BMV-712 Smart Settings

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sfrank2010 asked Feb 11, '24

BMV-712 Smart Settings

I have a new Four Wheel Camper with dual 6 volt batteries rated to 224 amps. To charge them I have a RedArc BCDC1225 powered from the alternator, 180 watts of solar, and of course shore power.

I honestly have no clue what I'm looking at even after reading the manuals several times. I've included some screenshots of the settings and the most recent few hours and day of charging to hopefully give more context. Charge efficiency, peukert exponent, and tail current are all foreign to me. Below is our normal usage.

At night we run the furnace and some floor LEDs. The furnace pulls 2.5 amps, and in Colorado in winter it essentially runs constantly all night. The LEDs are negligible power, but in a 10 hour night we'll see around 25 amps used. Knowing that 50% of the total amps is usable, is 25 amps per night a huge draw on the battery? In the morning the battery percentage shows about 91%, but I'm not so sure of that's accurate with my settings. And with this, the low voltage alarm set to 50% goes off every night, but that's with the draw of the furnace on it.

With the RedArc, it can charge at a max of 25 amps, but with an AGM battery it's not all going in. I have the efficiency setting at 85%. I would expect the truck to give it nearly all that, but it seems to top around 20 amps only for a very short period of time then quickly tapers off and hovers around 4-8 amps. Next you can see the solar seems to get around 4-5 amps, but again it's only for a short period of time even with direct sunlight at noon and quickly stabilizes around 1-1.5 amps. The RedArc shows it's at a float charge which would seem to make sense with all of this, but I find it hard to believe I'm depleting only 9% or so at night then it can bulk charge to about 97% in 30 minutes off the alternator.

New to the forum, so thank you in advance for all the help. I'll surely do what I can to help answer questions as they come up and have a confident answer.

battery charging BMV Battery Monitor smart solar set-up help

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kevgermany answered · Feb 11 at 10:59 PM

Your tail current is set very low, preventing the BMV from synching to 100%.

Try setting it up a little. 1-2% would be better.

Charged voltage is also too low, probably needs to go higher, especially if most charging is solar. If most is grid/alternator, it should be just below float, probably 13.6V, otherwise for solar about 14.2V.

Other settings look good.

Do you know the C rating at which your battery capacity is measured? At low discharge rates the capacity rating is too low. You're discharging at a low rate and this may confuse the SOC calc.

But saying that, the graphs show a healthy system. But a low battery voltage, indicating undercharging and a misleading SOC display. SOC display tends to drift. For this reason it automatically resets to 100% when voltage, tail current are met for the charge detection time. Doesn't look as if your system reaches 100% on the BMV.

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kevgermany answered · Feb 12 at 04:37 AM

Confusingly there are two ways of stating the C rating. C20 means 20 hours to discharge the battery. The other way is current as a decimal fraction of capacity. E.g. 0.1C.

Your battery capacity is at the normal C20 (20 hour) rate. Your use described is closer to the C100 rate. To avoid confusion, probably better to stick with 224Ah capacity. It will be better when you get higher loads.

The data sheet says charge at 14.7V, float at 13.0V. So my guess was a little off. Better to set charged voltage higher, say 14.6V. This will work for solar and other charging. You may need to tweak in future. What you're doing is matching the BMV to your system and local conditions.

You need to check the charger voltage settings. I think they may be too low. Absorption should be 14.7V.

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JohnC answered · Feb 12 at 08:18 AM

Hi @sfrank2010

The optimistic SOC you complain of is at least partly caused by too high a Peukert. There's a calculator on the Victron website, but to save you the trouble..

1.07 is quite low, and also affected by Temperature (which Mr Peukert couldn't quantify). I also have GC2 pattern batts (FLA while yours are AGM) and use 1.10.

If your batts are cold then they can alarm on V with even light loads. Ignore them once you gain confidence in the SOC. I don't alarm until 10.5V, too low for you maybe, but that's for decent loads. They bounce back when load removed.

You need to decide whether to sync under Absorb or Float. The settings are completely different. In your pics I can see a Tail happening under Float at ~1.4A. That's equivalent to like 0.7% at whatever the V is. Sync in Absorb has to happen before dropping to Float. And you may not want that.

Once you decide on a Peukert setting, try adjusting Charge Efficiency so that you get pretty close to 100% SOC without actually needing to sync. This isn't for impatient people, could take a while. Call it a hobby.. :)

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Answer 1 · 2024-02-11T23:56:39Z

sfrank2010 answered · Feb 11 at 11:56 PM

DC224-6 Specs.pdf

Changed the tail current to 1.5% based on your recommendation. Thanks for that.

As far as charging goes, the vehicle sits for around 8 hours during the day, and in winter that's hit or miss for inclement weather and obviously low sun angles. The truck is running for about 1.5 hours a day which I assume is more of a charge than solar even in solars best days. Would it be a bad idea to just set it at 14.2 volts all the time?

