solar charge controller vs battery size

Newbie growing pains...

My wife who lives and works near West Palm Beach, Florida and has experienced first hand hurricane exposure and the lack of power immediately after the storm. She claims the worst part of being without power is the lack of movement of air in the house (3rd floor condo). So I purchased from Costco a solar system that is composed of two 80 watt solar pannels (160 watts, 9.34 amps) with a 30 amp solar charge controller. I purchased from a marine store two new sealed deep cycle 55amp each batteries to go with system. I hooked all the pieces together (batteries parallel) and put the system to a dry run test by attaching a ten inch, 12 volt fan and record the results. The batteries were fully charged before this test and the fan worked well for three days, and then slowed down considerably. The solar pannels were connectd through out these days,(direct sun aprox 5-6 hours day) so I was suprised by the drop in power on third day. My question is, did I mis-match battery size with the size of controller? I am assuming that the controller had to wait until the batteries fell less to than 30 amps before the solar panels became effective? Sure wished I knew what I am doing. (Sorry that I did not know about N Az Wind & Sun before purchasing this system) Thank you in advance for your thoughts.


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: solar charge controller vs battery size

    tis better to be a newbie with growing pains than to be an oldy with the same pains blown out of proportion.  :-)
    the panels or pvs(photovoltaics) as we call them were giving power for 5-6 hrs wereas the load was constant over 24hrs. if say it was 6hrs of charge at 9.34amps(probably less), this is about 56ah, but the load was 24hrs and it therefore took 3 days to wipe out the batteries. 55x2=110ah capacity. add to that the 6hrs per day of charge to that for all 3 days. 56x3=168ah+110ah=278ah is the total used in the 3 days. divide by 72hrs is 3.861amps draw from this fan. you should note that you shouldn't take your batteries below the 50% depth of discharge as this takes away from battery life and could damage the batteries. that gives you 55amps to work with from the bank without any other charging. now that means you have 55/3.861=14.245 hours of capacity to utilize that fan with this battery bank safely.
    odds are you aren't getting the full amperage speced on the pvs due to stc ratings being high for most of the usa. ptc is closer and figure this to be about 90% of the stc ratings. your 9.34amps is now maximumly about 8.4amps and add to that any cloudy times taking away from the full sun you'd have normally gotten and you can see it's all derating. 6hrs x 8.4amps = about 50amphours(ah). this would change the general calculations in figuring the amp draw of your fan, but i did it using the figures you gave just to show you that even though your figures would be too high it shows how easily it could drain your batteries when the fan's on constantly. electric motors are high drawers of electricity in general. even the small ones drain much from batteries. see if you can read the actual amps the fan is rated for so as to give you an even better idea of how long your batteries will last for. without an amphr meter in line for overall power in versus out you can't be sure of the charge in the batteries. voltages of the batteries when at rest after a few hours can be an indicator of the state of charge the batteries are in.
    bring your batteries up to full charge without loading them down to ward off sulphation of the plates for now. try tracking your power usage better and use conservation or risk killing those batteries. if you need more charge power during the day then buy another pv or pvs(same pv) and if the battery capacity isn't high enough get more batteries(same battery), but do it now while they would be similar in age and usage. doing the battery additions later can cause the new battery to be drawn down and equaling the oldest or most worn out battery present. in other words all of the batteries will be as good as the weakest one.
    ps. i thought it was spelled suave, not swave. :?
  • crewzercrewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: solar charge controller vs battery size


    A PV module’s STC Imp rating doesn’t suffer like the Vmp in high ambient temperatures. In fact, the Imp can even rise a bit, but it’s a small increase, and I recommend ignoring it for most applications.


    Average solar irradiation for this time of year in West Palm Beach is indeed in the 5-6 hr/day range, assuming no shade and correctly angled PV modules. On average, you can count on 9.34 A x 99% x 5.8 hrs/day = ~54 Ah/day, assuming your sealed batteries are VRLA (AGM or gel) and they’re not fully charged. If, for example, the fan draws 3.5 A, that’s 3.5 A x 24 hours/day = 84 Ah/day, and, as you witnessed, the batteries will gradually discharge (84 Ah/day - ~54 Ah/day = ~30 Ah/day discharge) .

    Note, however, that average daily solar irradiation drops to, say 5 hours/day in October, when you're still in the hurricane season.
    My question is, did I mis-match battery size with the size of controller?
    Matching the PV array power to the battery bank capacity is more important. Your 9.34 A array would be well matched for a battery bank in the ~100 Ah to 200 Ah range. Considering the constant fan load, I’d suggest that you consider adding at least a third 55 Ah battery, for a rated capacity of 165 Ah, and you could probably even add a fourth for 220 Ah.

    The full 220 Ah battery bank could run the fan for up to five or six days early in the season, but perhaps just four or five days late in the season. These periods would be reduced if a power loss is accompanied by cloudy weather. An operational option would be for the batteries to run the fan and power a 12 V fluorescent lamp during the evening, and maybe even a low-power 12 V LED lamp at night. After that, the fan (and/or lights) would need to be disconnected and the batteries recharged. This type of deep-cycling is not something you’d want to do to the batteries on a regular basis, but it’s OK to do for the occasional hurricane as long as the batteries are otherwise maintained at full charge.
    I am assuming that the controller had to wait until the batteries fell less to than 30 amps before the solar panels became effective?

    Assuming sufficient Sun, the controller should be delivering power to the battery bank no matter what their state-of-charge (SOC). If they’re full, the float charge will be small.

    Jim / crewzer
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