Greetings and Off-grid advice requested!

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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #32
    Re: Greetings and Off-grid advice requested!
    Cariboocoot wrote: »
    The good news is you don't need 30 Amps to charge 170 Amp hours of battery. One of those panels on an MS MPPT 15 would do fine.

    Can we talk about this a little more? How do you know/calculate how much solar power/charging amps you will need to recharge a battery bank of a given capacity? Are there some simple rules of thumb or will this be another math lesson for me? :)

    Thanks again for all the great advice/information!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #33
    Re: Greetings and Off-grid advice requested!

    Simple rule of thumb: if you have enough potential peak charge current it will work.
    That current is, for most batteries, between 5% and 13% of the battery's Amp hour capacity (at the "20 hour rate"). 10% is a nice in-the-middle figure to use and easy to calculate. So we get 170 Amp hours * 10% = 17 Amps peak current as the "middle number".

    15 Amps on a 170 Amp hour battery is (15 * 100 / 170) 8% which falls within the parameters.

    Less than 5% and the battery may not charge. More than 13% and you're spending money on panels you don't necessarily need (even though most batteries can usually take more current). And remember that is a net charge rate: any loads drawing while charging will reduce it. Usually this is not a problem.

    This doesn't mean you must or even will achieve this current level, just that if the potential is there the battery should be able to recharge in one good day of sunlight.

    As with everything else there are exceptions to the rule. For instance the 5% rate will work on a system that is used only at night because there are no loads drawing while charging. Or you may want greater than 13% capacity if you're in a location that has a lot of bad weather that needs to be compensated for.

    As a rule, 10% peak charge current and 25% depth of discharge limit with 4 hours of equivalent good sun per day works out to a balanced system that will work in most instances. But it is always necessary to consider the specifics of the installation, including location and usage.
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #34
    Re: Greetings and Off-grid advice requested!

    Thanks for the explanation.

    I guess my main goal now is to figure out the power draw so I can properly engineer this system. Feel like it's probably a reasonable draw as RV's are designed to go unplugged/boondocked for a day or two at a time I'm sure.

    Can I assume, even if these Interstate batteries are already somewhat damaged from ill treatment, that I'll still be able to get an accurate power draw reading (with a DC Clamp like Bill posted)? Last time I was up (to retrieve them for the winter) they still had enough juice to run the blowers for the furnace so I assume they are at least somewhat functional.

    Cheers!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #35
    Re: Greetings and Off-grid advice requested!

    The current draw @ Voltage should give you a fairly accurate number for power used even if the batteries aren't 100%. What you're looking for, though, is not a momentary reading but a reading over time: accumulated Watt hours. This means the current is going to change as different loads turn on and off throughout a day. Somewhat more difficult to measure, especially with a clamp-on meter (which needs to be re-zeroed every now and then). But you could use it to test the individual current draws of everything you use by measuring with each item on individually, then calculate how long each item is used per day and come up with totals that way.

    This is where a battery monitor can work better in that it will measure total Amps out and accumulated Amp hours (if wired to ignore any charge current coming in). Kind of expensive ($160 for a Trimetric), but as close as you can get to a "DC Kill-A-Watt meter". With the old batteries it will not give you an accurate SOC reading.
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #36
    Re: Greetings and Off-grid advice requested!
    BB. wrote: »
    For a "good enough" DC Current Clamp meter (really a full function DMM), this one is a really good price from Sears.

    Hey Bill,

    Just FYI, that unit is no longer sold, which I discovered when I added it to my cart! EDIT: Ugh, had a link to "replacement item" here but, it was the same item... :(
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #37
    Re: Greetings and Off-grid advice requested!
    Cariboocoot wrote: »
    The current draw @ Voltage should give you a fairly accurate number for power used even if the batteries aren't 100%. What you're looking for, though, is not a momentary reading but a reading over time: accumulated Watt hours. This means the current is going to change as different loads turn on and off throughout a day. Somewhat more difficult to measure, especially with a clamp-on meter (which needs to be re-zeroed every now and then). But you could use it to test the individual current draws of everything you use by measuring with each item on individually, then calculate how long each item is used per day and come up with totals that way.

