Dual battery controller Q

cptdondo
cptdondo Solar Expert Posts: 103 ✭✭✭
I have an RV with 2 battery banks. One bank consists of 2 Trojan T-135 batteries and the other bank consists of a single 12V battery. I have a Blue Sea selector switch that lets me pick Bank I, Bank II, or Bank I+II.

Now I want to add Solar into the mix. I'm looking at one or two 135W panels; something like the Kyocera Solar KD135SX.

Which controller do I need to use? I am a bit restricted in that 1. Cost is an issue (of course) and 2. I need a remote panel (the controller will be buried under a closet floor, as that's the closest place to the batteries. It is not easily accessible.)

I found this: SunSaver Dual Battery Controller. It even does dual battery banks.

I am wondering if I can use it with Bank I+II connected in parallel, in other words with the two outputs "shorted". Also, is the PWM controller significantly less efficient than MPPT?

Thanks.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Dual battery controller Q

    I believe the MorningStar dual controller will work in your system with your A/B/A+B switch just fine. [however, I may be wrong. -BB]

    However, assuming your vehicle battery is a typical "car batter", you would probably not want to do the A+B very often. Car batteries do not last long if deeply cycled and will use a lot of water if charged heavily/long periods of time--so monitor the water level in the car battery even if maintenance free.

    For a small system, PWM controllers are fine.

    You would only want to look at a MPPT controller, typically, if you have a long wire run from the solar array to the battery bank (say you park under trees/shade but run the panels 50+ feet away in full sun--staked to ground to prevent wind damage/theft). Then you can put the panels in series (Vmp 35+ volts) and use much smaller cable/wire to run the remote panels.

    Here is a nice thread with video from Kevin in Calgary Canada that shows designing and installing solar PV in a small RV trailer (he did use a small MPPT controller + remote panels for his installation).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Dual battery controller Q

    Welcome to the forum.

    A PWM controller is not significantly less efficient than an MPPT type, especially on a small system (under 400 Watts, say). The MPPT advantage comes with larger systems, higher Voltage arrays, cold temps, et cetera. However there are small MPPT controllers like the Rogue and the Blue Sky and the 15 Amp Morningstar unit.

    Your two battery sets aren't evenly matched, which becomes a problem for charging them from one source. The right set-up for the two Trojans is bound to be too much for the single 12 Volt, as the T105's would want an Absorb Voltage of 14.8 and the 12 Volt (model and capacity unknown?) probably lower Voltage - and less Amps.

    The SunSaver Duo http://www.solar-electric.com/modubachco25.html is not meant to have the two batteries connected together. Quote the blurb: "This product will charge two separate and isolated batteries at the same time".

    Frankly you shouldn't be connecting two mismatched banks together ever anyway. Is the 12 Volt the "starting" battery tied in for emergency purposes? A few more details and maybe we can figure something out.
  • cptdondo
    cptdondo Solar Expert Posts: 103 ✭✭✭
    Re: Dual battery controller Q

    OK, to clarify:

    Both banks are deep cycle batteries. The "normal" mode of use is to use Bank I (Trojan T-135) and hold Bank II (single 12V deep cycle) in reserve. When the voltage on Bank I drops below "safe" levels, switch to Bank II. At that point, I have may be 1 day left before i have to charge up, using the engine alternator, generator, or another source.

    The vehicle also has a starting battery (Bank III if you will). There is an ACR (automatic charging relay) which will connect the hot bank to the vehicle alternator when the ACR sees charging voltage.

    I don't normally connect Bank I + Bank II but if the switch allows it it will happen.... I could replace the bank selector switch with just a Bank I and Bank II selector if that would let things work better.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Dual battery controller Q
    The SunSaver Duo http://www.solar-electric.com/modubachco25.html is not meant to have the two batteries connected together. Quote the blurb: "This product will charge two separate and isolated batteries at the same time".
    Note that "Isolated" is not really the correct term... The batteries do have to share a common ground and have to be the same voltage bank (both 12 volt).

    http://www.morningstarcorp.com/en/support/library/SunSaver%20Duo%20Manual.pdf

    I would probably set the RV bank to "Flooded Cell" and the vehicle battery to "Sealed" so that it charges at reduced voltage (less water usage). If you set different charging voltages--you should not do the A+B except for emergency use.

    You could probably also set for (90/10) charge current sharing... Hopefully, your vehicle battery will not need much power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Dual battery controller Q

    Obviously the negatives are common, but I suspect that "isolated" in this application means "don't connect the positive outputs of the charge controller together". Although I shouldn't think it would lead to more than "confused" charging. It's normal practice to attach two controllers to one bank without trouble; I'm more concerned about the different charge requirements of the two battery banks.

    Anyway, we're not talking about hooking to the starting battery but two different size deep cycle banks.
    The next question is: how different? Could be 225 Amp hours vs. 100 Amp hours, which is pretty far apart for charge parameters.

    Possibly with 270 Watts of panel on the Trojans you won't even need Bank 'B' except in a dire emergency (like the sun stops shining so you have to start the gen anyway). Two 135 Watt panels should put you at the minimum 5% charge rate; four is best for the T105's. With two you might see 14 Amps at best, probably less. RV applications tend to not have the best insolation of panels.
  • cptdondo
    cptdondo Solar Expert Posts: 103 ✭✭✭
    Re: Dual battery controller Q

    OK, that sounds like a plan....

