medium size newbies battery bank size off grid

blueskies

please correct me if i am wrong

as a newbie i destroyed batteries discharging them too low

BUTT (extra t intended)

regardless of the battery bank capacity and chemistry , IF YOU MAINTAIN 80% CHARGE (off golf cart bats) the size of the battery bank is (relatively*) not important as long as it is not too SMALL.

mike95490

Too large of bank, and you cannot effectively charge it with a small PV - the electrolyte in flooded batteries does not get stirred up. Or it may begin to sulfate in the days needed to recharge.

BB.

It depends on chemistry too... Typical deep cycle flooded cell lead acid batteries need to be cycled to keep the plates in proper condition.

It seems that cycling a FLA d.c. battery once a month between 75% and >90% state of charge is good for the battery bank.

If you just "float" a FLA d.c. battery bank, they tend not to last as long and/or have poor performance under current/load (this may be a stretch on my part).

There are batteries made for floating (100% to 85% cycling--Very long life as long as you don't "deep cycle" them).

There is also the question of what your loads are? The batteries are kept at 80% state of charge (which is an issue in itself, typically you use a hydrometer to monitor state of charge--Voltage and even current shunt measurements are, at best, only estimates of state of charge).

If you have steady state DC loads (filimant lamps, perhaps universal/brushed DC motors, heating loads, etc.), the battery does not really do much of any thing.

However, if you have, for example, a 120 VAC 60 Hz AC inverter... The current draw looks like a 120 Hz sine squared wave from (think of a 120 HZ sort of sine wave that goes from zero amps to 100% peak amps) as the AC inverter draws power from the DC Battery bus to supply the 120 VAC 60 Hz loads.

Now, the battery is acting like a "chemical" capacitor. It is getting charged/discharged a 120 times a second. So-called, "micro-cycling".

And the battery terminal voltage (say it varies 1 volt--not usually that high--just for illustration). Say your battery is fully charged and at 12.8 volts. And you have this 1.0 volt voltage pulse at 120 Hz (because of the 120 Hz current draw from your inverter).

If the battery voltage ranges from 12.8 to 13.8 volts, you are "micro cycling" above the charge/discharge line.

If the battery is cycling from 12.4 to 13.4 volts, you are "micro cycling" charging and discharging 120x per second.

And if the battery is cycling 11.8 to 12.8, the battery is only discharging.

To know what is happening to the battery, you need to put a scope on the battery terminals and see what your micro cycling is--It is always charging, actually cycling, or something else such as discharging for an hour, then recharging for an hour).

Obviously, in the case of an AC inverter, too small of battery AH capacity, and you will have too wide a range on the "ripple" voltage and actually be cycling (charge/discharge) 120x a second.

And these "ripple currents" are a source of heating inside the battery (as well as in a capacitor--I^2*R losses).

And as Mike says above, too large of AH battery bank, and you end up not being able to properly recharge the battery bank (FLA deep cycle batteries typically recommend a 10% minimum rate of charge, although 5% does seem to work "well enougH' for summer / weekend cabins (batteries "age out" before they "cycle out").

It is not easy to design the "optimum" system. Everything needs to work together, and if you try to do something different (like balance solar array power to load power, and not cycle the battery bank), one of the other design rules seems to bite you.

For example, what happens if somebody turns off or on your DC loads--Solar chargers seem to have a 1-2 second reaction time--And during that time the battery will need to sink (the extra solar current) or source (the starting load surge) current.

So, you still may need a relatively big FLA battery bank to sink/source the current spikes. Which means the battery capacity is "wasted" the rest of the time.

AGM and Li Ion batteries may be a better fit for this type of application. They have lower internal resistance and may supply those current spikes with smaller AH capacity (FLA, C/2.5 current surge capacity. AGM down towards C/1 surge, or lower, capacity, and Li Ion can do even better).

-Bill