goal is to obtain maximum recharge via alternator in minimum amount of time.

I've been on the road for a month now and have been getting well more than half my recharging from the alternator vs the 200 watts of solar. Both together is key for a well working system.

Ive got 5 gaĺlons of 95 degree water heated by inverter and heating pad ready to go after this next surf session.

Ive got a single group 31 battery rated at 130 amp hours new closing In On 300 cycles now. It will easily gobble up 60 alternator amps over my doubled 4 awg cable, and no way in heck would I have hot rinse water if I did not have all this copper to feed a thirsty hard working battery. And 40 miles.of.driving daily.

3/0 gauge is not excessive.

Alternator capable of high amperages at idle.speed are more at risk of overheating. If the vehicle is not moving. Air movement is pretty critical for the life of this component, and a dedicated cold air intake can not only increase alternator longevity. But also increase charging amps too.

Low and slow battery charging is fine if you have grid power and days to top them up. When the next discharge cycle begins in a few hours. The single most important thing is getting the battery to accept all it can with high voltages, and traditional ampacity charts and limitation theories are the enemy.

For example the guy who says a battery can only accept 15 amps so 10 awg is more than enough. This guy is s serial committer of batterycide e specially if the bad advice is jumped on by those fearful of copper prices, and spreads around the internet as gospel.

Buy a little more copper now. Or buy a lot more lead later to go with the badly operating electric al system and the frustration that goes with that.

 

[Mark Alan]

Once you get to the "_/0" the wire size gets larger as the numbers increase. 3/0 is a huge cable and way larger than most of us would ever have a need for. 4 awg is more than sufficient for connecting an auxiliary battery to your alternator. The largest cable I have in my system is 2/0 from my house batteries (2 6 volt trojans) to my 1800 watt inverter/charger.

 

[highdesertranger]

stern I like that, "buy a little more copper now. or buy a lot more lead later". so true, so true. highdesertranger

 

[stealthywon]

I never thought of a dedicated cold air intake for the alternator Sternwake thanks for the tip.

I tried googling alternator cold air intakes and didn't find anything vehicle specific.

Did you fabricate yours or modify one that was used for a differfent purpose?

 

[Optimistic Paranoid]

Sternwake has years of practical experience to back up a thorough understanding of electrical and solar theory. A lot of us here take his posts as gospel on these subjects.

Regards
John

 

[SternWake]

While ive acquired the components to make a CAI for my alternator, ive yet to install it.

Mine pulls air through from the back. Which is also inches away from the exhaust manifold. I added a heat shield between the two. This was good for 5 extra amps at idle.

I wish the temp probe.on my multimeter were longer, as I am curious how hot and how fast it gets hot.

While my battery monitor reads.alternator current into battery, I cannot easily see it from drivers seat. But I do notice that within a mile or 2 with a depleted battery max voltage falls to 14 when moving slow, whereas it usually seeks a satisfying 14.9. I believe this is due to heat. As at highway speeds it maintains 14.7+ while.amps.required.to hold that taper

However it is still close to the maximum the battery can take, but a CAI would make a measurable difference in sub 5 mile drives.

The op.has stated that they were not afraid of the price.of copper. This is ultimately cheaper than using 6 awg as the goal is getting the battery as close as possible.to full. Every time the vehicle.is driven.

And thin wiring is like forcing a sprinter to breathe through a cocktail straw.

Give the battery the option.of deciding how much it can take from a variable rate charging source. Best way to do this is with seemingly extravagant amounts of copper.

4 awg is the minimum one should use.

Id love 3/0 awg copper with thick walled ancor lugs . The cable terminations are just as. If not ultimately more important than the copper thickness.

properly terminating thick wires is an art form. Often it is best to pay an artist. Genuinedealz has top quality copper and makes custom cables for an excellent price.

With short drives. Thick copper is the battery's best friend.

Exactly how.beneficial will vary between vehicles as the voltage regulator function has the ultimate word. Influencing the VR to do ones bidding is done with thick copper. Much thicker than ampacity charts will lead you to believe.

 

[MikeRuth]

Thanks to all the comments, and I should have stated AWG 1 or AWG 2 and not the 1/0 etc. I've seen the problems with inverters with under sized wire and I do understand basic electronics and electricity. But it never hurts to ask the folks that have been there and done that. Thank you all.

I'm not so sure just how often I will let them charge from the Alternator. It will be my preference to use shore power as often as I can and I'm also going to have Solar panels on the roof. Only 200 Watts but that will be enough for when it's sitting during the week, which will happen more often than not for a while to keep them topped off.

Very good suggestion on the CAI. I never thought of that and it would be pretty easy to fab something up. If memory serves my 2007 E-150 does not have a fan on the ALT. so the CAI would be a really good idea.

Mike R

 

[SternWake]

The fan is likely internal to the case of the alternator.

Mine pulls air from the back and exhausts it out the front, so in theory I could have the alternator pull cold air in through a dryer vent tube without an additional fan, if i were to make a custom shroud allowing for the wires and cables to still pass through without chafing..

Only 200 Watts but that will be enough for when it's sitting during the week, which will happen more often than not for a while to keep them topped off.

Do note that if the batteries are not cycled overnight, then the solar controllers settings do not need to hold the battery at high absorption voltages daily for 2+ hours like would be necessary had the battery been cycled below ~75%.

Basically the deeper the cycle the more important it is to hold ABSV for a while each day and as always battery manufacturer absorption voltage recommendations are usually a lot higher than the lawyerly imposed limits many controllers leave the factory with.

If they are not cycled at all then holding ABSV for more than 15 minutes every day for a week, is abusive to the battery.

Some controllers will hold ABSV x amount of time
Some controllers will hold ABSV until amps required to hold ABSV fall below a certain threshold
Some controllers have adjustments for both. Like hold it for 2.5 hours or if amps required to hold 14.8 taper below X, revert to float voltage

Some controllers are just junk which allow no customization and are a serious compromise and will likely undercharge deeply cycled batteries and overcharge batteries sitting and floating daily which were not cycled the night/day before.

For many the compromise is no big deal, and most are unaware, and the batteries might still live an OK lifetime if the electrolyte levels do not fall below the tops of the plates.
But
That compromise drives me a bit nuts, and special needs batteries, can fall flat on their face after a dozen undercharge cycles.

I have one such battery. The SOB needs 14.9ABSV and then 15.3v topping) daily after a cycle to 70% or less.

If I could not adjust Absorption and float voltages this battery would have been recycled long ago. It is now a year old and getting worked hard nightly and easily meeting my overnight needs. But it has lost capacity as is expected at 300+ cycles.

Until Lifepo4 becomes affordable, we have to deal with the petulant lead acid batteries and their proper care is all about the proper and prompt recharge, and no 2 batteries are exactly the same as to their specific requirements, and the requirements change as they age.

 

[MikeRuth]

Stern Wake thanks for that info, I had not purchased a controller yet so now I will make sure I have the control to set those parameters.

TC, Mike R