Noob fusing question on small system

thewiz1969thewiz1969 Registered Users Posts: 4
I have what might be a dumb fusing question:

Equip:

4x35 watt W Solar panels (Imp:1.95a Isc:2.21a) parallel into a Steca 8.8 charge controller
3x15 watt Thunderbolt panels (Imp: .75a) parallel into Thunderbolt CC (harbor freight kit)
6x35 AH 12v AGM batteries in parallel

(ignore the approx. c/20 charge rate - I am adding another 4x35/Steca string shortly)

400 watt PWM inverter - probably upgrading to Samlex SSW-600-12A pure sine wave 600w inverter this week - inverter is connected to one of the charge controllers' inverter output, but will be changed to direct from battery bank on upgrade. Batteries are wired "method 2" with CC's/future inverter on opposing positive and negative leads of the battery bank (positive at the beginning of the bank, negative at the end).

Using this small setup to power summer outdoor kitchen area and for light winter lighting and outage backup. Our utility bill in summer puts us over 300% of baseline and dramatically increases our bill when we add loads. Going to add strings/batteries until we get to where it will power pool pump and all outdoor needs. Yes, I will dump the harbor freight kit as it expands - it will go to help charge my boat battery while camping 8-).

my question(s):

if my Steca CC has an internal 10a electronic fuse and the Thunderbolt CC has an internal fuse, and the inverter is internally fused do I need to fuse anything else?

Any other good/bad observations? (without worrying about the load requirements - I have a choice to take loads off grid as we increase capacity)

Thanks

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Noob fusing question on small system

    Welcome to the forum.

    If we want to get technical about it, the panels with more than 2 parallel connections should be fused. But they are of such small current capacity it probably isn't a big deal. No one is inspecting this. :D

    If you are sure both charge controllers have output fuses built-in there's no reason to add more. Accessibility might be an issue, though.

    Six 12 Volt batteries in parallel? Well there's a problem in itself. The current sharing will never be equal no matter how carefully you wire it. Ideally there would be a fuse on each and every one of these too, as 'loose' wires off batteries can supply quite a zap before they reach any fuses internal to controllers or inverters.

    If you're having trouble with utility bill size you might want to put some effort and money into conservation. This small-scale solar is definitely not very economical, so you may save on the electric bill but over-all you're spending more money. Another thing to consider (although it demands a large capital investment) is GT solar.

    You are going to find (or have already) that expanding a small system is not easy, but it is very frustrating. Take a look at what your end goal is, and focus on that. Putting money aside to pay for the larger scale equipment will return better value than buying stuff you'll eventually discard anyway.
  • thewiz1969thewiz1969 Registered Users Posts: 4
    Re: Noob fusing question on small system

    Hi coot, and thanks for the reply.

    I was intending on using the Outback bus bars to help equalize the battery charge/discharge. I read a bit on batteries before I started and I intended to create 2 strings of 3 35ah batteries to the bus bar. Then I'll add 105ah or 110ah batteries when I add larger panels to get me to around 600. When I rewire to the bus bar I will fuse each battery/string as you suggested. I read a post somewhere about using bus bars with equal wire length helping to equalize the draw/recharge. Hopefully I'm not misunderstanding?

    I can look this up - but since we're conversing - what size fuse should I use on a 105ah battery or battery string?

    As for utility bill affordability and conservation - it has a lot to do with choices... A/C in the Sacramento Valley summers and my pool pump. Just getting my pool pump off the grid (and not adding new outdoor kitchen loads to grid) will take me well below the 300% baseline where the electricity is most expensive. We're finishing the outdoor kitchen/gazebo setup in the next 60 days and the small system will run it until we replace the pool pump. Then (based on the current pump req - but modified for the newer efficient pump ratings) we want to add something like 4x235watt panels with a Morningstar Tristar MPPT 45 or 60 (I haven't done the math yet) with the approx. 600ish ah of battery capacity. This will reduce my bill significantly during the 4-5 months of the year when it skyrockets to $500 - $600/month since we will be under baseline (much higher marginal cost of energy over baseline). The pool pump is on a separate sub panel, so disconnecting this non-critical load from utility will be easy with no need for a transfer switch.

    On the cost/panel size - the 35watt panels are $1.30/watt after shipping (vs $1.05 shipped for the 235watt panels) so they aren't completely over priced like I see a lot of smaller panels. I chose the smaller panel size so they could be mounted on a small roof (think gazebo).

