Battery Specific Gravity

Estragon · November 2017

@mcgivor - yes, that's what I was referring to as well... transition to float based on current at Vabs. OP seemed to be asking for clarification on the capacity rating (ie C/20 vs C/whatever).

The comment on float time was in response to a separate question, so probably should have put in a separate answer with quotation of OPs question for clarity.

@jonr - agreed, but 1-2% of C@20hr is a reasonable starting point. IMHO, starting at 2% of C@100hr may be too high for a newish bank, and terminate absorb too fast. I'd rather set lower and increase if/when I see the absorb timing out without getting to end-amps. YMMV though.

BB. · November 2017

You are all correct... The "magic number" for battery capacity we use is the 20 Hour discharge rate (i.e., discharge a battery from 100% to 0% state of charge).

If your battery supports a 5 amp discharge for 20 hours that is 5 amps * 20 hours = 100 Amp*Hour capacity.

The termination of charging (for solar, typically the transition from Absorb ~14.75 volts for a flooded cell lead acid storage battery to Float voltage of ~13.6 volts or so) is the 2% (sometimes 1% or less, especially for AGM type cells) is when the battery is near 100% full. Falling to 13.6 volts just keeps the self discharge in "balance" with a small charging current. Also, there are various chemical reactions over time that the proper float voltage can reduce and give you a longer battery life.

The problem with a typical solar charge controller is that they only "know" their output current. They cannot determine the difference between current going to the battery bank or to loads on the DC bus. So, a backup up is typically to limit Absorb cycles to ~2-6 hours (depending on battery type and depth of discharge).

A few vendors (Midnite, possibly Outback, and a few others) have added a precision resistor to the battery bus (on the negative bus terminal). This shunt measures the current going into/out of the battery bank itself (regardless of DC bus loads/charging sources).

Midnite then can actually be programmed for a ~2% Absorb "cut off" because it knows the charging current going into the battery bank itself. The charge controller will fall back to 13.6 volts and support both battery float current and variable DC loads on the battery bus. Quite cool.

Note that there are more details to be watching. AGM batteries typically have very low leakage currents (0.1% to 0.01% or even less). Flooded cell lead acid batteries have lower leakage current when new and higher leakage current when old (true too for AGM and other lead acid type cells). Call it ~0.1% to 1.0% leakage current (deep cycle fork lift type batteries have higher end of the range for leakage current).

And, one rule that I read somewhere years ago for a company that maintains large Telco and similar battery banks, if your "leakage current" is 2% or greater--It is time to replace the battery bank. The risk of battery failure or even chance of fire is greatly elevated with >2% constant current flow (float charging).

-Bill

oliline · November 2017

mcgivor said:

You have to consider the sun sets every day so the absorption will terminate as the ability, or availability of output diminishes later in the day, using an end amps of 2% is to prevent overcharge, but if the loads exceed that then the absorption will not terminate, hense the time limit. Depending on time of year the time limit can be adjusted to suit loads and solar availability, there is no one setting fits all, careful observation of battery gassing will indicate where settings need to be. Once in float, the PV will support the loads, usually a recharge voltage set-point is programed to prevent over discharge before the sun sets, it's a fine ballance that is specific to each system, only the operator can see where these transitions and setpoints need to be.

Going back to my question...
Let's say the Abs to Flo Current is set to 2% of the capacity of the battery at C20 to prevent overcharge. Let's say "the loads exceed that current and do not allow the absorption to terminate".
Why would it be the problem? Can the absorption not be continued next day?

oliline · November 2017

If we put a time limit on the abs the charge controller will jump to Float but the the battery will not be full anyway! Is it not better to everyday try as long as possible to accomplish the 2% Abs to Flo Current. I don´t see the point of cutting the absorption process using a time limit really...

Estragon · November 2017

Providing the controller sees actual current going to the battery (ie not confused by loads), yes, terminating absorb with end-amps should mean never timing out. Older batteries may never get to below 2% though, so end-amps would need to be upped a bit.

Battery Specific Gravity

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