Optimal charging current

kevinjones · February 2017

@westbranch:

"Are the 2 , each with a dead cell, the ones out of Service?"

no :-/

The numbers correspond to their physical configuration. The upper and lower portion of each column wired together in series, all the columns wired together in parallel.

Okay.

westbranch · February 2017

Why are those 'dead' cells still in service, they are probably the bubbling ones...?? No? If not, ??

Yes I got the configuration, just wanted to be clear... I would suggest that you number each 6v battery, so that as time marches on you will have a long term history of it. all 24 of mine have a unique number... easy to keep things in order..

BB. · February 2017

The "dead cells" can either be "open" or "shorted" or "other"...

Open, they simply prevent any current from flowing through the strings.

Shorted, they can actually subtract 2 volts per shorted cell from the battery string voltage... You may be paralleling a 12 volt and a 10 volt battery together (the 10 volt battery will discharge the rest of the bank).

Other--The cell may have some voltage, but basically will not allow much current through--Limiting charging/discharging of the rest of the cells in that battery string. Those dead cells may boil (and run hot) or do nothing (low to near zero current flow).

That is also why I like a DC Current Clamp Meter:

http://www.sears.com/craftsman-digital-clamp-on-ammeter/p-03482369000P

You can figure out what each battery string does relative to the rest during charging/discharging cycles.

When you have a string that has less than ~50% of the current flow as neighboring batteries, then that string is not much of anything during discharge.

If you have a string that is drawing lots of current during charging--Either the other strings are "lazy" or that string has (for example) a shorted cell. Again, not a good thing.

-Bill

Estragon · February 2017

Can't do cut & paste properly on this phone for smartguage link so try googling smartguage and parallel - the article should be found.

The way the pos/neg posts are paralleled means there will be differences in resistance as described before. Better ways of wiring are shown in the article. One of these would be to bolt your existing cables to a common heavy guage copper bussbar.

In any case, your SG readings show a badly balanced bank. Breaking the bank up, ideally to individual batteries if you have a 6v charger, but into 12v strings otherwise, and charging individually is best. EQing the whole bank will be hard on the already charged cells (watch water closely) and take longer to bring low/dead cells back than if you do separately.

Taking apart will also be a good time to clean and retorque all connections. If you have to reassemble before changing to a better parallel setup you may want to rotate string locations so that the different resistances are hitting different strings. The weak cells may have some permanent damage so the idea is to make sure they see the least resistance after rotation.

BB. · February 2017

http://www.smartgauge.co.uk/batt_con.html

-Bill

kevinjones · February 2017

"http://www.smartgauge.co.uk/batt_con.html"

Sorry, but I beg to differ with the author's findings regarding method #2, in saying that the load to the batteries is not balanced.

Assuming Ideal connections, suppose the resistance for each interconnecting link is 1 Ohm (just as an example).

Numbering from the top,

Battery #1 sees 3 ohms to the positive connection, 0 ohms to the negative connection - total 3 ohms
Battery #2 sees 2 ohms to the positive connection, 1 ohms to the negative connection - total 3 ohms
Battery #3 sees 1 ohms to the positive connection, 2 ohms to the negative connection - total 3 ohms
Battery #4 sees 0 ohms to the positive connection, 3 ohms to the negative connection - total 3 ohms

Each battery sees 3 ohms additional load due to the interconnecting links. Regarding current flow, the only thing that matters is the total resistance, it matters not whether the resistance is between the positive or negative connection.

Photowhit · February 2017

I would normally agree whole heartedly, but he did take some direct measurements. It's certainly interesting.

BB. · February 2017

I believe his is saying that the design is balanced--But there is (relatively slightly) differing current flow because of the batteries themselves (batteries are not identical--i.e., real world).

-Bill

Photowhit · February 2017

Bill, There were measured significant differences;

"After this simple modification, with the same 100 amp load....

The bottom battery provides 26.7 amps of this.
The next battery up provides 23.2 amps.
The next battery up provides 23.2 amps.
The top battery provides 26.7 amps."

...the author goes on to suggest a single post or bus bar solution.

While it doesn't make sense, I have seen others make this same observation. Perhaps it's the inner batteries being warmer, though I would think they would contribute more not less. Perhaps it's some field disturbance from measuring the inner vs outer batteries. If it was a minor variant WTH, but I'd say this is significant.

BB. · February 2017

That is not a large difference between the high and low current flow (~10%).

And these same batteries where originally wired ladder style with:
Battery internal resistance = 0.02 Ohms
Interconnecting lead resistance = 0.0015 Ohms per link
Total load on batteries = 100 amps
The bottom battery provides 35.9 amps of this.
The next battery up provides 26.2 amps.
The next battery up provides 20.4 amps.
The top battery provides 17.8 amps.
So the bottom battery provides over twice the current of the top battery. It is not clear if these are measured or simulated results--Or if the two sets of measurements are done with the same batteries... But I do believe the results should be close to real life.

