Battery cable size for 48V system

2twisty · December 2014

Right now, I have 8 GC2s wired to 48V. In the next few days, I will be picking up 8 more to double my bank size (my original 8 are only a few months old, so I think I can get away with adding batteries) Batteries are 208AH each. My loads are not huge, but many run all day -- I got the larger inverter for the occasional large surge load. Most of the time, my loads are under 1kW, and for about 80% of the time, they are under 500W.

Anyway, my question is about the cables to connect the batteries. Right now, I've got 4AWG cable, and I know that's probably not large enough--but it was what I could afford when I first assembled the system.

The batteries will have 1-2" between them for ventilation, so each series interconnect will be approximately 8-10". The length from the combined battery bank to the inverter will be about 10', since my batteries will be in a shed outside on the other side of a wall. I have 4/O cable that I will use for that 10' run (I used to be on 12v and bought this cable but never used it). For the series connections between the batteries in each string, what size? 2AWG? 1/O? 2/O?

I can afford to run 2/O (barely), but if something smaller is fine, I don't want to waste money on the cable.

I can't seem to find a good thread that discusses this for 48V systems.

Cariboocoot · December 2014

Re: Battery cable size for 48V system

At full continuous power your 3648 will pull about 80 Amps. 4 AWG would actually handle that for the connections between batteries.

The surge rating is double that current, but momentary. For a few seconds at 7200 Watts the 4 AWG would handle it, but it would be better to go up in size. 1/0 will handle approximately 2X the current of 4 AWG.

MarkC · December 2014

Re: Battery cable size for 48V system

2twisty wrote: »

Right now, I have 8 GC2s wired to 48V. In the next few days, I will be picking up 8 more to double my bank size (my original 8 are only a few months old, so I think I can get away with adding batteries) Batteries are 208AH each. My loads are not huge, but many run all day -- I got the larger inverter for the occasional large surge load. Most of the time, my loads are under 1kW, and for about 80% of the time, they are under 500W.

Anyway, my question is about the cables to connect the batteries. Right now, I've got 4AWG cable, and I know that's probably not large enough--but it was what I could afford when I first assembled the system.

The batteries will have 1-2" between them for ventilation, so each series interconnect will be approximately 8-10". The length from the combined battery bank to the inverter will be about 10', since my batteries will be in a shed outside on the other side of a wall. I have 4/O cable that I will use for that 10' run (I used to be on 12v and bought this cable but never used it). For the series connections between the batteries in each string, what size? 2AWG? 1/O? 2/O?

I can afford to run 2/O (barely), but if something smaller is fine, I don't want to waste money on the cable.

I can't seem to find a good thread that discusses this for 48V systems.

If I understand correctly, you will have two sets of series connected 6 volts (8 batteries) that are in parallel connections to each other for a total of 16 batteries. If so, do you have enough 4 AWG to run two sets of the 10 ft (four total runs) to the inverter breaker/fuse? You could then run a short section of larger cable from the breaker/fuse to the inverter - although likely not actually necessary. The negative side run both cables to a common lug.

I'm using 4 AWG for a similar run from a single set of 4 - 12 volt golf cart batteries for a nominal 4000 watt 48 V inverter. (with a second parallel set as a spare battery set!)

http://assets.bluesea.com/files/resources/newsletter/images/DC_wire_selection_chartlg.jpg

2twisty · December 2014

Re: Battery cable size for 48V system

ok. Since my surge loads rarely exceed the normal full output of my inverter, and because I already have one bank of batteries interconnected with 4AWG, I think I will use 4AWG between the batteries and use the 4/O that I bought and haven't used for the link between the bank and the inverter. It will be overkill, but since I will be going 10-15', it will help keep the losses down.

Using the 4AWG for the batteries will keep my wire cost down since I will only have to buy wire for the new string, and since it's 4AWG, it will keep the cost in check, too.

Thanks for your input.

2twisty · December 2014

Re: Battery cable size for 48V system

OOOH! Bonus!

I just realized that the 4AWG wire that currently goes from my batteries to the fuse block is long enough to be recycled into interconnect cables. WOOT! Looks like I don't need to buy wire at all! I'll use the 4/O for that job in the new arrangement. Right now, my batteries are all inside my living space and I only have 8. There's no way I can handle 16 of them in this 12x20 cabin, and besides, it's not a fantastic idea to have sources of flammable gas in your living space.

I plan to pour a concrete slab outside the house on the other side of the wall for the batteries. I'll then build a little shed over them.

Cariboocoot · December 2014

Re: Battery cable size for 48V system

Just a nota bene: the risk of "inflammable gas" from batteries is often exaggerated. Even under EQ charging conditions (highest Voltage, most rapid gassing) they do not produce enough to be hazardous in an open environment. The problem usually occurs when people try to protect against this hazard by isolating the batteries in a compartment; there the density of the gas can build to dangerous levels. Ironic, no?

This is also why those "run your car on hydrogen" scam devices do not work; the electrolysis process does not operate fast enough to supply any appreciable amount of hydrogen.

2twisty · December 2014

Re: Battery cable size for 48V system

Right. I have always been skeptical of that warning, but I was concerned that 16 Electrolysis devices might make enough to cause a problem. The bigger concern is floor space. 8 batteries take up a good amount of space, and 16 would be double that. Living in a 12x20 cabin means that you have to be extremely careful about how much space something takes up.

That's the biggest reason that I want to get them out.

