Inverter Battery Bank - Lug vs SAE Terminal connections

[Vincentc]

Moving ahead with the inverter project, I bought the big one and have a Magnum 2812 with battery monitor and remote control, auxilary AC panel, 250 amp DC fuse, DC switch and 4 Deka East Penn / Samsclub 230 amp 6v golfcart batteries. I have re-read Nigel Calder and built a battery box.

I am about to order the battery cables to connect the batteries in series / parallel and connect them to the accessories and inverter.

The batteries have both the traditional big post and 5/16" threaded posts. Since we are dealing with a lot of amps:

Is it better to connect with SAE terminals to the big post or use lugs connected to the 5/16" posts?

Where I am making two connections to the same battery post is there an advantage to connecting one cable with a terminal and the other cable with a lug vs stacking two 5/16 lugs? Stacking would probably simplify the cable runs. So would 2/0 cable.

Magnum recommends 4/0 battery cable for the cable to the inverter, would that apply to the connections between the batteries as well? The inverter will be mounted within a foot of the battery box, which will be closed and vented to outside the hull.

"Life is full of tough choices, isn't it" _ Ursula, from The Little Mermaid

Or am I wasting time on unimportant stuff?

Thanks

 

[Scottbee]

I have a similar set up... and a fair amount of experience with invertor configurations. So I'll chime in with my opinion. I am sure there will be other opinions, and some may disagree with my position and experience. It happens.

I am confident that you'll be just fine with the smaller gage cable for the battery interconnections, for two reasons: the run length is substantially shorter than your main run, and the interconnects carry less current than the main.

Personally I went with the 5/16" lug terminals, and I stacked them. Just make sure that the bottom lug is directly in contact with the lug base (no washer) and do not use any kind of washer in-between the stacked lugs. You can use a SST washer between the top lug and the nut. And use nyloc nuts instead of wingnuts (for all connections). I also have a habit of liberally slathering everything with dielectric grease during assembly.

It sounds like you are going to have your cables fabricated by somebody else. That's OK, but I made my own. I prefer solder-potted terminals over the swaged units. I feel that they give better conductivity and don't have voids for corrosion to grow. But I believe most fabricators use a swaging tool.

 

[Boatsb]

There is no need for 4/0 cable. Also I would not put smaller cables on the jumpers. Use all the same size.

 

[Scottbee]

I forgot to ask: Are you going to fuse the batteries?

 

[Vincentc]

Thanks for the input. 2/0 cable and lugs sounds good to me.

A fuse is incorporated in my plan. The positive battery cable goes to a 250 amp fuse, then to a DC switch, then to the inverter.

I recall that the Trojan web site also mentions soldered connections. Is that soldered and crimped or just solder?

I plan on getting the cables from Genuinedealz which has tinned battery cable. I will check on their attachment process.

 

[Scottbee]

If you're going to run a 2812 then a 250A fuse would be undersized. At full continuous load the 2812 draws about 375A if I remember correctly.

Personally I really like the Blue Sea Battery Stud Terminal Block fuses... and they're what I use on my rig. But I believe their largest block fuse is 300A, which would still be too small for your application. I see that Cooper Bussman has fusible links in the 400A rating.

I'm surprised that you're thinking about putting a switch in the line. Don't see it getting much (if any) use and it would be a likely source of voltage drop and general problems.

As far as the lugs are concerned, mine are just potted (soldered). Nothing wrong with swaged and soldered... it just wouldn't work with my assembly method.

 

[Boatsb]

Scott. a 300 amp fuse will be fine for a 2800 watt inverter if it is truly only drawing 2800 watts. 3000 watts at 12 volts is a 300 amp load. If your thinking it is not enough replace it with a slightly larger one but to get too big is not safe. 350 maybe the next size up.

A battery switch is not only a good idea but a safety item. It will break the hot line if someone has to work on the inverter. I believe it is required by ABYC, the coast guard for inspected vessels and most insurance companies but what would I know? I haven't prepared a bout for inspection in months.

Vincent,

Well made battery cables are probably not easily bough on the internet. you never know what they pass as quality. Just because its tinned does not mean its good. Remember the battery connection is already a weak point on the terminal dont make it worse. Look at the Lifeline 8d battery attachment points for a comparison.

If you put the inverter in as a charger you will need breakers and switches for safety. If its in passthrough you will need more of the same. Plan it out and have someon who knows electricity look at the drawings before you start.

 

[Scottbee]

Boatsb,

The Magnum 2812 specs say that the max continuous input current is 373A at an inverter efficiency of 88%. Agreed, running at 2800W this current wouldn't actually be required, but Magnum appears to be accounting for their 3900W surge capability.

Personally I have never seen a disconnect "switch" used with a high power inverter... but your experiences are certainly different than mine. If it is a requirement that who am I to disagree?