Flooded cell lead acid batteries can have a 20% variation in capacity and state of charge just in normal use--Seeing variations in matched current flow/wiring setups seems to be within normal usage.

-Bill

kevinjones · February 2017

"I believe his is saying that the design is balanced--But there is (relatively slightly) differing current flow because of the batteries themselves (batteries are not identical--i.e., real world)."

If inbalance were caused by the batteries themselves, it wouldn't matter how they were wired, there would still be inbalance. What we are dealing with, is the optimal wiring configuration, in which case, only has to do with the path of the wiring.

Having had a career in Electronics and Instrumentation, it would be very difficult to convince me that an ideal version of #2 wiring configuration in his example would result in an inbalanced load. (And any discrepancies due to less than ideal conditions will show up in any configuration, though it could be possible if the cards were just right, that the discrepancies in the wiring might happen to balance out the discrepancies in the batteries.)

Vic · February 2017

Hi kevin..,

Not wanting to dwell at all on the nature of the batteries on this system ...

Sometimes, particularly it seems, on 12 V systems, batteries are added to the system over a period of time. Of course this in, and of itself, usually creates imbalances of its own. Am not saying that this was the case with this system.

Agree, that wiring will probably not be the be all, and end all for any/all imbalance.

Just the physical size of this battery bank, and likely temperature variations across this bank, plus, the probable inability of selecting one battery that is representative of the entire bank, on which to place the necessary BTS/es can be a real challenge. Having accurate charge and Float voltages is very important for battery health, and the temperature effect on these voltages is fairly high for many off-grid Flooded battery banks (as they are often in unconditioned spaces).

Also, of course, ageing of batteries will not leave many strings that were once-balanced, balanced some months later.

Having a battery bank that is composed of many, many batteries (in parallel at that), will; give the manager of the system an opportunity to measure current balance and SGs often, and probably often, move batteries around to try to maintain balance, and/or to charge batteries and strings individually in order to try to bring lagging batteries in some semblance of balance with others.

Measuring the voltage of individual battery voltages, ideally with high charge or discharge currents, can often give a quick indication of approximate balance, when compared to the voltage of other batteries in the bank.

Please be very careful, when connecting and disconnecting battery cables to move or individually charge batteries, due to energy and possible explosion hazards that can exist. Sometimes a small mistake, like dropping a wrench, etc can cause a surprise.

FWIW, not to try to be too preachy. Best of luck, Vic

kevinjones · February 2017

@Vic - not preachy at all, just good information. Thanks.

I have inherited the responsibility of maintaining a large battery bank that was set up years ago for a small community. We had no idea the amount of care that was involved back then. I am finding out it is not just a matter of wiring them up, hooking up charge controllers, and then checking the water periodically.

I have ordered a clamp-on ammeter as was suggested.

kevinjones · February 2017

Just a quick question, it was suggested that they may not be getting cycled deep enough. Even if that is the case, I still need to keep a float charge on them at all times, right?

Vic · February 2017

Hi Kevin, Thanks for the messages.

Most Lead Acid batteries should be cycled below 90% State Of Charge (SOC), before recharging them fully. And cycling them to 80 or 75-ish percent SOC is actually good for Flooded batteries.

Had read that Thread on Optimal Charge current (or whatever was its exact title), several times, and did not really know how folks got the idea that the batteries were not being cycled deeply enough, other than the possible assumption that each and every cell was fizzing during Absorb, perhaps due to overcharging.

Mike 9????? mentioned that Absorb should not be extended past one hour, which would only be the case for a very lightly-cycled battery. Just to make certain that you know, a Flooded battery that is cycled to about 80 % SOC would probably require two or more hours of Absorption, perhaps three hours, when using recommended Absorb voltage.

Managing batteries can take quite a lot of time, and parallel strings will take some additional time.

Many folks believe that AGM batteries require NO maintenance. This is not true, they just take a different kind of maintenance, primarily due to the difficulty in knowing what is the actual SOC the batteries.

You will probably end up culling a few batteries, and this will help reduce the number of strings, unless you absolutely must have that Capacity.

Please DO stop back to let the group know how you are doing, as this is a friendly and knowledgeable group.

GOOD LUCK, Thanks, Vic

westbranch · February 2017

@Kevinjones, what is/are the load/s that have to be supplied and the amperage?
We know need to supply 12V but what are the limitations in this puzzle from your perspective? Just thinking about other ways to supply 12V that have less issues...

Optimal charging current

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