Ideal voltage for 12v battery charging

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freddyuk
freddyuk Registered Users Posts: 11 ✭✭
in Off Grid Solar & Battery Systems #1
Is it true to say that a 16.5v (vmp) solar panel would be more effective at charging via PWM into a 12 volt system than a higher voltage say 18v.?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #2
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    Re: Ideal voltage for 12v battery charging

    It all depends...

    If you have a simple system (colder weather, shunt/dump charge controller, short/heavy wiring, battery bank in controlled climate, etc.)--One could argue that a 16.5 Vmp panel may be slightly more efficient (more wattage) when charging a 12 volt lead acid battery bank (maybe 8% over an Vmp~18.00 volt panel).

    However--given that we try to design systems that work over temperature/time/cost effectively/with minimum maintenance--A 16.5 Volt Vmp panel based system will be more work, less reliable, and probably more expensive (I don't know of any high volume/low cost 16.5Vmp panels out there--Not that I am in the business, it is just 32 cell panels are not really standard anymore for larger systems).

    15.0-16.5 volt panels used to be used in smaller/commercial systems (like off grid traffic warning lights/signs/etc.--As I understand) and were sold as "self regulating" panels (no charge controller).

    Such a system may cost a bit less to build (no solar charge controller), but batteries will probably not last as long and not be useful for use in a cabin/home with daily loads (and deeper cycling).

    Of course, there are other technologies/chemistries other than lead acid batteries--But for what we typically see around here, I do not see any need for a 16.5 volt Vmp panel/array. Get a PWM charge controller (not too expensive), and more ~17.5-18.6 volt Vmp panels (check $$$/Watt panel pricing--Higher volume production panels tend to be 1/2 the cost or even less than the smaller/non-standard panels out there these days).

    In the end, do a couple different paper designs and see which one will best meet your needs/cost requirements.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
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    Re: Ideal voltage for 12v battery charging

    Nope.

    In fact the opposite may be true. Here's why: a solar panel is a current source. It will put out its maximum current into a dead short. It will maintain that current up to the Voltage point known as Vmp is the load warrants it. But the Voltage it puts out is subject to another factor: heat. As the panel heats up the Voltage goes down. So the factor Vmp rating may be 16.5 but once the panel gets hot (as in subjected to direct bright sunlight when it makes the most power) the Voltage goes down. More Voltage is lost in the wiring from the panel to the controller and battery due to resistance in the wire. You can minimize it but you can't eliminate the drop. So by the time you measure Voltage at the battery the 16.5 Vmp panel may not be capable of producing sufficient Voltage to charge that battery. If the best it can manage is 13.5 Volts the battery never goes above that level and thus doesn't get charged. The 18 Vmp panel has more 'Voltage overhead' to work from. Under the same circumstances of V-drop its output at the battery would be 15 Volts; enough to completely charge the battery.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
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    Re: Ideal voltage for 12v battery charging

    fyi, the old us 64s were 16.5v vmp and i'd imagine there's still plenty of them out there yet. it may be more cost effective if you are in a colder climate, but more efficient overall i'd say no.
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  • freddyuk
    freddyuk Registered Users Posts: 11 ✭✭
    #5
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    Re: Ideal voltage for 12v battery charging

    It is for UK climate so never ambient "hot". Temperature coefficients:
    Power: -0.43 %/K
    Current: +2.50 mA/K
    Voltage: -2.12 mV/K
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
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    Re: Ideal voltage for 12v battery charging
    freddyuk wrote: »
    It is for UK climate so never ambient "hot". Temperature coefficients:
    Power: -0.43 %/K
    Current: +2.50 mA/K
    Voltage: -2.12 mV/K

    Ambient temperature is not solely what drives panel temperature. They heat up because they are being hit by sunlight (we hope). Operating temperature may still be 40C even on a 25C day.

    If you look at the two theoretical panels in question you see a difference of three cells in series raising the Vmp from 16.5 to 18 (approximately 0.5 Volts per cell). The current may be identical between the two. Let's say it's 7 Amps. So the lower Voltage panel is (16.5 * 7) 116 Watts and the higher Voltage panel is (18 * 7) 126 Watts. At the battery using a PWM controller they are equal in power outlet providing there is no loss in Voltage high enough to drop the 16.5 Vmp panel to an unusable output level. From that POV the 18 Volt panel is less efficient because you are getting no more real power from it than you would the other panel.

    So with no need to overcome excessive V-drop and no MPPT (about 0.5 Amps more in this example) to take advantage of the higher Wattage panel there is no benefit from the 18 Vmp. Just be sure that is the case, though. For example at that current level 20 feet of 16 AWG would lose about 1 Volt. 30 feet and the lower Voltage panel would be in trouble for providing enough V to charge with. Normally you'd use larger than 16 AWG; just using that as an example (large enough to handle the current but small enough where V-drop becomes a concern).
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