Tristar MPPT to power 12v devices from higher voltage battery bank.

animatt
animatt Solar Expert Posts: 295 ✭✭✭
While kind of over kill I was wondering if a tristar mppt controller could be used to down convert the high voltage battery array to power 12v loads.


I bought 2 tristar mppt 45a model. This was to have a spare in case something happened. I also bought a cheap 24v to 12v converter as well.

So this inquiry is to have the tristar as a backup in case the normal converter lets out the magic smoke, or does not work as planned.

Although an expensive converter the tristar would make a very efficiency 24v to 12v converter. Very high efficiencies and very low idle power consumption.

Would be nice to have the one mppt control being a backup to either piece if either were to fail. (converter or charger controller).

The basic idea would be to hook up the 24v battery array in this case to to solar input of the controller and have the output of the controller hooked up to a 12v dc panel.

Obviously I do not think it would be that easy as I think you would have to limit the current from the batteries to the solar input terminal. And I believe the controller powers up via its battery output terminals. Maybe that would require a simple ultra capacitor on the output line to always have the controller charged. It could be simple if controller was setup for it but have no idea, and did not see anything in the manual about it.

IN reality a dc-dc inverter with 45a output is expensive to begin with so the tristar not such a bad price.


Anyone heard anything about something similar to this or have ideas?
I am going to be traveling in a motor home and would like to have some redundancy.


Matthew

Comments

  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    that will work,as you desribe, first MPPT charges 24 v battery, second MPPT charges 12 v battery from 24v
     
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  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    Little more detail.

    This will be in a motor home with ~470ah at 24v main battery bank and a single starting battery.

    I would like to pull from the 24v battery bank and convert to 12v and power a 12v electrical circuits. These circuits are not in anyway connected to the 12v starting battery.

    How do you limit the 24 battery array from over currenting the 2nd mppt(converter). Solar panels have limited current while battery could push mega amps at the controller.

    It would be nice to power the 12v loads without an additional 12v deep cycle battery. but as stated above a ultra cap could probably handle that.

    So questions how to limit 24v battery amps. Is it all dependent on the charging parameters? Basically custom set points just set it to float all the time at 12.8v? Would this allow the varying 12v dc loads to pull appropiate amps from the "converter"

    matthew
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    The MPPT magic is what limits the charging output of the controller. So you CAN feed a MPPT controller from an oversize array or a battery, and it will use it's internal max amps to limit the current that it outputs.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    I knew you could oversize arrays but I figured there would be a limit to the over sizing. Which a battery array would be massively over sized. Good to know that works.

    So connect my 24v battery array to the mppt solar input terminals.(max input to controller would be about 15a)

    Now load side. Like stated above i would like something just strong enough to power the controller up. Once controller is up a little bit of power recharges that 12v source power but the bulk of the power goes to loads. Say I want my tv on (1.1a) and lights in the bedroom(~1-2a more) so in total I would be using 3 amps at 12v dc. I would like the setup to draw ~1.5a 24v battery array and power loads. Things change and load goes to 6amps. mppt converter would now be drawing ~3a from the 24v array. All loads go away, I would like the mppt to pulll just enough power from the 24v and move it to the 12v side so mppt controller can continue to run.
    Basically thinking on the output side of mppt controller having a small 10ah battery or a capacitor wired in parallel with the loads.


    Would the above scenario be possible or am I stuck with having a separate full size 12v battery to run off of, and just have the converter charging the batteries?

    Could I just program controller to always be in absorb mode. right around 12.5. Higher load pops up voltage pulls capacitor down for a second and the controller ramps up a bit? Or is there better logic to use?

    matthew
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    The MPPT charge controller really will handle it all for you. Just be sure to set the maximum times and all will be well.

    The key to not blowing up is that the MPPT charge controllers aren't just switches that get turned on and off, as with PWM charge controllers. There's enough smarts, and enough isolation, that the 24 volt batteries are never directly connected to the 12 volt battery.
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    "Just be sure to set the maximum times"

    From that I am understanding you are considering the loads pulling directly from the battery. And the mppt controller is just keeping the battery charged.

