# Wiring Battery Bank

Solar Expert Posts: 121 ✭✭✭
Hello to all! I am in the wiring design phase and have a few basic questions that I hope to get advice on.

I will have 4 T-105 batteries. They are each 6 volts and 225 amp hours. In order to achieve a 12 volt system I will have to wire them in series to increase the voltage...correct?
In order to do this I connect the four batteries + to - then run the + inverter cable from the positive battery terminal to the + terminal on the inverter and the - inverter cable from the negative terminal to the - terminal on the inverter...correct?

How many battery interconnect cables will I need...3?
Will I need only a pair of inverter cables?

I hope that I am clear on what I am asking.

• Banned Posts: 17,615 ✭✭
Re: Wiring Battery Bank

You will need both series and parallel connects:

Two 6 Volt batteries in series = 12 Volts.
Two banks of these in parallel will double the Amp/hour capacity.
Vis:

(-) Battery 'A' (+)<->(-) Battery 'B' (+)

(-) Battery 'C' (+)<->(-) Battery 'D'(+)

(-)
Inverter
(+)

That's three battery interconnections altogether.
But note; There must be a fuse between the battery bank and the Inverter.
Also, for best results the (-) and (+) Inverter connections are made to diagonally opposite corners on the battery bank. To correct the above example. the Inverter should be connected to (-) on Battery 'C' and (+) on Battery 'B'. A charge controller for the solar panels (or other charge source) would then be connected to the (-) on Battery 'A' and the (+) on Battery 'D'.
• Solar Expert Posts: 121 ✭✭✭
Re: Wiring Battery Bank

Thank you Cariboocoot! That clarifies it...I figured a series/parallel connection would be better.

I see the two connections: Battery 'A' (+) to Battery 'B' (-) and
Battery 'C' (+) to Battery 'D' (-)
Sorry for being so dense, but where is the third battery interconnect?

Does the breaker go on the (+) line from Battery 'B'?
I read that this should be less than 18" from the battery...correct?

A breaker is needed between the charge control and the battery...correct?

Do they make DC breakers that connect in-line?
• Solar Expert Posts: 27
Re: Wiring Battery Bank

Crystal, They make fuses for the charge controler to battery and from battery to inverter fuses. Don't froget the ground
• Solar Expert Posts: 334 ✭✭
Re: Wiring Battery Bank

Hey Crystal,
You do want a circuit breaker or a fuse between the Charge controller and the battery, and it's also good to have one between the charge controller and the PV panels. For this reason it may be better to just have a small box like the midnite baby box or similar, you could mount both breakers/fuses right next to each other and still have room for another breaker for your battery -> inverter wire. Just remember that each power source needs to have breaker or fuse to protect that wire run: PV to Charge control, battery to charge control, battery to inverter, etc...

As for the interconnections I actually would count four: battA->B series connection, battC->D series connection, and then the positive parallel connection (B+ -> D+) , and then negative parallel connection (A- -> C-). So what you in essence are making is two 12V batteries: A+B and C+D, and then tying the positives together and negatives together (paralleling). Since you have two parallel batteries make sure that you hook the Positive and negative leads to opposite corners of the battery as Marc said.

Depending on how you wire things, you could simply have positive and negative buss bars that all DC components (chargers, inverters, DC load center?) would connect to and from those bars you could run just one set of heavy gauge wire to the actual batteries: current would flow one way while charging or the other when discharging. I find this makes a nicer cleaner installation at the batteries by eliminating a mess of extra wires and multiple connections on the terminals. I do believe code says you need a fuse/breaker within 18" of the battery, but I would double check with your local inspector if it will be inspected otherwise just keep the first wire protection as close as possible to the battery.
• Banned Posts: 17,615 ✭✭
Re: Wiring Battery Bank

Sorry for the confusion: after 6 PM I can't count right!
Four interconnects: one between each battery in the series stringing, and one each for connecting the (+) and (-) of the two banks.

Fuses are important. You need at least one between the batteries and the inverter and one between the charge controller and batteries. Some may advocate a separate fuse per battery bank, and some even recommend fusing between the batteries in series as protection against accidental shorts.

