Floating a 12V battery from 48V bank

Greets,

I have a system with some heavy-duty safety-of-life requirements and I want to specify that the monitoring system runs from a 12 volt battery that is being maintained in a "standby" mode directly from the main 48 volt battery bank.

The plan is to use a Morningstar SS-MPPT charge controller with the PV inputs connected to the battery bus and the battery outputs connected to a 33Ah 12V AGM battery.

The monitoring equipment and wireless radios will then be powered directly from the 12V battery bus.

To insure that the "real" system is up and running, the "PV Voltage" will be monitored an a sequence of errors reported should the "real" system die due to low 48 volt bank voltage.

My understanding is that the SS-MPPT will work fine in this configuration.

Has anyone here used an SS-MPPT in this configuration, or am I on crack?

Comments

  • mike95490mike95490 Solar Expert Posts: 8,500 ✭✭✭✭✭
    Re: Floating a 12V battery from 48V bank

    I'll concur and say it should be fine.

    You may want to protect that tiny 33Ah battery with a fuse, so the MPPT won't dump it's full output into it. Maybe a carefully selected series resistor, at low current, it won't have much drop.

    Although once charged and floating, it should never "need" full charging power.

    And check the voltage presets of the MPPT, match the battery specs, if it's AGM, there is no ez adding water to a dried cell. Disable EQ.
    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 ,

  • FullpowerFullpower Solar Expert Posts: 69 ✭✭
    Floating a 12V battery from 48V bank

    Surepower model 41010C00
    48 volt DC nom to 13.8 volt [email protected] 10 Amps
    about a $140.
    http://www.smithae.com/products/surepower_converters.html

    MPPT not required.
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Floating a 12V battery from 48V bank

    I'v got a Xantrex C35 ( PWM ) doing it right now on a 24V system to 12v output. They seem to work fine on any old cheesy power supply.
  • BB.BB. Super Moderators, Administrators Posts: 29,837 admin
    Re: Floating a 12V battery from 48V bank

    The fuse will not be needed to reduce current flow--Just normal short circuit protection (fuse to 48 volt battery bank; fuse to 12 volt battery bank).

    Cacading requirements for safety can be really difficult. Keep it simple. Perhaps a "deadmans switch" to a transmitter with Lithium primary cells if the watchdog fails?

    Of course, there is the transmit/polling method--As long as the system is transmitting green, then everything should be OK. If it stops transmitting, then something has failed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Floating a 12V battery from 48V bank

    PWM controllers are out. It will either blow up the series pass device or if it has current sense and then it will just immediately shut off the series pass device. Some may have linear regulator mode with temp sense regulation on series pass.

    The only issue I see with the sunsaver MPPT is it will go to its max battery charging current capability of 15 amps during any bulk phase with input connected to 48vdc battery bank. This is a bit much for a 33 AH battery.
  • mike95490mike95490 Solar Expert Posts: 8,500 ✭✭✭✭✭
    Re: Floating a 12V battery from 48V bank
    Fullpower wrote: »
    Surepower model 41010C00
    48 volt DC nom to 13.8 volt [email protected] 10 Amps
    about a $140.
    http://www.smithae.com/products/surepower_converters.html

    That's just a DC-Dc convertor. Not a battery charger.

    But Julie - here's a thought - What about a 48V - 12V DC-Dc convertor, and power the gear right off it. 4 marine batteries gets you 48V cheap. No 12V battery to fuss with.

    My $100, 120 watt 48-12V DC-DC converter slideshow.
    http://www.naturalstudies.org/Photo_galleries/LED_driver/
    A 94% efficient converter, used to down-convert a solar charged 48V solar battery bank, to power 12V LED drivers for a Burning Man project. I built 20 of these up, and the user also uses the XW inverter in the trailer, as household power at home in Joshua Tree.
    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 ,

  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Floating a 12V battery from 48V bank

    RC in FLA you sure about this? you have tried it and seen failures?PWM controllers are out. It will either blow up the series pass device or if it has current sense and then it will just immediately shut off the series pass deviceWhy I ask is I have used a Chinese copy of an old Steca PWM charger , Its imputs connected to an 18v DC supply and used it to regulate the voltage to a 12v battery. no dramas happened. Only did it for a few months ,but it worked
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Floating a 12V battery from 48V bank

    If a 48V DC to 12V DC converter is used the 12v battery will soon be destroyed as it will keep delivering the maximum current it can into the battery even when its fully charged.
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: Floating a 12V battery from 48V bank
    john p wrote: »
    If a 48V DC to 12V DC converter is used the 12v battery will soon be destroyed as it will keep delivering the maximum current it can into the battery even when its fully charged.