    This is where a battery monitor can work better in that it will measure total Amps out and accumulated Amp hours (if wired to ignore any charge current coming in). Kind of expensive ($160 for a Trimetric), but as close as you can get to a "DC Kill-A-Watt meter". With the old batteries it will not give you an accurate SOC reading.

    Thanks. During the day I will not really be using any loads other than what the RV itself draws, ie. DC fridge, whatever other standby stuff is on. We really just use the batteries for lighting in the evenings. So while the RV is "on", there will be a pretty steady load from whatever it uses inherently 24hrs a day, then in the evening we will have some small 12V lights on from dusk/sundown till bedtime (up till midnight i guess).

    Sometimes in the late fall/early spring we will fire up the propane furnace which does use electric fans/blowers and probably draw a fair amount of power.

    So what I am hoping now to do is get the DC clamp amp-meter, take the interstates up, and get a few readings during the day while everything is in "standby", then another few readings with 3-4 lights on, and then fire up the furnace and get a few readings when the blower is going. This should give me some reasonable data points to work from?

    Cheers!
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #38
    Re: Greetings and Off-grid advice requested!

    Thanks Bill,

    I wasn't sure if I could post links to non-sponsor store stuff.

    http://www.solarenergydc.com/index.php/en/products/batteries
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  • Rybren
    Rybren Solar Expert Posts: 351 ✭✭
    #39
    Re: Greetings and Off-grid advice requested!
    Rossman wrote: »
    Hey Bill,

    Just FYI, that unit is no longer sold, which I discovered when I added it to my cart! EDIT: Ugh, had a link to "replacement item" here but, it was the same item... :(

    Keep trying. 9 times out of ten, the link will show that the unit isn't available for international shipping or some other issue will block the sale, but every once in a while it will work. It took me a number of tries, but I eventually got the sale to go through. (I bought mine in September)
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #40
    Re: Greetings and Off-grid advice requested!

    Rossman; your plan should be reasonably close to an accurate load measurement. Just always make allowances for those days when there might be a bit more power used!

    For some reason certain American web sites hate Canadians! :p They read the IP and go "Oh no! Loonies!" :p
    Of course if they'd just offer us the same stuff on the web sites up here ... :grr
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #41
    Re: Greetings and Off-grid advice requested!
    Rybren wrote: »
    Keep trying. 9 times out of ten, the link will show that the unit isn't available for international shipping or some other issue will block the sale, but every once in a while it will work. It took me a number of tries, but I eventually got the sale to go through. (I bought mine in September)

    You can avoid discrimination on the basis of IP address location by doing your browsing through a proxy. (I won't go into the details here, but it is quite simple.) But when you get all the way to the end of the sales process, you will probably still have problems putting in a shipping address. It would let you see the prices and details though.
    SMA SB 3000, old BP panels.
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #42
    Re: Greetings and Off-grid advice requested!

    Thanks for both of those posts, Rybren.
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #43
    Re: Greetings and Off-grid advice requested!
    Cariboocoot wrote: »
    Rossman; your plan should be reasonably close to an accurate load measurement. Just always make allowances for those days when there might be a bit more power used!

    For some reason certain American web sites hate Canadians! :p They read the IP and go "Oh no! Loonies!" :p
    Of course if they'd just offer us the same stuff on the web sites up here ... :grr


    Yeah, Canadian sites are the worst. I usually get stuff of amazon.com and have it shipped to sister-in-law's place in Vermont, then periodically drive down and pick up.

    Am actually now wavering between the clamp amp meter and the battery monitor. If I am going to want the battery monitor later on when I get the proper system installed it might be better use of cash to forego the amp meter and go straight for the monitor...