    To go more extreme example. I want a super small battery. Imagine 8 AA rechargeable batteries in series for 12v. Basically the AA batteries just keep the controller powered. So basically the converter is passing current directly to the loads rather than to the batteries. The AA batteries would not be able to hold a 10a 12v load but since in my purposed situation it is wired in parallel(load and 12v batts) the mppt converter picks up the load on the fly.

    Anything wrong with my extreme example? Other than battery maybe a little to small. would a 5ah 12v battery work?

    What would the programming look like. That is really want I am stumbling here. I come to the conclusion that 100% of time in absorb mode of say 12.8v would do the trick as the controller will want to keep the voltage constant? Atleast until all loads go away. Once loads go away controller is pushing how many amps into the small 5ah batt. What happens to charge rate?
    Can the mppt controller deal with small battery/capacitor. With such a small battery/capacitor voltage could drop very quickly on it in float stage, but seems it will stay in float until 30 minutes were spent below a trigger voltage. What would the amps while in float mode?
    1/2 amp would be c/10 rate.
    The manual I am reading does not seem to technical not sure there are ones that go into more detail about charging.

    While the controller I am sure is very smart just wondering about these things. Think you did bit of independent work on the controller so hopefully you know some of these things. Are there any more technical documentations around.
    I know you do not care so much about midnite solar but definitely like their interaction with non standard questions and documentation. Although I like a fanless design even better :)

    thanks
    matthew
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    Your loads and their surge, will determine the size of the battery. I'd suggest a small 12V wet cell motorcycle battery, cheap, and it would handle the loads for the second it may take the charge controller to respond.

    The desired configuration would be to keep the battery in float, and block out Equalize cycles.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    As I read the manual the float mode is constant voltage but no idea how the current is handled. They are vague in how that happens.

    No load draw on battery what would be float charge on 10ah batt? Now a 3a load comes online what does the converter respond and provide the extra 3a after a second or two? I continue to ramp up to 15a of usage? WOuld it stay in float providing 15a? Would it continue to ramp up current up to controller limit (my case 45a)? In that case as long as the battery can pick up the surge it is fine?

    My loads realistically only come one in fairly small increments. The only larger load is the 12v water pump which is max load of around 7amps. The rest of the loads come on line roughly .25a -1.0A at a time. I do have a furnace as well that probably draws 8-10a but not going to be using it.

    Realistically with water pump on I would be hard pressed to break 15a, and realistically would probably be under 10a. 12v loads or led lighting. 12v tv, 12v water pump, maybe a 12v fan in the future.

    Water pump off I would probably be under 5a most of the time.

    Not sure if I would want to adjust "Absorption Extension Voltage" and "Float Exit Timeout", or just leave settings and pick 5,6,or 7 (if using wet cell) battery charging preset.


    Thanks for the help.

    Did you suggest wet cell for cost? vs agm? I really have not preference. It is whatever works and lasts on permanent float charge.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    In Float mode a charge controller feeds only as much current as is needed to maintain that Voltage level. If loads increase, so does current. If it can not maintain the Voltage due to heavy loads or insufficient sun it drops out of Float. It is not charging; it is maintaining.
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    thanks for the clarification.

    So if you had a big enough battery array the controller would float at ~13.5v at 45A or 60A depending on model.

    Is there a lower limit to this. Could the controller float a 1ah battery? This last 1ah batter question is just out of curiosity.

    matthew
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    There would be a lower limit due to the effect of the current fluctuation. Sort of like putting a capacitor in a circuit to stabilize Voltage; insufficient capacity leads to greater Voltage fluctuation. The greater the battery capacity compared to the load current, the less fluctuation. I'd try to make the battery Amp hour capacity 5 to 10 times the continuous load current.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    I would suggest the numbers that Marc/Cariboocoot typed... If you are looking for an extreme minimum battery size, I would use C/2.5 (i.e., 2.5x the maximum surge current you would expect to see for the load).

