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Old 01-17-2021, 05:11 PM   #1
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Battery Monitor Pitfalls (or, don't be lazy like me.)

Just throwing this out there for people who install a battery monitor and then get lazy because it seems to be so great.

I have a Victron BMV-712. Love it. Makes tending to the batteries simpler and more reliable. But . . .

I don't ever plug in. I'm either charging on solar panels, running my generator (hardly ever), or charging as I go down the road.

But I'm set up mostly for spring/summer/fall RVing. 400 watts of panels that don't tilt - which works well except when the sun angle drops too far - like in the winter months, when I'm getting maybe 150 watts from my panels at peak time.

Here in Arizona for the past month, with nights in the 30's, I'm not getting enough sun, plus I'm running my furnace blower a lot. Gives me an energy deficit, and so I have been running the generator every three days or so for a couple of hours.

So I watch my BMV, and run the generator until a bit past where it tells me "100% SOC." And I've started having strange problems with electric over the past week. Had an issue just last week where the genny died because the chassis battery was drained, which shouldn't happen.

So I happen to be watching my BMV the other day as the genny was running, and it said "-20 aH", then "-19 aH", and then - blip! - "0.00 aH", along with "100% SOC". It jumped from 18 amp-hours deficit to completely charged.

It synchronized itself 18 amp-hours early. So, I've been charging up to a state of charge that has been ratcheting down in actual charge every day, until I finally started seeing issues from low charge.

Now I'm reading up on "charged voltage" and tail times to see if I can't make this go away. I got lazy - I was depending on the BMV to tell me when I was charged, without doing any other checking, and it bit me.

So now I wait for an hour after turning off the generator, and check the battery voltage again - the step I was skipping - and invariably end up running the generator for an hour more after it initially told me my batteries were charged. It then re-synchs at the higher charge.

SO, this long tale is just to make people aware of the synching issue with BMV's. Hope it helps someone.
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Old 01-18-2021, 03:30 PM   #2
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You do not mention what type and how many AH of batteries you have
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Old 01-18-2021, 04:30 PM   #3
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I sync my batteries about once a month to account for the gradual drift in SOC display.
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Old 01-18-2021, 07:12 PM   #4
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You do not mention what type and how many AH of batteries you have
Sorry. I have two of the Renogy type 8d AGM batteries - the big heavy ones - 200aH each.
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Old 01-21-2021, 01:19 AM   #5
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It synchronized itself 18 amp-hours early. So, I've been charging up to a state of charge that has been ratcheting down in actual charge every day, until I finally started seeing issues from low charge.

astrocamper nailed it. To amplify a bit ... it sounds like you are not "synchronizing" the BMV often enough. Read section 2.2 on page 10 of your manual. It is essential to fully top off the battery frequently to keep the BMV in sync with the actual SOC. This means take the battery up to 14.4V and through a complete absorb stage. You will see the BMV hit 100% before the battery is fully charged. The BMV will then sit at 100% while the battery continues to charge. Once the battery is fully charged, the BMV counts down from 100% as the battery charge drops from 100% during use (discharge).

The reason this is necessary is because the BMV has to estimate the Ah used internally in the battery (as heat) and that estimation is rough at best.

There are some BMV settings that default to values that may not be best for your batteries. These are the settings that the BMV uses to estimate the Ah used internally from what it sees at the battery terminals. One is the Peukert Exponent and the other is the Charge Efficiency. Ideally you will get these from the battery manufacturer and enter them into the BMV. You will still want to synchronize once a week or more often, but will accumulate less error between top-offs.
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Old 01-21-2021, 07:58 AM   #6
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The underlying problem, for me, is that I use solar or the generator, and never ever plug in, and so it's a rare day (now, in the winter) when I can get my batteries "fully charged." Running the genny that extra five or six hours to get the last of the float charge packed in just doesn't happen.

And so I can synch daily, but I can't synch accurately. That's where the ratcheting comes into play.
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Old 01-21-2021, 09:33 AM   #7
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With my LiFePO4 batteries my drift is in the opposite direction. I hadn't synced since my last camping trip in October. I did a sync last week when I could see from the MPPT charge controllers all going to float that my batteries where full when the BMV had them at 82%. I switched off the inverter load for a couple of hours and let controllers sit at float and then did a sync.
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Old 01-21-2021, 01:47 PM   #8
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The underlying problem, for me, is that I use solar or the generator, and never ever plug in, and so it's a rare day (now, in the winter) when I can get my batteries "fully charged." Running the genny that extra five or six hours to get the last of the float charge packed in just doesn't happen.