As for fusing the PV array, it depends on what that array is. Generally speaking, one string of PV's doesn't require fusing, but if you parallel strings you need to fuse each. Your battery bank would want approximately 480 Watts of panel, and this can be accomplished either with parallel panels, series connected panels, or a combination of series/parallel. If you have panels in series you will need an MPPT type controller to 'down convert' the higher array Voltage to system Voltage.
Re: Wiring Battery Bank

Using fuses at the positive end of each battery string and between any loads (like battery charger to battery bank) that are rated no larger than the wiring (or device) can support.

For the inverter, you probably would want to use a switch+fuse or Circuit Breaker to turn if off when AC power is not needed. Inverters generally draw ~6 watts of power when "idling" with no loads and that is a significant amount of power for 24 hours per day x 7 days per week.

Breakers are nice--but are usually terribly expensive at the high current / DC ratings required for an inverter. A fuse+marine type transfer switch is much less expensive (try to only turn the switch off when your AC loads are already turned off--high DC current switching will wear out the switch overtime).

There are inverters that have "standby" modes where they only turn on if there is a ~6 watt or greater AC load--you could get by without a DC side switch--but I would still recommend one (turning things off when you are gone).

Our host lists lots of components for wiring--their site will give you a good idea of what is available.

Large Fuses and Fuse Blocks
Large Breakers
More Breakers and Bus Bars
350 Amp Marine Switch

As always--the Internet safety warning... Lead Acid Batteries can output a seriously dangerous amount of current (1,000's - 10,000's of amps into a dead short). Also, they generate hydrogen gas when charging (explosive) and are filled with fairly strong sulfuric acid (eats holes in clothes, hydrogen explosion can drive acid into eyes, etc.).

Typically, wrap all metal tools with electrical tape when you work on the battery bank to reduce the chances of accidental electrical shorts. Remove all jewelry (rings, bracelets, watches, necklaces, etc.).

If something falls across a bank connection--it is very possible for the tool to be welded to the terminals/cables and very difficult to remove.

Proper cable termination (crimp connectors, clamp connections, etc.) must be made solid so that cables cannot pull out of their sockets. Also, poor electrical connections can overheat and fail/cause fires.

Get professional help if you do not know, for sure, what you are doing.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 121 ✭✭✭
Re: Wiring Battery Bank

All this information is useful...Thank you!
As for fusing the PV array, it depends on what that array is. Generally speaking, one string of PV's doesn't require fusing, but if you parallel strings you need to fuse each. Your battery bank would want approximately 480 Watts of panel, and this can be accomplished either with parallel panels, series connected panels, or a combination of series/parallel. If you have panels in series you will need an MPPT type controller to 'down convert' the higher array Voltage to system Voltage.

We will have 2 Evergreen ES-200-A. This will be 400 watts and we have a generator when the batteries need to be charged more. The plan is to connect Panel 'A' (+) to Panel 'B' (-) with a 10 foot MC cable, then use a 3 foot MC cable and cut it in half to use both ends to come off the panels and splice to higher guage wire to the charge control.
Would you use splice bolts or wire nuts? What are the pro/cons in these two options?
Is there any problems in this design?

We will be getting the Rouge 30 amp charge control...according to the manual it will 'down convert' 24 volt to 12 volt.
Don't froget the ground
What guage wire is suggested for 400 watts?
Depending on how you wire things, you could simply have positive and negative buss bars that all DC components (chargers, inverters, DC load center?) would connect to and from those bars you could run just one set of heavy gauge wire to the actual batteries: current would flow one way while charging or the other when discharging. I find this makes a nicer cleaner installation at the batteries by eliminating a mess of extra wires and multiple connections on the terminals. I do believe code says you need a fuse/breaker within 18" of the battery, but I would double check with your local inspector if it will be inspected otherwise just keep the first wire protection as close as possible to the battery.
I have been in contact with NAWS. They suggested using wire nuts to splice rather than busbars. I realize that it would be much cleaner and nicer to have the busbars and they are really not that expensive. What guage wire would you run to the batteries from the busbars?
What are the pro/cons of these two options?
NAWS also suggested the MNBabybox since the Samlex 300watt inverter only requires a 40 amp breaker.
Breakers are nice--but are usually terribly expensive at the high current / DC ratings required for an inverter. A fuse+marine type transfer switch is much less expensive
I was looking at the 40 amp and smaller Outback breakers...I have not found small fuses on NAWS. I don't believe that I will need bigger than 40 amp breaker for my projected system...unless I am mistaken.

