Hi Ray and Drake, Ray, I'm not sure I follow your statement that MPPT does not do much for a battery. With today's higher voltage modules, there can be a dramatic improvement in the amount of power reaching the battery by using MPPT charge circuits. Simply put, MPPT circuits convert all the available PV power (remember power is expressed as watts or the ability to do work) into current and voltage. The higher the differential between battery and array voltage, the more current that can be produced by MPPT circuits. A non-MPPT circuit simply connects the battery to the PV array thus pulling down the operating voltage and wasting power.
Consider a single Evergreen 205 watt module. The Vmp is 18.2 volts and Imp is 11.4 amps. If you use a non-mppt controller like the old C-40, the operating voltage of the module will be just above battery voltage. If the battery is at 12.4 volts, you will only capture about 145 watts of the available power. The module can do 205 watts of work so you are wasting 30% of the power. As battery voltage rises and PV modules heat up, the difference will decrease but there will most always be a benefit with well designed MPPT circuits. So, Drake, you are wise to use MPPT if there is sufficient differential voltage. Since it is a percentage of improvement, the larger the PV array, the greater the benefit. A PWM controller is a method of controlling voltage, not pulsing off sulfation. When a battery reaches 100% SoC then the lead sulphate has been recombined into lead, lead dioxide and sulphuric acid. There are pulsing circuits available but the PWM in a charge controller does not perform that function. So, why is PV power an excellent battery charger? Because it uses Starlight power! But you knew that. Larry Crutcher Starlight Solar Power Systems (928) 342-9103 Mailing Address: 11881 S. Fortuna Road, #210 Yuma, AZ 85367 Retail Store & Shipping Address: 2998 Shari Ave. Yuma, Az 85365 ----- Original Message ----- From: "R Ray Walters" <r...@solarray.com> To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org> Sent: Wednesday, March 24, 2010 10:51 PM Subject: Re: [RE-wrenches] PV excellent battery charger > MPPT doesn't really do that much for the battery, it just makes the PV modules more effective. > The real magic is: > A) the 3 stage charging (compared to traditional single stage AC chargers) > B) Temperature compensation > C) PWM that helps pulse off sulfation > D) Slower charge rates let batteries charge more fully without damage from overheating. > > R. Walters > r...@solarray.com > Solar Engineer > > > > > On Mar 24, 2010, at 7:04 PM, drake.chamber...@redwoodalliance.org wrote: > > > Can anyone tell me why PV is considered an excellent battery charger? I > > think it is, only after the addition of a MPPT charge control. Otherwise, > > the battery drags the PV voltage below Vmp. > > > > One of the NABCEP objectives for the entry level class is to explain why > > PV modules make excellent battery chargers and show this through the IV > > curve. > > > > Thank you, > > > > Drake _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
