Sealed Lead Acid

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jwax
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Sealed Lead Acid

Post by jwax » Sun Oct 13, 2013 7:30 pm

Many of my projects require a SLA rechargeable battery. All too often, one develops a failure mode of not able to re-establish full terminal voltage, and unable to deliver anything near its amp-hour rating under a light load.
Is there a fix for these damaged goods? I could understand failure after hundreds or thousands of charge/recharge cycles, but some fail after a few dozen cycles.
Are they repairable? Desulfation?

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Re: Sealed Lead Acid

Post by Rodney » Tue Oct 15, 2013 2:05 pm

Most of these early failures are due to improper use. You must NEVER discharge to deeply and always charge at not more than 0.1C. Always recharge as soon as the voltage falls to 11.9 volts on a 6 cell battery and use a charger that will not overcharge.

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jwax
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Re: Sealed Lead Acid

Post by jwax » Tue Oct 15, 2013 4:29 pm

Understood Rodney. Is there a regeneration method/process? Looking for a fix for the bad batteries.

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Re: Sealed Lead Acid

Post by dacflyer » Wed Oct 16, 2013 4:31 am

i have problems with gel-cells going prematurely bad, i believe from too light of a load.
i have a solar light that uses 15 watts of power, stays on all night, and should have several days of reserve.
it is a 75AH and has a 75watt solar panel the charge regulator also has a low voltage shut down.
but the battery seems to loose capacity after about a year. last week we had over 9 days of lousy to no sun at all
my yard light lasted maybe 2-3 days then went out, i resorted to putting the battery charger on it.
after less than 1 hr the battery showed full charge..but when i removed the charger the led volt meter on the charge controller went down 3 pegs. so the battery is pretty much done.
i am assuming that too light of a load is not good for gel cells, either that or i got a dud battery.

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Re: Sealed Lead Acid

Post by G8RPI » Sat Oct 19, 2013 12:36 pm

To answer the question, no there is no way to revivie these batteries.
"Gel-Cell" is a very generic term fo spill-proof batteries. There are many different types of different applications. Even the same size from the same manufacturer can be made with different chemistry. The most common application is standby power for lighting and alarms. These sit for long periods on "float" charge and are seldom asked to give power. They often fail after only a few cycles of use. Batteries designed for cyclic use tend to cost more but last longer. It is important to charge to the correct voltage for the SPECIFIC battery you are using. A difference of a tens of millivolts per cell can make a big difference. Check the manufacturers datasheet and any application notes.

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frhrwa
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Re: Sealed Lead Acid

Post by frhrwa » Sat Jan 25, 2014 8:43 am

what do you guys think of those pulse type chargers. I've been using them on my acid batteries (jeep, trailer, tractor, VW, etc) and they seem to work much better than the older regulated straight shot charger.. I've had a few that were not chargeable with the regulated, so I put the pulse on and it revived them to where they work great.. wondered if that would work pulsing the charge into the Ni-Cad?
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Re: Sealed Lead Acid

Post by haklesup » Mon Jan 27, 2014 5:59 pm

Rapid charge and discharge might partially rejuvenate some battery types but there is no reliable way to revive a sealed battery. Rapid charge/discharge can build a lot of heat so has to be done carefully. The pulse charger may allow the higher charge currents but at a duty cycle that avoids the heat and gas build up that is the real danger. In theory you might be able to break down the (lead) oxide deposits that form between the plates.

In many applications, newer LI-Ion and Li Polymer batteries will hold as much energy as some SLA batteries at lower cost, weight and better reliability. However if you need more than 2Ah you are probably still looking at these cells. The market is full of small USB equipped Li-Ion power packs, often with charge regulators and control chips built in. Especially places that sell stuff cheap like woot and nomorerack are good places to find these things (unless your local store still has a bunch leftover from Christmas). Most of these are 5V unless you look at the e-Cigarette modified batteries, some of these are whopping powerful and have programmable V regulators with digital displays in something a little bigger than a lipstick. I've seen batteries coming close to 3Ah and more in a rectangular format.

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Re: Sealed Lead Acid

Post by MrAl » Sun Feb 09, 2014 11:59 pm

Hi,

My experience with this has been that high current pulsing does help but not that much. I have to admit though that i did not have a very scientific approach i just built a pulsing circuit using a high current MOSFET and microcontroller driver/pulse generator. I did get a nice high current pulse for a short time period and repeated that over and over for a couple days. This was using a 6v lead acid battery.

It seemed to raise the ampere hours capacity, but not by enough to make it worthwhile. It was something like 1 ampere hour increase. That was not enough as it was a 4 ampere hour battery.

It could also be that i had discharged it too low, which damages the battery too. Perhaps better care in the usage would have made it last longer.

The real problem is that we can not see inside the battery to see what is happening. We cant examine the plates before and after. If the battery case was clear plastic we might be able to tell what was going on when we did experiments like this, otherwise we have to rely on other people's posts about how to do this properly and if it really works or not.

We may be able to cut part of the top off so we can see inside, but that gets dangerous so i never attempted to do that. If we could see the plates we might be able to tell. It's so hard to test because in order to know we have to discharge the battery and calculate the ampere hour capacity that we got from that discharge.

Aside from all that, i dont think we can get as many cycles from a Lead Acid as we can from other technologies. In fact i think we are lucky if we get 200 full discharge and recharge cycles. If we dont discharge too much we can get lots of cycles, but that's not saying much because discharging only a little doesnt do us much good except for purposes like starting the car in the morning.

We can also look at cycle life vs depth of discharge. If we get 200 cycles with a deep discharge then we might get 400 cycles with only 50 percent discharge, and 1000 cycles with only 30 percent discharge for each cycle. If we discharge too low though we might damage the battery and then it might not work very well at all after that.
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frhrwa
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Re: Sealed Lead Acid

Post by frhrwa » Mon Feb 10, 2014 7:59 am

dad used to empty lead acid batteries when they went bad. rinse them out, then cut carefully along the ridge around the top and remove the top portion with the plates hanging.. he'd clean the plates real good, clean out the case, then use some sort of silicon glue he got at Nellis AFB to put the top back on.. fill them back up with battery acid, the batteries were good for another 6/7 years or better..
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haklesup
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Re: Sealed Lead Acid

Post by haklesup » Tue Feb 11, 2014 2:06 pm

Now that's old school Yankee thrift. You don't see that much anymore.

Not advisable anymore since the stuff you rinse out is fairly toxic to the environment and should not be flushed or put in drains. That plus the amount of labor your dad used probably equivalent to the cost of a new battery in todays dollars. An SLA might not be so simple either

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Re: Sealed Lead Acid

Post by dacflyer » Thu Feb 13, 2014 8:47 am

what actually gets damaged in the battery from too deep of a discharge ?
is it chemical or mechanical damage ?

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jwax
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Re: Sealed Lead Acid

Post by jwax » Fri Feb 14, 2014 7:57 am

During the discharge cycle, lead sulfate builds up on the plates. If not properly recharged, the lead sulfate coating turns into hard crystals, which are more and more difficult to remove by recharging.
That buildup reduces the amp-hr capacity of the battery.
An ideal recharger would completely convert all the lead sulfate back into lead and sulphuric acid.

Years ago, I did try the "reclaim" process described by frhrwa. Didn't work for me, I think because I had no way to remove the hard sulfate buildup.

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frhrwa
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Re: Sealed Lead Acid

Post by frhrwa » Wed Feb 26, 2014 12:57 am

I cannot for the life of me remember how he cleaned the plates, but man, when he got done, they looked brand new.. putty knife? one of these days, or nights, it'll come back to me..
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Re: Sealed Lead Acid

Post by dacflyer » Fri Feb 28, 2014 12:50 pm

i wonder if,,,once you take the battery apart, if you put it into a ultrasonic cleaner, if that would clean the sulfate build up off.
i wonder also if using a fine wire brush to scraps that crud off would work ?

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Re: Sealed Lead Acid

Post by haklesup » Fri Feb 28, 2014 9:07 pm

I'm not an expert but this is the way I understand it: (and to expand on jwax answer)

Lead sulphide is just a product of the electrochemical reaction that drives the battery, it is essentially reversible as long as the crystals are very small and surface area is very large (actually ratio of surface area to volume). When you charge, the process dissolves most of the tiny crystals turning them back into metallic lead and ionic sulpher in the acid solution in an electroplating like reaction. I say most because the process is never perfectly efficient and over time, a plaque of larger crystals develops on the lead plates. Some of this dissolves when charged but more and more remains stuck over time. Deep discharge can allow very large crystals to grow that can never dissolve and may even short the plates. Keeping the battery between 20% and 80% (Hmm, Maybe that's for NiMh) with frequent use will extend life to maximum be keeping the crystals in the optimal range.

As this process continues, the Pb-S plaque hinders electron flow and encourages the conditions that allow it to grow. Gradually this prevents full charging and reduces maximum current flow. Batteries are often modeled as an ideal source in series with an internal resistor. In this case the chemical reaction is the source and the Lead sulphide crystal layer is the internal resistance (plus others in the model but in this example it dominates). When the internal resistance of a battery rises above a certain level, the battery is considered dead and if when recharged it does not recover, it is considered really dead.

In the case of a car battery with a simple plate configuration the lead sulphide will come off like frost from a freezer, it may come off in chunks but you will need to scrub to get it all off. Its a rigid crystal and the lead plates can bend so potentially this can be used to remove it. Ultrasonic cleaner would probably be helpful and may even replace the scrubbing. Naturally not all of the lead is left after this process, nor do they have the same surface area so the battery will not perform the same. When you turn in your batteries, even the lead in the lead sulphide is recovered.

Gel cells may have different chemistry and replenishing the electrolyte is not straight forward like it would be in a wet cell car battery. Furthermore the plates may have a more complex geometry in an effort to tailor the characteristics of the battery to its application that prohibit disassembly and cleaning.

I'm less clear on what happens during overcharging and how that damages the battery.

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