Connecting a battery directly to a panel

Sun Dog
Sun Dog Solar Expert Posts: 115 ✭✭
in Off Grid Solar & Battery Systems #1
I appreciate that connecting a battery directly to a panel is frowned upon by many but I am still looking for a way to keep my spare 4V battery alive. I have been using a 6V automotive battery charger but if the ambient temperature is greater than ~18C it overheats and shuts down.

I have 5 x 15W 12V panels that I would like to use. Is there the possibility, real or theoretical, that when the panel is small relative to the battery that the load of the battery could cause a dead short in the panel? In my case it would be a 4V @ 1280Ah battery and the panels mentioned above.
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  • jcheil
    jcheil Solar Expert Posts: 722 ✭✭✭
    #2
    Re: Connecting a battery directly to a panel
    Sun Dog wrote: »
    I appreciate that connecting a battery directly to a panel is frowned upon by many but I am still looking for a way to keep my spare 4V battery alive. I have been using a 6V automotive battery charger but if the ambient temperature is greater than ~18C it overheats and shuts down.

    I have 5 x 15W 12V panels that I would like to use. Is there the possibility, real or theoretical, that when the panel is small relative to the battery that the load of the battery could cause a dead short in the panel? In my case it would be a 4V @ 1280Ah battery and the panels mentioned above.

    Unless I am missing something, your problem is you are trying to charge a 4V battery with a 6V (or higher in the case of your solar panels) charger. That is likely going to overcharge if not damage the batteries in no time (or blow them up). And the fact that you are not using a charge controller for the solar means that it will overcharge that battery for sure.

    You could get a super cheap, made in china, $10 solar controller (I do that for my lawn mower battery maintainer panel), but I don't know of any that will work at 4v (or even 6v). Smallest I have ever seen was 12v. Perhaps they do exist. Without one, you are going to overcharge your battery or worse.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Connecting a battery directly to a panel
    Sun Dog wrote: »
    I appreciate that connecting a battery directly to a panel is frowned upon by many but I am still looking for a way to keep my spare 4V battery alive. I have been using a 6V automotive battery charger but if the ambient temperature is greater than ~18C it overheats and shuts down.

    I have 5 x 15W 12V panels that I would like to use. Is there the possibility, real or theoretical, that when the panel is small relative to the battery that the load of the battery could cause a dead short in the panel? In my case it would be a 4V @ 1280Ah battery and the panels mentioned above.

    Panels have no problem with dead shorts. They will only produce their Isc rating (current short circuit) and they can in theory do this indefinitely. This requires not only a dead short on the output but also full insolation.

    Can you connect a panel to a battery with no controller? Yes, and if it is small enough in respect to the battery size there will be no problem; putting out 1% of capacity in current for a few hours per day is highly unlikely to result in an overcharge.

    A 15 Watt "12 Volt" panel produces about 0.85 Amps. Many an automotive battery maintainer is based on this. You have five such panels so that's a bit more than 4 Amps of current. On 1280 Amp hours that is nothing. In fact it is so little it will probably do no good.

    Your 6 Volt charger is overheating because it is trying to charge a 4 Volt battery to over 7 Volts. That is not going to work. Probably better if you put some current limiter on it and just check and charge as needed.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #4
    Re: Connecting a battery directly to a panel

    Use a fuse/breaker to reduce the chance of fire if "something" goes wrong.

    If the battery is in otherwise good shape, then a 1% rate of charge for float charging should be fine:

    1,280 AH * 0.01 * 17.5 volt Vmp (enter your panel Vmp here) = 224 Watt of Vmp~17.5 volt panels
    1,280 AH * 0.01 * 30.0 volt Vmp (enter your panel Vmp here) = 384 Watt of Vmp~30.0 volt panels

    Should work fine... Get whatever is the least expensive Vmp and Wattage panel(s) you can.

    Check after a month and make sure the panels are producing the desired current. They should be OK--But vendors seem to not really want to have their panels run near Isc.

    You could add a power resistor:

    17.5 volt Vmp - 5 volt charging = 12.5 volt drop
    V=I*R
    R= V/I = 12.5 volt drop / 12.8 amps charging = 0.98 Ohm power resistor (or heater element)
    P= 12.5 volts * 12.8 amps = 160 Watt minimum power resistor

    Would I use power resistor(s)--Maybe, if I could find some cheap at a surplus store. In theory, will stress the solar panels less.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: Connecting a battery directly to a panel

    To follow up on Marc's and Jay's comments,

    You could also put a power resistor (similar calculations) between your AC charger and the battery cells.

    And, if you want, use a 24 hour lamp timer to set 1-24 hours per day of float charging--Depending on how many Amp*Hours per day you need to keep the cell properly floated.

    A "going bad" cell may need 2% or more float current. A good cell may need 1% or even 0.1% current. Colder batteries will last longer and need less current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Sun Dog
    Sun Dog Solar Expert Posts: 115 ✭✭
    #6
    Re: Connecting a battery directly to a panel

    Great information guys. Bill, I have no idea how you can think and type so fast, impressive.

    I had also wondered about adding something like this http://www.ebay.com/itm/DC-to-DC-Converter-18-48V-to1-16V-buck-step-down-power-supply-voltage-Regulator-/141284248907?pt=LH_DefaultDomain_0&hash=item20e5328d4b I thought it would allow me to manually regulate the voltage and that it would also boost the current. Does it look at all useful?

    When connecting a large 4V battery to a small 12V panel will the output voltage of the panel sag down and if yes does this mean that the current will rise or will I never get more than the 0.85A that Marc mentioned?
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  • Sun Dog
    Sun Dog Solar Expert Posts: 115 ✭✭
    #7
    Re: Connecting a battery directly to a panel

    Bill, I am not familiar with power resistors. In addition to doing some reading, where does one buy these? Digikey, Mouser or ???
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  • Sun Dog
    Sun Dog Solar Expert Posts: 115 ✭✭
    #8
    Re: Connecting a battery directly to a panel

    No idea if this is a good or bad price but I found this 1 ohm 160 watt power resistor http://www.ebay.com/itm/Ohmite-dividohm-1156a-160-watt-1-ohm-adjustable-power-resistor-free-shipping-/321009902049
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #9
    Re: Connecting a battery directly to a panel

    Probably an OK price--That is an adjustible resistor (sliding collar in the middle).

    You will find some big aluminum units at computer surplus stores sometimes.

    Attachment not found.

    I don't know why I could always find so many large power resistor in our local surplus stores--I never needed them in shipping products (waste of energy/money). :confused:

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