My personal problem with the switch is voltage drop. A 12V high current system has relatively small margins to play with. I have spent countless hours chasing down .1V drops... often due to silly things like SST washers between terminals, iffy contactor points, bad swaged cables, corrosion, and faulty fusible links. I like to keep cabling systems simple... while still being safe and meeting codes/requirements.

24V and 48V systems run lower currents and have larger margins, making them less susceptible to the above "issues", but they have other challenges that need to be addressed.

 

[Vincentc]

Interesting and helpful discussion.

I plan on having an electrician review and assist with the final install, problem is, the information here appears better suited for boat specific issues than what I hear from electricians. Forum information usually requires some sorting out, and differences of opinion are expected.

The 2.8kw inverter is significantly oversized for my intended use. I intend to connect only 3 AC legs, the refrigerator breaker and the breakers for the port and stbd outlets. Continuous use would be less than 200 watts with occasional use of a coffee pot, microwave or induction hot plate. I have measured each and they are 900w, 1200w and 1600w respecively and share one duplex outlet. Therefore, I could only plug in two of those appliances at a time if I wanted to. Power for those legs coming from the main breaker, after the generator/shorepower selector switch, would pass through the inverter and go to the auxiliary breaker.

I suppose someone could plug in a hair dryer, but that usually follows a shower and the generator is needed for the water heater.

I started with a plan for a 1000w inverter, was convinced to go to 2000w, but the Magnum power curve indicates the 2800w unit is more efficient at 200w, and cost about $100 more than the 2kw.

I have a blue sea fuse holder which will handle 300a fuses. I can swap out the 250 fuse for a 300a fuse, just in case, and still be under the 330a capacity of 2/0 cable.

From reading the Magnum manual it is my understanding that a DC switch is recommended. I have a Blue Sea 3 breaker AC panel to connect to the main panel breakers and then connect to the wires for the refrigerator, and two outlet circuits.

Thanks

 

[GJH]

Do not hesitate to contact Magnum for help with your installation ; you should ask them all the same questions you are asking here and default to their advice. I have found them to be very responsive and helpful, one of the biggest reasons to go with that brand.

Ok, now, I'll chime in, having just spent some time on my inverter system (all Magnum) in the process of replacing my battery bank. First, read the manual! There is very good, comprehensive and specific advice in there about how to do a lot of what you are asking about. The new manuals are far more complete than when i bought mine 5 years, so I downloaded the current version.

Second, just forget about the idea of soldering the connectors. A properly swaged crimp is a FAR superior in this application. These are heavy cables, and it is on a moving platform. Let GenuineDealz do it right for you.

So now some opinions (the above was something stronger than a mere opinion). My batteries are in series to form a 24v bank, so not exactly like yours. If I were you, I'd use the use the automotive type posts for the positive and negative leads to the inverter. Superior connection IMO. These frees up the studs for your temperature sensor and the ground back to the ground bus, and various other things that might get added on to the system on both the positive (for instance the power to the BMK in your case) and negative side. Similarly, where you are connection the two 12 volt banks in parallel, why not have one cable on the post and the other on a stud? Cleaner and a better connection. Apply anti corrosion (I use Corrosion X) AFTER making the connections, per the Magnum manual.

Good choice getting the battery monitor, very handy.

 

[Vincentc]

Thanks George,
Good thought regarding Magnum customer service.
Regarding swaged vs soldered, the Trojan Battery web site has some pretty strong words and pictures regarding this issue:

"Connectors that are mechanically crimped to the cable’s end will create
a high resistance area. At high discharge rates, this resistance can create
enough heat to melt the terminal post on the battery and/or melt the
adjacent insulation on the cable."

see http://www.trojanbattery.com/pdf/WP_BatteryCableGuide_0512.pdf

I think I will look for someone who will crimp and solder. Wonder if I can solder the crimp connection after it is crimped and before the shrink tube is heated?

 

[Robby]

Make sure you have the tempsensor that will shut the whole thing down.I've see boiling batterys recently and its a scary site.Make sure you mount the inverter/charger on a shelf that can be easily seen and monter it reguarly.

 

[Boatsb]

Thanks George,
Good thought regarding Magnum customer service.
Regarding swaged vs soldered, the Trojan Battery web site has some pretty strong words and pictures regarding this issue:

"Connectors that are mechanically crimped to the cable’s end will create
a high resistance area. At high discharge rates, this resistance can create
enough heat to melt the terminal post on the battery and/or melt the
adjacent insulation on the cable."

see http://www.trojanbattery.com/pdf/WP_BatteryCableGuide_0512.pdf

I think I will look for someone who will crimp and solder. Wonder if I can solder the crimp connection after it is crimped and before the shrink tube is heated?

