question

mogal avatar image

BMV712 - Amps vs Percent?!

Hi All,

I just can't figure this out and it's driving me nuts. There are similar posts but never a working solution?

I have two 6v Golf Cart batteries connected in series. 230 Ah. System settings and wiring below.


Victron reports 10.4 amps used, and says that's 97.2% of 230 amps. (is that not 95.4%?)

But when you take their 97.2% of 230, that SHOULD BE 6.44 Amps used?

HOW DOES THAT WORK?! What am I missing here?


What I've done:

- Fully charged the batteries to 100%.

- I then let them let sit for 2 days to stabilize voltages (Completely disconnected all power leads - only thing connected was the BMV 712 - just the little power wire)

- Battery read 12.8v after the two days (The little BMV power wire was still connected)

- I did as the manual states to set zero current:

A zero current calibration is (almost) never needed. Only perform this procedure in case the battery monitor shows a current while you are absolutely sure that there is no actual current flowing. The only way to be sure of this, is to physically disconnect all wires and cables connected to the side of the shunt. Do this by unscrewing the shunt bolt and remove all cables and wires form that side of the shunt. The alternative, which is switching off loads or chargers, is NOT accurate enough as it does not eliminate small standby currents.

- To me, it should not matter the state of the battery when you set Zero Current?

(There is NO current flowing as the battery is disconnected)


Can someone explain it like I'm 5? LOL

So far everything else seems to work like it should, but I'm still confused as to what my battery state really is?

Battery reports full when I believe it to be from the converter (and also the MPPT charger on the solar reports the battery to be full as well)


At the VERY LEAST, what is more accurate, the percentage, the amps used or the voltage?

(So far, I've been following Amps and watching the Voltage reading and trying to make an educated guess that way) Voltage will read 12.1v when the percent still reads in the low 70's? Is that not empty? So confused :(

To me the percentage is so far out of wack that it make ZERO sense. This battery monitor DOES NOT help me monitor my batteries :(

-----

BATTERY DETAILS:

The battery is manufactured by East Penn MFG also called (DEKA) its a GC15.

Two batteries connected in series will be 230Ah. (130Ah useable)

- Reserve Capacity at 25A is 448 Minutes

- RC at 75A is 120 minutes

- The Peukert’s coefficient for most use cases for this battery is 1.24


What is the recommended Bulk, Absorb, Float, Equalize and maintainer parameters?

- Bulk charge to 2.35 volts per cell and hold for Absorption. (14.1v for two in series)

- Float charge at 2.25 volts per cell. (13.5v for two in series)

- To compensate for battery temperature not at 20°C:

- subtract 0.005 V/cell for each 1°C above 20°C;

- add 0.005 V/cell for each 1°C under 20°C.

- An Equalization charge can be done at 15.5 volts.


Max bulk charging current is <=30A per 100 Ah (C20). So for this 230 Ah battery, that's 69 amps. (If charging a pair in series, it's still 69 amps.)


battery-wiring.jpg


victron-settings.jpg


victron-settings1.jpg


BMV Battery Monitorbatteryvoltagesetup
victron-settings1.jpg (296.3 KiB)
victron-settings.jpg (294.8 KiB)
battery-wiring.jpg (277.0 KiB)
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2 Answers
JohnC avatar image
JohnC answered ·

Hi @Mogal

"HOW DOES THAT WORK?! What am I missing here?

Can someone explain it like I'm 5? LOL"

Ok, I'll set you a challenge.. In Victron Connect change your Peukert Exponent to unity, (1.0). It will do an instant correction to your SOC to make it more closely match your Ah drawn. It's reversible, so you can test this.

Would you be happier there? Maybe, but there's a reason for it. You've specced your batts at 230Ah, presumably the C20 rate, then you've drawn down those Ah at a far lower rate, let's say C100. And if your batt maker gives a C100 capacity it will be higher.

What's happening is your BMV is compensating for the non-C20 discharge rate, and adjusting the SOC to be useful against the C20 capacity you've entered.

There's a Peukert calculator on the Victron website, and you may get a better feel for the basis of this 'adjustment' if you try using it.

Using V and/or Ah alone is for bunnies. SOC calculated correctly is far more accurate. But you have to set it up properly, which you can better do once you get beyond any mindsets hindering you.. :)

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Thanks, I looked a little closer into the calculations there. After a little more digging around, I found the C rates through some listed questions:

the 448 minutes RC rating is at 25A. The RC at 75A is 120 minutes.

And entering this into the Peukert's Calc and get a value of 0.55?

Doesn't seem right when the recommended value is 1.24?

I'll start playing with these and see if I can narrow it down a little better <shrugs>


Using V and/or Ah alone is for bunnies. Maybe so, but really, that's what I'm finding I have to do with this device?! The percentage go so far out of whack so quickly, that your only alternative is to read voltage and amps to get and educated guess!! (percentage is useless?)


Thanks... Maybe I can get the percentages a little closer? But at the very least, can I assume the amp reading is correct? I need at least ONE reliable constant here lol.

Is this your battery?

https://www.nationwide-battery.com/deka/gc15.php

Using this peukert calculator with the values of the battery in the link above you get a peukert of 1.25.

Capacity:
@5 hrs: 174 AH
@20 hrs: 230 AH

@Mogal

The Amps can be assumed to be correct. That's the purpose of a shunt. Volts too. But Ah is actually meaningless in terms of Energy. We differ in terms of what is useful.. I don't want my SOC to 'match' my Ah, but you do. So be it, I'll stick with Wh matching SOC (which isn't reported short-term by the BMV). I don't even look at Ah.

I use similar batts to yours but strung for 48V. And I do congratulate you (or whoever) for the BMV parameter setup. Similar to mine, but I do tweak them a little to suit the seasons. As you may do too.. Temp changes things as well..

Sorry, it just drives me nuts when I look at the numbers... it just doesn't line up?

Why have a percentage when it doesn't seem to follow the numbers?

If I went off the percentage only, I would have screwed my batteries.


I watched my batteries dip below 12v when the percentage says I still had 72% left!

So YES I would like the percentage to follow the numbers (why wouldn't you?!)


I guess I was hoping for a quick glance at the meter and just know what the batteries are at.

Nope, each time I open the app, I'm sitting there staring at numbers doing math in my head before I can determine what my battery is at.

@Mogal

Yeh, looking back at my VRM data for this morning, I too cracked the 48V (=your12V), at 76.5%SOC. The SOC dropped further after that to 74.8, but V rose to 48.4. A light load (just ~1% A/Ah), so I know V was going the 'wrong' way, and SOC more indicative of the truth. The reason - temperature, just 8degC. And I know this won't happen in summer, when my batts are far more responsive.

Ah isn't recorded in VRM, because it's meaningless without V.

I was once never a big fan of SOC either. I grabbed a Smartshunt to better control my mppt Absorb time. Then I found that this damn thing was a genius once tuned in. Once I cleared my preconceptions it showed me I wasn't working my batts as hard as I thought I was. And I'm happy about that..


harold avatar image
harold answered ·

To say it as simple as possible:

If you discharge with higher amps, you use more of the stored capacity

If you discharge "lighter", you use less of the available capacity


So it is not a liniar relation!

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