# Small system questions

Registered Users Posts: 3

I’m trying to determine how long it will take for two 100W panels to recharge a battery bank. The bank consists of two 55 AH 12V AGM batteries = 110 AH.

If I de-rate the panels then 200 W x 0.8 = 160 W.

Assume 4 hours of full sun per day: 160 W x 4 hours = 640 WH.

640 WH / 12v = 53.3 AH.

110 AH bank discharged 25% (trying to be conservative) = 27.5 AH used.

If the bank likes a 1/10th charge rate, then 110 x 0.1 = 11A. The two panels have a 5.75 A ISC x 2 x 0.8 = 9.2 A. Would this be a good rate of charge?

If the MPPT charge controller needs to bulk charge, absorb charge, and float charge, how long does all of that take?

If the bank needs 27.5 AH replaced and the panels produce 53.3 AH in the day, is that one afternoon of good sun enough time for the charge controller to complete the 3 phases of charging?

Thank you for helping me to understand this.

Sincerely, David

• Solar Expert Posts: 8,066 ✭✭✭✭✭

are you after minimum cost, or maximum battery life?

Not every day is 100% sunny.  You have to plan to be able to recharge  fully on 1 sunny day after 1 cloudy day.

Or risk battery life or run a generator on the cloudy day.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 3,120 ✭✭✭✭✭
You want to look at Imp, not Isc. Isc is the short circuit current, Imp is current under load, which will be a bit lower.

Using C/10 as a rate of charge isn't so much a matter of what the bank likes, it's more a question of time avaliable. What the bank does like is being held at a high enough voltage for long enough to absorb a full charge on a regular basis. A lead acid battery generally needs 3 hours or so at absorb voltage. With an AC charger, you can charge at a low rate overnight to get to absorb voltage (which is roughly an 80-85% state of charge) and hold it there. With solar, you'll run out of sun at a too low charge rate.

With a C/10 (10%) charge rate, and a 75% SOC, it should take about an hour to reach absorb voltage, then 3 hours or so with current dropping off to around a 1-2% rate to absorb. In your example, what will likely happen (assuming proper panel tilt, typical climate, etc), is as the sun hits the panels voltage into the controller will drop a bit as they get warm and current will gradually rise to ~ 10a into the batteries around noon. At a 75% SOC in summer, you might hit absorb voltage before noon, and never see 10a. After noon, potential current output will gradually diminish, but if the batteries have hit absorb voltage that's okay, because as they absorb the charge they'll take less current anyway.

The C/10 number is just a rule of thumb to cost effectively balance the ability to get to absorb voltage most days for most of the year. At higher latitudes, C/10 will be more than enough in spring/summer, but not enough in fall/winter.
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter