DC disconnects/fusing etc questions

Re: DC disconnects/fusing etc questions

santaigo wrote: »

Small off-grid cabin in Baja that will be used for 6-8 days at a time, about 4 times a year. Here's a drawing of my simple set-up;

Just my two cents worth--You need enough solar panels to keep the battery bank "floating"--any extra panels are going to be expensive when the place is only used 4x per year.

Using a generator to charge the bank (AM charging, use batteries for balance of day/night) will probably be more cost effective until somebody lives there nearly full time.

Notice that I start with two 195-watt, 13amp panels but will eventually expand to four: the charge controller and the inverter are sized to handle that. My question on this thread is switching and fusing. I am a putter-arounder sort of guy and hate to spend a lot of money on off-the-shelf stuff for the 4kw system designs NOB: for example, this is my PV combiner box w/ #6 from the panels and #4 to the charge controller:

Really need to know more about Battery Bank voltage and if this is a MPPT or PWM type charge controller and if the panels will all be in parallel or series/parallel connected.

A simple exterior box with a grounding buss cut in half and a cover plate and for about $10 I've got it.
I do not want to buy a Midnite Solar or Xantrex DC disconnect for $200+.
1: I have in-line maxi fuses for fuses #1 and #2: any issues or recommendations for these?

With more than two panels in parallel, you should have a Series Fuse or breaker (see panel specs. for rating) in the + lead of each panel string... 4 strings, for fuses (typically in the combiner box).

You really do not need Fuse #1. You will design the system to carry the short circuit current (typically 1.56x Isc of your solar array) and since solar panels are current sources--a short circuit does not cause a huge current flow (like shorting a lead acid battery would).

For fuse #2--it should be 1.25x the Amp rating of the charge controller (if MPPT) or 1.56x the Isc of the solar array (if PWM controller). Check the charge controller installation manual for details.

2: Is a disconnect or switch between the panels and CC really necessary; and if so, what do you use (remember, this will eventually carry about 60 amps if I stay at 12v). I can get switches from the auto supply stores cheaper, by a long shot, than from solar supply sources.

Code requires a switch, and many people like to have one. But--no, normally a switch is not used for anything. Any switch rated for DC current/voltage of your array will be fine. Note that you should not plan on pulling fuses as a "under power" disconnect. Some fuse holders will arc and damage themselves if you pull the fuse holder under load.

3: For the "Circuit Breaker" in the drawing, I'm thinking of using a Perko on/off switch and a 150amp fuse that sits on the battery terminal that is often used in marine applications (I am using 2/0 wires for battery connects and to the inverter). Suggestions here will be appreciated as I only know A/C and am uncomfortable with D/C. Note: All 2/0 runs will be under 5' - fer sure.

Using marine hardware for solar systems is a good source. Pretty much the same requirements if your battery bank is 24 or 12 volts. If you had a 48 volt system--many of the marine hardware is not rated for 48-60 VDC use.

4: Grounding has really got me confused: can I use the grounding buss in the A/C panel for the inverter and the CC? In other words, can I have one grounding rod that everything, the PVs, the CC, the inverter, the load center is all tied to? If so, I'm thinking of putting a grounding buss on my make-up board, a 4'X4' piece of plywood, bond that to the load center buss and have one run to the ufer ground. By the way, per lots of input from Baja solar 'experts', I am not bonding the neutral buss to the ground buss in the load center.

One big issue with AC and DC grounding.... Depends on the inverter... Many MSW inverters (modified square wave) will short circuit if you "ground the neutral" output--and that neutral ground block is connected back to the DC battery ground block (MSW inverters tend to not be isolated between DC input and AC output).

If this is a TSW (true sine wave inverter), most of them have transformer isolation from DC input to AC output and you can bond the AC neutral to earth ground and to DC earth ground at a common point (do it one place in the system--typically done at the earth ground rod).

5: I do not quite understand this drawing: the perko switch and the fuse in #3 above takes care of the breaker in the positive run; but what is the shunt doing? It looks like it's just a pass-thru to me and the negative and grounding busses are bonded together. Why is the hair on the back of my neck standing up?

I am not understanding the "Perko Switch" and #3 fuse question.

The shunt is usually used to measure current into and out of the battery bank. You connect this to a battery monitor and it will totalize the Amp*Hours going into and out of the battery bank. The Battery Monitor will estimate the state of charge in your battery bank between 0 and 100% state of charge--I highly recommend using a battery monitor any off grid system.

By the way--A 1,000 AH battery bank (by the way, what is your bank voltage) is pretty large (and dangerous in its own write--think large arc welder) and if you are using parallel battery strings, each string should have its own 100-300 amp fuse (depending on your needs/design) to prevent a battery short circuit somewhere causing a fire.

Also, have you a good handle on your loads? You can design a large system (battery bank and AC inverter) to power anything you want....

But, for a vacation home used ~1 month out of 12--You may be better off designing the Battery Bank and Inverter to manage a relatively small amount of loads (tv, computer, lighting, radio, fan) and plan on using a genset to power your heavy loads (microwave, cooking, power tools, etc.) just when you need their use.

Battery Banks fail over time--and depending on lots of issues (cost, quality, maintenance, use, etc.) the battery bank may last you from 3-8 years (very good batteries may last your 15+ years)--You may have a heart attack every time you have to change out the bank.

Since batteries are a consumable--at this time--you may be better off minimizing battery bank size and bite the bullet on generator use (fuel, run time, noise, smoke). If you can swing a quiet genset (Honda euX000i family or equivalent)--they are very quiet and pretty fuel efficient. And it may save you some money for now.

-Bill

Re: DC disconnects/fusing etc questions

Yea, covering the panels or waiting until dark is OK...

Also if you have a breaker on the charge controller output--shut down the output and there is no current flowing through the solar panel input.

Series is the + to - to + to - "stacking of cell" -- Think of a three cell flashight--those batteries are in series. Voltage adds, current remains the same.

If you have a pickup and want extra battery for running a winch and such... You would take two 12 volt batteries and connect + to + and - to -... Here voltage remains the same and the current adds (note a 12 volt battery is actually 6x 2 volt cells in series).

A 220 AH 12 volt battery bank if it was going to charged by solar panels... 1% is the minimum for float charging. For operation from solar array, around 5-13% is a good rule of thumb. Assuming a 0.77 panel+charger derating:

• 220 AH * 14.5 volts charging * 1/0.77 derating * 0.01 rate of charge = 41 watts
• 220 AH * 14.5 volts charging * 1/0.77 derating * 0.05 rate of charge = 207 watts
• 220 AH * 14.5 volts charging * 1/0.77 derating * 0.13 rate of charge = 538 watts

So--the above gives you a good range of solar panels--Of course, we would have to know your actual loads too to figure out the minimum amount of solar panel needed to keep up with your daily loads (watt*hours, amp*hours) of usage too.

I would also suggest you look at the Morningstar 300 watt TSW inverter... It it would be a nice inverter for a small cabin--and it has "search mode" which makes it real nice for powering CFL's and such for wondering around the middle of the night and having to turn on the inverter.

MSW inverters tend to overheat small wall wart transformers and such.

-Bill

Re: DC disconnects/fusing etc questions

I don't know... Solar Guppy may--or you will have to contact a retailer or Xantrex.

-Bill

Re: DC disconnects/fusing etc questions

With solar PV systems--Conservation is the number one place to start.

Choosing lights and appliances that use as low power at practicable for your application--And usually, that means buying a meter to do your own power measurements (trust, but verify). A Kill-a-Watt meter for 120 VAC, a DC-Watt*Hour/Amp*Hour meter for DC, and/or a current clamp meter (note; sears.com is down for me right now) are all good places to start. And for the battery bank, I really like Battery Monitors.

12 volts is very difficult to get a lot of power through your copper wire, without using very large diameter wire (expensive) and it is very difficult to send 12 volt power any distances without voltage drop issues (12 volt battery, perhaps down to 11.5 volts under load, and a 1.0 volt drop takes you to 10.5 volts--few devices will operate correctly below that voltage).

Another issue is that it is not really 12.0 volts... It is 10.5 volts to 15.0 (or even 16.0 volts) for an Off Grid battery system. Many 12 volt appliances are designed for cars/RV's where the battery voltage is nominally around 13.8 to 14.2 volts... And if you operate the appliances outside that range, they may not work correctly or even fail (like DC adapters for laptop computers, etc.).

The downside with inverters--Electrical noise; a real problem if you are a HAM radio operator or like to listen to AM radio at night (you are out in the middle of no-where)--Inverters and even charge controllers can be a real pain.

Also, there are losses with inverters... They are around 85% efficient or so--But if you run a large inverter with no, or very light loads, they will consume a fair amount of power just being "on" (6 watts for smaller inverters--30 or even 60 watts for very large inverters is not unusual with no load). Some inverters have a "standby or search mode" which can reduce standby power a bunch... Basically turn on once a second for a few cycles looking for 6+ watts of power. For a 12 volt system, the MorningStar TSW 300 watt inverter is a great unit for small solar PV systems.

Sometimes, it makes sense to have two inverters... A small TSW inverter to recharge your cell phone, laptop, run a small TV, etc... And use a big old cheap MSW inverter when you need to run some power tools.

In the end, you need to look at your power needs (peak and average loads) to figure out your battery bank base voltage... My two cents--1,200 watts or less, 12 volts is OK. ~1,200 to 2,400 watts--look at 24 volt bank. If you are over 2,400 watts, look at 48 volt battery bank.

-Bill