Battery voltage drops while charging?

[samcrut]

I have a little $40 (12v 1A) BatteryMinder trickle charger that has the desulfation pulsing stuff going on in it. I'm using it on two 12v deep cycle batteries in parallel that I'm pretty sure went into storage without a charge and caked up. I know they haven't had much use over their life.

Anyway, I noticed something a bit ago. When the BatteryMinder is charging the battery, my meter shows about 6.5V, up from 5V a few days ago, but the interesting thing is that when I disconnect the charger, the voltage in the battery RISES to about 8V and stays there. Any idea what would cause that behavior?

I have a 60W (12v 5A) charger showing up here shortly that I'll use on it to try and heat it up a little more aggressively to melt the sulfer with a good stout charge and then I'll put the BatteryMinder back on it, but I'm trying to wrap my head around how a battery bank would show a lower voltage while being charged and then a higher voltage when disconnected.

[Sunking]

Give up you are wasting your time. 6 volts on a 12 volt battery is a boat anchor. You cannot wake the dead.

[inetdog]

I have a little $40 (12v 1A) BatteryMinder trickle charger that has the desulfation pulsing stuff going on in it. I'm using it on two 12v deep cycle batteries in parallel that I'm pretty sure went into storage without a charge and caked up. I know they haven't had much use over their life.

Anyway, I noticed something a bit ago. When the BatteryMinder is charging the battery, my meter shows about 6.5V, up from 5V a few days ago, but the interesting thing is that when I disconnect the charger, the voltage in the battery RISES to about 8V and stays there. Any idea what would cause that behavior?

I have a 60W (12v 5A) charger showing up here shortly that I'll use on it to try and heat it up a little more aggressively to melt the sulfer with a good stout charge and then I'll put the BatteryMinder back on it, but I'm trying to wrap my head around how a battery bank would show a lower voltage while being charged and then a higher voltage when disconnected.

A. You do not "melt the sulfur".
1. The temperatures involved would be far too high.
2. The problem is sulfate, not sulfur. It is next to impossible to force the sulfates back into solution once they have crystalized. In some cases you might be able to physically dislodge them.

B. With a voltage like 6 or 8 volts, the chances are that you have one or more shorted cells in addition to the sulfation. A desulfater or a charger cannot do anything to rescue that. (A demossing device might possibly help, but the odds are low.)

Unless you are doing this just for the educational aspects, trying to revive those batteries is, as Dereck said, pointless.

And for your intellectual curiosity, try this hypothesis: The voltage can rise while sitting when a cell has become reversed and is actually developing a voltage in the wrong direction during charging. Then as the voltage of that reversed cell decays you will see more voltage on the series string based on the other non-reversed cells. There are all sorts of chemical and physical oddities that can take place in a dead battery, and I have not spent a lot of time investigating them!

[samcrut]

Give up you are wasting your time. 6 volts on a 12 volt battery is a boat anchor. You cannot wake the dead.

I've seen a few people who have managed to revive from such a low state. Granted it takes weeks to do, but it's not impossible. Sure, we'll be putting new batteries in the RV to replace these, but it's not any skin off my back to leave these plugged in for a month or so and see if the pulse charging does anything for it. It was at 5v when I started and it has come up some since I plugged it in, so until it quits improving, I'll let it keep slow cooking. I don't have any major expectations. More of a scientific curiosity than anything else.

[billvon]

Anyway, I noticed something a bit ago. When the BatteryMinder is charging the battery, my meter shows about 6.5V, up from 5V a few days ago, but the interesting thing is that when I disconnect the charger, the voltage in the battery RISES to about 8V and stays there. Any idea what would cause that behavior?

Soft short on the battery charger. My guess would be:

1) Battery discharged.
2) Cheapo battery charger connected. Overcurrent ensues.
3) Battery charger output stage blows out. Now shows odd behavior, often sucking power from the battery backwards through diodes.

This shouldn't happen, of course, but some cheaper battery chargers have lousy output stages.

Second, less likely option:

1) Battery discharged.
1) Smart (6/12/24V) battery charger connected. Charger decides that the battery is 6V due to low voltage. Charges to only 6.5-7V.

[samcrut]

A. You do not "melt the sulfur".
1. The temperatures involved would be far too high.

Yeah, I know that. I meant "to reintegrate the sulfate into the electrolyte." More electricity, more electrons chipping away at the crystals, one atom at a time. I'm sure a little heat in there wouldn't hurt chemical reaction though, but no, I wasn't expecting to blowtorch the sulfate off the plates. =)

2. The problem is sulfate, not sulfur. It is next to impossible to force the sulfates back into solution once they have crystalized. In some cases you might be able to physically dislodge them.

