Odyssey AGM and solar experience?

[inetdog]

.... Constant Current and Constant Voltage charging. Fact is they are both very similar, only difference is the voltage set points.

So for a properly configured charging system of either kind, the end voltage / Float voltage will be exactly the same (for a particular battery type)?
How does the fact that solar PV is only available for a limited number of hours at a time come into the picture?
What happens if you are still in the first Stage when the insolation falls off? (With or without additional load on the battery before the next day comes around?)

[Sunking]

So for a properly configured charging system of either kind, the end voltage / Float voltage will be exactly the same (for a particular battery type)?

Exactly the same.

How does the fact that solar PV is only available for a limited number of hours at a time come into the picture?

Solar power is extremely extremely limited and you need the fastest charging algorithm to make the best use of the short time you have in a day. Time is not a problem from commercial power sources.

What happens if you are still in the first Stage when the insolation falls off? (With or without additional load on the battery before the next day comes around?)

You know the answer to that. Your battery sits less tan 100% charged up which is not a good thing.

[PNjunction]

That makes it much clearer to me now. Thanks!

Batteries are gulping down 8amps solar as we speak.

While freaking out, I ran across an interesting page referencing Linden's Battery-Bible, (which was graciously pointed to in another thread on SPT - thanks again Sunking!) in regards to fast charging sealed VRLA's, that kind of put my mind at ease - basically stating that a properly functioning charger that doesn't go haywire with super high voltages should be safe:

http://www.powerstream.com/SLA-fast-charge.htm

Believe me, I'm not trying to sand-bag any thread here with later web cross-references!

Before any reader jumps the gun, I do NOT advocate going beyond a manufacturer's specified ratings.

[inetdog]

That makes it much clearer to me now. Thanks!

Batteries are gulping down 8amps solar as we speak.

While freaking out, I ran across an interesting page referencing Linden's Battery-Bible, (which was graciously pointed to in another thread on SPT - thanks again Sunking!) in regards to fast charging sealed VRLA's, that kind of put my mind at ease - basically stating that a properly functioning charger that doesn't go haywire with super high voltages should be safe:

http://www.powerstream.com/SLA-fast-charge.htm

Believe me, I'm not trying to sand-bag any thread here with later web cross-references!

Before any reader jumps the gun, I do NOT advocate going beyond a manufacturer's specified ratings.

Warning: That article does not mention the importance of monitoring the battery temperature during any fast charging process.

1. The desired fixed voltage, if you use one with a current-limited charger, is dependent on the battery temperature. Very hot or very cold batteries require different (temperature compensated) voltage settings.
2. If you do not monitor the battery temperature, you risk thermal runaway, in which the internal battery voltage goes down as temperature increases and therefore either the current increases or you continue charging after you reach full charge (which heats the battery even faster and causes it to vent and expire.

For fast charging (above the guaranteed-safe-reabsorbtion-rate current) you have to monitor not just the ambient temperature as most controllers do, but the temperature of the batteries themselves using a remote temperature probe attached to the charger.

If it is a series string, monitor the battery which has the least cooling, but also check the others periodically. Do not leave any battery on fast charge (above the Manufacturer's limit, or even at it the first time through) unattended.

"Stay cool, my friends!"

[PNjunction]

Whoa! Thanks inetdog - I overlooked that.

In fact, I'm off to get some temp probes and perhaps an IR meter right now! You just saved my butt, and most likely my house.

[russ]

Whoa! Thanks inetdog - I overlooked that.

In fact, I'm off to get some temp probes and perhaps an IR meter right now! You just saved my butt, and most likely my house.

The IR meter is not terribly accurate - it can be go - no go or is it getting hotter but to get a precise correct temp isn't easy.

[PNjunction]

Thanks Russ - good to know about the relative IR inaccuracy. I lost the temp probes for my Fluke 87V, so I picked up an Extech EX470 multimeter from RS on short notice that has IR and a type-k thermocouple. I won't be taking it into my work lab - but it beats a shirt-pocket model that's for sure.

To let you know where I'm going with this - The Odyssey's are doing fine so far with at least .4C or more - no sweat. But now I'm going to experiment with the typical Powersonic ups-style agm's, and see if I can't take them up to .4C or maybe even .5C at the most without blowing vents or going into thermal runaway. Got the temp meters and the facemasks ready. Powersonic says NO to anything above .3C at any phase of the charge cycle, but I've got to find out first hand I guess. SAFELY that is.

[russ]

Comments within the text in bold

V I picked up an Extech EX470 multimeter that has IR and a type-k thermocouple. I bought a cheap Extech MiniTec 26 - it is a couple of degrees C off. It is repeatable so no problem for me but I suggest you check your against a known temperature. Type K is perfectly suitable.

To let you know where I'm going with this - The Odyssey's are doing fine so far with at least .4C or more - no sweat. But now I'm going to experiment with the typical Powersonic ups-style agm's, and see if I can't take them up to .4C or maybe even .5C at the most without blowing vents or going into thermal runaway. Got the temp meters and the facemasks ready. Powersonic says NO to anything above .3C at any phase of the charge cycle, but I've got to find out first hand I guess. SAFELY that is.

[inetdog]

One thing that I also take issue with is the statement that no oxygen or hydrogen are being produced until the cell voltage and SOC reach some hard and fast limit. This is not necessarily the case, since IMHO irregularities in the electrodes could still result in some localized electrolysis even though the cell is not yet gassing violently. That should be handled by the recombination catalyst, but it still will not result in a zero pressure on the vent.

Comments from the experts?

[Sunking]

Comments from the experts?

You have to reach the Gassing Voltage of about 2.39 volts per cell before any gassing occurs. This is why voltage regulation is mission critical when charging VRLA battery types. This is why monitoring the temperature of VRLA batteries is important to monitor to prevent thermal runaway. In the event a cell fails shorted (common failure mode), the voltage drops and fools the charger into thinking the battery needs more charge current and this causes the remaining individual cell voltages to rise above the gassing point and where heat begins to build up causing the voltage to drop even lower which causes even more current and heat. The effect is regenerative feedback and called Thermal Runaway. If left unchecked can cause a violent explosion and fire.

Thermal runaway is not as serious of an issue with flooded lead acid batteries because of the large thermal mass encountered in FLA batteries. Nor can a FLA explode from over pressure.

[PNjunction]

Ok, guess I'm drifting away from Odyssey's, but did a test today on a general purpose Powersonic 7ah agm with currents larger than the common 0.3C to check for heating and/or gassing. Objective was to take it to 0.5C, which was met, and test was a success - although this is just one time, and long-term results are yet to be seen.

Test subject: Powersonic PS-1270 (12v, 7ah) agm. Discharged to 50% DOD as per the "watts up" meter and verified again after 12 hour rest. Known-good unit - not pulled from a shed 10 years ago powering a weedwacker and rejuvenated with a 50ah wheeled charger. I treated this new battery nicely for about 50 cycles.

Test Gear: Fluke 87V and Extech 470 multimeters with IR and type-K thermocoupler.
(Yep Russ, the IR consistently reads about 5 degres higher than the thermocouple. Also why didn't I listen to Sunking and get a real shunt earlier?? )

Panel: 60 watt , 12v nominal

Charge Controller: Morningstar Sunsaver 6 PWM, unjumpered for 14.4v. Ambient temp compensation only. 12-foot round trip #18 gauge from CC to panel. 3-feet round trip #18 from CC to battery.

Environment - 85 to 105 degrees fahrenheit from start to finish of test. All gear outside and in the shade, except for panel. Panels ramped up to 160F pretty quickly. No clouds, and panel ground mounted, but angled towards sun a few times. Body temp at wrist 95F. Felt like being on the surface of Venus.

Results: (note that the battery temperature never rose more than 5 degrees above ambient - even at the 0.5C rate, so I'll leave ambient temp out of the table.

Time | Charge current | Terminal voltage | Battery temp |

1045am | 3.45A | 13.46V | 85F
1100am | 3.38A | 13.72V | 85F
1115am | 2.50A | 14.00V | 90F
1130am | 1.68A | 14.12V | 91F (this has just crossed below the recommended 0.3C rate)
1145am | 1.05A | 14.20V | 93F
1200pm | 0.78A | 14.22V | 95F
1215pm | 0.64A | 14.23V | 95F
1230pm | 0.49A | 14.18V | 97F
1245pm | 0.40A | 14.18V | 99F
1300pm | 0.35A | 14.13V | 101F
1315pm | 0.30A | 14.08V | 105F
1330pm | 0.29A | 14.09V | 105F

and so forth climbing it's way down until I ran out of good insolation.

The temp probe and the IR of the Extech was fun to use. I searched all over to see if there were any hot spots developing (front, back, sides, rear, bottom). There were up to maybe 2 degree differences in temp depending on where I measured, the hottest being near the terminals, and generally the top. 2-3 degrees difference maybe all around with IR.

The battery didn't immediately complain and seemed not to notice that it had been over 0.3C for about 45 minutes. No hissing, gassing, whining or tantrum thrown!

Thing is, this is a one-test shot, so again long-term, I don't know. But now that I'm armed with some temp probes, I plan on testing further at the 0.5C rate with my larger powersonics.

[Sunking]

The battery didn't immediately complain and seemed not to notice that it had been over 0.3C for about 45 minutes. No hissing, gassing, whining or tantrum thrown!

You never reached the gassing voltage.

[PNjunction]

GREAT point which I didn't catch!

Btw, the 7ah test battery is holding at 13.21 volts SOC after a 9 hour rest. Doesn't say much until I load test it, but it is a data point.

You mean 2.45v/cell, or 14.7 volts right? Unfortunately, my charge controllers only go as high as 14.4v. Aside from the safety issue not being fully tested, at this point I'll just be walking-down my capacity it looks like.

However, I will rerun the test later this week with a 3.5a NOCO charger that does go to 14.7! Again this will be done with safety first in mind. Maybe I'll just do the NOCO on it monthly if the safety test with it passes.

Edit: YIKES - you just made me realize I'm not reaching gassing voltage with solar on my expensive Odysseys either! Guess I shouldn't quit my day-job huh?

[Sunking]

You mean 2.45v/cell, or 14.7 volts right?

No Sir 2.38 to 2.40 vpc. Set your charger to Absorb 14.4 and Float @ 13.6

[PNjunction]

Right on about the 14.4 absorb and 13.6 float. Although I think my CC's temperature compensation is playing into that at nearly 100F. (I really need to go with remote temp sense to do it right rather than just ambient).

Maybe I've spent too many hours in the sun testing yesterday - my brain feels like lead-sulfate now. I think I see what that guy is pointing out and why a single word of "overcharge" aka absorb is significant to me here:

During the bulk phase, I'm well below the absorb voltage setpoint - in practice, I'm in the "float" voltage arena during the bulk process, where larger amounts of current beyond the common 0.3C shouldn't be a huge issue. By the time the battery reaches the fixed absorb setpoint where the CC is now in control, I'm already well below 0.3C. So two controllers are in charge here - in bulk the battery runs the show until it reaches absorb, then the CC takes over signaled by reaching 14.4v (or whatever you have set it to). The CC is now overcharging the battery on purpose, but nowhere near the current levels seen in bulk.

Then again, maybe that's in a perfect world? Poor quality manufacturing may be the limiting factor to 0.3C ? (ie local hot-spots from manufacturing abnormalities acting as a thermal runaway trigger, and not so much from a pure engineering standpoint) ??

I'm not trying to prove a point, just trying to um, absorb it all in my head if you will.