Charger

Re: Charger

Certainly, Northern Arizona Wind & Sun has more experience than I regarding their product and solar in general. Here is my understanding/reasoning regarding my recommendations/rules of thumb:

The 5% minimum works out fairly well as ensuring batteries are recharged in a reasonable time (if C/20 = 5%) that is about two days to recharge a battery bank that has been recharged 50%. Also, as batteries age, they tend to have faster self discharge (an old forklift battery--worst case--can be in the 1-2% rate of self discharge per day--A 5% rate of charge for 5-7 hours per day will just keep up with self discharge--Obviously getting time to replace the battery).

10%--C/10 rate. You can recharge your 50% down battery in about a day. Also Trojan (and perhaps a couple others) recommend 10% minimum rate of charge.

13%--~C/8 rate. For batteries in heavy industrial use (i.e., forklifts that are discharged by 80% or more per shift), C/8 is (usually) the maximum rated current without overheating the battery (or needing forced air cooling). Also, batteries are "less efficient" at charging/discharging at higher rates of charge (apparent capacity of battery bank falls with higher discharge rates).

~25% (or 30%?) maximum rate of charge per NAWS... From my guess, if you don't deeply deep cycle the battery bank, not huge problem with overheating. However, it would be a good idea to have a remote battery temperature sensor/monitor the battery bank to make sure everything goes as planned.

C/2.5--C*0.40 maximum surge/discharge rate for flooded cell. I use this number as the maximum discharge rate for the "average" deep cycle battery. During discharge, if you pull more than ~C/2.5 worth of current, you run the risk of collapsing the battery voltage and not being able to (reliably) start your loads (well pump, power tool, etc.). I would guess that I would not recommend a battery charger at this current level (if the charger does not have a voltage controlled output--just current control).

Again, just starting rules of thumbs to get us "in the ball park". If you have other needs or other than standard setup (i.e., AGM batteries--support discharge rates upwards of C*4--such as running a UPS), then we can certainly discuss and make suggestions towards optimizing the best for your needs.

Regarding the typical 5-13% rate of charge--Another reason is to use your system to gather the most "cost effective" power... I.e., if your system recharges your battery bank to 100% by 10am every morning--You have a lot of extra solar power.

Likewise for using an over-sized genset. Yes, you can recharge at 25-30%--but that will only be for a short period of time (during bulk phase). Once the battery reaches the "set point" (say ~14.5 volts), the "typical" charger will shift over to "absorb" charging--The actual current will begin to back down over the next 2-4 hour period (if you are going to ~95-100% state of charge). During the absorb stage, the generator begins "unloading" and running at way less than 50% of rated power output--Which, usually, is less fuel efficient.

So, I like to recommend a smaller genset+charger rate of charge so that you can run the genset in a more fuel efficient range (>40% or so of rated output). Yes--you can run a 25+% charge rate, but you will end up using lots of extra fuel for the balance of the charging cycle.

To see all of the factors that affect genset/charger selection, this thread has some really nice information:

Question about battery charger selection with EU2000 generator.

-Bill