I like to do as many next to impossible things as possible.

There is sulfer in sulfate, and technically it's lead sulfate we're dealing with (PbSO4), not free-standing sulfate (SO4) as I understand it, but everybody ignores the lead and abbreviates it. I was just ignoring the oxygen too.

B. With a voltage like 6 or 8 volts, the chances are that you have one or more shorted cells in addition to the sulfation. A desulfater or a charger cannot do anything to rescue that. (A demossing device might possibly help, but the odds are low.)

Ooh! Demossing? What would that be? I haven't heard of that one. A new toy?

I know my odds are low, but if it works, I get the batteries back. If it doesn't, it doesn't.

Unless you are doing this just for the educational aspects, trying to revive those batteries is, as Dereck said, pointless.

DING DING DING! Bingo. This is just for scientific educational experimentation. Seeing if I can make garbage useful.

[samcrut]

Charger decides that the battery is 6V due to low voltage. Charges to only 6.5-7V.

Ooh. I hadn't thought of that possibility.

[inetdog]

I was just ignoring the oxygen too.

Ooh! Demossing? What would that be? I haven't heard of that one. A new toy?

Ignoring the oxygen is fine except for breathing.

Three things that can short out a cell internally are:

1. Mossing, which is most often used to describe the accumulation of floating crud at the top of the electrolyte which eventually bridges across between the plates in a cell, but is also used occasionally as a synonym for dendrite growth.
2. Dendrite growth, which is a network of hair-like or snow-flake like (choose your analogy) thin bits of pure lead which grow during extended low rate charging. Eventually they can poke holes in the separator and short out the cell.
3. Stuff falling off the plates and piling up on the bottom till it reaches the plates. Some disassembly required.....

From what I have seen from the battery fan(atic)s' posts, the dendrite growth is minimized by throwing in an occasional high current discharge pulse during rejuvenation. That really does burn out the dendrites. In some cases even after the dendrites short out the cell you can melt them back by a very high charging current applied just to that cell (cutting open the top of the battery to get access to the internal busbars between cells.)

Battery fan(atic)s are a fascinating group of people and you will find more specific advice, circuitry, etc on their forums than you will here.

The crud on top is another question. Avoiding it in the first place seems to be the preferred strategy.

[PNjunction]

... I don't have any major expectations. More of a scientific curiosity than anything else.

What you might expect is that even if you can wake the dead, it won't carry any sort of load at all other than a voltmeter. Ie, what was once an abused 100ah battery is magically transformed into a 1ah battery after resurrection. Good physical exercise though.

[samcrut]

Well I'll be damned. I got my Battery Tender in the mail today. That's the stronger 5A charger I was talking about earlier. So I plugged it in and since it uses the same quick connect cable type as the BatteryMinder Plus I've been using I just popped the rings that were already on the battery into the cable and the charge light didn't come on. Hrrm? I grabbed my volt meter. Now granted, this isn't a legit reading since it was connected to the charger, but that SOB came up TWELVE!!! I disconnected the charger cable and it's since dropped a little to 11.5v, but considering it was 5v when I started out a couple of weeks ago.... I REALLY didn't expect that AT ALL. I was hoping for 7v, but honestly expected it to still be at 6.5v. It's 2 batteries in parallel, so I unbolted the bridge cables and got 11.5v on each. I'm psyched!

Alive. ALIVE. IT'SSS ALIIIIIIIIVE!!!

Granted, you could be right about the capacity being compromised. I have to do some testing on them to see how they hold up, but still, you have to admit, this is pretty cool. From now on, any time someone says "Awww, it's 6v. It's dead. Throw this one away and get a new one." I'll be saying "Can I have that?" =)

I'll keep the BatteryMinder Jr on the bank for a while and let it keep reconditioning them. They're 115AH EverStart 27DC-6 batteries so even if they are compromised somewhat, there's lots of potential in there.

I know I got real lucky here, but I know these batteries haven't been through a bunch of charge cycles over the 2+ years they've been in the RV. They've been very lightly used and the main issue is that they weren't charged up when they got put away last time. It was an ideal scenario for a desulfator to come through. If they do hold a charge, I'll be getting a solar panel to keep charging them and then run an inverter off of them to make a solar powered UPS for my computer.

If they do come back with solid amp hours, that $43 BatteryMinder Jr just paid for itself 4-5x over. WOOHOO! NEVER SURRENDER!

[inetdog]

If they do come back with solid amp hours, that $43 BatteryMinder Jr just paid for itself 4-5x over. WOOHOO! NEVER SURRENDER!

Don't count your chickens before they hatch! What you have is a gratifying intermediate result. Your real time to celebrate (or even declare your BatteryMinder paid off) will be when you are able to do the load tests. I wish you continued success!

[samcrut]

Don't count your chickens before they hatch! What you have is a gratifying intermediate result. Your real time to celebrate (or even declare your BatteryMinder paid off) will be when you are able to do the load tests. I wish you continued success!

Absolutely, but it's at least gotten out of the realm of utter hopelessness.

I connected a little air compressor to the bank to load test it a little and it dropped off pretty quickly as I anticipated, but it did run for about a minute before it started to slow down. That ain't nuthin. "Something" is way more than I had a few days ago. Those compressors run about 15A@12v.

[inetdog]

Absolutely, but it's at least gotten out of the realm of utter hopelessness.

I connected a little air compressor to the bank to load test it a little and it dropped off pretty quickly as I anticipated, but it did run for about a minute before it started to slow down. That ain't nuthin. "Something" is way more than I had a few days ago. Those compressors run about 15A@12v.

Definitely. Now 1 minute at 15 amps is a whopping 1/4 AH. But definitely a lot better than the batteries could do when you started!

If you can figure out a way to measure the individual cell voltages, it will tell you whether the current is dropping off as the result of a single low cell or all of them together. If it is a single cell, you can either give up immediately or try a strong equalizing charge for awhile (after more conditioning, not immediately.)

I can't recommend it unconditionally, but I have experimented with measuring the partial cell voltages by connecting one meter lead to a stainless steel rod which I dip into the battery acid through the filler hole (don't let it touch the plates!) and other lead to either the + or - battery post. One obvious drawback is that you are, in effect, measuring half a cell, one and one-half cells, etc. so you have to massage the numbers a bit to make sense of them. But they do give you a good indication of a single really bad cell. And it does not involve drilling holes in the battery top to get access to the inter-cell buses.

[samcrut]

I'm hitting it with the higher amp Battery Tender tonight to see how far that will take it. I also have a hydrometer being shipped to me from Amazon so I can test each cell based on the acid content, but that won't be here for another week or so.

The cell readings are pretty strong, 1.6-2.0v between each cell, with the exception of the NEG terminal to the first cell on both batteries. Those 2 places are reading under .5v. Pretty sure that's not good, but I'm not giving up until I've let that BatteryMinder Jr work them over for several weeks, maybe a month.

Readings:

Battery 1
+ POS 1.7 1.9 2.0 2.0 2.0 1.6 .44 NEG -

Battery 2
+ POS 1.7 2.0 2.0 2.0 2.0 2.0 .39 NEG -

[inetdog]

...with the exception of the NEG terminal to the first cell on both batteries. Those 2 places are reading under .5v. Pretty sure that's not good, but I'm not giving up until I've let that BatteryMinder Jr work them over for several weeks, maybe a month.

Readings:

Battery 1
+ POS 1.7 1.9 2.0 2.0 2.0 1.6 .44 NEG -

Battery 2
+ POS 1.7 2.0 2.0 2.0 2.0 2.0 .39 NEG -

That is the part I mentioned in passing as needing some adjustment. Notice that you have 7 voltage readings and only 6 actual cells in the battery.
The voltage between electrolyte dips is composed of the voltage of one "side" of one cell and the voltage on the other "side" of the adjacent cell. The sum of the two half-cell voltages at either end is roughly equal to the voltage of one cell, but it is made up of components from both end cells.

That is also why I said it should only be used as a rough indication of a really bad or shorted cell. If two adjacent voltage readings are low compared to the others, that indicates that the cell which contributes to both of those readings is bad. The only reading in your list that suggests a problem to me is the 1.6 volts on the second reading from the right in Battery 1. And it suggests that the - end cell in that battery is weaker or still at a lower state of charge than all of the other cells in the two batteries. When you get a hydrometer or refractometer to measure the electrolyte, you may notice a difference in that cell too.

A reading set like this:

1.7 2.0 2.0 1.7 .40 2.0 .40

indicates that the second cell from the right is shorted, since the readings on both sides of it are low.