Here's the list of components:
2 - Evergreen ES-200-A
20 amp breaker
#4 welding cable
Rouge 30 amp charge control
40 amp breaker
#10
4 - T-105 batteries
4 - #2/0 interconnect cables 12''
Samlex 300 watt inverter
40 amp breaker
1 - pair #2/0 inverter cables 5'
IOTA 55 amp battery charger w/IQ4
20 amp breaker
#6
Trimetric 2020 battery moniter w/ 100 amp shunt
3 amp fuse
#16
MNBabybox
#6 for ground
MC cables
1- 30'
1- 3'

Any suggestions or problems you see with this please let me know.
Thanks again!
• Solar Expert Posts: 334 ✭✭
Re: Wiring Battery Bank
Crystal wrote: »
All this information is useful...Thank you!

We will have 2 Evergreen ES-200-A. This will be 400 watts and we have a generator when the batteries need to be charged more. The plan is to connect Panel 'A' (+) to Panel 'B' (-) with a 10 foot MC cable, then use a 3 foot MC cable and cut it in half to use both ends to come off the panels and splice to higher guage wire to the charge control.

I'm not sure why you're ordering extra MC cables, for the interconnection the Evergreen panels have MC cables already so all you need is 1 single MC cable that is double the length of the distance from the PV panels to the nearest J-box location. Any excess MC cable (which is only 10 awg) would introduce excessive voltage drop. You would probably want to keep this distance to 15' or so if possible, otherwise you'll start loosing more watts before they even get to the #4 awg wire.
Crystal wrote: »
I have been in contact with NAWS. They suggested using wire nuts to splice rather than busbars. I realize that it would be much cleaner and nicer to have the busbars and they are really not that expensive. What guage wire would you run to the batteries from the busbars?
What are the pro/cons of these two options?
NAWS also suggested the MNBabybox since the Samlex 300watt inverter only requires a 40 amp breaker.

For general purpose splicing of 2-4 wires of the same size wire nuts would make a lot more sense. When you start getting a lot of wires, and of a wide range of sizes then a bus bar starts to make a lot more sense. If you can make all your DC input and output wires connect at one set of bus bars (after proper fusing/ breakers), then that one set of heavy gauge wires to the battery terminals.
As for the size of the wire, it would depend on the distance from those bus bars to the battery terminal and the Max charging OR discharging current from your system: in your case that would be the Iota charger's current + your combined PV input. Normal sizing for PV wires would be the ISC of your modules times 1.56, this is for the overall ampacity that the wires need to be sized for; in your case by the time you factor in Voltage drop considerations based on the IMP of both modules combined + the Iota charger you should be well covered for the ampacity rating too. I notice that you are listing a 5' inverter cable, so if you have a 5' run to the battery terminal you could use #2 awg and keep your voltage drop low and meet ampacity minimum sizing.
Crystal wrote: »
Here's the list of components:
2 - Evergreen ES-200-A
20 amp breaker
#4 welding cable
Rouge 30 amp charge control
40 amp breaker
#10
4 - T-105 batteries
4 - #2/0 interconnect cables 12''
Samlex 300 watt inverter
40 amp breaker
1 - pair #2/0 inverter cables 5'
IOTA 55 amp battery charger w/IQ4
20 amp breaker
#6
Trimetric 2020 battery moniter w/ 100 amp shunt
3 amp fuse
#16
MNBabybox
#6 for ground
MC cables
1- 30'
1- 3'

Any suggestions or problems you see with this please let me know.
Thanks again!

Again I don't see the need for both sets of those MC cables, just one set to run from the PV output leads (MC connectors) to the J-box where you can join them to your #4 awg.

If those 2/0 inverter cables are premade withe terminal lugs then I don't think you will be able to use that terminal lug with your inverter. That wire size is WAY overkill for that inverter. Back to the bus bar idea; if your inverter is somewhat close to those bus bars you could just run a short stretch of #6 awg wire from the inverters DC terminals to the DC bus bars (or a larger guage if you need to run much more than 5' or so).

Other observation that I have is that 1 baby box is not nearly enough for doing a bus bar set up as I was describing, and is likely a bit too tight for just the DC circuit breakers and wiring. Check out some wiring diagrams and count the number of wires that will need to be managed; just on the DC side you'll have: pv-> charge controller breaker, charge controller-> battery breaker, Iota charger -> battery breaker/fuse, and battery -> Inverter DC breaker, and a DC loads breaker if needed.

Then you'll have the AC breakers to house as well: Generator input, Inverter, and any AC circuits that you'll want to have and perhaps a breaker or fuse for the Iota charger too. Make sure that your inverter will NOT be able to supply power to the same loads that your generator will power while the genny is on (a transfer switch is the most "idiot proof" and safe way to do this), and make sure that your Iota charger does not get powered from the inverter (just the genny). One simple and cheap way to do this is to plug the Iota charger directly into an outlet that is wired directly to the generator input only (separate from the main AC circuits powered by both the genny and inverter).

Enough for now, I'm sure others will have further observations.
HTH,
HB
HTH,
HB
• Solar Expert Posts: 121 ✭✭✭
Re: Wiring Battery Bank

Hillbilly thanks for all the pointers.
However, I am still unclear about a few things.
I'm not sure why you're ordering extra MC cables, for the interconnection the Evergreen panels have MC cables already so all you need is 1 single MC cable that is double the length of the distance from the PV panels to the nearest J-box location. Any excess MC cable (which is only 10 awg) would introduce excessive voltage drop. You would probably want to keep this distance to 15' or so if possible, otherwise you'll start loosing more watts before they even get to the #4 awg wire.
I am not quite sure how MC cables connect to the panels.
I realize that the Evergreens already have the MC cables...they have two (male and female) coming off each panel...right?
The MC cable connects Panel 'A' (+) to Panel 'B' (-) which will be 10 feet apart (in my situation). We want to wire the panels in series to increase the voltage.
So, if my two panels will be placed 10 feet apart I should get a 20 foot MC cable?
I plan to splice the larger #4 awg wire for the run to the charge control.
If anyone can explain this MC cable wire design would be greatly appreciated.
I notice that you are listing a 5' inverter cable, so if you have a 5' run to the battery terminal you could use #2 awg and keep your voltage drop low and meet ampacity minimum sizing.
This is good to know (I thought that the inverter cable was required). So, I can connect the batteries to the inverter with #2 welding cable rather than the special #2/0 inverter cable. I suppose I would then get the proper sized terminal lugs.
The #2/0awg is not overkill for the battery bank interconnects, is it?

Thanks again!
• Solar Expert Posts: 334 ✭✭
Re: Wiring Battery Bank
Crystal wrote: »
Hillbilly thanks for all the pointers.
However, I am still unclear about a few things.

I am not quite sure how MC cables connect to the panels.
I realize that the Evergreens already have the MC cables...they have two (male and female) coming off each panel...right?
The MC cable connects Panel 'A' (+) to Panel 'B' (-) which will be 10 feet apart (in my situation). We want to wire the panels in series to increase the voltage.
So, if my two panels will be placed 10 feet apart I should get a 20 foot MC cable?
I plan to splice the larger #4 awg wire for the run to the charge control.
If anyone can explain this MC cable wire design would be greatly appreciated.

You are correct in your connections then, but I would not use a 10' span between your panels. Again this MC cable wire is #10 in size, so a 10' series connection and you're already just over a 2% voltage drop. If there is any way to avoid this long span I would install the panels closer together, otherwise this kind of defeats one of the main benefits of going with the MPPT charge controller and running the panels in series (higher voltage, less wire losses).
Crystal wrote: »
This is good to know (I thought that the inverter cable was required). So, I can connect the batteries to the inverter with #2 welding cable rather than the special #2/0 inverter cable. I suppose I would then get the proper sized terminal lugs.
The #2/0awg is not overkill for the battery bank interconnects, is it?

Thanks again!

You do need some wire from your inverter to your battery terminal, or bus bars if you go that route. As far as what size wire that depends again on how far you'll run it. The battery interconnect may well be a bit on the large size, but I see no reason to go any smaller on battery interconnects (the wire is a short piece so it's not going to save you much money to downsize this). Overkill is good on battery interconnects.
BTW that book I had recommended has voltage drop charts in it that would allow you to consider all sorts of different installation options and what corresponding wire size would be needed (among lots of other good info too).
Good luck
HB