House battery plan: reasonable or foolish?

[Tim Moses]

I happen to have two Marine Deep Cycle batteries (they came with the bus) and a Tripp Lite inverter/charger (of unknown origin). I have a vague idea of how I can hook these up, but I would like to run it by someone else to be sure I'm not doing something stupid and I have a few questions.

2 845A Interstate Batteries batteries - http://goo.gl/eflW0
Tripp Lite 1000 watt APS1012 inverter/charger - http://www.tripplite.com/shared/product-pages/en/APS1012.pdf

Here's the text version of what would be a whole lot better as a schematic...
Bus chassis -> neg on battery 1 -> neg on battery 2 -> neg on inverter.
Pos on inverter -> fuse and fuse panel -> pos on battery 2 -> pos on battery 1.
Ground on inverter -> bus chassis.

Basically, wire the batteries in parallel to get 1690A at 12V, right?

Questions:
1) I couldn't find a listing of amp hours for those batteries. Can that be calculated (or guessed at) from the specs on the website?
2) What size fuse do I need between the inverter and the batteries?
3) What's the formula for how many amps I could safely draw and for how long?
4) Same as #3, but when plugged in and charging batteries?

Thanks for whatever help I can get!

[Sean]

...
Here's the text version of what would be a whole lot better as a schematic...
Bus chassis -> neg on battery 1 -> neg on battery 2 -> neg on inverter.
Pos on inverter -> fuse and fuse panel -> pos on battery 2 -> pos on battery 1.
Ground on inverter -> bus chassis.

1. I recommend you wire both positive and negative from the inverter all the way back to the batteries, rather than use the chassis for the return.

2. Make sure you connect the batteries "on the diagonal", meaning you take the positive output from one battery and connect the negative of the other to ground, with straps connecting both positives together and both negatives together.

3.  System catastrophe fuses should generally go on the negative side of the batteries, not the positive.  So the master fuse goes between the negative of the first battery and the ground post, and the load negative(s) go to that same post.

Basically, wire the batteries in parallel to get 1690A at 12V, right?

Yes, you will wire them in parallel, but unless you are using these to start the engine, the cranking amps is not really relevant, so you will never have 1,690 amps from these.

Questions:
1) I couldn't find a listing of amp hours for those batteries. Can that be calculated (or guessed at) from the specs on the website?

These are about 105 AH at the 20-hour rate, so this would give you a total of 210 AH. Source:
http://www.interstatebatteries.com/cs_eStore/content/product_info/marine_f.asp
Remember, though, that these are hybrid batteries, not deep-cycle, so their performance will not match true deep cycles of the same size.

2) What size fuse do I need between the inverter and the batteries?

The inverter manufacturer specifies a fuse of 225 amps, per the spec sheet you linked.  However, it is more important that you not fuse higher than the ampacity of the wire you are using; to go to a 225a fuse you would need to be using 00 (two ought, also written 2/0).  The inverter itself is rated for a draw of 95 amps, so figure your wire size and fusing accordingly.

3) What's the formula for how many amps I could safely draw and for how long?

What do you mean by "safe"?  Are you talking about running the batteries down, or not overloading the inverter or the wiring?  The inverter spec says you can draw 1500 watts for an hour, or 2000 watts for a few seconds.  1000 watts is the continuous rating.

4) Same as #3, but when plugged in and charging batteries?

Again, what do you mean by safe?  Also, if you are plugged in to shore power, it is better to switch the loads over to it as well, rather than continuing to power them from the inverter.

-Sean
http://OurOdyssey.BlogSpot.com

[TomC]

Sean-why should the fuse go on the negative side of the battery?  Good Luck, TomC

[bevans6]

I don't know the answer to that either.  I've seen arguments that by fusing the ground you don't need to insulate the fuse/holder, since it is at ground potential.  But to me it's only at ground potential if the fuse in intact, as soon as the fuse is gone one side is at supply potential.  I also want to be able to remove all supply potential from the load device.  If I fuse the ground, then supply potential still exists in the entire positive wiring run and into the load device, which could have other ground opportunities through loads or control wiring or whatever.  Easier for me to remove the positive source rather than open the ground path.  I have read that Coast Guard requires the fuse to be in the positive path, but I really don't have a good source for that, just marine discussions and FAQ's.  The idea of fusing the ground connection seems to be in low DC voltage situations only, AC supply requires bonded grounds and fuses positive, as does DC over 50 volts as far as I know.

So call me confused on this too!