    I can't see this happening because although the converter may keep trying to deliver it's maximum current, the battery will first have to accept it! A fully charged battery will only accept more charge/current if the source voltage is raised.
    If the source voltage is fixed at 13.8v (which in this case it is, since this is a converter and not a multi stage charger), and the battery's voltage is currently low, say 12v, the voltage difference between the converter and battery will enable current (limited by the source/converter) to flow and charge the battery, however if the battery's voltage is 13.8v, i.e. fully charged a 0v voltage difference will not enable current to flow and therefore the battery will not get overcharged.
    I have been using this set-up for quite some time (24v --> 12v) and I do have voltage and ampmeters to monitor what's going on.... basically what I have just stated.
    The converter is a simple 7812 followed by a couple of 2N3771 transistors to boost the output.
  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Floating a 12V battery from 48V bank
    Brianellul wrote: »
    I can't see this happening because although the converter may keep trying to deliver it's maximum current, the battery will first have to accept it! A fully charged battery will only accept more charge/current if the source voltage is raised.
    If the source voltage is fixed at 13.8v (which in this case it is, since this is a converter and not a multi stage charger), and the battery's voltage is currently low, say 12v, the voltage difference between the converter and battery will enable current (limited by the source/converter) to flow and charge the battery, however if the battery's voltage is 13.8v, i.e. fully charged a 0v voltage difference will not enable current to flow and therefore the battery will not get overcharged.
    I have been using this set-up for quite some time (24v --> 12v) and I do have voltage and ampmeters to monitor what's going on.... basically what I have just stated.
    The converter is a simple 7812 followed by a couple of 2N3771 transistors to boost the output.

    The quibble is not that it won't work (but is possibly harmful on fully discharged small batteries with low internal resistance without current limits) but that a fixed voltage charger is a compromise between the float voltage and the absorption voltage that's about a volt higher, and is needed to fully recharge a battery after use. You're right, it won't be overcharged if lightly used but will be undercharged after use.
  • RCinFLARCinFLA Solar Expert Posts: 1,280 ✭✭✭
    Re: Floating a 12V battery from 48V bank
    john p wrote: »
    RC in FLA you sure about this? you have tried it and seen failures?PWM controllers are out. It will either blow up the series pass device or if it has current sense and then it will just immediately shut off the series pass deviceWhy I ask is I have used a Chinese copy of an old Steca PWM charger , Its imputs connected to an 18v DC supply and used it to regulate the voltage to a 12v battery. no dramas happened. Only did it for a few months ,but it worked

    You may be using a power supply that has a max current limiter.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: Floating a 12V battery from 48V bank

    Brianellul You might not dsee how it can happen but it will, If the converter is set to 13.8v in less than one week if the 12v battery is only lightly used it will be dried out if it is standard lead acid battery. If the converter can output over 10a it will kill the battery. I have seen this happen in many cases where people have used a 13.8 dc supply permantly connected to a 12v battery and soon had failure where there was little use of the battery.

    IF it was ok to use 13.8 power supplies then why do people use properly and more expensive to make multistage battery chargers??

    if the converter voltage is set to 12v the battery will never get charged at all.
  • BrianellulBrianellul Solar Expert Posts: 95 ✭✭
    Re: Floating a 12V battery from 48V bank
    john p wrote: »
    Brianellul You might not dsee how it can happen but it will, If the converter is set to 13.8v in less than one week if the 12v battery is only lightly used it will be dried out if it is standard lead acid battery. If the converter can output over 10a it will kill the battery. I have seen this happen in many cases where people have used a 13.8 dc supply permantly connected to a 12v battery and soon had failure where there was little use of the battery.

    IF it was ok to use 13.8 power supplies then why do people use properly and more expensive to make multistage battery chargers??

    if the converter voltage is set to 12v the battery will never get charged at all.

    Hi John

    I mentioned the 13.8v since this was previously mentioned in the preceeding posts. I agree that it's too high to leave the battery on this voltage. The system I have built at home, the voltage is set to 12.7v/12.8v which is just enough to leave the battery fully charged and ready for use. I'm following the same principle as UPS batteries which are kept topped up ready for use. This is on the same lines as the original post...
    I'm not implying that multistage chargers can be replaced by power supplies.... but to keep a battery topped up, you don't really need a multistage charger but a 12.7v fixed voltage power supply should do the job well enough. In my case my 7 AH battery has been setup in this way for the last 5 years.
  • nsaspooknsaspook Solar Expert Posts: 396 ✭✭✭
    Re: Floating a 12V battery from 48V bank
    Brianellul wrote: »
    Hi John

    ... but to keep a battery topped up, you don't really need a multistage charger but a 12.7v fixed voltage power supply should do the job well enough. In my case my 7 AH battery has been setup in this way for the last 5 years.

    Have you load tested your battery?
    I assume this is a small gel-cell type with a float voltage of 2.25V to 2.30V/cell. http://www.power-sonic.com/images/powersonic/literature/SLA_Batteries/20110920-TechManual-Lo.pdf
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Floating a 12V battery from 48V bank

    The only problem with Julie's original premise is the potential of the CC to put its full 15 Amps out if the battery should be discharged, as Mike mentioned.

    The simplest solution is to use a larger capacity battery, unless space restraints make this impossible.

    It would be ironic to cook the battery that's meant to power the system to warn you when something has gone wrong.
  • tallgirltallgirl Solar Expert Posts: 413 ✭✭
    Re: Floating a 12V battery from 48V bank

    Thanks for the feedback. I think adding a third layer of backup is more likely to cause a needless failure -- I think Lion chemistries have their place, but I'd rather go with NiMH, if I were going to have a backup battery other than lead acid.

    As regards various charge controllers' ability to handle DC-to-DC conversion -- please keep in mind that most plug-in power supplies are current limited. A lead-acid bank, of the size being planned, is more like an infinite current source than a current limited one. Using a current limiting resistor is a needless load -- the SS-MPPT needs to be able to buck an infinite source from 48 VDC to 12 VDC. My understanding is that the design of the charge controller means it can. If someone knows otherwise, please set me straight -- I'd rather not have a phat resistor in there making it work.
  • tallgirltallgirl Solar Expert Posts: 413 ✭✭
    Re: Floating a 12V battery from 48V bank
    Brianellul wrote: »
    Hi John

    I mentioned the 13.8v since this was previously mentioned in the preceeding posts. I agree that it's too high to leave the battery on this voltage. The system I have built at home, the voltage is set to 12.7v/12.8v which is just enough to leave the battery fully charged and ready for use. I'm following the same principle as UPS batteries which are kept topped up ready for use. This is on the same lines as the original post...
    I'm not implying that multistage chargers can be replaced by power supplies.... but to keep a battery topped up, you don't really need a multistage charger but a 12.7v fixed voltage power supply should do the job well enough. In my case my 7 AH battery has been setup in this way for the last 5 years.

    That strategy doesn't work, as anyone who's worked around UPSes for 25 years would be more than happy to tell you. Which is why I'm telling you that it doesn't work ;)

    Batteries really =do= need to be exercised to some degree.
  • BB.BB. Super Moderators, Administrators Posts: 29,837 admin
    Re: Floating a 12V battery from 48V bank

    Yes, the MorningStar has been tested (and function may even be on the data sheet) by Solar Guppy and one or two others here to connect the input to a higher voltage battery bank and down convert to a lower voltage battery bank.

    The only problem (that I have read here) was there was a bug in the older firmware that prevented the status LED's from reporting status (float/absorb?) correctly unless the input "went away" once a day (solar panels at night).

    There is a later firmware that fixes the bug (display issue only, does not affect functionality of the controller--that I have heard of). That was something like a year or so ago in some thread here (if you need to look it up).

    The only other issue is 15 amps into a 33 Amp*Hour battery bank...

    A quick look at the ModBus programming document (PDF) for the SunSaver MPPT controller seems to indicate a couple of programmable registers that you can limit Watts and/or current into your battery.

    Lastly, I believe that this controller will read "hot" (internal temp sensor) and it is a VERY good idea to get the remote battery temperature sensor option.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tallgirltallgirl Solar Expert Posts: 413 ✭✭
    Re: Floating a 12V battery from 48V bank

    Bill,

    I'm not too concerned about dumping 15 amps into a 33Ah AGM -- they are routinely rated to handle C/3, and C/2 would only happen on those very few occasions when there would be little or no charge in the batteries in the first place.

    The other issue, and this is especially true going into AGM batteries, is that the length of time for that very high rate of charge is unbelievably short -- my guess would be they'd reach Absorb at something like 50 percent SOC at C/2. From there the current drops extremely rapidly. At C/2 you're recharging 1% SOC per minute, more or less, and the acceptance current drops like a rock from 50 percent to 80 or 90 percent.
  • tallgirltallgirl Solar Expert Posts: 413 ✭✭
    Re: Floating a 12V battery from 48V bank
    The only problem with Julie's original premise is the potential of the CC to put its full 15 Amps out if the battery should be discharged, as Mike mentioned.

    The simplest solution is to use a larger capacity battery, unless space restraints make this impossible.

    It would be ironic to cook the battery that's meant to power the system to warn you when something has gone wrong.

    There's no chance a 33Ah bank is going to kill the bank that's feeding it. For one thing, the load is on the order of 0.5 to 1A at 12 VDC. But since this is the "warning" system, it =must= be functional so it can notify service personnel to come out and fix whatever is broken with the main bank.

