Is it possible for batteries to loose half their capacity?

CALLD · October 2014

Ok, I'm officially pissed-off with my battery bank now and almost willing it to die so I can justify a new purchase to my wife.........

I have two separate banks - one 306AH silver calcium bank and one 100AH old-school leisure battery bank. The 100AH is 2 months older.

The two banks operate independently - the 100AH bank is sort of used as a reserve bank if and when needed. The 306AH bank is the primary. They have both been cycled approximately the same amount throughout their life and been kept at a similar SOC.

The 100AH bank is performing very well, just as well as when new if not better!

The 306AH Silver Calcium bank has been rubbish pretty much from the start. I have never managed to get more than half the rated capacity from it without the voltage sagging so low I just can't bring myself to push it any further.

I bought them up to 110% yesterday with a 4 hour boost charge at 30.4v until the charging current had tapered off to 3 amps (0.01C) and let them rest for 3 hours before performing a discharge test overnight. By the morning 125AH had been removed, voltage was reading 23.7v with a 5 amp load. The SOC could be estimated to be over 60%. I applied a 120 Amp load for 6 minutes to check the voltage sag, the voltage dropped to 21.9v after 6 minutes and then returned to 23.5v after the load was removed. After a rest period of 1 hour the voltage was at 24v.

Now I do know that 24v open circuit resting voltage = about 50% SOC, which is close to what they should have been, but the issue here is how they perform under load. These batteries appear to have a very big voltage sag under load compared to other batteries I've tried and tested. They also have very high charging voltages so it works both ways. Their internal resistance just seems way too high. Sulfation was my first guess, but after leaving them idle for 2 days after charging the open circuit voltage was 26v so that leaves me scratching my head. Could it be loss of active material on the plates? Wish I could open them up and see for myself...

I'm not the battery expert, but after reading hundreds of pages of literature including the 1454 page 3rd edition of "handbook of batteries" I've concluded that these batteries should be performing much better than they are. But none of the literature at my disposal covers the specific type of battery I have. Silver Calcium grid alloys are used primarily for SLI batteries and is poorly suited for cycling applications, yet these batteries I have are specifically marketed as "suitable for cycling duty" which makes my head hurt even more. The manufacturer does not give any useful info on discharge profiles other than specifying the "end voltage" of 1.7vpc. We all know what a discharge curve looks like or should look like. And if they are to be trusted then one could assume by the time a 306AH battery is reading anywhere below 24v with a 5 amp load on it, it is fairly flat. A 120Amp load (C/2.5) should not cause the voltage to sag from 23.7 to 21.9 on a battery with over 60% SOC, a sag to around 23v would have been more acceptable.

So now the question, when the batteries are still working - just as badly as they were when I got them, how do I justify spending $2000-$3000 on a new proper deep cycle bank to the wife?

D

jcheil · October 2014

Re: Is it possible for batteries to loose half their capacity?

CALLD wrote: »

So now the question, when the batteries are still working - just as badly as they were when I got them, how do I justify spending $2000-$3000 on a new proper deep cycle bank to the wife?
D

Without her knowing, set up a random timer that cuts the power for 10 mins every few hours, especially at times when SHE really needs it.
After a few days of that, she'll be helping you pick out the new batteries herself

Photowhit · October 2014

Re: Is it possible for batteries to loose half their capacity?

CALLD wrote: »

I bought them up to 110% yesterday with a 4 hour boost charge at 30.4v until the charging current had tapered off to 3 amps (0.01C) and let them rest for 3 hours before performing a discharge test overnight. By the morning 125AH had been removed, voltage was reading 23.7v with a 5 amp load. The SOC could be estimated to be over 60%. I applied a 120 Amp load for 6 minutes to check the voltage sag, the voltage dropped to 21.9v after 6 minutes and then returned to 23.5v after the load was removed. After a rest period of 1 hour the voltage was at 24v.

So you applied a 125 ah load to a 300 ah battery bank that was at 60% SOC? or drawing at a rate of over half or the remaining capacity? I suspect you've just got unrealistic expectations.

You should look at your complete system, I suspect you don't have a large enough system to meet your needs. You should reevaluate your daily loads and size a system properly to meet them.

Cariboocoot · October 2014

Re: Is it possible for batteries to loose half their capacity?

Applied a 120 Amp load to a 300 Amp hour battery bank and the Voltage sagged and you're surprised? That's 40% discharge rate: nearly double the maximum that should be applied. You do know that "300 Amp hours" is the capacity over 20 hours and that would be 15 Amps steady discharge? If you increase the rate of discharge you decrease the capacity at the moment the load is applied. If the relative SOC is depleted before the heavy load is applied the effect is more severe because you are starting out with less capacity (like using a smaller battery).

It's not the batteries that are at fault here. Either you've got your numbers confused or as photowhit said your expectations are unrealistic.