Someone needs to send that to the FAA and the toasties. Mechanically crimped connectors are the industry standard. Heating the cable hot enough to solder will burn jback the insulation.

I use double crimps and 3m glue lined shrink over it. So far no failures in more cables than I care to think of. By the way the crim tool is expensive but worth it if you do it often. I thought about the hydraulic one but decided it was too much to cary into a bilge.

 

[Scottbee]

I don't have a dog in the hunt or a horse in the race... so this is just a sharing of opinions and experience. Not trying to "win" anything.

Back in my sordid youth I was engineer at a power supply company that was under contract for the Aegis Class destroyers. Had to learn a lot about the Mil Specs of the day, and what it took to keep the military happy. Didn't really enjoy it much. Anyway, we did a fair amount of work terminating cables of this size. We had multiple high-current failures of swaged terminals (that had met go, no-gauge criteria) and petitioned for a change to solder-pot terminals. That resulted in a helluva lot of electrical, thermal, and mechanical testing. The "slip-in" solder pot method was finally chosen as the preferred method, and it is personally what I have used ever since. Mechanically it was stronger (in tension) than the swaged configuration, and from a resistance/conductance perspective it was far superior.

As has been noted, soldering AFTER swaging is problematic because the requisite heat will usually torch the insulation. You can try to heat-sink the cable to eliminate or reduce this, but it requires a semi-custom clamp heatsink and makes the soldering more difficult (but not impossible).

The solder-pot method is pretty simple and pre-fabricated solder slugs (with flux) properly sized for the cable and terminal are inexpensive and readily available. They are color coded corresponding to cable gauge. You simply secure the terminal (I use a small vice), drop in the slug and then heat (with a torch) until the solder melts. Slowly and carefully slide the bare cable end into the "pot" while continuing to apply heat. Stop when you see the solder begin to "wick" up the strands and deplete the "pot". Remove the heat and allow the assembly to cool. The net result will be a VERY strong mechanical joint with good conductivity.

I'm also a BIG fan of the battery monitors. That wasn't an option with my inverter, so I had to go with an external unit. I have a Victron BMV and I find it to be an invaluable tool.

 

[Scottbee]

Happened to find this online with a quick search:

http://www.youtube.com/watch?v=SXDkNMDDrBs

It's fairly similar to the method that I described. The only change that I really make is that I slather on some sealant before installing the heat shrink to ensure that it is hermetic.

Enjoy!

 

[Robby]

Someone needs to send that to the FAA and the toasties. Mechanically crimped connectors are the industry standard. Heating the cable hot enough to solder will burn jback the insulation.

I use double crimps and 3m glue lined shrink over it. So far no failures in more cables than I care to think of. By the way the crim tool is expensive but worth it if you do it often. I thought about the hydraulic one but decided it was too much to cary into a bilge.

Scott when they cut the wiring and cables out of my Cessna 172 of corse I had to change the cables but I butt spliced everything else back to gather and five years later no problems.While reading the Zantrex installed Manual a day or so ago they said butt splices were ok---

 

[gbharrington]

Happened to find this online with a quick search:

http://www.youtube.com/watch?v=SXDkNMDDrBs

It's fairly similar to the method that I described. The only change that I really make is that I slather on some sealant before installing the heat shrink to ensure that it is hermetic.

Enjoy!

I've been following this thread with interest as I'm facing the same issue in a few weeks, so I'm grateful for the discussion and especially so for the explanation and description of "pot soldering". I believe that will be my prefererred method going forward.

 

[Boatnut]

Just to bore everyone by getting back to the original question.

Either the bolt or the stud mount will work satisfactorily, the difference is in the quality of the connector, and the use of the proper crimping tools.

Soldering is always an option. Just remember soldering is a good conductor, but has little mechanical strength. The cardinal rule of soldering is, frist a strong mechanical connection then a proper solder connection.

Crimp connectors as most things come in good and bad. Home cheapo as usual leads the race for cheap lousy crimps. Go to a good electrical supplier or battery shop. Thomas and Betts are a quality connector, as are the Amp connectors, and plugs.

Typically if the insulation is melting when you solder, you are using cheap wire, or too much heat. You are better off with a big 100 watt iron than a torch.

 

[Scottbee]

Typically if the insulation is melting when you solder, you are using cheap wire, or too much heat. You are better off with a big 100 watt iron than a torch.

I don't think a 100W iron is going to put much of a dent into soldering a 4/0 terminal lug onto a battery cable. The cable is conducting the heat away faster than the 100W iron can supply it.

 

[Boatnut]

I don't think a 100W iron is going to put much of a dent into soldering a 4/0 terminal lug onto a battery cable. The cable is conducting the heat away faster than the 100W iron can supply it.

Works for me, have you tried it?