Renewable Energy Questions/Discussion > Renewable Energy Q&A

Dead AGM Batteries

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MadScientist267:
Sounds like the meter definitely took a dump and that certainly explains that end of it.

Battery voltage "normal" under no load but unable to supply current is symptomatic of a damaged cell in the battery somewhere if all of the connections and wiring check out. More than likely what you'll find is that under load, one will still be trying to hold it's end up, while the other one goes silly, possibly even completely "mirroring" the good one with an inverse voltage across it's terminals.

I wouldn't use the electronics present to try and test for these conditions because the unpredictability could cause damage to the electronics themselves.

Rather, disconnect everything from both batteries and give them a simple moderate load such as a headlight across each one independently and see if it can hold it up, and what the terminal voltage is while it's doing so. The one with the issue should reveal itself by not providing enough power (or any at all from the sound of it) to light the headlight up, and the meter will confirm the degree of the sag.

On to the cause from how I perceive it...

I think they serve a rough life as 75% down is pretty significant for lead acid, and while there are many factors that contribute to cell damage in batteries, they're not all "equal". You're in a portable environment there for one. Discharged cells are more fragile and while AGM is physically more rigid in construction, they are still susceptible to vibration and shock... The lower the SoC, the more pronounced this is. There's a good chance there's a crack in the internal electrical structure somewhere in a cell, which because they're all in series, the limitations of the weakest cell affects the entire battery.

I screwed up somehow with the math on the theoretical "2C charging rate"... Crossed the streams... Lol I was tired when I was writing that... But you're still looking at C/5... and this is still another strong potential factor because it exacerbates even minor differences in cells (unavoidable), which leads to runaway imbalance. Weaker cells will reach both charged and discharged states sooner than the stronger ones, which ages them quicker yet because they spend more time at both ends. Sealed cells are particularly vulnerable to overcharge as excess gassing causes pressure to build, and even tho some have catalytic recombination devices in them (to turn hydrogen and oxygen back into water), these can easily be overwhelmed by excessive rate... And unlike in a flooded cell, once it's lost, the water is gone forever.

Extreme rates also can cause warping and buckling of plates due to expansion from hot spots that form... Not only leading to cracking but even internal shorts... I probably don't need to explain where that can lead...

However since you asked, yes an "event" would refer to, worst case, catastrophic failure of a cell... Explosion, etc... Best case, internal damage resulting in a shorted cell that essentially is just "bypassing" itself... And anything in between. In any case, the early demise of the cell in question, and by extension, the battery it's part of.

Charge rates need to be limited to allow the chemical reactions in the cells to be able to keep up with the current flow causing them to happen. This is what the "absorb" stage is about in charge controllers. The current is allowed to taper off while the voltage is held steady because as the cells reach full, the number of places these reactions can occur become less and less, and the places where they've taken place in full, the energy gets expended splitting water, making heat, and causing other unwanted reactions that slowly but permanently degrade [namely positive] plate material.

Typically, lead acid is designed for C/10 charging rates, and while some excursion above this can be tolerated, and in some cases even designed in for a specific battery's purpose, higher than this is generally not a good thing.

So while you're not looking at 2C (which would be *20 times* this "standard rate"!), C/5 is still very high to be using regularly. If the controller has means to limit (it may not, being only PWM), you may want to consider dialing it back some to limit those peaks. Unfortunately, this is of course a double sided coin... The controller doesn't know the difference between the battery and an ongoing external load... So its a balance. With the whole "2C" thing behind us, which I again apologize for... Sigh... You may be able to just "ignore" that for your use case. Just keep it in mind for general purposes.

Discharge rates are a little different because the source of the energy is the battery itself and there's a degree of self limiting that occurs, but it still can be problematic. Depending on the physical construction of the plates and cells, the rate is limited by some intentional combination of the plates and the electrolyte, with the battery's intended purpose in mind. Also, during discharge, gassing is *much* less a factor and so more of the plate area stays in contact with the electrolyte, helping reduce localized hot spots by way of improved cooling. The reaction during discharge is also endothermic (you'll never notice the cooling effect however because ohmic heating overtakes it).

In a battery that's meant to crank an engine, capacity limitation is imposed by the plates, with an "excess of acid" (relative to plate mass), as they're designed to deliver a lot of current for a short period and then be immediately recharged.

Deep cycle has much more plate than acid relative to a cranking battery but with a lower instantaneous surface area in contact with the electrolyte, since peak current delivery isn't as important as the ability to deliver power over long periods of time. This reduction in active area increases the internal resistance of the cells however, so when an attempt to draw large amounts of current from a deep cycle cell takes place (relative to the cell capacity), it generates more heat, which just as in charging, can result in plate warping and other identical threats that too high a rate can cause there.

The so called "Marine/RV" batteries are a hybrid between these two extremes, and for anything outside of small light loads (again relative to the capacity) for comparatively short periods, and starting smaller engines (outboards, generators etc), they're generally considered "not great at either one". They can't deliver quite the punch that a "true" cranking battery can for rolling an engine over in the cold, and they don't stand up to the deeper cycling as well as a true deep cycle can either. This doesn't mean they don't have their uses, they do, but it's a bit more limited than it would appear, and so they endure a little more abuse overall by being selected for jobs they're not particularly suited for. The price tag is usually the alluring factor...

Hope this helps some... I know it was a little drawn out, but figured I'd go ahead and cover all the bases on this one. These "little black boxes" we all rely on are a lot more complicated than they'd appear on the surface, unfortunately.

One other thing comes to mind with the advent of the pics... That's a pretty simple and clean layout... But other than the jumper, the wire sizes there look a little anemic for the loads you're saying are in use... May want to look into beefing them up a little... To reduce losses even if they're holding up safety wise.

Steve

lighthunter:
Defenitely an interesting problem you have. If the solution wasnt found in a bad connection, it is possible for an AGM to have a severed connection inside the battery case. It actually happens quite a bit with the jump starter packs. They demand so much current from a tiny battery that something gives. Ive never taken one apart but its possible they design them with a one-time fusible link of sorts or something just goes wrong with the connection, im not sure. One way to make trblshoot easy, use 4 alligator clip type jumper wires and attach a tiny (automotive bulb) across each agm. This will give you a voltage indication of each battery under load. Let us know how it goes, sounds fun!  :)

BTW, i like your setup, good idea! I may have to do that for the lighter work, take some hours off the stihl and easier on my body than limb work with an ms391.

OTG:
Thanks MS267,

With my head screwed back on (I had "$#!+-to-do" that weekend and the compounding failures honestly threw me a bit off-kilter - I also lost my Stihl air filter, so I had to improvise with a rag, a real PITA morning...) I did some more comprehensive testing last weekend and the picture is much clearer now...

The meter is basically cactus - V readings are wonk... sometimes right (especially at lower V like AA batteries, or when flicking between Auto/Manual ranges) but mostly wrong (and all over the place); Cap & Temp readings are bung (all 0.00); but A and Resistance are OK (hooray). Pity... it's only 4 years old and has been a relative "Garage Queen" of a meter. Spose I'll just use it for cl-Amp meter duties now...

I tested each battery with a small 12V 1.5A Flood Light, and as you guessed again (do you play a lot of Charades or something!? ;) ) one battery was fine under load, the other's V went from a steady 13V, to erratic/constantly changing readings (as low as a few V, high as about 80V and everything in-between) with the light constantly dimming in and out. So I have a buggered battery - what do you think these symptoms indicate - damaged cell or two?

I hooked up some car batteries to test the CC and Panels (individually & together) and the V/A of these components all look ok - so I don't think they're part of the problem.

And yep, fair cop on the DoD point - the (silly) reason for it is the CC gives an audible alarm at 75%/50%/25% (fixed/not user-programmable), so it was just an easy marker to tool-down on the bigger jobs/with longer use. Low SoC/vibrations & shock could well be a factor too, as the pattern of use is basically drive somewhere, do the work, drive back - often via a bumpy track (not to mention, the old 4M40 Diesel comes to a pretty rough stop when you cut the ignition :P ). A definite lesson learned and I'll be changing my max-DoD behaviours. The terrain I can't do much about, I'll just keep taking it easy on the rough stuff.

Thanks for clearing up the charge rates (no biggie!!!  :D ) and the detailed explainer was definitely of help. Indeed, as "Marine/RV" hybrids I'll be more mindful of their limitations in future.

RE the wiring - it's standard 4mm Solar Power Cable (60A Max) and 0 Gauge Power Cable (proper tinned stuff - 300A Max) - so definitely safe/within-spec but to your point I'll throw in a second run of power cable in-between the batteries to further support the heavier loads.

Indeed, these "little black boxes" do just-work (the good kind :-) ) "like magic", and so we usually take them for granted - particularly the less electrickerally minded folks like myself.

Thanks again MS267!

OTG:
And thanks guys, I've been pretty happy with the "Solar Truck" so far (minus this battery hiccup).

I've got a bit of tough ground to cover (basalt country) and lots to do (fencing, remote infrastructure, clearing weeds (big cactus/lantana), more fencing, etc). I didn't want to lug around a genny and I hate having them constantly running for intermittent work (e.g. welding/jackhammering), and so the Solar Truck was born.

It's basically an old Meter Box, with 40mm Angle Bar welded on each side for mounting over the rear-view window (it was broken so I replaced with a flat sheet of colorbond), with an MDF offcut bolted inside holding the CC, Array/Battery CB's, and DC Loads fuses/bus bars for the Cigarrette Lighter outlet on the side (I use this for worklights now and then, charging phones, etc...). I used 50A Anderson plugs to terminate the battery/solar leads for easy removal/swapping of bits if need be (this has actually come in REAL handy at times - using additional leads, I can charge car batteries off the array and I also used the array for a while to run my Solar Bore Pump while I was still getting it's permanent panel in place (we needed the water, damn drought!)). The Dual Battery box is just some 75x75 4mm (I think) SHS cut up, glued together, and bolted to the tray. The array is mounted on 25mm SHS/40mm Angle bar, cut/glued flush together to easily slide the panels on from the back. Cables enter the tray via the ABS Cable Entry Glands. The inverter I keep in the shed and it sit's on the breadcrate when in use, held in place by the bit of broken tie-down strap (I was going to make a mount but haven't got around to it yet). I had cut the head of a SS bolt, and used two seperate nuts for the Solar/Inverter Cables, so I could easily take off the inverter without touching the solar terminations. That's a longwinded brain dump, but that's about it...

Other then some Sikaflex, the Cable Gland, and the Electrickery components, the rest was cobbled together from offcuts and freebies - so it worked out pretty cheap in the end.

My main concern was panel damage (e.g. cell cracks, interconnect breakages, etc...) but so far so good. Also the truck lives outside, so I'm mindful to park at an angle for water run-off on the panels and when using it, I also park for maximum solar gain (especially considering the panels lay flat).

If I could turn back time and magic up an old trailer frame I might have rigged it all up on that, which would free the truck up for less 'considerate' driving. But I can't do either and so Solar Truck it is... which really has been invaluable so far and allowed me to get lots done, genny/dinosaur juice free (not counting the truck itself)! Also, as good as the MS250 is, I very much loathe the 2 Stroke fumes, not to mention fueling it up and all for mostly small jobs. I've very much enjoyed using the corded electric one and not smelling like an old lawn-mower at the end of it. Plus it's much easier to swing around. And your 391's probably twice the chainsaw mine is lighthunter - I can imagine ti would be a real lump for limb work!  :D

Also I'm not sure about the battery one-time fusible links... if anybody else out there knows It'd be interesting to hear.

Otherwise, I'll be grabbing new batteries shortly and will have the truck back to work in no time!

MadScientist267:
It's a nice looking setup OTG... And would seem to do the job for the most part exactly as you need it to, which is what matters the most.

As for the charades... Haha Naa. Just had and otherwise seen my fair share of battery failures over time, not the least of which was my rig recently. There's a bunch of things that pull in polar opposite directions when the mobile thing gets involved... And it becomes a "best balance" situation to try and keep everything happy.

Batteries being what they are, heart of any system and finicky about what they need... Tend to take a lot of hits and sometimes there's just a "to be expected" factor. Rough and tumble environment is certainly one contributor.

The panels on the other hand, they're surprisingly robust in that service ain't they? Lol I remember thinking I needed to protect mine in all kinds of ways and feared for quite a while that they'd suffer damage from this and that... But while I wouldn't go out of my way to abuse them, they do stand up to quite a bit.

Out of curiosity, what mounting scheme did you use for them? I only ask because I chose a "floating" system and just kinda wanted to compare notes. Simple as it turned out, it was one of the most thought provoking aspects of my entire design and I was always curious what others come up with for the unique aspects that mobile brings to the table.

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