Brian

[Sean]

Tom,  a catastrophe fuse on the ground side protects you from shorts that would happen "upstream" of wherever the fuse would be on the positive side, including the positive battery posts themselves.  More than one person has gotten in trouble by swinging a wrench in the battery compartment and shorting the positive buss bar to the frame, and the more batteries you have in parallel the worse this can be.  Having the fuse between frame and negative protects against this.

Those of us with 24-volt systems don't have this option if we have 12-volt loads tapped off the center.  That's because blowing a ground-side fuse can put reverse polarity across the 12-volt loads, some of which might be damaged.  We instead need two fuses, one each for the 24 volt and 12 volt feeds, and these should be as close to the batteries as possible.  And definitely remember to turn everything off and then disconnect the ground wire before working on the bank.

Note here that I am talking about main or master fuses, not individual load fuses, which should go in the positive line to the load.  But usually inverter/chargers do not have separate load fuses and instead rely on the master fuse for protection.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

...  I've seen arguments that by fusing the ground you don't need to insulate the fuse/holder, since it is at ground potential.

This is true, but I would not consider it the main reason for a ground-side main fuse.  The insulated covers for the class-T style holders are a couple bucks at most.

But to me it's only at ground potential if the fuse in intact, as soon as the fuse is gone one side is at supply potential.

Well, it would be more proper to say that, if the loads are "on", supply potential exists across the holder -- the frame would actually be at supply potential.  Importantly, main disconnect switches go on the positive side of the battery, and you should open the disconnect before servicing the fuse.

I also want to be able to remove all supply potential from the load device.

Correct  -- see above.  Individual load fuses, as well as main disconnect switches, all go on the positive side.  Only the catastrophe fuse goes on the ground side.

If I fuse the ground, then supply potential still exists in the entire positive wiring run and into the load device, which could have other ground opportunities through loads or control wiring or whatever.

No.  Once the catastrophe fuse opens, the frame is no longer at battery-negative potential, and the positive-fed loads are free-floating.  With no way to complete the circuit to battery negative, there is no possibility for any load return to find its way to "ground" other than the frame, which is now bonded to battery positive.  From an electrical standpoint + and - on a battery are arbitrary concepts, and electrically there is no difference in putting the fuse on one or the other.  It is choosing to bond the frame to one of them that defines "ground potential" and opening the frame connection changes that immediately.

Easier for me to remove the positive source rather than open the ground path.

You should not use the fuse as a disconnect switch.  Again, the main disconnect goes on the positive side.  Using individual load fuses as disconnects is also fine, but they, too, belong on the positive side.

I have read that Coast Guard requires the fuse to be in the positive path, but I really don't have a good source for that, just marine discussions and FAQ's.

The Coast Guard has no involvement in the electrical design of marine vessels. UL, CE, CSA, and various insurance underwriters do (e.g. Lloyd's), and many pleasure boats in the US follow a voluntary set of standards promulgated by the American Boat and Yacht Council (ABYC).  But that's not really relevant -- boats have a whole different set of issues.  Many things found on a boat would not pass muster under the NEC, just as many things on a bus will not meet ABYC standards, and there are places where the standards conflict with one another.  I am familiar with both, and discussing why things are a certain way on a boat but different on a bus or in a house could fill a book.

Hope that clears things up a bit.

-Sean
http://OurOdyssey.BlogSpot.com

[bevans6]

Actually, as soon as I gave my head a shake and realized the fuse to ground you are advocating was on the ground connection of the battery and not the load, all made sense and I was in complete agreement with your point.  My poor brain wanted to know why is he putting the load fuse on the ground side, which confused the heck out of me, and of course you were not.

What is your thought on a ground side fuse for loads like starter motors?  Or house banks that can be tied to assist in starting when needed?

Brian

[Sean]

What is your thought on a ground side fuse for loads like starter motors?  Or house banks that can be tied to assist in starting when needed?

Fusing the starting system is a completely different subject.

Since you brought up boat standards, an interesting note is that the starting system is the one place on a boat that no fuse is required, and generally there are no fuses in the starting system on a motor vehicle, either.  Some vehicles incorporate a sacrificial piece of wiring known as a "fusible link" to serve as catastrophe protection in the main battery circuit, which would include starting, but it is uncommon.

As far as tying house batteries to the starter, you really need to think hard about how much current might be carried on that link.  We opted not to fuse the house-to-chassis intertie system, so a short in that system could lead to a catastrophic failure.  OTOH, I can start the bus with the house batteries alone, should the chassis batteries be completely dead.  A fuse that could handle that much current would have to be either very high amperage, or very slow blowing.  (On a side note, were I to do it over again, I would not have separate banks at all, and use the house batteries to start the coach 100% of the time, since I have a separate small 12v battery to start the generator.)