    Now can you guys help me out with the rest of the stuff?

    I prewired the RV with #8 from just below the roof to the central electrical box. From there it's #2 to the batteries and it's only a short run. I also prewired control wiring from the electrical box to the electrical control panels.

    I'm assuming the charge controller will go into the main electrical box and the remote will go with the other panels.

    I don't quite understand all of the connectors, junctions boxes, etc. that the newer panels use.

    What do I need to hood the 2 panels together? I'm assuming I need a junction box of sorts that's roof mounted, that accepts the wiring from the panels, pierces the roof, and connects to my #8 wire. The roof is fiberglass so there's no problem putting a hole in it and sealing it watertight. Can someone point me at a suitable parts list?
  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Dual battery controller Q

    The 90/10 is a preference I think, not an absolute. So it'll split 90/10 until one or other is full, and then send 100% to whichever still needs charging.


    If the panels have a junction box, then you would run some wire (say a pair of #10) out of each panel's j-box, and connect both + from the j-boxes to the #8 + and both - from the j-boxes to the #8 - and then connect the other end of the #8 +/- to the SSD input.


    If the panels have MC4 connectors (locking, waterproof connectors) then you would use one MC4 male "Y" to parallel the + and one MC4 female Y to parallel the -. You would then need to buy a short MC4 extension cable, and cut it in half.

    The Ys are like this:

    http://www.solar-electric.com/mumc4colam.html


    The MC4 extension has one male end and one female end, so when you cut it in half, you have two cables, one which connects to the male Y, and one which connects to the female Y. On both, the other end is just cut, so you can strip it and splice it to the #8.

    The extension which you will cut in half to go from MC4 to the #8, is like this:

    http://www.solar-electric.com/6fomc4so2exc.html

    And don't forget the MC4 toolset:

    http://www.solar-electric.com/ditoformc4co.html



    You can mount a box and run the cut off MC4 extension in to it to go through to the #8, but you don't have to. If you did do that, you would want a box with a gasketed cover, like a "direct burial" box. Something like this...

    http://www.google.com/products/catalog?q=plastic+junction+box&hl=en&safe=off&client=opera&hs=Lra&rls=en&channel=suggest&prmd=ivns&bav=on.2,or.r_gc.r_pw.&biw=1332&bih=601&um=1&ie=UTF-8&tbm=shop&cid=1312749238055274850&sa=X&ei=YbFBTriTOfTYiAKvktGaBQ&ved=0CGUQ8wIwAQ

    ...although 4" deep is really more than you need.


    Then, before you mount the box, you put put a "nipple" out the back of the box to go through the hole in the roof.

    Something like this, except that your box would be surface mounted instead of recessed:

    fig2.jpg

    You probabaly also won't need the 90 on the inside end of the nipple.

    The nipple could be a short piece of PV pipe glued to a fitting which goes through the hole in the back of the box.

    Basically like this, except with a short piece of pipe out the back instead of just an open hole:

    hottub_power800x590.jpg



    The "gotchas" are to make sure that where the wire enters the box that it is sealed to keep water out, and that all around the hole in the roof is also properly sealed. (Put a bead around the bottom of the box before you screw it down.)

    Also to make sure that on the inside, where the wire comes out of the nipple, there is no sharp edge on the nipple to rub the wire.



    Parts list would be:

    Some 2-wire #10 (if the PV have j-boxes)
    or
    1 male MC4 Y
    1 female MC4 Y
    1 short MC4 extension (f the PV have MC4 connectors)

    1 J-box

    2 cable entry fittings to grip the wire where it comes into the box and keep water out (one for each of the cut MC4 extension wires, or one for each 2-wire #10 cable from each PV):

    fitting.jpg

    1 short piece of PVC pipe
    1 PVC male adapter (to go through the hole in the back of the box) w/lockring

    pvc-male.gif

    Some approprate crimp connectors to connect the #8 to the wire coming in from the roof.


    You also need a 30a fuse or breaker somewhere in the + run between the PV and the charge controller. I'd probably put a 30a Bussmann Type 2 "Modified Reset" in the j-box on the roof and connect the #8 + to one side, and the incoming + wire from the PV to the other side:

    http://www.wiringproducts.com/contents/en-us/p1151.html

    I would also put one of those breakers in each positive output line from the charge controller to the batteries.


    Not much to it really.
  • cptdondo
    cptdondo Solar Expert Posts: 103 ✭✭✭
    Re: Dual battery controller Q

    Thanks. I got most of that; I missed the 30A breaker.

    I got a roof-top mount j-box from Amsolar made for RVs. Looks nice if pricy.

    Need to get a 30A inline breaker, though. My batteries have a 100A fast-blow on the ground, which have demonstrated their usefulness several times. The 100A fast-blow fuses blow long before the 30A thermal breakers do and work wonders when you do something boneheaded like hit the starter with the charging relay forced on. (I use a fuse-to-ground as a last-ditch effort at protection. The + side goes to the typical fuse block.)