    As I said, this is a safety-of-life requirement. If the main bank is in danger of reaching LBCO, something is seriously wrong and the main function of the system is severely compromised and people could wind up dead if it doesn't get fixed.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Floating a 12V battery from 48V bank

    Julie;

    If the 33 Amp hour 12 Volt AGM is a Concorde, they claim to be able to handle a charge rate up to CX4. In that case the potential harm from hitting it with 15 Amps in the unlikely event it because seriously depleted is nullified. If it were to occur, the 45% peak charge current would push the Voltage up very quickly, and the current would drop likewise. I wouldn't expect a scenario where the little battery would be fed big current for very long, if ever.

    I think your plan would work. ;)
  • tallgirltallgirl Solar Expert Posts: 413 ✭✭
    Re: Floating a 12V battery from 48V bank
    Julie;

    If the 33 Amp hour 12 Volt AGM is a Concorde, they claim to be able to handle a charge rate up to CX4. In that case the potential harm from hitting it with 15 Amps in the unlikely event it because seriously depleted is nullified. If it were to occur, the 45% peak charge current would push the Voltage up very quickly, and the current would drop likewise. I wouldn't expect a scenario where the little battery would be fed big current for very long, if ever.

    I think your plan would work. ;)

    Plenty of AGMs will take Cx1 or even Cx2 -- my lawnmower is a 17Ah pair of 12 VDC batts. I recharge with my car charger at 25 amps all the time and after years of doing this, the lawn mower keeps on mowing.

    What seems to make it work is the acceptance charge drops like a rock. I don't have a small bank with all my toys hooked up, but next chance I get, I'll hook something up and post the charging curves. It's really pretty slick -- the voltage rapidly hits Vabsorb and from there the current just goes down. Fast.
  • firerescue712firerescue712 Solar Expert Posts: 93 ✭✭
    Re: Floating a 12V battery from 48V bank

    I am using the MPPT15L off a 24v bank and charging a 12v bank for lights and small loads. It works flawlessly. I have been running this for several weeks. Once a week, i disconnect the MPPT15L and let the 12v batteries drain to 80% mark. Then, I reconnect the MPPT15L. The 12v batteries go through the absorb phase, then float until the next week. The 24v bank discharges to between 24.1 and 24.4 volts overnight. It charges fully by noon each day.
  • BB.BB. Super Moderators, Administrators Posts: 29,837 admin
    Re: Floating a 12V battery from 48V bank

    Depending on how you measure the 24.1 volts, loads, bank temperature, etc... that may be below 50% state of charge (need to measure battery resting voltage--3+ hours of no load/charge--at ~75F to estimate state of charge).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • firerescue712firerescue712 Solar Expert Posts: 93 ✭✭
    Re: Floating a 12V battery from 48V bank

    I was going by the input reading of the Rogue 3024 between the 24v and the 12v battery banks....and the other Rogue 3024 reading to the 24v battery bank from the pv modules. I understand the at rest reading, but if the reading shows 24.1 on both charge controllers, this is the voltage under load. If the battery is put to rest, the reading should climb. The same principle applies to pressurized water systems. The reading at flow is residual. The reading at rest is static. It is close enough to use to monitor the systems as they charge and discharge throughout the 24 hour day.
  • BB.BB. Super Moderators, Administrators Posts: 29,837 admin
    Re: Floating a 12V battery from 48V bank

    Correct... Except that batteries can take 3-12 hours or so to reach "static" voltage since it is both a chemical and physical equalizing of stuff inside the battery.

    Just trying to make sure that you are running the battery(ies) in the range you want--Taking them to dead even once or twice can kill them. Deep cycling (below 50%) is not the worst thing in the world... Say you get 500 cycles of life from deep cycling (instead of 2,000 cycles for shallow cycling)--Do this once a week and you are still in the ~10 year cycle life--Other things may kill them faster.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • firerescue712firerescue712 Solar Expert Posts: 93 ✭✭
    Re: Floating a 12V battery from 48V bank

    Thank you, Bill. I appreciate you looking after us newbies. I have 6v golf cart batteries to abuse. The Concorde PVX4050HT's will be here Friday to replace them. I have a unique system...mainly just testing the waters. I got the second Rogue in today to replace the Morningstar MPPT15L between the 24v and 12v batter banks. So far, everything is great. I might have to reduce the float voltage on the 12v bank to keep from placing such a drain on the 24v bank overnight. I have the usual "vampire" loads run off the 12v batteries through a Morningstar 300 inverter. I run my office computer and monitor during the days and early evenings off this, too. I am slowly adding to the system to increase the load shift from grid to off-grid. I am not trying to go totally off grid.

    I hope Tallgirl sees this discussion. I am sure she has already done whatever she was looking for.

    Have a safe evening.
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