Homebrewed Electricity > Storage
Desulphation (Mad Science!) Experiments
joestue:
Periodically i can get UPS batteries for free. the last batch of batteries were 5AH AGM cells float charging at 13.5 volts in a 70F environment for 5 years. they are sigmastek, from batterysharks. one or two of them went open circuit and failed during discharge, a third one leaked a little bit i believe but can't prove it (they are mounted on their side and i can't remember if i cleaned out the battery tray 5 years ago). 10 in series for 120 vdc. 7 of them have replaced my truck battery in a 4.3L s-10 for the last 6 months or so, they seem to have enough capacity to crank the engine for 30 seconds, which is marginal but good enough for me.
another batch of 5ah 12v batteries that had similar use case as the others, but are potentially from 2006, have been float charging ever since. they measured 13.2 volts open circuit when i had them sitting on the ground outside for about 6 months. i believe i discharged them through a 100 watt 12v lightbulb and they held up 11 volts or so. anyhow, after 6 months they still measured 13.2 volts but no capacity. could short circuit them and no significant current. milliamps.
so i tried charging them backwards. for a few minutes, perhaps tens of minutes there was no significant current flow into the battery. then some of them started getting hot. internally hot far faster than i was pushing current into them (backwards). Two out of 8 got hot enough to burst some of the cell dividers. they remained hot for some time, hot enough to burn your fingers.
i don't think they shorted out internally because when they were getting hot they decided to go from -15 volts back to +11 volts or somewhere around there.
so something was going on. anyhow, some of the others that didn't burn up had hot spots which is indicative of broken plates, so i threw them all away.
Ungrounded Lightning Rod:
--- Quote from: joestue on July 06, 2018, 02:40:39 AM ---... after 6 months they still measured 13.2 volts but no capacity. could short circuit them and no significant current. milliamps.
so i tried charging them backwards. for a few minutes, perhaps tens of minutes there was no significant current flow into the battery. then some of them started getting hot. internally hot far faster than i was pushing current into them (backwards).
--- End quote ---
Sounds like your batteries were still charged - but with a layer of sulphation on the surface of at least one of the plates. Good (or decent on all but one cell) voltage, but high resistance.
Thunderhead's approach left a small load on his cells until the whole of each plate was discharged - both + and - plates essentially solid PbSO4. Then he charged them backward, turning the lead sulphate on the positive plate into lead and on the negative plate to lead oxide - the reverse of the usual situation (and usually a good way to break up the plates and kill the battery).
Story goes that sulphation is irreversible after a while because led sulphate goes through three forms with time. First it's amorphous. Then it forms a crystal structure that is difficult, but not impossible, to react back to lead - or to lead oxide - with electrochemistry. Then, over a few months, it changes to another structure that is flat out impossible (or nearly so) to react (and which also may mechanically break up the plates as the crystals become large).
Thunderhead assumed that it's actually only ONE of the two reactions (to lead or lead oxide) that's not possible on the last crystal form - and that, by reverse-charging the battery somewhat, he can break up the crystals and get their material back into service. (Let's presume he's right...)
But you didn't first fully discharge the plates. Instead, you reverse-charged them when there was still a large forward-charged mass under a layer of the crystalized lead sulphate. So what you did, on the hard-to-react plate, was to convert the skin of lead sulphate to a plate of the opposite type. Your reaction started on the surface and worked inward, reducing the thickness of the sulphate insulating coating. Eventually the insulation layer became negligibly thin and the reverse-charged layer broke through. Now you had a single plate with layers that were charged each way, connected across, first the break-through hole, then eventually across the whole surface. That's a fully charged cell with the two sides shorted by a VERY large connection, discharging itself rapidly.
It got hot? I'm surprised it didn't explode!
--- Quote ---another batch of 5ah 12v batteries that had similar use case as the others, but are potentially from 2006, have been float charging ever since. they measured 13.2 volts open circuit when i had them sitting on the ground outside for about 6 months. i believe i discharged them through a 100 watt 12v lightbulb and they held up 11 volts or so. anyhow,
--- End quote ---
And here you did it again - the batteries (or the portion of their plates that isn't stuck as crystalized lead sulphate) are nearly fully charged, because you didn't let them go until they had zero unloaded voltage.
--- Quote ---Two out of 8 got hot enough to burst some of the cell dividers. they remained hot for some time, hot enough to burn your fingers.
--- End quote ---
Same mechanism as before. But then, if a cell divider really ruptured enough to connect the electrolyte in adjacent cells, you have ANOTHER form of shorted cell: With the electrolyte connected you have a charged plus plate from one cell connected by a thick hunk of lead to the charged minus plate from the other cell, immersed in the same electrolyte bath. LOTS of power there.
--- Quote ---i don't think they shorted out internally because when they were getting hot they decided to go from -15 volts back to +11 volts or somewhere around there.
--- End quote ---
And I tend to agree - on all but maybe one of the cells. Once the insulating layer was reacted to opposite-plate type and the two layers shorted, the plate discharged until it the reverse-charged part and a corresponding forward-charged part were discharged, leaving the remainder of the forward-charged plate (and ditto on the equivalently-charged plate on the other side). +11 volts might be an all-but-one-cell battery, with the missing cell corresponding to a ruptured separator taking a pair of plates out of service and making a single cell out of the remaining two plates and the two cells worth of partly depleted electrolyte.
joestue:
these batteries had been float charging so long there is no liquid electrolyte inside them. when two of the batteries got hot, they got hot enough to bubble out from the case and break the connection between the outer shell and the cell divider. but i don't believe they had enough liquid in them to short out one cell to another, and they had 11 volts on them while they were very hot. the whole battery fairly uniformly was similar temperature, not just one side where the cell dividers let loose of the case. anyhow i really didn't notice or hear any of the valves open to release the gas, so i think the battery got hot enough that the plastic was so soft it bubbled out (i think the valves let gas out at 5 psi internal)
anyhow the point being some chemical reaction went on that caused internal heat to be dissipated in excess of the total amount of power that i pushed into the cell.
another issue here is the calcium oxide layer that forms on the positive plate between the grid and the active material (this is actually a better explanation than sulphation due to self discharge). charging the battery backwards would likely puncture through that layer which would liberate calcium sulphate which apparently is already added to the positive plate to increase discharge performance specifically at low temperature
here's another interesting paper i just found
http://cecri.csircentral.net/2081/1/22-1988.pdf
thunderhead:
--- Quote from: Ungrounded Lightning Rod on July 07, 2018, 03:08:09 AM ---
--- Quote from: joestue on July 06, 2018, 02:40:39 AM ---... after 6 months they still measured 13.2 volts but no capacity. could short circuit them and no significant current. milliamps.
so i tried charging them backwards. for a few minutes, perhaps tens of minutes there was no significant current flow into the battery. then some of them started getting hot. internally hot far faster than i was pushing current into them (backwards).
--- End quote ---
Sounds like your batteries were still charged - but with a layer of sulphation on the surface of at least one of the plates. Good (or decent on all but one cell) voltage, but high resistance.
Thunderhead's approach left a small load on his cells until the whole of each plate was discharged - both + and - plates essentially solid PbSO4. Then he charged them backward, turning the lead sulphate on the positive plate into lead and on the negative plate to lead oxide - the reverse of the usual situation (and usually a good way to break up the plates and kill the battery).
Story goes that sulphation is irreversible after a while because led sulphate goes through three forms with time. First it's amorphous. Then it forms a crystal structure that is difficult, but not impossible, to react back to lead - or to lead oxide - with electrochemistry. Then, over a few months, it changes to another structure that is flat out impossible (or nearly so) to react (and which also may mechanically break up the plates as the crystals become large).
Thunderhead assumed that it's actually only ONE of the two reactions (to lead or lead oxide) that's not possible on the last crystal form - and that, by reverse-charging the battery somewhat, he can break up the crystals and get their material back into service. (Let's presume he's right...)
--- End quote ---
That is exactly right: I left a 55W bulb connected to them for about a week before attempting reverse charge. At the end of that time the voltage shown on the terminals -- without load -- was millivolts.
For what it's worth the four batteries I did last summer overwintered quite well. They seem to be performing like, well, normal batteries.
Ungrounded Lightning Rod:
--- Quote from: thunderhead on July 08, 2018, 09:32:22 AM ---I left a 55W bulb connected to them for about a week before attempting reverse charge. At the end of that time the voltage shown on the terminals -- without load -- was millivolts.
For what it's worth the four batteries I did last summer overwintered quite well. They seem to be performing like, well, normal batteries.
--- End quote ---
Great!
As I read it, what you did to your (85 Ah) rated batteries was:
* Discharge to zilch at 5A or less over a week or more (including some extra time to insure things were really flat)
* Reverse charge at 4A in 7 daily increments of about 40 Ah (presuming a 5 solar-hours rating for summer in Scotland), producing a slow (and reverse) forming charge of 280 Ah, about 3.3 times the rated capacity.
* Discharged into a load and rechargedf (that panel again? or something else?) a couple times to estimate capacity.
* Discharge to zilch at 5A or less over a week or more (including some extra time to insure things were really flat)
* Trickle charged them forward through a bulb (with an unspecified wattage), probably less than 4A and less than 5 hours per day.
It will be interesting to see whether/how that can be simplified and/or optimized.
* Are the sleep periods in the reverse forming useful?
* Can we take out the "test while reversed" cycles or do they do something useful (like break up bigger crystals)?
* Can the forward charging and reforming go faster?
* Do additional discharge-and-slow-recharge cycles in the forward direction help with the battery reforming?and so on.
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