And so I can synch daily, but I can't synch accurately. That's where the ratcheting comes into play.
Agree. Been there, done that. This problem is maybe the main reason that I switched to LiFePo4.

It's often not recognized, even in the how to books on solar, that lead acid batteries require about half again more solar than LiFePo4 for just the reasons you point out.

Here's a trick .... if you anticipate that the solar is not going to top off your batteries by late afternoon (i.e., you start the day down more than about 25%), run the generator first thing in the morning. Your converter should start in bulk mode and as such will deliver it's full rating to the batteries potentially getting you through the bulk stage in an hour or two. You then have most of the solar day, about six hours, to get you through the absorb stage. By late afternoon the solar may only be producing a few amps, but at the tail of the absorb stage, that's all you need.
The solar won't be well utilized after the first hour or so of absorb, but at least it's quiet and needs no babysitting.

More solar is always better, but unless you have enough solar to get your lead-acid batteries through the bulk stage before 10:00 am, it's not enough. Since you don't get all that much sun before 10:00 am, it requires an inordinate amount of solar to properly charge lead acid batteries.
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Old 01-21-2021, 02:48 PM   #9
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I don't think you need to sync daily or even reach 100% SOC each day. If you do it a couple of times a week you would be fine. I agree with the previous poster that you would be better served running you generator a few times a week in the morning to get through the bulk stage so your solar can finish it off to 100%.

Obviously you should do this on days when the forecast is for sunny weather if your goal is to reach 100% and sync.

I have seven 100 amp-hour BattleBorn batteries and my solar doesn't have issues with reaching 100% SOC.

If you have enough solar for you usage you can make lead acids work well. I ran a pair of Trojan T-105 batteries for about 10 years in my small 5x8 cargo trailer. The 240 watts of solar was enough to top them off by mid day since my overnight usage was low. I sold the trailer to a friend and the batteries are still going strong as long as the new owner keeps the water level topped off.
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Old 01-21-2021, 06:26 PM   #10
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Agree on the running-the-genny-early so that solar can fill in the absorb time.

I simply don't have enough panels (or tilt) for winter. I have no problem operating spring/summer/fall - I can hit that 100% daily. And it's not that I'm synching too often - I'm not synching, the BMV is, automatically, so the ratcheting effect is involuntary. I've now set tail current and detection time such that the BMV may well never automatically resynch - and that will work out better for me.

(Until I address the lack of panels, of course.)
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Old 01-22-2021, 02:44 PM   #11
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Since the SOC can drift off fairly quickly in lead-acid batteries, it would be good to keep an eye on voltage as backup to the SOC.

I'm not clear on your explanation of the automatic resych. The BMV resync is not a specific event. It simply stops at 100% when charging continues beyond that point.

Astro, I've not heard of the SOC reading drifting low with lead acid or LiFePo4. Makes me wonder if your BMV has a peukert constant or charge efficiency that could be set better.
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Old 01-22-2021, 02:56 PM   #12
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I'm not clear on your explanation of the automatic resych. The BMV resync is not a specific event. It simply stops at 100% when charging continues beyond that point.
Running the generator, I can sit here and watch the "aH" readout go from (say) -22.5 to -22.4 to -22.3 to . . . 0.00, in one quick blip. The BMV has synched at that point, and is telling me I have packed in all of the amp hours that I took out. SOC then reads 100%. But I'm still in absorption on the converter, and if I turn off the generator and wait an hour or so, voltage indicates that my AGMs are at 12.6, which is decidedly not fully charged. If I then run the generator another two hours, I can get them up to 12.8. If I ran the generator another four or five hours, I could get them up to 13.0, where they ought to be. Takes a lot of gas to get up there. I did that yesterday, to see what happens. Then, I resynched manually.

But my BMV was telling me that I was there hours ago. And I wasn't. I've been playing with the settings, and I've been making that amp-hour-deficit readout smaller - but it's still making a jump from some number over -10 amphours down to 0.0.
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Old 01-22-2021, 03:42 PM   #13
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Running the generator, I can sit here and watch the "aH" readout go from (say) -22.5 to -22.4 to -22.3 to . . . 0.00, in one quick blip. The BMV has synched at that point, and is telling me I have packed in all of the amp hours that I took out. SOC then reads 100%. But I'm still in absorption on the converter, and if I turn off the generator and wait an hour or so, voltage indicates that my AGMs are at 12.6, which is decidedly not fully charged. If I then run the generator another two hours, I can get them up to 12.8. If I ran the generator another four or five hours, I could get them up to 13.0, where they ought to be. Takes a lot of gas to get up there. I did that yesterday, to see what happens. Then, I resynched manually.