If you use a ground-side catastrophe fuse on the house system, then that fuse will protect the intertie from that end (so it had also better be large enough), but be aware that while the intertie is engaged, the catastrophe fuse is essentially bypassed since the chassis battery will be grounded to the frame, meaning the chassis alternator could pump fault current into the loads.

In short, the whole fusing picture becomes more complex when dual battery and charging systems are involved and interconnected, and it requires careful planning.  Such a setup might be cause for having a positive-side catastrophe fuse on the house system rather than a ground-side one.

-Sean
http://OurOdyssey.BlogSpot.com

[eagle19952]

[/quote]
snip....(On a side note, were I to do it over again, I would not have separate banks at all, and use the house batteries to start the coach 100% of the time, since I have a separate small 12v battery to start the generator.)

-Sean
[/quote]

This what I have and am happy for it,I am glad you feel this way.

[jjrbus]

Another consideration is the charging capability of the Tripp Lite. The Tripp Lite I had was a low amp single stage charger and it would take a loooooong time to charge the batteries. Not something I would do with a generator

[kevink1955]

The OP was told in one of the first posts to run both the Pos and Neg from the inverter directly to the batterys and not use the chassis for ground, I agree with that but now if you put a fuse in the negitive battery connection to the chassis you have no protection for the inverter and it's feed wires.

Cannot have it both ways, you need a plan and then have to stick with it

[Brassman]

The catastrophe fuse is first after negative post, no matter what else you do.

[Tim Moses]

Thank you for all the responses. Sean, nice response time, within an hour after my midnight post.

1. I recommend you wire both positive and negative from the inverter all the way back to the batteries, rather than use the chassis for the return.

2. Make sure you connect the batteries "on the diagonal", meaning you take the positive output from one battery and connect the negative of the other to ground, with straps connecting both positives together and both negatives together.

3.  System catastrophe fuses should generally go on the negative side of the batteries, not the positive.  So the master fuse goes between the negative of the first battery and the ground post, and the load negative(s) go to that same post.

I think I was describing the same thing except for the fuse. I'll try it this time with a picture. Is this where the fuse should go? How about the rest?


Sean,
Based on your recommendation at the bus rally, I have been looking at some electric refrigerators. I'm trying to do the math to see how much a refrigerator will draw and how long I can run it with this setup.

Say I pick a cheap, 350kWh/yr apartment refrigerator. I know there are more efficient refrigerators in that range, but I'm just picking that number to work out the calculations. Also, I'm not factoring in the loss of conversion from DC to AC. It's not the part of the calculation I'm wondering about.

Convert both to watt hours.
Refrigerator: 350kWh/yr = 39.95Wh (on the average)
Batteries: 210Ah x 12V = 2520Wh
To find the amount of time I could run the refrigerator off the batteries, do I simply divide the batteries' Wh by the refrigerator's Wh to get about 63 hours? This is assuming nothing else is running off the batteries. Am I doing that right?

[viento1]

snip....(On a side note, were I to do it over again, I would not have separate banks at all, and use the house batteries to start the coach 100% of the time, since I have a separate small 12v battery to start the generator.)

-Sean
[/quote]

Thats what I did aswell, seems to work just fine and makes use of all that space. The monster Alternator puts out enough current for the inverter to run the AC without me wasting extra fuel on the Genset

[Oonrahnjay]

(snip)
Convert both to watt hours.
Refrigerator: 350kWh/yr = 39.95Wh (on the average)
Batteries: 210Ah x 12V = 2520Wh
To find the amount of time I could run the refrigerator off the batteries, do I simply divide the batteries' Wh by the refrigerator's Wh to get about 63 hours? This is assuming nothing else is running off the batteries. Am I doing that right? 

   I haven't checked your math but first thing is you must not (in practice) draw your batteries down more than 50% SOC (State of Charge).  So that knocks you down to about 31 hours, following your example.  Also, refrigerators are odd in their wattage use.  It varies a lot depending on air temperature around them, and any other conditions (are you cooling off warm contents, is the door opened often, is the storage space full or empty, etc.) that alter the %-age of the time that the compressor runs.  Typically the compressor runs a portion of the time, during which the compressor brings down the temperature to a given level, then switches off.  Over a period of time, the temperature will rise to a point where the thermostat kicks on the again.  Changes in the relative on/off time will make a difference in the wattage draw.  You really can't make a good estimate based on the kW/h/yr rating.