I attached the spec sheet of the battery. I guess 0.05 for C rating?

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Answer 2 · 2024-02-12T10:32:29Z

sfrank2010 answered · Feb 12 at 10:32 AM

Alright new settings... 14.6 volts for charged voltage and 1.10 for peukert exponent. I'll admit I couldn't find the calculator on the website with my mobile device and have been mildly busy with a toddler. Thay calculator looks easy enough to plug and play with the spec sheet.

Patience, or a hobby, probably isn't my strong suit, but hopefully this can get me dialed in or at least closer. Tonight I'll have shore power after about 2 hours of driving, so in the morning tomorrow I'll do a manual sync to 100% just before unplugging. That should have it at 100% I would assume. There's certainly some drift at this point with incorrect settings, but the long drive plus 8+ hours of AC charge should get a known value, right?

Thanks for the help. Will let y'all know what I see!

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JohnC ♦ commented · Feb 12 at 11:38 AM

@sfrank2010

One little thing.. The 'Charged V' setting in the BMV is the V at which the BMV needs to at least be before it will sync. It doesn't control charge at all.

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Answer 3 · 2024-02-12T14:23:44Z

sfrank2010 answered · Feb 12 at 02:23 PM

Right. The BCDC1225 controls the charge. It has an indicator for each charging state. So how do they communicate? Or do they communicate at all? I'm guessing they don't, and if I wanted to sit and watch both at the same time I could probably do a setting adjustment with that. Extremely tedious and not practical however.

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Answer 4 · 2024-02-12T16:33:43Z

sfrank2010 answered · Feb 12 at 04:33 PM

One more point of confusion.. The state of charge is probably off bases on previous settings and not having a solid 100% charge and sync. The volts spiked up to 14.3 when I turned on the truck this morning and remained there for an hour. With the truck still on, it dropped off to 13.3 seen here. What causes this? With the charged voltage set above 14, I don't think I would ever see a sync with this situation? I have it set at 14.3 for now. Looking at the history I have never seen it get to 14.6 even with the spec sheet saying 14.7. The other picture shows the amps over the same period. They taper off from around 20 to about 7. I would expect this as it gets near full and begins to enter float, I think.

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JohnC ♦ commented · Feb 13 at 05:10 AM

@sfrank2010

The drop in Charge V is your charger dropping from Absorb to Float V. Both charge V's *should* be temperature compensated, so won't match the target settings. I mentioned earlier that you have to decide whether to sync under Absorb or Float, now you can see why.

Your batteries are fully charged when they can't accept current. Pb's always accept some (the Tail), but it varies with V. You need to find a value you can use. Might be (say) 1.5A in Float (0.7% Tail), and maybe 8A (3.6% Tail) in Absorb. The trouble with using Absorb is you have to catch it early, and it will sync to 100% when only actually 98% or so. Fine if you can live with that.

Those figures are from both my own and your graphs, which match surprisingly well. When setting a 'Charged V' in the BMV you have to also allow for Temp Comp in the charger - this gets a little finicky in Float and hot weather.

This might all sound too hard, but once you understand what's happening it's fairly simple really.

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sfrank2010 JohnC ♦ commented · Feb 13 at 05:49 AM

So that being said, if using float the manufacturer recommends 13.6 volts as charged, and 14.7 for absorb.

Sync using absorb.. 14.4 (0.3 less) for around 8 amps with the bigger tail current. Duration maybe 10 minutes?

Sync using float.. 13.3 (0.3 less) for much less tail current due to float charge being less. Using a lower voltage is fine so long as the tail current is low, right? Because if input is much higher that 13.3 I'd run the risk of an early sync if my tail current was higher?

I haven't studied so deeply into a subject this hard in decades, so I apologize again profusely for the probably really dumb questions.

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JohnC ♦ sfrank2010 commented · Feb 14 at 04:07 AM

@sfrank2010

Ha, they're not dumb questions. Advisors were thin on the ground when I started fiddling with this stuff. Learning the hard way ain't the best way, trust me.. Someone once said to me 'Batteries are Evil'. He was right. They're also expensive, and you pay dearly for careless treatment. I talk to mine nicely nowadays (instead of swearing), monitor them, and treat them as gently as I can. They respond, but they're still Evil..

..My rant for today.

Basically you seem to understand what I'm saying. That 13.3V is ok, but it's down at the level where you can get false syncs from passing cloud. That's not a gamekiller, just be aware of if and why it can happen. Then adjust as necessary.

I'd drop that 14.7V Absorb target to 14.4V. They'll still charge fine and they'll run far less risk of overcharge. Temp Comp is important, and you may not even have that. Batt makers aren't famous for explanations, but 14.7V is too high, unless they spend their days in snowy weather.

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sfrank2010 JohnC ♦ commented · Feb 14 at 11:44 AM

I fully agree that batteries are evil. The manufacturer responded a couple of times to my emails, however he suggested I reach out to victron because he doesn't know much about battery monitors. That's fair.

