Battery Bank Voltage Tap for low burden control

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Brendan
Brendan Registered Users Posts: 7 ✭
in Off Grid Solar & Battery Systems #1
Hi All,

Practically speaking, would it still be generally frowned upon to tap 24VDC from a 48VDC battery bank where the daily burden is likely to be no greater than 2Ah. This draw would only be in times of grid-outages also.

This would be to provide coil power to a DPDT relay in a grid tied battery backup system, using the Sunny Island grid-presence function.

The coil could be AC powered and tapped onto the utility source, but then the coil is running pretty much all day everyday, rather than just when the grid goes out.

Any input would be greatly appreciated.

Thanks,

Brendan.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #2
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    Re: Battery Bank Voltage Tap for low burden control

    It certainly sounds like a fine idea to me. If it sis only going to be uesd during outages, there is almost no room for harm, besides 2 ah is not enough draw to worry about, especially in which part time use. Just my opinion however, I let others opine.

    T
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #3
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    Re: Battery Bank Voltage Tap for low burden control

    Personally im against it, I think its bad drawing anything from part of a battery bank. I have seen it done many times and it can cause problems. its hard enough to keep a string of batteries identical under ideal conditions.
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    #4
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    Re: Battery Bank Voltage Tap for low burden control

    It is okay unless you are doing extended float periods.

    It does not take much leakage on lower batteries to upset the voltage dividing balance between batteries. This will expose upper batteries to higher voltage float which over time will degrade them.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #5
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    Re: Battery Bank Voltage Tap for low burden control

    Couldn't you just use a 48V relay instead? The sunny island has an internal connection which supplies 48V and 600mA for driving high power relays.
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  • Brendan
    Brendan Registered Users Posts: 7 ✭
    #6
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    Re: Battery Bank Voltage Tap for low burden control

    I am using a Magnecraft DPDT power relay, and they only come with 24VDC max DC controlled coil.

    I tried to research some good alternatives but came away with little.

    I'm always open to suggestions!

    Thanks,

    Brendan.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
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    Re: Battery Bank Voltage Tap for low burden control

    What are you controlling, maybe we can find another relay that works.

    IMHO , 2a is NOT trivial, when it's for more than an hour, you would surely get the battery bank un-balanced and need a couple long absorb cycles or an EQ cycle to straighten it out.
    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 ||
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
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    Re: Battery Bank Voltage Tap for low burden control

    if you were to do that you'd need a way to charge each 24v section of the string separately. that's most likely impractical so maybe a voltage converter or if you are electronically inclined you can build a regulator.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #9
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    Re: Battery Bank Voltage Tap for low burden control

    Since this is an "emergency" solution, one could simply pu t a 2 amp load on the other setoff batteries for the duration of the outage. A waste of power, but a solution to keep the batteries even.

    Tony
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  • Brendan
    Brendan Registered Users Posts: 7 ✭
    #10
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    Re: Battery Bank Voltage Tap for low burden control

    Just to clarify, this is not a 2A load. It was a 2Ah/day burden.

    The actual load is 86mA.

    It is controlling the output power routing of a Sunny Boy, between the MDP and a backup loads panel.

    Thanks,

    Brendan.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #11
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    Re: Battery Bank Voltage Tap for low burden control

    Or put a small resistor in series with the relay?
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  • Brendan
    Brendan Registered Users Posts: 7 ✭
    #12
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    Re: Battery Bank Voltage Tap for low burden control

    I was thinking of a crude solution like a potentiometer, seeing as the coil resistance is approximately fixed. It should be easy to calculate a simple resistor network on paper but whether it manifests itself in a product that can be sourced is another thing.

    TBD with that one. If I found the correct pot, could we just put it into a JB and field wire to it without any issue? Are there any NEC provisions for this kind of component, more typically found in a manufactured device?

    Thanks,

    Brendan.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
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    Re: Battery Bank Voltage Tap for low burden control

    As to the issue of unbalancing the battery bank, what's the average load on the whole bank? 83 milliamps isn't much of a load. If the whole bank has to put out 1 Amp, then the balance is upset. If the bank has to put out 100 Amps - the relay load is insignificant. You're likely to suffer more imbalance from unequal resistance in wiring.

    If you're really concerned, buy two relays and wire them in series and run them off the 48 Volts.
    But be sure to check the relays Voltage range, as a charging "48 Volt" system may exceed 60 Volts.
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  • Brendan
    Brendan Registered Users Posts: 7 ✭
    #14
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    Re: Battery Bank Voltage Tap for low burden control
    stephendv wrote: »
    Or put a small resistor in series with the relay?

    My response was silly. The coil is rated at 277 ohms. I could try match that and put it in series with the coil.

    Very obvious - good thinking.
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  • Brendan
    Brendan Registered Users Posts: 7 ✭
    #15
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    Re: Battery Bank Voltage Tap for low burden control

    All things considered, without resorting to soldered connections, I think it's best to obtain a DIN rail mounted AC to 24VDC regulated power supply and mount it into a JB and be done with it.

    I don't want to chance it when it comes to the potential of battery imbalances.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #16
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    Re: Battery Bank Voltage Tap for low burden control

    I think you could also get relays with coils powered directly from AC. e.g. http://www.rowse-electrical.co.uk/p-251-3rh1122-1af00.aspx
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #17
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    Re: Battery Bank Voltage Tap for low burden control

    Couldn't you just put a step down transformer for that low amp load alone to get to 24V?
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  • techntrek
    techntrek Solar Expert Posts: 1,372 ✭✭✭
    #18
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    Re: Battery Bank Voltage Tap for low burden control
    Cariboocoot wrote: »
    If you're really concerned, buy two relays and wire them in series and run them off the 48 Volts.

    That would be my solution, too. Or instead of 2 in series, use the 2nd relay on the other 2 batteries - but still not as much of a guarantee of an even draw as going in series and using the 48 volt connection.
    4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
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  • Les Nagy
    Les Nagy Solar Expert Posts: 121 ✭✭
    #19
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    Re: Battery Bank Voltage Tap for low burden control

    Another option is to use 40 diodes in series to get a 24V voltage drop. 0.6V drop per diode.
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  • nsaspook
    nsaspook Solar Expert Posts: 396 ✭✭✭
    #20
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    Re: Battery Bank Voltage Tap for low burden control
    Les Nagy wrote: »
    Another option is to use 40 diodes in series to get a 24V voltage drop. 0.6V drop per diode.

    Or two 12v 5W Zener diodes in series. http://www.parts-express.com/pe/showdetl.cfm?partnumber=1N5349&utm_source=googleps

    They should handle your 100ma load easily. Instead of regulation at the load they can be used in series before the load to reduce the voltage. If used this way be sure to have a minimum current load at all times. (a few milliamps)
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