CALLD · October 2014

Re: Is it possible for batteries to loose half their capacity?

Jcheil: hahaha brilliant idea!

Well I suppose my expectations may be a bit high, funny thing I've just noticed is all of a sudden they are giving much better readings today! Yesterday the voltage dropped under 25v within half an hour on a 15amp load, today it's still at 25.4v after an hour on the same load...?!?!. Do batteries have a sense of humour? Or are they just afraid of ending up homeless this soon...?

The difference is that yesterday the started the day at 90% Soc and were kept at 30.4v boost charge for 4 hours.
Today they started the day at 50%soc and spent 2 hours at 29.8v absorb, then 2 hours at 28v before discharge.

Can a lower charge voltage yield a better discharge curve? Is it possible that yesterday's high voltage charge filled the electrolyte with gas bubbles that reduced contact area with the plates?

Who knows, I will perform the same test tomorrow morning and see if anything changed...

D

Cariboocoot · October 2014

Re: Is it possible for batteries to loose half their capacity?

CALLD wrote: »

Can a lower charge voltage yield a better discharge curve? Is it possible that yesterday's high voltage charge filled the electrolyte with gas bubbles that reduced contact area with the plates?

D

Sort of. Not so much Voltage as current, although higher Voltage for Absorb is needed for mixing and yes it generates bubbles which can affect SG readings, especially if taken immediately. The gas should disipate and SG stabilize overnight.

A couple of us tested charge acceptance this year and found some interesting results. Unfortunately lacking the funds to do a full-study we kind of ran into trouble attempting controlled discharge on active systems. In other words we can't accurately check and verify what the experiments were indicating.

Following that disclaimer what we did find was that lower current over longer time showed better charging, in that more of the power went to replenishing the batteries rather than heating them up at the higher charge rate. The most curious result was improved charging using scale current in relation to the SOC; in other words not starting out with 10% of the full capacity current but rather 10% of the reduced capacity current. Id est: 10% of a 200 Amp hour battery is 20 Amps, but if it is at 50% SOC it's 100 Amp hours so the 10% current is 10 Amps. As SOC increases, so does current. Chargers aren't designed to do this and it was a bit tricky manually (done in stages) but the results, although scientifically imperfect, were interesting.

Too low or too high of Voltage, current, or time will result in less-than-best results. It's a bit of a dance, but most systems have enough tolerance built in that minor variations aren't even noticeable.

With a 300 Amp hour bank you should expect an average draw of 30 Amps maximum and a peak draw of 75 Amps. A 'steady draw' of 15 Amps should provide the maximum Watt hours of approximately 3600. Change the draw, change the capacity.

CALLD · October 2014

Re: Is it possible for batteries to loose half their capacity?

Cariboocoot: thanks very insightful! I think I never really took into account how different loads affect capacity despite being aware of peukerts law.

I also forgot that soc affects internal resistance. The other thing that happened today was the batteries had been charging at a higher current than yesterday due to being at a lower soc.

Yesterday was cloudy, the batteries were at 90% soc at sunrise and as a result they were charging at a very low current.
Today was sunny, they were at 50% soc at sunrise and were charging rapidly at a higher current.

Yesterday they entered absorb of 30.4v at 10amps.
Today they entered absorb of 29.8v at 26.5amps.

As a result the batteries were 4°C warmer after charging than they were yesterday. The higher temperature. Could be one of the reasons for better performance, and maybe it also helped to dissolve some sulfate off the plates? All speculation for sure!

PNjunction · October 2014

Re: Is it possible for batteries to loose half their capacity?

Did you get any charging material from the manufacturer? You may STILL be too low in charge voltage.

Consider this - A silver-calcium battery is an additional grain refinement over Calcium-Calcium. Essentially, the silver is an additional help to deal with high underhood vehicle temperatures, where the major advantage of super low self discharge rates of calcium-calcium would be nullified.

BUT, unless you have actual manufacturer's charging recommendations, consider this:

The NOCO Genius G26000 12v charger has a "special" charging mode for calcium-calcium. It is at 16.5V !! This APPEARS to be the same voltage as the "EQ" mode it can supply to other batteries, but don't confuse them. Here, in calcium-calcium mode, the 16.5v is not current limited like the other 16.5v eq mode. They have big warnings not to confuse them, and apply the calcium-calcium charge mode to anything else.

In other words, they don't want a user mistakenly applying the 16.5v calcium-calcium mode to any other battery that they just want to do a high-voltage EQ to - to do that, even though it is still 16.5v, they MUST use the boost mode, and not the calcium-calcium mode.

Check out the operators manual on the 26000 where they specify the 16.5v voltage for normal calcium-calcium charging:
http://www.geniuschargers.com/downloads/manuals

Another reference to 16.5v:
https://www.youtube.com/watch?v=lE_DUDBJTTA

Warning - I don't know how much different, other than an additional grain refiner of silver to a calcium-calcium battery may make when it comes to the absorb voltage setting - but it has me thinking that it is possible that you are STILL too low in absorb charge voltage unless you reach 16.5v (nominal 12v battery).