So using 14.4 volts for absorb rather than a lower voltage for float would cause a slightly early sync, but it would be minuscule especially compared to a potential false sync with the lower voltage? I'm not entirely sure what you mean by catch it early, but I do believe I am beginning to pick up what you're putting down otherwise... Attempt to sync at the end of absorb rather than beginning of float?

Another random thought I had. Say it was set at 13.3 volts with tail of say 2 amps for easy math. Solar is charging at 4.5 amps and is maybe in absorption. Then let's say I have a draw on it of 3.5 amps with a net of 1 amp. Could this cause a false sync, or is it smart enough to know just the input for charging state and not take into account any draw?

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JohnC ♦ sfrank2010 commented · Feb 14 at 12:45 PM

@sfrank2010

Re the random thought.. When A drops to that 1A, V will drop sharply. But it might take a few minutes to get below 13.3V. That's where the Charged Detection Time comes in. I think the default setting is 3 minutes, but I found it necessary to use 5 min to avoid false syncs. This can also happen early in the day under solar charge when a cloud rolls over.

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Answer 5 · 2024-02-13T04:09:51Z

sfrank2010 answered · Feb 13 at 04:09 AM

Was at 93% when pure solar charging started. Not many amps as you can see. Then driving for about 1.5 hours getting good voltage but minimal amps (maybe absorption or float). Then after 5 minutes with shore power showing 14.6 volts and less than 2 amps it jumps from 96% to 100%. Unfortunately the BCDC1225 doesn't show what phase of charge with shore power, only vehicle and solar. So when shore power is removed and the vehicle turned on, the BCDC1225 flashes once every 2 seconds indicating float charge. All that being said, it shows float and victron shows 100% so long as the settings are accurate or at least close, we should be pretty good I think? Currently set at 14.3 volts, 1.1 peukert, 1.5% tail current, and charging time 15 minutes (longer time felt better), and charging efficiency 80%.

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JohnC ♦ commented · Feb 14 at 03:25 AM

@sfrank2010

A and V together on a graph would be easier to see how they relate, but you can still see it on those.

You've added shore power at the end, V exceeded 14.3, Tail low, so it synced. But you can also see before you did that, a 'Tail off' to around 1.4A, so it was actually ready to sync even earlier but couldn't because V was too low. It wouldn't have synced at all if you hadn't added Shore.

Having 3x different charge sources makes it more difficult to set this up. Your solar (presumably the one with fluctuating A, perhaps a PWM controller) seems to be set with a very low Float, which is likely too low to arrange reliable sync. Given you may not always be travelling or have Shore, I'd set it's Float charge target higher (13.6?) and then consider what your sync Tail should be.

In a setup like this I'd set it to sync in Float state.

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Answer 6 · 2024-03-20T14:00:45Z

sfrank2010 answered · Mar 20 at 02:00 PM

Going to revive an old thread. Back to the camper now and have the volts and amps together. It's the same time-line just got different timestamps to show the value better. The initial spike to 14.45 was with shore power applied. The battery was most likely very full before being plugged in, so the amps makes sense. I have the tail set to 0.7%. What's confusing is the voltage drop to 13.86 then again to 13.28. The furnace and some LEDs were all that ran through the night. Then the drop later to 13.28 and hovering around there is confusing again. All still shore power. Amps telling me it's now in float, but what are the volts telling me here? I have the charge level set to 13.2 and 10 minutes with that tail. The last image has a description of what's going on at that stage, and I'm completely lost, again. :)

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Answer 7 · 2024-03-21T04:48:07Z

JohnC answered · Mar 21 at 04:48 AM

@sfrank2010

I interpret the drop to 13.28V as a drop from Float to Storage level. Storage, at least in Victron terminology, a 'nothing' level to neither charge nor discharge. Usually applied when the batteries are considered full, so sync should really be set to happen before then.

The 1.8A while in Absorb suggests the batts are already full (as you said). The 1.3A in Float is well along a slow taper, so typical of a suitable sync point. Your 0.7% Tail is like 1.56A, so sync would have happened.

Your 13.2V Charged V is maybe a bit low, you could raise that to nearer the Float V. 10 min might be a bit long with solar and passing cloud, could be a bit shorter.

The V rise when solar arrives is likely an independent charge cycle taking off. Not much A happening to do that, so your batts are still well up in charge.

All looks fairly normal to me..

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sfrank2010 commented · Mar 21 at 02:38 PM

Roger that. So a 13.6 volts in float should work for it then? Or even possibly 13.7 or 13.8 since it remained above 13.8 during that charge time.

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JohnC ♦ sfrank2010 commented · Mar 22 at 05:00 AM

@sfrank2010

Raise it as far as practically possible. But keeping in mind that the Temperature Compensated value for the Float setting may get lower if your batteries warm up. It must be lower than that.

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