    Thanks everyone again for all the helpful advice!
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #44
    Re: Greetings and Off-grid advice requested!
    Rossman wrote: »
    Yeah, Canadian sites are the worst. I usually get stuff of amazon.com and have it shipped to sister-in-law's place in Vermont, then periodically drive down and pick up.

    Am actually now wavering between the clamp amp meter and the battery monitor. If I am going to want the battery monitor later on when I get the proper system installed it might be better use of cash to forego the amp meter and go straight for the monitor...

    Thanks everyone again for all the helpful advice!

    Keep in mind that although the battery monitor will give you your basic condition information with the least effort once your system is stable, the clamp-on will give you the details about panel output, CC input, inverter input etc. which you will want when getting started. And it is almost essential if you will be paralleling battery strings. A battery monitor with shunt will only give you a measurement in one fixed location.
    SMA SB 3000, old BP panels.
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #45
    Re: Greetings and Off-grid advice requested!

    Thanks, inetdog, good to know. Maybe I will put the amp meter on my Christmas list and see what is forthcoming!

    As regards to the battery monitor, I made a modified image of how I think this would be wired? I am also adding a easy DC on/off switch just to make life easier. Did I get this right?

    Attachment not found.

    Thanks!
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #46
    Re: Greetings and Off-grid advice requested!
    Rossman wrote: »
    Hey Bill,

    Just FYI, that unit is no longer sold, which I discovered when I added it to my cart! EDIT: Ugh, had a link to "replacement item" here but, it was the same item... :(

    In the US--It does add to cart just fine--And they keep reducing the price (sale or clearing out item???)--Right now, it is $51 ($60 list).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #47
    Re: Greetings and Off-grid advice requested!

    Regarding the wiring of the shunt... Normally you put it in the negative lead from the battery to the negative common bus (and chassis/safety ground, if you have one).

    It does not matter in operation (I don't think--might depend on the meter being used if it can take zero volts or 12 volts with respect to ground)--But the better reason to connect to the ground side is the two voltage sense wires that come of the shunt--And the shunt itself. If the shunt is at "ground potential", even if a wire is shorted or something metal falls across the shunt, you will not get a dangerous short circuit.

    When I had to use remote sense wiring on my computer systems--I had to put a fuse (or high value resistor) between the voltage source and the meter to prevent high short circuit currents from causing a fire.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #48
    Re: Greetings and Off-grid advice requested!
    Rossman wrote: »
    Thanks, inetdog, good to know. Maybe I will put the amp meter on my Christmas list and see what is forthcoming!

    As regards to the battery monitor, I made a modified image of how I think this would be wired? I am also adding a easy DC on/off switch just to make life easier. Did I get this right?

    Thanks!

    Pretty much got it. The shunt goes on the negative line and the battery monitor looks at "both sides" of the shunt. It will also need a wire going to (+) to power it.
    Usually the ON/OFF switch would be on the positive side, but there's no reason it can't be on negative (common practice on auto applications is to disconnect negative before doing any service work as it guarantees no power available - sometimes there's more than one positive connection).
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #49
    Re: Greetings and Off-grid advice requested!
    Cariboocoot wrote: »
    Pretty much got it. The shunt goes on the negative line and the battery monitor looks at "both sides" of the shunt. It will also need a wire going to (+) to power it.
    Usually the ON/OFF switch would be on the positive side, but there's no reason it can't be on negative (common practice on auto applications is to disconnect negative before doing any service work as it guarantees no power available - sometimes there's more than one positive connection).



    Nice, thanks!

    Then when I eventually upgrade and put the 6VDC batteries + charge controller + solar panel, it would be like this?

    I assume I will need grounding + fusing on wires A/B/C/D somewhere?

    Attachment not found.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #50
    Re: Greetings and Off-grid advice requested!

