Wire size to tie-in to second battery bank

Starting the homework to install a second bank of batteries. Unfortunately the closest possible location could have a wire run back to the existing house batteries of upwards of 15 feet by the time I allow slack for battery tray movement, etc. (Could be as little as 10 feet, still working on routing and measurements.) This is not a function of bins that are full of stuff (which of course they are), it's a function of the fact that the closer bins cannot be used for flooded battery storage, e.g., inverter/equipment bin, electrical bin, water bin, etc.

Second battery bank will consist of 4 batteries of similar/identical capacity as the house batteries in the tray: about 480AH. I'm thinking the highest charging current would be battery charger (100A) ? 2 so 50A. The highest theoretical load would be max inverter load (2000W?12V=170A ish) + (other 12V loads so say 20A max) ? 2 = 95A. It seems like this would be the extreme worst case scenario for perhaps 5 min periods at most, with real world load numbers probably half of that. This is my crayon math, so some likelihood my assumptions are way off and/or I have failed to consider other issues.

My plan, once wire routing and length is figured out, is to get the cables made at a welding shop.

My questions:

1) How's my math? What am I missing?
2) Using my/your math, and these approximate distances for now until I get better measurements, about what wire size am I looking at (besides BIG! )

It's occurred to me that the distance and amperage involved may mean double cables but would really like to avoid that if at all possible for both cost and install/routing reasons.

The long-term plan is to add solar, but it seems to me if the super high load issues are handled correctly I will be more than ready to tie-in solar anywhere in the setup.

I'm committed to flooded batteries as my house (flooded) batteries are new. Plan is to place second set in marine battery boxes (found this source: http://batterymart.com for boxes and vent kits). Looks like two of these: http://www.batterymart.com/p-hm426-dual-6-v-battery-box.html would work.

Thx

This may help http://www.wirebarn.com/Wire-Calculator_ep_41.html

Thanks. Looked for calculator and everything I found was for high voltage

Looks like 2 gauge for 2% drop and 2/0 for 1%, so it's doable with a single, large, cable

Should be doable and since you are planning on a solar install at some point, I would go for the least voltage drop, if you can get it to 1% or less you will be doing yourself a favor.

That's what I was thinking too. If I'm going to go to all the work and expense, no sense in scrimping on wire size

Right off the top of my head...
Does the bay you are planning on putting the new battery bank into have sufficient venting? Otherwise, you may be building a time bomb.

denmarc said:

Does the bay you are planning on putting the new battery bank into have sufficient venting?

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Short answer, no. For that reason the battery boxes will be installed with vent kits that vent overboard

While it is not quite optimum, I would not be afraid to use AGM "valve-regulated" batteries in the second bank and avoid the venting concern. Also excellent for zero maintenance - no water and no corrosion worries.  A second "bank" that is all AGM and wired in parallel isn't a bad case of mixing battery types. I've done it with a previous coach myself, in a situation very much like your describe.

Gary RV Roamer said:

While it is not quite optimum, I would not be afraid to use AGM "valve-regulated" batteries in the second bank and avoid the venting concern. Also excellent for zero maintenance - no water and no corrosion worries.  A second "bank" that is all AGM and wired in parallel isn't a bad case of mixing battery types.

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Interesting. Last thing the battery guy at Battery Warehouse told me (and everything I've read) was that I couldn't mix AGM and flooded; maybe he meant within a bank. This raises a few questions:

1) The Xantrex (and I assume other) charger has a specific AGM charge routine; what's different about the AGM vs. flooded charge routine? I presume you're suggesting using the flooded routine to charge the entire mixed flooded/AGM bank?
2) I was given advice to do an equalize charge monthly if I boondocked with deeper discharges, and every other month if primarily full hookup with only occasional boondocking. Pretty sure I recall that AGM are never equalized, so I suppose the procedure would be to disconnect the AGM bank cables (not a big deal based upon the wiring I'm planning), equalize the flooded then reconnect the AGM - yes?

Gary RV Roamer said:

I've done it with a previous coach myself, in a situation very much like your describe.

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What was your setup and how long did you operate in that fashion? Did you have a sense (or better, some sort of data/proof via the charger, e.g., calculated capacity) that you didn't suffer degradation of either the flooded or AGM? (I get that you wouldn't suggest it if it had been bad, just asking.)

Indeed, it would be vastly easier and I would say cleaner overall to go your suggested route - maybe even cheaper or close since I have to come up with and probably build a sealed, forced air vented box to make the install work. Had a talk with the people at batterymart.com today about their battery boxes and vent kits. Boxes are not sealed. Vents have to be installed vertically to allow hydrogen to naturally rise (of course, the floor would be above my second battery box so that was a deal killer right there). At HueyPilot's suggestion I called Discount Solar  (they didn't answer), so I ended up speaking with Solar Bill. They've dealt with my exact issue, their solution: custom built sealed box, 4" inlet hose with computer fan running 24/7 (what if it quits/should I have two? What's the current draw?), then exhaust hose vented overboard. Aye yi yi. Not what I had in mind. The relatively simple add-on ain't so simple no more.