2twisty · December 2014

Re: Battery cable size for 48V system

I was wondering about using electrolysis as an opportunity load. Compress and store the H2 and O2 and then use them to run a generator or some other combustion-based tool.

I don't have much for opportunity loads in my setup, so I was trying to think of things I could do with the surplus I'm going to have in the summer. I am setting up my system to make plenty of power in the winter months, so I should have power falling out of my ears during the summer.

BB. · December 2014

Re: Battery cable size for 48V system

2twisty wrote: »

I was wondering about using electrolysis as an opportunity load. Compress and store the H2 and O2 and then use them to run a generator or some other combustion-based tool.

I would stay away from it. Hydrogen gas is a difficult one to work with. The molecules are so small, that they cause hydrogen embrittlement for standard plumbing and tanks. And require very large tanks and/or very high pressure to store any significant quantities.

Add the issues of separating the oxygen from the hydrogen during production--An explosive mix--I would not--Unless you plan on a lot of study and engineering work.

The few hydrogen generator systems I have seen (not that I know anything about them), use low pressure and only keep a small amount of gases inside the equipment--Basically generate the gasses on demand (like for hydrogen welding of aluminum).

-Bill

Cariboocoot · December 2014

Re: Battery cable size for 48V system

2twisty wrote: »

I was wondering about using electrolysis as an opportunity load. Compress and store the H2 and O2 and then use them to run a generator or some other combustion-based tool.

I don't have much for opportunity loads in my setup, so I was trying to think of things I could do with the surplus I'm going to have in the summer. I am setting up my system to make plenty of power in the winter months, so I should have power falling out of my ears during the summer.

Yeah I think maybe you just missed the part in my post about how the H and O become a problem if they are confined to a closed space. Bottling them up would fall under that heading. You may have also missed the part about the run-your-car-on-water stuff not working due to the slow nature of the process; even if stored it will take an awful long time to get enough gas to even fire a motor. Never mind that you can't just feed pure H into a gasoline motor and have it work without issues. Sorry I didn't mention the low-return-for-complexity aspect of this.

Surplus power is better spent doing real work like pumping or heating water. Much simpler to achieve, safer, and far better payback for the investment.

zoneblue · December 2014

Re: Battery cable size for 48V system

These sorts of questions are always solved by going back to fundamentals. In this case its ohms law.

Work out what the voltage drop is going to be with the proposed cable and decide whether its acceptable to you. Then double check the current isnt going to blow the ampacity rating.

This one works as well as any, just remember to put the one way distance in.

http://www.calculator.net/voltage-drop-calculator.html

mike95490 · December 2014

Re: Battery cable size for 48V system

With the large wires, be SURE you use a good hydraulic crimper for the lugs. Since you are doing a bunch of them, it should be worthwhile to buy a crimper. Ebay has a 10 ton for about $60 new, from a store.

2twisty · December 2014

Re: Battery cable size for 48V system

I'm only making a few connections with the 4/O. I have a crimper for the 4AWG ones. It's not hydraulic, but it makes good crimps.

I have a friend with a hydraulic crimper, so if his will do either the 4/O or the 4AWG, I'll borrow it to be sure.

For the 4/O crimps, I have the hammer type and a 5lb sledge that I can whack them with.

706jim · December 2014

Re: Battery cable size for 48V system

Interesting to see that you live in such a small place.

My previous cabin was 12 x 16, so even smaller than yours.

Anyone who has ever lived in this tiny of a space can appreciate the value of floor space.

Those batteries have gotta go outside!

oil pan 4 · December 2014

Re: Battery cable size for 48V system

All 4 gauge wire was not created equal.
100 amps continuously is on the ragged edge of what 90'C THHN 4 gauge wire can handle as long as the wire is not in an environment above 25'C. Lower temperature 60'C 4 gauge UF wire can only handle 70 amps.
In welding, 4 gauge is used for up to about 150 amps but only intermittently.

Don't compress HHO gas.
Some stupid HHO experimenter tried compressing and storing HHO not too long ago. When the experiment reached its only logical conclusion the compressor tank exploded and killed that guy and injured 2 other people.

2twisty · December 2014

Re: Battery cable size for 48V system

Well, firstly, if I were going to collect HHO, I would store the H2 and the O2 separately. I wouldn't be stupid enough to store it combined. Secondly, I'd use proper gas bottles to do it. I would not store it in an air compressor tank. Thirdly, I would have these tanks at least 150' from my living space.

That said, it was just an idea for how to use excess energy production. Heating water isn't really an option for me since I use on-demand propane water heat. I don't have a storage tank to divert to. It's more likely that I'll run a window AC unit as an opportunity load, since the time of year when I will have the excess is also the time of year when it's the hottest out here in the West Texas desert.

Space certainly is a premium here! In our 12x20 cabin, we have myself, my wife, 3 pomeranians and 2 cats. The entire perimeter of the cabin is taken up with stuff: shelves, bed, poo bucket, TV, fridge, stove, sink table and freezer. by moving those batteries outside, I get a BIG hunk of floor that I can use!

westbranch · December 2014

Re: Battery cable size for 48V system

tank exploded and killed that guy

Darwin at work again! Too bad he injured others.

oil pan 4 · December 2014

Re: Battery cable size for 48V system

http://www.vcstar.com/news/1-killed-2-injured-in-explosion-in-simi-valley-ind

Battery cable size for 48V system

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