Perhaps this might be the problem - I'd certainly look into it. Maybe you are sulfating. See if you can get more accurate info on that battery. I see 14.8v being bandied about on the internet, yet a respectable charger manufacturer uses 16.5v in regular absorb mode for these. Hmmmmm

Also, like mentioned before, you are HAMMERING them with too high a discharge rate as well.

WARNING - additional thought - since this charger is primarily designed for an automotive application, where after charge a normal alternator voltage is mainly used, which is much lower than 16.5v certainly - is it possible that the Silver-Calcium needs to see a 16.5v charge every now and then, but NOT every day? In other words, perhaps it does want to see an "EQ" every now and then, but the normal current-limited EQ as used with flooded (and certain agm's able to handle it) won't do the job properly, and with calcium-calcium, the ONLY way to do that properly is to do it with a normal "absorb" at this high voltage? Aka - the Noco's "special" calcium-calcium mode?

That's my train of thought anyway. I'm starting to think that the confusion over voltages varying between 14.8 to 16v is that perhaps either the vehicle is modified to actually charge at 16.5v, but run the electronics lower with a regulator, OR you run them normally at 14.8v, but every once in awhile, they like to see a full charge to 16.5v (out of the vehicle I would think!)

CALLD · October 2014

Re: Is it possible for batteries to loose half their capacity?

PNjunction: yes the whole 16.5v thing has had me scratching my head for a long time already. The manufacturer does not give a cycling or EQ charge voltage for these batteries, just a float voltage of 13.8v. I think these batteries were primarily designed for standby applications like UPS etc.

I have noticed that gassing becomes audible at 29.4v, and becomes vigorous at 30.4. I have once tried taking them to 31.2v and the gassing became so loud I immediately changed the voltage back down for fear of damaging something...

The reassuring readings I got last night were less impressive this morning, 23.5v with a 5amp load and 140AH removed. But from what I've heard so far on this thread the conclusion is to accept it and live with it. No point in fixing something that ain't broke yet.

The original concern came from numerous C/20 discharge curves I've seen in battery literature, where the 2v per cell line is only crossed at around 40% SOC. Perhaps that just ain't so for these batteries. Also, in others literature 2v per cell is stated as the resting voltage for 50% soc. In that case 23.5v under light load is perfectly acceptable IMO for a 50% soc battery.

In the mean time, I'll avoid using the batteries to make coffee when they are at less than 70%.

D

PNjunction · October 2014

Re: Is it possible for batteries to loose half their capacity?

Smart move keeping an eye on them when they started vigorously gassing.

I'm assuming you have two 12v silver-calcium batteries in series. Are they balanced to start with? Any problems with wiring infrastructure? Did you get them new or used? Is it possible to charge each one separately with a 12v nominal charge setup, and then put them back together?

CALLD · October 2014

Re: Is it possible for batteries to loose half their capacity?

PNjunction wrote: »

Smart move keeping an eye on them when they started vigorously gassing.

I'm assuming you have two 12v silver-calcium batteries in series. Are they balanced to start with? Any problems with wiring infrastructure? Did you get them new or used? Is it possible to charge each one separately with a 12v nominal charge setup, and then put them back together?

That's right they are 12v batteries. 6x102 ah in 3 parallel strings branch-linked with equal lengths of wiring. They were all brought to 100% with a charger before being placed in the bank. They were all purchased new. Concern is as always that they carry an equal current. The wiring attempts to achieve this but because not all batteries have identical internal resistance I have to accept some unequal flow. I've measured the voltages of each individual battery while the bank is under heavy load (120 amps) and have found a variance of 0.067volts between the highest battery and the lowest battery. Under light to moderate load the difference is less than 0.010volts.

During bulk charge the voltages are within similar tolerances up to 28volts and after that some differences begin to emerge. At 30v I find one side of the bank to be at 15.2 volts and the other side to be at 14.8volts. This happens every time and is always the same side of the bank that is higher. It makes me scratch my head because during discharge the bank is pretty equal.

PNjunction · October 2014

Re: Is it possible for batteries to loose half their capacity?

Are they flooded or sealed? What exactly is the manufacturer and model number if known?

If flooded, does the hydrometer indicate any SG's that are way out of line? You may also want to invest in a handheld IR temperature gauge that you can pinpoint to check terminal temperatures, cable temps, and even individual cell temps if you have the room to spot the outliers.

Are you connected in a simple "ladder" type setup (not recommended), or on the "diagonal" across the bank?

Just throwing out stuff off the top of my head which is hard to diagnose this far away.

Is it possible for batteries to loose half their capacity?

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