But my BMV was telling me that I was there hours ago. And I wasn't. I've been playing with the settings, and I've been making that amp-hour-deficit readout smaller - but it's still making a jump from some number over -10 amphours down to 0.0.
There's something wrong somewhere. Or something we are overlooking.

There should not be a sudden drop in current in absorb mode until current is down to an amp or two. I'm guessing the absorb process is being halted by a timer that switches the charger to float mode prematurely. Some three-stage converters or chargers do have timers that limit absorb time. Ideally, in absorb mode, you would watch the current drop exponentially (fast at first then slower over time) until it hits roughly an amp or two. Only then will current drop quickly as the converter switches to float mode. The drop you are seeing at higher current suggest a timer is preventing full charge. Absorb mode normally takes four to six hours.

The BMV may indeed hit 100%, but if the battery is not being fully charged due to premature switching to float, then it is not being properly reset. It's only reset properly if you get the absorb tail current down to an amp or two. \

Resetting the BMV manually does not help. You are telling the BMV to read 100% (that's all the manual reset does) when the battery is not at 100% SOC. Manual reset should only ever be used in a new installation where you know the battery is at 100% and the BMV is not reading 100%. After that, hitting manual reset can only set the BMV higher than the actual SOC.

Back to the absorb mode problem ......... If running the generator again adds more charge by staying in absorb mode for a while and to a lower absorb current, you are surely seeing an absorb timer giving you just a couple of hours before switching, again, prematurely, to float. Each time you start the charge process you get further down the absorb exponential curve. I could see your system needing three trips through absorb to get down to a couple of amps if a timer is cutting each one short.

What you should see when you apply the charger to a lead acid battery that is below about 70% charge is 1) a bulk stage where the converter/charger is at its maximum current and voltage is rising over time to 14.4V; 2) the converter/charger switching to absorb mode and holding 14.4V; and 3) when absorb "tail current" gets down to an amp or two, the voltage dropping from 14.4V to 13.6V indicating the converter has switched to float mode. If you aren't getting this full process, then you are not getting a full charge. You really need to get rid of that time. Can you access it?

Where are you seeing 13 volts as being appropriate? 12.6 volts can be full charge on an older battery. Most are 12.7, at least when new. These are no-load resting voltages. Float is around 13.6 with the converter running.

Yes, it's an industry secret that generators are terrible at charging lead-acid batteries. Nobody sits through the necessary hours and so lead-acid batteries don't last as long as they should. Timers on absorb mode add to the problem. It's just the way it is.

I hope this helps.
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Old 01-22-2021, 08:00 PM   #14
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I hope this helps.
To start: Immensely. Thank you. My basic knowledge of bulk/absorb/float and the concomitant watts/amps/volts changes is too thin, and it's hard to find good explanations by searching the net. Yours are great.

A few topics:

13.00v for AGMs as being "fully charged" - all the tables I find specific to AGMs list 13.0 as being the sign of a full charge. Before I got to Quartzsite - with my flat panels trying to see sun at 54 degrees, plus running my furnace fan during the 32 degree nights - I've always hit 13.00. That's my true signal of full charge. I simply don't have enough solar-panel power - from lack of panels to handle the furnace, plus the lack of tilt - to hit my recharge needs here in Q in winter. Need to make some additions. I hate generators.

You spoke of problems in my absorption phase - as in perhaps my converter settings need to be reviewed. I think you've hit the nub of the problem, except I'm starting to think the converter is set just fine, but my knowledge of where I am in regards to charge is deficient and not matching reality - and that the rest of my systemic deficits are combining to create this issue.

I've been chatting with one of the Victron gurus. His advice - which I'll follow tomorrow - has to do with how my too-small solar collection is figuring in to this problem.

Tomorrow, I'm going to shut off my charge controller before I start my generator. The weather here has been "partly cloudy" - which means I get full sun/shade/full sun/shade patterns all day. He thinks the sun/shade time is triggering my tail detection - voltage is dropping in response to the clouds, and the BMV takes that as a completion signal. (I have tried lengthening that tail current detection time, but apparently not enough.)

So, we'll see tomorrow. And the combination of what you told me and what he told me makes me think I'm finally getting a handle on a topic I didn't really need to understand in spring/summer/fall. So, thanks!
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