    I do want to do GT, but it will be a couple of years before I get the backyard projects done and address GT. The secondary, and very important to me, reason for this system is some form of backup power during the winter. We've lost electricity enough times that I want some basic backup to run fish tank pumps, LED lights, cell phone chargers, etc. Full GT/battery backup will probably be cost prohibitive for me.

    Thanks again for responding to my noob questions!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Noob fusing question on small system
    thewiz1969 wrote: »
    Hi coot, and thanks for the reply.

    I was intending on using the Outback bus bars to help equalize the battery charge/discharge. I read a bit on batteries before I started and I intended to create 2 strings of 3 35ah batteries to the bus bar. Then I'll add 105ah or 110ah batteries when I add larger panels to get me to around 600. When I rewire to the bus bar I will fuse each battery/string as you suggested. I read a post somewhere about using bus bars with equal wire length helping to equalize the draw/recharge. Hopefully I'm not misunderstanding?

    Well, you got me confused.
    If you put batteries in a string it implies series connections which ups the Voltage. So three 12 Volt batteries in a string would be 36 Volts at the same Amp hour rating as one.
    Maybe you should read through this bit: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power
    I can look this up - but since we're conversing - what size fuse should I use on a 105ah battery or battery string?

    Wiring and circuit protection sizing depends on the expected maximum continuous current draw. That's about the only answer I can give, other than that a battery can dump its current quickly if allowed to (short or heavy load). The faster the rate of discharge, the lower the actual Amp hour capacity.
    As for utility bill affordability and conservation - it has a lot to do with choices... A/C in the Sacramento Valley summers and my pool pump. Just getting my pool pump off the grid (and not adding new outdoor kitchen loads to grid) will take me well below the 300% baseline where the electricity is most expensive. We're finishing the outdoor kitchen/gazebo setup in the next 60 days and the small system will run it until we replace the pool pump. Then (based on the current pump req - but modified for the newer efficient pump ratings) we want to add something like 4x235watt panels with a Morningstar Tristar MPPT 45 or 60 (I haven't done the math yet) with the approx. 600ish ah of battery capacity. This will reduce my bill significantly during the 4-5 months of the year when it skyrockets to $500 - $600/month since we will be under baseline (much higher marginal cost of energy over baseline). The pool pump is on a separate sub panel, so disconnecting this non-critical load from utility will be easy with no need for a transfer switch.

    Well you're talking to the wrong guy here. My front yard is several million gallons of glacial lake so I don't need a pool or A/C. :p But I'm sure some of the guys have mentioned better pool pumps before.
    On the cost/panel size - the 35watt panels are $1.30/watt after shipping (vs $1.05 shipped for the 235watt panels) so they aren't completely over priced like I see a lot of smaller panels. I chose the smaller panel size so they could be mounted on a small roof (think gazebo).

    Space limitations aside, wiring up more and more small panels to get the same Watts can be problematic too. More connections = more potential failure points. And as mentioned before they should technically have fuses on them. If you were in the area of 235 Watts all out of small panels (six or seven) - the lack of fuses could be an issue. Small doesn't scale-up well.
    I do want to do GT, but it will be a couple of years before I get the backyard projects done and address GT. The secondary, and very important to me, reason for this system is some form of backup power during the winter. We've lost electricity enough times that I want some basic backup to run fish tank pumps, LED lights, cell phone chargers, etc. Full GT/battery backup will probably be cost prohibitive for me.

    Normally we say "generator" whenever someone asks "back-up?". But that predisposes you're around to start it. Fish aren't very good at flipping switches or pulling ropes, especially when they're asphyxiating. But when you do lose power, will solar work? Often it goes out during storms - and panels don't charge when there's no sun. Just something else to think (worry) about.
    Thanks again for responding to my noob questions!

    Not a problem. It's why we're here. Most of us learned the hard way, while the industry was growing and changing. It still is, so we're still learning - as fast as we can.
  • thewiz1969thewiz1969 Registered Users Posts: 4
    Re: Noob fusing question on small system
    Well, you got me confused.
    If you put batteries in a string it implies series connections which ups the Voltage. So three 12 Volt batteries in a string would be 36 Volts at the same Amp hour rating as one.
    Maybe you should read through this bit: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    My bad on nomenclature - by string I meant 3 35ah batteries in parallel to mimic one 105ah battery then connected to the bus bar (in parallel with the other 105ah batteries). The leads from the bus bar to the battery (or the 3 battery set) would be the same length. This (from what I've read) helps balance the charge/discharge evenly across the batteries.