The difference is typically about 0.1v in max charge voltage. Some AGM makers act picky about the difference, while others don't seem to make a big deal of it. The 2004 edition Xantrex inverter/charger in my current coach does not have a separate AGM charge profile, and my coach came factory equipped with AGMs. It charges them just like they were flooded cells.  The original set lasted only 4 years, but the replacements (Trojans) are coming up on 8 years now and I plan to use them another year. Maybe they would last even longer with a AGM-specific charger, or maybe not? AGMs are sensitive to overcharging, but no 3-stage charger should ever do that anyway.

In a previous coach I had two 6v GC2's in series, and I added a Group 27 AGM deep cycle in parallel in a separate bank. The charger was an Iota 3-stage with only one charge profile, but it had dual battery circuits (each bank had its own wire direct from the charger). I ran that way for three years and then moved the AGM to my bass boat as a trolling motor battery and it lasted another two fishing seasons of light use.

Mixing batteries isn't ideal, but if the charger is a decent one and all the batteries in good condition, it usually works OK.  As with many things, if you know what you are doing and pay attention to any possible warning signs, you can skirt around the general rules of thumb.

Trojan carries both AGM and Gel, both advertised as sealed, no maintenance deep cycle. What's the difference? Neeeever mind...

"Gel Cell Batteries must be recharged correctly or the battery will suffer premature failure. The battery charger being used to recharge the battery(s) must be designed or adjustable for Gel Cell Batteries. If you are using an alternator to recharge a true Gel Cell a special regulator must be installed."

Gel is more sensitive to charging issues than AGM, which is one of the reasons they never were very popular (the other was $$).

Many new cars come with AGM batteries these days, and they live their entire lives with an engine alternator pushing 14+volts at them. Even my 2006 Acadia is equipped with an AGM starting battery.

You've sold me. While doable, the whole power vented, custom sealed battery box plan was getting to be a bit much. Plus, I'll be ready for those half price lithium batteries in 5-8 years.

When you get everything figured out, hear is a good place to order your custom battery cables. The link is to a custom 2/0 cable build but they have heavier if you want.
http://www.bestboatwire.com/2-0-awg-custom-battery-cables
Bill

WILDEBILL308 said:

When you get everything figured out, hear is a good place to order your custom battery cables. The link is to a custom 2/0 cable build but they have heavier if you want.
http://www.bestboatwire.com/2-0-awg-custom-battery-cables
Bill

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Good to know about this resource.  Another source is http://www.amsolar.com/ When I installed my solar system, I ordered some custom cables from them and they were very good.

Gary RV Roamer said:

Mixing batteries isn't ideal, but if the charger is a decent one and all the batteries in good condition, it usually works OK.

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Just had a very interesting discussion with Battery Warehouse on "mixing" a bank of flooded with a bank of AGM. He concurred that while not ideal it wasn't an absolute no-go. Takeaway points:

1) Charger should be set to AGM charge profile. He stated that it's not good to charge AGM at a flooded rate (too fast) but OK to charge flooded at AGM rate, will just be a little slower
2) Both flooded and AGM should be equalized every couple of months however... flooded equalizes at much higher voltage than AGM so absolutely should not equalize AGM on flooded profile. Conversely, shouldn't equalize flooded on AGM profile because the voltage is too low and it just won't be effective. So equalizing should be done in 3 steps: First, disconnect flooded bank leaving AGM bank hooked up and equalize on AGM profile. Second, disconnect AGM bank, reconnect flooded bank, switch to flooded profile and equalize. Finally, switch back to AGM profile and reconnect AGM bank so all batteries are now connected. Little bit of a pain but only needs to happen every couple months.

Gary RV Roamer said:

As with many things, if you know what you are doing and pay attention to any possible warning signs, you can skirt around the general rules of thumb.

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Glad you brought this up Gary as I would not have considered this. Given your comments and those of those in the business, feeling like I'll be OK with this setup. By the time I get cables and batteries (and probably marine boxes just to make a clean install) will have close to $1500 in it so need to be confident it's the right way to go.

No offense to anyone, but too much work for me.
RVing is suppose to be fun. Not so complicated as to what to set your battery charger at with mixed battery banks.

One bit of bad advice I have seen when batteries are paralleled is "You can use smaller cable cause they share the load"

Normally.. Yes, they do just that.  But batteries have several failure modes. One is HIGH RESISTANCE.

THis, in fact, is a COMMON failure mode.. So when it happens that battery (or pair if six volt) will not share the load and ALL the load will be on the other battery.. Thus the connecting cable has to be up to the job of hauling the FULL LOAD.