    "A" going direct to the battery is going to bypass the current shunt--So charging will not be taken into account. Battery Monitors, typically, are connected so that all loads/sources go through the shunt so it can estimate the battery bank's state of charge (all negative connections should be on the "left" side of the shunt).

    Normally, you would have a common battery + bus connects. All wires leaving that point should each have their own fuse/breaker for the rated capacity of the wiring (or less). Note, that a fuse/wiring should be rated 1.25x the maximum continuous current you expect (10 amp load * 1.25 NEC safety factor = 12.5 minimum fuse/wire rating).

    For most systems (negative ground), you do not need to place a fuse/breaker in the negative lead (with negative grounded to chassis, the negative wiring never gets much above zero volts--so there is no short circuit energy available).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #51
    Re: Greetings and Off-grid advice requested!

    Thanks Bill.

    Revised diagram attached. As I can only assume the existing RV wiring is correct, I should only need to fuse positive wire B? (the other wire - red - coming off battery positive goes to the regular RV wiring).

    Attachment not found.

    Cheers!
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #52
    Re: Greetings and Off-grid advice requested!

    Further to what Bill said, there appears to be a connection between the (+) of the "house" batteries and the (+) of the far right battery which is probably the starter battery?
    You don't want the two systems connected together. If you keep each separate, than neither will draw the other one down and the two different charging profiles (between the deep cycle and automotive) will not "interfere" with each other. Then if you need to "borrow" some current from the "house" to get the motor started you still can.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #53
    Re: Greetings and Off-grid advice requested!

    Looking better every time. :D

    Oh, also fasten the batteries down; don't wedge them in the hole with blocks of wood. :roll:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #54
    Re: Greetings and Off-grid advice requested!

    If you have other sources such as the vehicle battery tied to the House Battery--Then the DC switch in the negative return may make less sense. You still have power flowing from the other battery/charging sources (it does stop the House Battery from participating).

    Normally, you would have a DC switch to the common + bus--And all of your loads would be connected there. This allows the charging sources to still charge the battery bank when the loads are turned off (say in storage). Turning off power to solar battery chargers is something I would avoid except for service (it may confuse some chargers).

    If you want the ability to turn off both loads and sources--There are A/B battery switches you can use:

    wind-sun_2239_4820473

    Connect Charger(s) to #1, loads to #2 and common to the battery + cable. Run 1+2 normally. Run #1 only when RV is in storage. Off when servicing.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #55
    Re: Greetings and Off-grid advice requested!
    Cariboocoot wrote: »
    Further to what Bill said, there appears to be a connection between the (+) of the "house" batteries and the (+) of the far right battery which is probably the starter battery?
    You don't want the two systems connected together. If you keep each separate, than neither will draw the other one down and the two different charging profiles (between the deep cycle and automotive) will not "interfere" with each other. Then if you need to "borrow" some current from the "house" to get the motor started you still can.

    I'm not actually sure of the full path of the positive house battery connection. I don't think it directly goes to the starter/engine battery positive, but back out those holes somewhere to the bowels of the machine. I'll make a note to try and figure out where this is going to. I guess the two systems are probably connected somewhere since the engine alternator will charge both systems, but I don't think it's within my ability to separate them. How bad is this going to be?

    As for fastening the batteries down, since I don't drive this RV am not too worried about movement! That ghetto job was done by whoever installed those batteries, probably the RV dealer where my mother had the beast sitting on consignment. I'll be sure to do a better job when I rig up the new system!
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #56
    Re: Greetings and Off-grid advice requested!
    BB. wrote: »
    If you have other sources such as the vehicle battery tied to the House Battery--Then the DC switch in the negative return may make less sense. You still have power flowing from the other battery/charging sources (it does stop the House Battery from participating).

    Normally, you would have a DC switch to the common + bus--And all of your loads would be connected there. This allows the charging sources to still charge the battery bank when the loads are turned off (say in storage). Turning off power to solar battery chargers is something I would avoid except for service (it may confuse some chargers).

