Solar System

1. My battery bank comprises 8 x 6 volt Surrette batteries, 350AH @ C/20, wired at 24 volts for 700AH. Given the bank is
5 years old and assuming I discharge the fully charged bank 40%, do I still have 420AH left or do I have something less?
(700AH x 40% = 280 + 420 = 700) If it is something less what can I expect as the remaining years go by? I hope to get
a minimum of 10 years from the bank. I only use the bank 5 months each year at the cottage from May 1 to Sept 30.
The Trace C40 trickle charges and equalizes every 30 days the remaining 7 months.

2. Is it important to know the discharge rate of a battery or battery bank in the overall scheme of things? I have simply been
discharging the bank as my usage demands. However, my maximum DOD has been 50% infrequently with the majority
being in the 25% to 30% range. I fully charge the bank frequently.

3. What is meant by battery derate and module derate?

As always, your assistance is appreciated.
Lone Ranger, Ontario, Canada


  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Solar System


    1) Sounds like your battery bank consists of eight Rolls/Surrette Series 4000 S-460 batteries wired in series/parallel to make for 24 V x 700 Ah nominal. Completing discharging a battery bank (100% DOD; 0% SOC) greatly reduces battery longevity. Accordingly, rule-of-thumb (ROT) battery guidance is to avoid discharging your battery bank below 50% SOC. In fact, typical guidance for daily discharges is in the 10% to 20% range. Following the 50% rule, if you take 280 Ah (40%) out of your battery bank, you really only have 70 Ah (10%) left to play with.

    The numbers above assume the batteries are healthy and are operating at 77 F (25 C). Battery bank capacity should be derated by ~1% for every degree C below 25 C (or ~ 10% for every 15 degree F drop below 77 F). So, if your battery bank’s temperature is ~10 C during a relatively balmy Canadian winter, the 700 Ah capacity should be reduced by ~15% to ~600 Ah, and the 50% useable capacity would be ~300 Ah.

    Battery capacity drops with age and use, even if they’re well maintained. Rolls/Surrette suggests the S-430 batteries have a useable life span of ~10 years or ~1300 cycles. They don’t define “cycle”, but I interpret it as a 20% discharge to an 80% SOC and then a full recharge. Deeper discharge cycles will reduce the number of useable cycles.

    As long as you are charging and maintaining your batteries in accordance with Rolls/Surrette procedures, your batteries should last the full 10 years. I would think that the seven months of rest would easily balance out your typical 25% to 30% discharge cycle. Assuming that you batteries get pretty cold, you should be using the remote battery temperature sensor with your charge controller.

    Here are links to Rolls/Surrette’s battery info and bulletins:

    Batteries whose operational capacity has dropped to 80% of original specification are considered to be candidates for replacement.

    2) Peukert’s Law basically states that a battery’s capacity is temporarily reduced as the load increases, and visa-versa. Accordingly, it can be important to know the discharge load on the batteries. However, it’s also important to consider a battery bank’s individual specs and loads.

    A battery’s C/20 capacity, or its 20-hour rate, describes how many Ah can be removed from a healthy and fully charged battery subjected to a constant current load for 20 hours at 77 F (25 C) until the battery voltages drops to 1.75 Volts per cell.

    For your S-430 batteries, the 350 Ah spec says that each will deliver 17.5 A for 20 hours. For your full bank, it should b able to deliver 35 A at 24 V nominal for 20 hours (actually a bit more due to Peukert). So, assuming 90% inverter efficiency, the question is, are you running 756 W (24 V x 35 A x 90%) loads on a regular and continuous basis? If no, then I wouldn’t worry about the battery loads.

    3) Battery loads and /or the ambient environment can affect battery performance. Useable battery capacity drops as its load increases, and battery capacity drops along with temperature. Both topics were addressed above.

    PV module derating is primarily an ambient temperature issue. PV module specs are based on so-called “standard test conditions”, or STC. The key module temp spec is 25 C. However, fully illuminated PV modules can operate at ~35 C above ambient, so ambient might have to be as low as -10 C for a fully illuminated, properly aimed and clean PV module to generate full power.

    Unfortunately, PV module output power drops as the module’s PV cell temperature increases. This loss manifests itself as reduced module voltage. A typical summer derating is to expect ~85% of a PV module’s nameplate power rating on a bright summer day. You can count on seeing full Imp (max power current), but, assuming an MPPT controller, Vmp (max power voltage) during the bulk charging stage will be ~85% to 90% of nameplate STC spec. Winter voltages are usually higher, and the MPPT controller will convert any "extra" voltage from the PV array into additional charge current (less the controller's internal losses).

    For a PWM controller like your C40, the PV array will operate at the battery voltage, so the derating could be even greater.

    Jim / crewzer