Basic safety questions

Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
I've collected the components for a small off-grid PV system to run a few lights and a garage door opener. Before I start installing things, I want to make sure I'm fusing and grounding properly.

One 12v 130w PV panel will be connected with 10awg cable (no fuse) to a 20A Morningstar PWM charge controller. The controller will be connected to two 6v 225ah golf cart batteries (series connected for 12v) with 10awg cable with a 30A fuse on the positive cable. The battery jumper (to link them in series) is 1/0awg. The battery bank will be connected with 1/0awg cables (2 feet one-way distance) and a 150A fuse on the positive cable to a Xantrex X-Power 1500w MSW inverter.

I read some threads here and think that I should be grounding (a) the aluminum frame of the PV panel, (b) the negative post of the battery bank, and (c) the case of the inverter, all to a common earth grounding rod. Is this correct?

Thanks in advance for helping keep me and my new equipment safe.


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Basic safety questions

    Normally it is a good idea to ground all the items you mention. But the inverter you have is MSW, and they often have grounding issues. Particularly neutral-ground bonding on the AC side causing feedback if the negative DC is also grounded. This is one of those "consult your installation manual" (and hope the info is in there) issues.

    I can't leave this without pointing out that one 130 Watt panel won't do much for charging 225 Amp hours of battery: 130 W @ 77% efficiency = 100 W/ 14.2 charge Voltage = 7 Amps, or only 3% of the Amp hour capacity, much below the recommended minimum of 5%. (Trojan actually recommends 10%.)

    Otherwise, although I haven't done exact calculations on it all, it looks like you've thought it out fairly well. You're just short about 285 Watts of panel (which would require more charge controller capacity as well). :roll: Sorry, but this is the #1 mistake people make.
  • Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
    Re: Basic safety questions

    The X-Power 1500 is an RV-style inverter; the manual says to ground the case to the vehicle chassis, where I assume the negative line from the battery is also grounded, so as long as I don't ground the AC, I should be OK (if I'm understanding "neutral-ground bonding on the AC side causing feedback" correctly). The manual makes no recommendations regarding grounding the battery bank; is there reason simply to skip doing this to avoid possible interference?

    About the shortage of panels: I entered into this project the kind of dumb way -- buying the minimum of hardware and then thinking if it doesn't work well, I'll add on (dumb, since buying the minimum is a good way to get things to not work well). I also got some hope from this page:, where regarding panels/batteries ratio, it reads:

    " If you have a pair of 220 AH golf cart batteries, or a single 4D or 8D 12-volt battery, you should be looking for something in the 85 to 250 watt range."

    I bought the 20A controller thinking that it could take one more 130w panel, and these two panels together would certainly be enough. (Could I put, say, two 130w and a 80w panel together in an array?) I may find out that this is not the case, eh? What should I be watching for? Is it simply a matter of the batteries dying in six months with no warning, or can I monitor battery voltage or something to determine if the charging input is sufficient?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Basic safety questions

    Ah, here we go with the "it all depends" answers. :p

    There's some variables that need to be taken into account, and sometimes you don't know what those variables are until you find out.

    For instance, there's the 5%-13% rule-of-thumb. That would mean you want a minimum of 11.25 Amps peak current to properly recharge the 225 Amp hour batteries. Can you recharge them with less? Yes. The problem is that if they don't get that occasional "jolt" to stir things up inside (literally) the batteries won't last as long as they might. Trojan, as I said, recommends a 10% rate for proper remixing.

    Now how do you get that 11.25 Amps? You might think it would be 11.25A * 12V = 135W. There's two things wrong with that. The first is that the actual target charging Voltage for a "12 Volt" system is 14.2-14.4 Volts. So we have 11.25A * 14.2V = 159.75W. Quite a bit different.

    The second thing is that a "100 Watt" solar panel doesn't put out 100 Watts. That is a factor "nameplate" rating, and in the real world it will produce an average output that's somewhat less than that. Typically we use 77% as a derating for panels, so the minimum 159.75 Watts becomes 207.4 Watts. Quite a long ways from 135 Watts, eh? It is this derating that will vary from site to site. My panels actually manage 82% due to high elevation, clear skies, and relatively low ambient temperatures.

    Pushing your charge controller to the most it could actually handle three of those 130 Watt panels:
    390 Watt @ 77% = 300 Watts / 14.2 = 21 Amps (the charge controller will only pass 20 Amps, regardless).

    Leave the AC output "floating"; don't bond the ground to neutral and don't add another Earth ground connection here. Just think of it as being in an RV. The point of grounding the PV frame and inverter case is to provide a path for current should something short out so that nothing gets "energized" and becomes a shock hazard.
  • Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
    Re: Basic safety questions

    Thanks, 'Coot. Where a person could discuss this stuff if this forum were not here is beyond me.

    This may sound really naive, but since this is the problem:
    if they don't get that occasional "jolt" to stir things up inside (literally) the batteries won't last as long as they might.

    why can't a person just pick up the batteries and give them a good shaking once in a while? I've read that the acid solution needs to bubble periodically or it gets stratified; seems just a physical mixing would do it. Or is it far more sophisticated of a process than I even begin to imagine?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Basic safety questions

    I over-simplified a bit. Not only does the charge current mix the electrolyte and prevent stratification, it also "forces" the sulphur off the plates and recombines it with the water, restoring the acidity. This prevention of sulphation is key to battery longevity.

    BTW, those batteries with built-in hydrometers ("magic eye", "Delco-eye") actually want you to gently rock the battery from side to side after charging from 'dead' to stir the electrolyte and get the "eye" to read correctly. :D
  • icarusicarus Solar Expert Posts: 5,108 ✭✭✭✭
    Re: Basic safety questions

    A L-16 trojan weighs ~ 100#. I'm not into picking one up and shaking it!

  • Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
    Re: Basic safety questions

    Yeah, and what if you had to shake it hard enough to knock the sulfur off the plates!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Basic safety questions

    A T105 only weighs 67 lbs, so they're easy to handle. :roll:

    *sigh* Used to be able to lift one with each arm. :cry:
  • mike95490mike95490 Solar Expert Posts: 8,068 ✭✭✭✭✭
    Re: Basic safety questions

    I love the simple and compact MRBF terminal fuse by blue sea (MRBF - Marine Rated Battery Fuse) I wish Wind Sun would carry these goodies.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
    Re: Basic safety questions

    I did look at those, actually because of a recommendation of yours, mike90045, in another thread. I ended up getting some car-audio type inline fuses mostly because of price, and because I don't really need to save space, and because I thought that in a pinch I'd be more likely to find replacement fuses locally.

    These are the fuse holders I have:

    So far I've wired the batteries and the inverter (I have yet to fashion a rack for the PV panel out of Unistrut). The inverter powers the garage door opener just fine, so I'm happy so far!
Sign In or Register to comment.