    If you want the ability to turn off both loads and sources--There are A/B battery switches you can use:

    wind-sun_2239_4820473

    Connect Charger(s) to #1, loads to #2 and common to the battery + cable. Run 1+2 normally. Run #1 only when RV is in storage. Off when servicing.

    -Bill

    Would moving the DC on/off as shown be a better setup?

    Attachment not found.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #57
    Re: Greetings and Off-grid advice requested!

    Separating the batteries is not a really big issue. Bill described the best case scenario, where the solar always can charge the house batteries and the heavy switch can disconnect it from the loads or connect both systems together if need be. Maybe they put a battery isolator between the two, maybe they didn't. Some more investigation is needed.

    One thing you can do is remove all the batteries, pull out all the fuses, and test the wiring with an Ohm meter to see what kind of connections you've got. If it's zero Ohms between the heavy (+) wires for the two battery banks then they are tied directly together. If there's an isolator (either electronic or relay type) there should be no continuity as the house batteries are only supposed to receive charge when the engine is running and the starting battery is charged up. That keeps the loads from discharging the starting battery.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #58
    Re: Greetings and Off-grid advice requested!
    Rossman wrote: »
    Would moving the DC on/off as shown be a better setup?

    Yup. :D
    Keeps the solar charging connected while the load draw can be disconnected.
    One caution: negative is sometimes connected to cases of things like charge controllers and inverters. Make sure there isn't an inadvertent path around that switch through the metal chassis of the RV and the case of the controller or something like it.
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #59
    Re: Greetings and Off-grid advice requested!
    Cariboocoot wrote: »
    If there's an isolator (either electronic or relay type) there should be no continuity as the house batteries are only supposed to receive charge when the engine is running and the starting battery is charged up. That keeps the loads from discharging the starting battery.

    I believe this to be the case because even when the house batteries run down the engine battery always has tons of juice to get the bus fired up and start charging the house batteries again.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #60
    Re: Greetings and Off-grid advice requested!

    Yes it would work to move the switch/Charge Controller grounding as show... However, the eningeer in me would tell you now you have, potentially, the DC negative of the battery at -12 volts (there are always a little bit of phantom loads/leakage current/something left on on the positive side--pulling the 12+ to "ground").

    Now, the two shunt sense leads are at -12 VDC and possibly exposing the small gauge sense leads to excessive current if there is ever a short.

    Grounding is a tough subject anyway (and, as you have probably read here, subject to religious discussions along the lines of how may angles can dance on an earth ground rod). When you add switches/fuses/etc. that make grounding into a "it depends" situation--It makes understanding the various fault possibilities very difficult to predict.

    My rule of thumb--If you ground bond something--Make sure it is a permanent/non-switchable/non-fused/non-conditional connection. Otherwise, the whole system becomes a "floating" power supply--Which in itself, is actually quite safe--But then demands other protections (like fuses/breakers in both +/- leads, understanding ground/power return connections for things like Car Radios/HAM/CB Antenna grounding, etc....

    Please, for your safety and sanity--Just hard ground the battery bank (on the left side of the shunt) and put a switch (or switches) plus fuses/breakers on the + Battery Bus Connection.

    Adding 12 VDC "car accessories" (like an AM/FM car radio for RV use) into the mix just makes alternative grounding schemes a nightmare (speaking as one guy that has in years passed worked on positive grounded cars with negative grounded radio--Guy set the radio/loud speaker system on a carpet over the transmission hump in an old Kaiser or Studebaker to reduce the chance of sparks).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Rossman
    Rossman Solar Expert Posts: 178 ✭✭
    #61
    Re: Greetings and Off-grid advice requested!

    Thanks Bill,

    I believe I have amended the diagram per your suggestion? In lieu of adding + bus bar to the diagram have simply noted two fuses on the positives leaving the battery bank.

    Cheers!

    Attachment not found.
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