    I believe you might be able to program the maximum charging current for the TS MPPT family of charge controllers (with the appropriate interface/software). If that is the case, and your power needs are not that great--I would suggest that you program the maximum charging current for the MPPT controller to the C/5 rate maximum--C/8 if you want to be a bit more conservative). With higher charging currents, a fully charge battery might not "buffer" a spike in charging current from the MPPT controller (i.e., you have a heavy DC load, then shut if off... The time constant for the MPPT to recognize the rising charging voltage until it reduces its output current may cause the battery to see a "voltage spike"--Have not heard of it being a problem for this family of controllers--but something to watch out for when running a very small AH battery bank).

    If your available charging current is >~C/8, then you should be using the battery temperature sensor to reduce the chances of thermal run-away (should only be an issue if for some reason you discharge the small battery pack below ~85% state of charge, and probably below 50% SOC, then hit it with a lot of charging current).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    Be very cautious using a battery bank on the input of any charge controller. These controllers are designed to be used with an inherently current-limited source (PV). A battery bank is capable of thousands of amps of current for the brief milliseconds it would take to smoke your controller if a fault occurred and the controller's current limiting didn't react fast enough. Also, for the typical buck controller, there is no isolation between input and output. At anything near 100% duty cycle, the input is essentially connected directly to the output. And if the top FET ever shorted due to a fault, you'd also have a direct in/out connection - until a breaker tripped, fuse blew, or a trace on the PCB vaporized.

    I would discuss this with Morningstar first, both to get their advice and find out what effect it has on the controller's warranty.

    Marc
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    would it be a bad idea use the motorhome starter battery for this task.

    24v battery bank is about 15 feet away from the front 12v starter battery. WIth all inverter and 24v devices very close to the batteries.

    Just parallel connect the 12v wiring and load wiring on the morningstar terminals, or to a small buss bar right next to the morningstar?
    The 12v fuse panel is with in a few feet of the 24v battery array. So the 12v battery wiring from battery to morningstar maybe 20ft depending on wiring path.

    I could imagine this would be a good thing to keep starter battery maintained. But not sure.

    How would the vehicle alternator affect things when engine was started? The battery maybe getting a +14v charge from the alternator. What would the mppt do seeing this 14v which is higher than its float voltage?

    Matthew
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    Marc's warning about over-current are entirely valid. It's a case where you would want a fuse/breaker on the second controller's input, preferably with all current ratings under the max for the controller. That way if the current gets too high for whatever reason the circuit protection should go before anything else. Nothing is 100% certain, though.

    Yes you could keep the vehicle battery charged this way. But you may need to play around with the charging profile a bit to find a set of values that doesn't output too much charge. As in limiting the Absorb Voltage to Float level so that all the charger does is "Bulk up and Float"; auto batteries aren't meant to have 14.8 Volts dumped into them for three hours straight. Check the normal charging Voltage of the system to start with; watch what it peaks at and where it levels out.

    The alternator's power should not be able to back-feed through the controller and do anything to the other batteries or wiring. For one thing it's an "uphill battle" in Voltage. For another the controller should 'see' the Voltage as fully charged battery and disconnect from the input.
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    Yeah was thinking of putting a 20 Amp breaker on the 24v input to the controller, and a 30-40amp on the output side. Everything running in the motorhome right now would not get past 30A. I am thinking a 40a breaker as it would allow quick battery jumping if for some reason motorhome 12v starter was low after extend period where the 24v to 12v converter has been off.

    Thanks for help. Will try contacting morningstar tomorrow.

    Matthew
  • tallgirl
    tallgirl Solar Expert Posts: 413 ✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.
    lorelec wrote: »
    Be very cautious using a battery bank on the input of any charge controller. These controllers are designed to be used with an inherently current-limited source (PV). A battery bank is capable of thousands of amps of current for the brief milliseconds it would take to smoke your controller if a fault occurred and the controller's current limiting didn't react fast enough. Also, for the typical buck controller, there is no isolation between input and output. At anything near 100% duty cycle, the input is essentially connected directly to the output. And if the top FET ever shorted due to a fault, you'd also have a direct in/out connection - until a breaker tripped, fuse blew, or a trace on the PCB vaporized.

    I would discuss this with Morningstar first, both to get their advice and find out what effect it has on the controller's warranty.

    Marc

    I believe a number of people have done precisely that. I've designed auxiliary power supplies for off-grid systems -- remote monitoring or telemetry -- that used Morningstar MPPT controllers as buck converters.

    Battery systems are a lot more complex than "a battery can source 5,000 amps to a bolted short". That may well be true, and certainly is for many respectable bank sizes (makes the AC resi electricians go "boggle" when I mention fault currents), but there are two opposing properties in a system like this. The first is the so-called "crack of the whip" -- the tendency of a battery, especially deep-cycle ones, to sag very badly under sudden extreme loads. The second is the opposite side of that -- voltage rise under heavy charging.

    All that said, a lot of people have successfully used a number of smaller Morningstar MPPT charge controllers for that task.
  • animatt
    animatt Solar Expert Posts: 295 ✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    I guess it even says it in the description here http://www.solar-electric.com/mosumpsochco.html

    Did not see it as I was looking at the larger controller I already have.

    The reason I am asking about the larger controller is I already have it. Having it around as a spare can replace a failing or poor performing 24v-12v converter i have, or a failing morningstar mppt 45.

    I like the flexibility there. Want some redundancy so I am carrying extra parts, but it is a motorhome and limited space. So have a multi purpose backup is nice.

    thanks for all the responses

    Matthew
  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.
    tallgirl wrote: »
    All that said, a lot of people have successfully used a number of smaller Morningstar MPPT charge controllers for that task.

    I don't doubt that people are doing it, I'm sure it can be done successfully, and I see that the forum's sponsor even suggests such a use. But I've never seen this recommended by any charge controller manufacturer (including Morningstar). Current limiting that is done in hardware is probably responsive enough to prevent a problem. Current limiting done in software -- maybe not. And if for any reason the controller decides to go full duty cycle (maybe a glitch, array sweep, whatever), it could be destroyed in an instant by any battery that you're likely to see in an RE system, unless external fusing is fast enough to open the circuit (which it probably would be). I guess the acid test would be to short the output of the controller and see what happens. But I'll leave that up to someone else to try :D .

    Marc
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.
    lorelec wrote: »
    it could be destroyed in an instant by any battery that you're likely to see in an RE system, unless external fusing is fast enough to open the circuit (which it probably would be).
    Marc

    Definition of semiconductors: Circuit elements that blow instantaneously in the event of a short circuit, to protect valuable fuses from damage.
    SMA SB 3000, old BP panels.
  • mbicha
    mbicha Registered Users Posts: 1
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    Hello,
    we want to do a similar thing on a ship, charge the 12V board battery with the (spare) Morningstar TS-MPPT-60 from the 48V 10AH battery bank.
    The big question is (as the engines run off the 48 battery bank too), can the TS be configured so that it stops charging by drawing current when the Array voltage (48V bank) falls for example below 48.5V, so ensure that it does not empty the 48V batteries too much.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Tristar MPPT to power 12v devices from higher voltage battery bank.

    Not that I can think of... Solar Charge controllers are designed to take as much energy as they can from their input--Sun varies from zero to maximum over the day--So, a controller Mfg. spends a lot of time to ensure they they don't do exactly what you are asking them to do.

    You could use a Voltage Controlled Switch turning on/off a latching relay (for example)--But that requires extra parts/expenses.

    Hmmm.. Midnite Solar's Classic MPPT charge controller has a programmable input curve intended for use with Wind turbines. I wonder if you could program their controller to turn off at low input voltage? You should probably contact Midnite directly or go to their forum.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset