Ending amps for batteries

photovoltaic · February 2013

I am attempting to calculate the ending amps setting to put into a Midnite Solar Classic for the East Penn Deka L16 batteries that I have.
The manufacturer charging specs for ending absorption says
Charge until change in current < 0.10A per Hr / Max Time: 12Hr

So I watched the charging amps and they held for quite awhile at 3.1 to 3.2 as the Trimetric Meter counted back from negative amps to zero.

Am I right to assume then that I should use 3.1 amps for the ending amps setting and put 12 hours for the Maxtime ?

Right now I have the Classic set to 4 hours Absorb time and absorb voltage 29 volts ( for my 24v battery system ) Previously I had it set for only 2 hours and according to the trimetric it never replaced all the amps taken out. I am only able to get out and see what it has been doing once a week.

Attached are East Penn Deka charging parameters for the model 8L16 370 amp hour batteries I have.

BB. · February 2013

Re: Ending amps for batteries

How many AH is the battery bank and is it "heavily" or "lightly" discharged (i.e., 25% or more AH rating per cycle--Heavy... Less than 15% AH per cycle--light?).

The typical numbers I have seen for end amps is around 1-3% of 20 Hour capacity (100 AH battery, 1-3 amps as end current).

Also, depending on battery type/age, the end amps could be closer to 1% (AGM in cool environment) to 3% (forklift/industrial batteries, getting old in >>77F environment).

As the many other comments are going on about industrial batteries and "proper" charge settings--I am tending back towards the question of how much water do you use and how much bubbling do you see on a "good day". Having to fill once a month is a fair amount of water usage. And "heavy" bubbling for more than 15 minutes is hard on a battery.

And, unless the system is unused (i.e., seasonal cabin, etc.)--I am not too concerned about float voltage. Most off grid folks using solar are not going to get much more than 8+ hours of day (in summer). And the other 16 hours, the battery is not getting charged, or is slowly being discharged.

It is a balancing point of long, lower current charging, vs shorter high current (higher voltage setpoint) charging that can shorten battery life.

Using a tracking system for the solar array--"Dave Sparks" likes because he can get useful charging current for more hours per day.

But for most folks with fixed arrays, they may be better off raising Absorb voltage (deeper discharge, higher absorb voltage). And use a remote battery temperature sensor to avoid over current during the last last 10% or or so of charging.

-Bill

westbranch · February 2013

Re: Ending amps for batteries

I would do as you are moving the Absorb time up in 1 or 2 hr increments a week at a time till you are happy with the results, whatever they are.

hth

photovoltaic · February 2013

Re: Ending amps for batteries

Yes the batteries are out in the cold shed now and usually not too heavily discharged when i am out on weekends. They are new in December. Bill - you just answered something I was wondering about - the range of Absorb voltage - I didn't understand how to correlate that with anything real. Since they are lightly discharged I will adjust it down towards the lower end or middle of the scale.

I put hydrocaps on the batteries and so far haven't added any water since they were new - but I did notice they weren't filled all the way to the edge of the split ring - which made it just about impossible for me to get a good suck of water in the hydrometer I bought since the end hose on it wasn't very long. So next warmer day I will try to top off all the batteries with distilled water and get some specific gravity readings on them when they are full for reference.

My other batteries are golf cart 24v on a separate system uses a Morningstar MPPT 60 amp controller and I used their setpoints for flooded lead acid. They have been performing excellent and the Trimetric meter AH always pretty much corresponded with the time it switched from Absorb to float.

I will see how the Absorb time change I did compares.

Both the controllers do have remote temperature compensation probes.

Thank you for the info - I will just keep watching and tweaking settings until I get it right.

Vic · February 2013

Re: Ending amps for batteries

Hi p v,

Well, first, I would ignore what the TriMetric is saying just now, as it needs to be calibrated to your exact system. Since you are asking about setting EA, it implies (to me) that you have not calibrated it to your system...

Many manufacturers of batteries spec EA to be 1 - 2% of 20 Hr Capacity, similar to what BB Bill said.

So you might want to set the EA to 1 %, or about 3.7 A if you have a single string of these batteries. Am GUESSING that these are Flooded batteries, if that is the case, you can actually measure the SG, and monitor for your target SG as speced by Deka. This value may be a bit too low a value. As you know, during Absorption, the battery current declines. As the battery becomes more fully charged, the rate of decline slows and slows. At some point the rate of change is so small, that the charge should be terminated. This is what the quote from Deka was getting at. The passage you quoted above seems poorly written, as a CHANGE in charge current less than 0.1 Amps per hour, fixed, regardless of the capacity of the bank, seems unmeasureable with common instruments. The Current Jitter on the output of the CC would far exceed this for a 370-ish AH battery bank IMHO.

The Surrette Flooded banks here need almost exactly 1.0% of C for EA, AND, EA is what we use to end Absorb here, FWIW.

If you can watch the charge progress near the end of Absorption, you will notice that the change in charge current will slow. If you DO have Flooded batteries, watch the SG of your Pilot cell/s this will heip you know what current to set EA as a starter. It is a good idea to monitor the SGs for several days following setting the EA value, to confirm that the set value is indeed correct.

At this point you could probably recalibrate the TM, as you would know the point that defined 100% recharged batts ... this value will probably change over time, but will be a good general quick status picture for the bank.

Good Luck, Vic

photovoltaic · February 2013

Re: Ending amps for batteries

Thanks for the info Vic. Yes they are flooded batteries. Along with all the other comments I am finally starting to understand how it all works.
I would think that someday one of the new controllers should be able to do some math or watch for some charge point and calculate the exact 1% or so to automatically change from absorb to float.

Vic · February 2013

Re: Ending amps for batteries

Hi pv,

Sorry, did not see your post while we were composing ours, so you answered some of my Qs before they were asked!.

Well the exact percentage of 20 Hr C will vary by battery type, construction, age, temperature, useage pattern etc. But, for my systems, EA is by far the best way to end Absorption. The CC usually does not know what loads, if any are on the battery bank (inverter loads, mostly), so even if the CC was looking for the long tail of current reduction that characterizes the battery becoming fully charged, the CC cannot differentiate between battery charge current vs load currents. The MidNite folks are developing an accessory for the Classic CC that will allow the battery current to be measured by a shunt (I presume), so at least the Classic should soon be able to monitor the batt current and terminate Abs for a very reasonable cost. These things always take a bit longer than we all hope, but this is on the horizon.

Have Fun, Vic

Ending amps for batteries

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