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Does there exist a relay with 2 inputs that triggers to the input with the lowest V

Discussion in 'General Electronics Discussion' started by jobbemann, Jun 11, 2013.

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  1. jobbemann

    jobbemann

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    Jun 11, 2013
    I need a switch (relay) that enables the following, but does this exsist? If not is it a easy build?
    I have 2 separated wired 12v battery systems in my boat. 1 battery charged by the alternator (powers starter motor, lights and main bilge pump) , 1 battery charged by a solar panel charging controller (mainly powers backup bilge pump).
    I would like to have a relay switch between the solar-charger and the 2 separate battery systems that connected the battery system with the lowest voltage to the solar-charger.
    That way the solar charger will charge both battery systems, but only one at a time, and always the one with the least charge, and still stop charging when both batteries are full (the charger should still stop charging when it senses that whatever battery is connected has good voltage).
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Yes, there are various ways to do what you want.

    First though, can you give some details of your solar charger.

    1. What is the maximum charge current?
    2. Will there be a problem if one battery is suddenly substituted for another battery, which the charger was previously charging? In other words, does the solar charger keep track of the condition of the battery and expect the same battery to be connected at all times?
    3. What problems will occur if the alternator is charging a battery that the solar charger is also trying to charge?
    4. Is your electrical system common-negative with all battery negatives connected together?

    My suggestion would be a latching relay (see http://www.digikey.com/product-detail/en/RT314F12/PB963-ND/1095287 which is rated at 16A and costs USD 4.66) driven by a small circuit that detects the voltage difference between the two batteries and drives the relay accordingly.

    A possible problem with this arrangement would be that battery voltage tends to droop after charging is removed, and this might cause the circuit to alternate continuously between the two batteries. This could happen because when a battery has been charged for a while its voltage will rise, so the circuit will switch the charger over to the other battery; the first battery voltage will then drop, so the circuit will switch back to the first battery, and so on for ever. What do you think?
     
  3. jobbemann

    jobbemann

    2
    0
    Jun 11, 2013
    First thank you for a great answer, and sorry for my bad spelling.. =)

    First though, can you give some details of your solar charger.

    Here are the details, lots of info on the e-bay page..:
    5A MPPT Solar Panel Battery Regulator Charge Controller 12V/24V Auto Switch TSR


    1. What is the maximum charge current?
    I cant find out reading the specs.., i would guess something just under 5Ah, but i realy have no clue...( maybe i dont understand them good enough to find it..

    2. Will there be a problem if one battery is suddenly substituted for another battery, which the charger was previously charging? In other words, does the solar charger keep track of the condition of the battery and expect the same battery to be connected at all times?
    Good question, what do you think?

    3. What problems will occur if the alternator is charging a battery that the solar charger is also trying to charge?
    Also good question, i thougt that the voltage always would be higher on the main batteries when charged by the 80Ah alternator, but maybe thats not as surtain as i thougt.. =( If the Voltage always is higher when the alternator is charging, then the relay you suggested shouldent choose to switch the solar-charger to that battery(?). Again, what do you think?

    4. Is your electrical system common-negative with all battery negatives connected together?

    no, the only connection between the two systems is that they are in the same boat (literally). There are no connections between them.
    Here is a schematic:
    ----------------------------------------------------------
    ...........(solar-panel)...........
    ..................|.....................
    .........(Solar-controller).........
    ............|...............|...............
    ..(battery).....(secondary bildgepump)...
    -----------------------------------------------------------
    ...................(80Ah Alternator)...............
    .............................|..............................
    .................(2x main batteries in paralell)......
    .....................|......................|.............|...............
    (2x primary bildgepumps)..(starter)..(lights and other stuff)
    ------------------------------------------------------------

    The reason i would like the solar-controller to also charge the main battery is to be shore that they get 100% charged up once in a while, and also to protect them if somebody forget to disconnect somthing that discharges them wery slowly, maby with help from the panel, they wont get totaly drained and die before the somthing sombody forgot to turn off is turned off.

    My suggestion would be a latching relay (see http://www.digikey.com/product-detai...963-ND/1095287 which is rated at 16A and costs USD 4.66) driven by a small circuit that detects the voltage difference between the two batteries and drives the relay accordingly.
    Wow, cool =) so this thing connects a charger to the battery with the least Volt?

    A possible problem with this arrangement would be that battery voltage tends to droop after charging is removed, and this might cause the circuit to alternate continuously between the two batteries. This could happen because when a battery has been charged for a while its voltage will rise, so the circuit will switch the charger over to the other battery; the first battery voltage will then drop, so the circuit will switch back to the first battery, and so on for ever. What do you think?
    Wery good point, but maby worth a try. it depends, if it happends twice a second i imagine it would be a big problem, if it happens once an hour, maby that wouldnt hurt to bad?

    - Lastly a follow-up question:
    I have one 220V Charger for 12V led-acid batteries.
    I have five led-acid batteries of different sizes and condition.
    I see that you can buy expensive chargers with the ability to charge several led-acid batteries simultaneously, but i only need to charge one at a time.
    Is there a relay/switch (or is it an easy build) that can connect the charger automatically to the next battery either when the battery is fully charged (maby the relay could be controlled to switch to the next battery when the green finish light lights up on the charger?) or even simpler, just jump to the next battery after a couple of hours and go round and round until all 5 batteries are fully charged? (would this hurt the batteries, resulting in overcharge maby?)

    - i dont expect you to answar all my questions, so thank you wery mouch eather way.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    No, it doesn't do the voltage comparison. Some more circuitry would be needed to compare the battery voltages and make the decision about which one should be switched to the charger. The latching relay is the part that actually DOES the switching. It has a common contact, which would be connected to the charger, and two "changeover" contacts; one would be connected to each battery.

    A latching relay has two "coil" inputs, and a pulse of current on either of those inputs will flip it to the corresponding state. The relay latches mechanically, or magnetically, so only a short pulse is needed to flip it to a new state, i.e. to change which battery is connected to the charger. So a latching relay doesn't consume any power, except briefly when it flips from one state to the other.

    I'm not sure that this will whole project will be workable. Here are my concerns:

    A. We don't really understand what's inside the charge controller. It's probably very simple but I don't know for sure.

    B. The charge current could be pretty high - the charger you linked to is specified at 15A even though you describe it as a 5A charger.

    C. Latching relays aren't available with very high current ratings - not from Digikey, at least. To switch a 15A charger, I would use a relay rated at 25A at least. This would be a large beast, which would require a continuous current to keep it in one state. You need a circuit with zero, or very low, standby current consumption, so it can be left connected without draining either of the batteries, so we may not be able to use a big relay as the switch.

    D. There are very high-current MOSFETs available that would do the switching, but that complicates the control circuitry a lot, and there is also the issue of "load dump" which happens when the engine is turned off - the battery voltage can rise to 70 volts or more for a short while, and this is likely to damage MOSFETs. (Relays are much more robust than MOSFETs.)

    The product you linked to, on eBay, is described as a 15A charger. So you need a relay rated at 15A or more. Preferably a LOT more. As I said earlier, this would mean a grunty relay such as one of these:

    http://www.digikey.com/product-detail/en/1432793-1/PB1773-ND/1236843 (30A, USD 2.88, requires 113 mA coil current, connections are made with automotive terminals)
    http://www.digikey.com/product-detail/en/1432868-1/PB1040-ND/1240171 (40A, USD 3.70, also requires 133 mA coil current, connections are made with automotive terminals)
    http://www.digikey.com/product-detail/en/ACNM1112/255-3335-ND/2804726 (30A, USD 4.31, coil current 53 mA, mounts directly onto stripboard).

    These relays require coil current to hold them in one state, and this would be a continuous load on a battery, which you want to avoid.

    The relay would be connected between the solar charger and the two batteries, to select which battery would be charged. These relays have an idle state, and an active state. In the idle state, the charger would be connected to battery A, which we might designate as being the standby battery for example. In the active state, the charger would be connected to battery B, the other battery. In the active state, you need to provide continuous current to the coil (133 mA for the first two I listed, and 53 mA for the third one).

    Perhaps we could connect the relay so that when you're not using the boat, the relay will always be in the idle state, and this would connect the standby battery to the solar charger, so no relay coil current would be needed. So relay coil current would only be needed to connect the charger to the main batteries. That seems like a good idea, but I'm not sure how we could do that. Perhaps if the solar cell voltage is too low, the relay could switch to the idle state. That should be fine because if there's no voltage from the solar cell, the solar charger can't charge anything anyway, so it doesn't matter which battery it's connected to.
    From looking at the eBay product page, I can't say for sure. Sealed lead-acid batteries are pretty simple to charge, so my guess is that there's nothing clever in the charger, and there wouldn't be a problem, but I couldn't be sure.
    Is it easy for you to open the charger and take a photo of the board inside? To attach a picture to a post, click "Go Advanced", and on the toolbar above the edit window, click the paper clip symbol. Browse to the file(s) you want to attach, and click Upload. File size limits are shown on the Attachments dialog. You'll probably need to rescale your image file to make it small enough.
    Yes, I think you're probably right. Also, we could hook the circuit into the ignition, so when the engine is running, the solar charger would always be switched to the standby battery. Or would that defeat the whole purpose of this project?
    Are you sure about that? Usually in automotive applications, and other applications when the chassis is made of metal (which I assume your boat is), one side of all the electrical circuitry (usually the negative side) is connected to the chassis. Often, components such as lights have only one wire feeding them; the return path is through the chassis.
    Are you talking about keeping the batteries topped up when the boat is not being used? Because I think the alternator will probably charge the main batteries very thoroughly when the boat is being used. It's only if the boat sits idle for days or weeks that you would want to switch the solar charger over to the main batteries.

    In that case it might be simpler to just make a time-delay-based controller. When the engine has been unused for, say, 24 hours, switch the solar charger over to the main batteries for 24 hours, then back to the standby battery for 24 hours, and so on. The cicuit could have a feed from the ignition circuit so it would know when the engine was in use. A simple behaviour like that might be less problematic than watching the battery voltages constantly, which could cause the following problem:

    Agreed. But it might be simpler to just alternate between the two batteries every 24 hours or so, while the engine is not being used. What do you think about that idea?
    It's definitely possible to do that, and I can draw up a circuit design for it, but it would involve quite a few components.

    As for whether that could overcharge the batteries, my gut feeling is probably not. Each time a battery is selected, the charger will probably start actively charging it, because its terminal voltage will have fallen below its float voltage while the other batteries were being charged. For a fully charged battery being actively charged, the terminal voltage should rise fairly quickly, so the charger would report completion and the circuit would switch to the next battery pretty quickly. Each battery would be getting a periodic top-up which it probably doesn't need. Whether this would cause a problem I really don't know, but I don't think it would.

    Batteries that are old and don't hold a charge well will probably take longer to return to the float voltage, so they will get longer bursts of active charging. Whether this is a good thing or a bad thing for old batteries, again I don't know.

    The design would need a big relay for each of the batteries, an LDR (light-dependent resistor) to detect the green LED, possibly a switch to select the number of batteries being used, and circuitry to control it all. I would probably use two or more integrated circuits (see http://www.digikey.com/product-detail/en/CD4017BE/296-2037-5-ND/67253) and the circuit would need to be built up on a piece of stripboard (for an example of a circuit built up on stripboard, see http://www.petervis.com/electronics...lashing Cable/Building a Flashing Cable 2.jpg).

    You might be able to find someone locally to build it for you - perhaps an electronics student, but it's not a quick job.

    Have a think about all of this, and let me know :)
     
    Last edited: Jun 13, 2013
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