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A quick question on backup power

dirtdiver

Hi everyone,

im making a model house with a solar panel and a rechargable battery.I've been looking on the internet for a while, but i cant really find out if what im planning to do is correct in terms of wiring.

Idealy you will have the external power disconnected and the battery will be charged by the solar panel and drained by the consumers (servo+ a few LEDs) and when the energy is not enought the external power will charge the battery/ or if the power network is down- rely on the solar energy to charge the battery alone.

 

Is this schematic applicable?

 

Im not sure if i have to put any diods around to limit the current's direction

 

**For some reason i cant insert an image so im attaching it as a file!

 

Thans in advance!

Attachments:
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  • 0

    PS I know about the diod is seris for the solar panel, just no tsure about the rest

  • 0 in reply to dirtdiver

    Hi,

     

    Unfortunately you've picked a relatively advanced topic, and it will be very hard to make this work (at good efficiency) without a fairly complex circuit.

    You could try to just stick a diode and resistor in series and then connect the solar cell to the rechargeable battery, and it may trickle

    charge the battery somewhat, but at low efficiency. I take it that the solar cell is fairly small if this is a model?

    If you want to do the advanced design, search for MPPT. An alternative may be to purchase a ready-made solar cell with battery, e.g.

    something like this possibly (no recommendation, I just googled this one).

  • 0

    I've been looking into this sort of thing a little bit, as I need to do something about the solar setup in my outbuilding (wife's art gallery) now that the charge controller seems to have died.

     

    From what I understand, as long as the maximum charge amps of the solar cell is at a rate small enough compared to the total battery capacity (ie, the equivalent of a trickle charge), you don't actually need any circuitry (other than that diode I guess) - the battery will just dissipate the extra energy in the form of a small amount of warmth.

    But I'm using basic lead-acid batteries in my setup, so that might not apply to your NiCd battery.

     

    I could also be very very wrong, and if so I hope someone will correct me before I blow up my wife's art gallery image

     

    Cheers,

    -Nico

  • 0 in reply to ntewinkel

    Hi Nico,

     

    I think you're right, I think some Lead Acid, e.g. possibly burglar alarm batteries, can be charged like this as long as you like. One thing possibly worth

    doing would be to have a cut-off that constantly monitors the battery voltage, and cuts off the solar cell (or reduces the charge current)

    if the voltage exceeds approx 13.8-14V (usually marked as the float voltage in the datasheet for the battery) for a 12V battery.

     

    For NiCd, some batteries (usually the larger ones, e.g. 'D') can be indefinitely trickle-charged (i.e. topped up) as long as the current is kept low (e.g. a series resistor), and probably trickle-charging like this doesn't apply to newer rechargeable type chemistries.

    If you wish to charge at a higher rate too (i.e. when the solar cell can supply the power, and when the battery does need more than trickle charging),

    then for both lead-acid and NiCd,  you know when the battery is charged by looking at dV/dT which will rise as a symptom of end-of-charge being reached (easy to monitor this with a dedicated IC I guess). If there are cold conditions, NiCd was preferred over Lead Acid for more efficient charging in the cold, but maybe manufacturers have resolved this nowadays.

     

    In the past some older charger ICs just relied on dV/dT, some on just time, etc., but a combination (including temperature) is good

    for safety, for NiCd and NiMH batteries. Modern ICs support the combination of charge termination conditions (with no charge termination, if the current is too high, even large NiCd batteries will eventually pop (I guess as you say, the heat doesn't reach the steady-state at this high level of energy) and has to go somewhere : ) I saw that occur once, not pretty!

     

    Li-Ion are super-sensitive and possibly best served with specialized charger ICs always.

  • 0 in reply to shabaz

    Thanks for the advice, Shabaz image

     

    I've decided to just buy a controller for my setup - easiest way to make sure all is well, and it turns out their not that expensive anymore.

  • 0

    Ok first you need a "deep discharge" battery. Your circuit really wont work. You will need a few things:

    1. an Inverter which will power your load. This should be about 48vdc input and 115vac output (you can geri-rig a large UPS)

    2. you need your cells on the roof, to go to your battery via a regulator. The diode that you guys where talking about is only so you don't back feed your system.

    3. you should have some sort of monitor (cpu) that looks at one cell for light/dark or a real time clock.

    4. you should also monitor the output voltage of the cells.

    5. you should also monitor the output of the system before the inverter but after the battery. (demand)

    6. you should also monitor the temperature of the battery's. (too hot is very bad and too cold you need to turn on heater.)

     

    I like your idea but you should also think of Hydrogen with fuel cells image.. (Hydrogen House on the web) very cool.

     

    enjoy my 2 cents

    Cris H.

  • 0 in reply to phoenixcomm

    For a small toy house, you shouldn't need a lot.

    Your cct doesn't give you much voltage headroom to efficently control it.

     

    I would tend to try 12v and then use a regulator to drop it down to 5v for your load.

    (There are plenty of cheap switchmode 12v to 5v USB style automotive regulators that handle 2.2A.)

     

    Solar panels tend to be high source impedance (not a lot of current but can have high open circuit voltage).

    Monitor the voltage and if it rises above 13.2 lead acid, or 13.6 for gell cell, then open circuit the solar panel.

     

    If you include some hysterisis then your relay or mosfet isn't going to die of oscillation.

     

    If you set the external charger output voltage to 13v then when the battery drops below 13 it will start supplying current, which will raise the battery voltage.

    If the solar is able to hold it above 13v, then the external charger will be idle.

     

    There are some good guides around for any size installation, and if its large then going to 48v reduces the R losses in the cabling and inverter.

    (It also means it needs 4 batteries of equal performance for each 'bank')

     

    Here is a good guide for calculating the capacity

    https://nicegear.co.nz/blog/what-size-solar-panel-and-battery-do-i-need/

     

     

    mark

  • 0 in reply to mcb1

    for my project is entirely for demonstration purposes so i used the shown schematic and it seems to be working (i used a diod to prevent current going back in the cell)