    After reading the link you posted, I will probably look at creating a 24v or 48v system when I bring on the larger array to power the pool pump. I was already considering the 6v wet golf cart batteries at Costco anyways. I hadn't decided on an inverter yet, so that makes sense. I have plenty of uses for keeping a small 12v system regardless of how I attack the pool pump.
    Space limitations aside, wiring up more and more small panels to get the same Watts can be problematic too. More connections = more potential failure points. And as mentioned before they should technically have fuses on them. If you were in the area of 235 Watts all out of small panels (six or seven) - the lack of fuses could be an issue. Small doesn't scale-up well.

    Assuming two sets of 4 parallel panels on their own individual CC's, it sounds like I should put a 10 or 15amp fuse on the positive side of the line from the panels to each CC?
    (Isc: 2.21a Imp: 1.95a per panel)
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Noob fusing question on small system
    thewiz1969 wrote: »
    My bad on nomenclature - by string I meant 3 35ah batteries in parallel to mimic one 105ah battery then connected to the bus bar (in parallel with the other 105ah batteries). The leads from the bus bar to the battery (or the 3 battery set) would be the same length. This (from what I've read) helps balance the charge/discharge evenly across the batteries.

    Nope. In fact it can make it worse. Nor would you get proper fuse protection that way unless you put a post fuse on every one (which you should). The only way to keep them equal is with same length wiring from each battery to common connection points (bus bars) for (+) and (-).
    Assuming two sets of 4 parallel panels on their own individual CC's, it sounds like I should put a 10 or 15amp fuse on the positive side of the line from the panels to each CC?
    (Isc: 2.21a Imp: 1.95a per panel)

    Parallel panels: the fuses on each one are to protect the wiring in the even one of the panels shorts and the others "gang up on it" with their Isc; feeding 2X (or more) the maximum current the panel can take. In that vein the series fuse rating is usually Isc * 1.25 * 1.25 rounded up. In this case 5 Amp fuse per panel.

    The output to the charge controller is another issue. Technically the panels can't put out more than their Isc, so as long as the whole circuit is designed to handle that no protection is needed. if you do install a breaker for disconnect purposes (or if the local inspector requires it) it has to be able to handle the full current.

    Yes, it's complicated.
  • thewiz1969thewiz1969 Registered Users Posts: 4
    Re: Noob fusing question on small system
    Parallel panels: the fuses on each one are to protect the wiring in the even one of the panels shorts and the others "gang up on it" with their Isc; feeding 2X (or more) the maximum current the panel can take. In that vein the series fuse rating is usually Isc * 1.25 * 1.25 rounded up. In this case 5 Amp fuse per panel.

    Thanks - going to look for waterproof 5a fuses now. Just an aside... if I were to consider the 4x35watt panels as one panel from a loss perspective (meaning if one 35watt shorts, I can accept the loss of all 4), is 4(2.21) * 1.25 * 1.25 =13.8125 = 15 amp fuse on positive lead to charge controller correct?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Noob fusing question on small system
    thewiz1969 wrote: »
    Thanks - going to look for waterproof 5a fuses now. Just an aside... if I were to consider the 4x35watt panels as one panel from a loss perspective (meaning if one 35watt shorts, I can accept the loss of all 4), is 4(2.21) * 1.25 * 1.25 =13.8125 = 15 amp fuse on positive lead to charge controller correct?

    Problem is: when one panel shorts the others feed their current to the shorted panel (path of least resistance).

    After that the protective fuse pops and the current will go back to the charge controller and be 3/4 the original max.

    Without anything shorting the panels act as one and feed 4X Imp to the controller at most, or 7.8 Amps. If the controller shorted and they fed full Isc that would be only 8.84 Amps. A 15 Amp breaker is about twice that capacity. It will work as a switch, but it would not be likely to trip under any circumstances. On these small scale system it's pretty difficult to get these things at just the right value. As it goes up in size the current multiplies exponentially so the difference between Imp of the array and Isc of the array becomes enough to trigger protection.
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