SOC to 100% too soon? Proper battery settings?

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goodguy5000hd asked Jun 10, '19

SOC to 100% too soon? Proper battery settings?

Hello. I just installed a BMV-712 (with temp probe) in a travel trailer that has 2 Costco flood batteries (link) in parallel which are about a year old and well maintained. After resetting the SOC to 100% (the trailer was plugged in for a week, no appreciable current), I unplugged the trailer and let the SOC run down to 90% over 1.5 days with a light load (lights, fan).

I then plugged the trailer back in to see how long I'd have to run my generator in the future to regain the lost 10% capacity. After only about 75 minutes, the BMV-712 reported an SOC of 100%, but the current was still showing ~+7A (gradually tapering downward), which indicates that the battery is still charging, and in fact not at SOC=100%.

I assume that something is wrong with my battery definitions.... the manual doesn't explain how to choose the correct numbers, so I did the best I could (see attached screenshot). The Costco page didn't have many specs, so I assumed that the equivalent Interstate battery is this one: (link).

I'm most interested in setting the right battery parameters for getting an accurate SOC value so that I never drop below 50%. Also, I'd like to know how long I'd need to run a generator during the day to regain recent losses (I don't see a "time to go" value on the app). Finally, I assume the "time remaining" value is for SOC = 0%, so I should always cut it in half for "time remaining" to SOC=50%?

Thank you for your help.

BMV Battery Monitor

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Answer 1 · 2019-06-10T03:32:37Z

Mark answered · Jun 10, 2019 at 03:32 AM

170Ah x 4% =6.8A, so if the charge current drops below 6.8A while the voltage is also above 13.2V then the BMV will auto synchronize SOC to 100%.

I checked the link for your batteries, but can't seem to find the Ah rating, so I'm assuming that the 170Ah you have entered is correct?

Anyway, as long as your charger is setup to continue charging in absorption phase until the charge current drops to ~1% of the Ah rating (1.7A) - then I would setup the BMV sync parameter to ~2% tail current & the charged voltage to whatever your charger absorption voltage is -0.2 to 0.4v.

Give that a go & see how you go...

Just bear in mind that if you set the sync parameters to aggressive then it won't ever sync - which can be even worse. So you need to find the right compromise for your setup and charger parameters.

The 'time to go' calculation is in relation to the 'discharge floor' SOC % set, not to 0% (unless thats what it's set to).

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goodguy5000hd commented · Jun 10, 2019 at 03:45 PM

[The site wouldn't post a "comment" so I put my response here....]

@Mark: Thank you for your time in answering the questions. I have a few more follow-up questions if you don't mind. I'm generally knowledgeable about battery charging, but have a few holes in my understanding.

I assume that the specs of 88AH is what is physically attached to my trailer, so I doubled that value (2 batteries in parallel) and backed off a bit for safety arriving at 170AH. Is that a reasonable approach? What is recommended from Victron?
I'm using the default battery spec "charged voltage" of 13.2v. Is that reasonable for my flooded parallel batteries? (The next day, while plugged in and trickle-charging, the voltage reads 13.63v.)
Just after I began re-charging the batteries from 90% SOC, I saw the trailer's charger amps slowly ramp down over time. The BMV-712 assumes that 100% SOC is achieved when the charging amps are below the tail current (4% or 2%), and the charge voltage is above 13.2v (the default I left as is for now). The next morning, I see that the batteries are still taking in some trickle current. How "real" is the assumed 100% SOC status given by the BMV-712 given that there's still charging happening? Is is really 95% or something? Should I set my discharge floor higher to compensate so as not to shorten the life of the batteries?
When I turn on my generator, or tow (attach to vehicle's alternator), is there a way to quickly find-out how long it will take to bring my SOC back to, say, 100% or 95%, or some level that's achievable in a realistic timeframe? I know that charging speed tapers off as the battery fills back up.
The app "history" tab shows a "max battery voltage" of 20.29V for my 12V system. Is that to be ignored? If it affects calculations or operations, is there a way to reset it?
The app "history" tab shows a min/max starter voltage, but I don't have an aux battery, and am using the temperature sensor. Is there a way to remove this, or shall I simply ignore it?

Thanks again!

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Answer 2 · 2019-06-11T02:23:44Z

1- Yes that's a reasonable assumption if you have no better info. You could possibly try to contact the battery manufacturer. Also note that the Ah rating varies for the same battery depending on the discharge rate, the battery capacity @ the 20h discharge rate should be entered into the BMV (some battery manufacturers quote Ah at the 100h rate, which can make their batteries appear better).

2- The BMV 'charged voltage' depends on your charger & the charge voltage settings. Is your charger a proper 3 (or more) phase charger or only a trickle charger? Also what is the max charge current it can provide? To properly recharge your batteries from a discharged state you really need to have a charger that has a bulk, absorption & float phase with programmable charge voltages & absorption phase duration/end trigger - the charge voltage during absorption phase would typically be higher @ ~14.4v for a 12v battery. If you have such a proper 3 phase charger then I would set the BMV 'charged voltage' a little under the absorption voltage (~14.0 to 14.2v). But if your current charger is just a trickle charger & never gets to that voltage then it obviously wont ever sync, so you need to set it a little under whatever the charger voltage is.

3- Even to maintain fully charged batteries at a 'float' voltage of say ~13.8v, the batteries will still require a very small amount of current to maintain the float voltage. The discharge floor setting has no effect on the SOC% calculation, just on the 'time to go' calculation.

4- No there is no easy way to calculate the required charge time from a given SOC%, as with a proper charger the 'bulk' phase only gets the battery charged to to only ~85 to 90% SOC - then the 'absorption' phase (which has gradually diminishing charge current) can take up to ~6h to finish off the rest. Ideally the 'absorption' phase should end when the charge current drops to ~1% to 2% of the battery Ah rating.

5- Yes I think that you can ignore the ~20v max voltage safely, but best to re-set it to get a useful real max V. I'm not sure how it ever reached this - maybe at some point the battery was disconnected while solar was still connected.

6- No unfortunately you cant remove or rename the min/max starter voltage section - I use the BMV B2 input as a 'mid-point' reading for the same battery bank, so the description is also incorrect for my setup.

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