Non rested vs rested battery SOC readings

Smitty77_7 · April 10, 2015

Lifeline says to take readings of SOC/OCV (Open Circuit Voltage) after a minimum of four hours of 'rest'. Is their any rule of thumb math to approximating these values with a 'non resting' battery?

The gent that helped me install our Battery Bank of AGM's, Solar Panel, and Magnum PSW with the BMK Meter option. Said that the values of the BMK of 'voltage VDC' would be usually be lower then the actual SOC/OCV, due to loads on the batteries. (And of course, temps too.)

Was wondering if anyone knew of a rule of thumb 'add a + rang of 'x to x' of the reading? Say a reading of 12.2 OCV, + .25-.50 would become 12.4 - 12.7 OCV if at rest.

Or, is their just too many variables due to loads on the battery bank?

We go into what the DW calls hibernation mode at night while boon docking. All unneeded power draws are turned off. We leave the salesman switch power onto the coach, and the Magnum MS2800 inverter remains on too, to feed the fridge. But all the little draws like computer and phone wall warts are off. We keep the heater off at night too, and I usually get up a bit earlier to turn the heat on.

When I look at the readings before turning the heat on, I'll get an OCV of 12.38 + or - .03, after a full night. (TV, DVR, Stereo DVD players all are idling over night (thinking of putting these all on a dedicated circuit to turn off overnight) as well as the big draw of the Samsung.)

Hope my question makes sense, TIA, and best to all,

Smitty

Chad Heiser · April 10, 2015

If you have a meter like the BMK or the Trimetric, then you know your battery bank's capacity in a percentage of full and do not need to worry about volts DC. I never look at volts DC unless I have a specific reason to. I pay attention to my SOC percentage and try to never let it go below 70%. I have a fairly large battery bank (928 amp hours worth) so it is pretty easy to stay above my preferred mark of 70%.

It is when you don't have a meter that gives you state of charge that the volts DC of the batteries becomes important. Switch your BMK display over to state of charge and get rid of the headaches of trying to calculate a resting volts DC reading.

70% is somewhat of an arbitrary number I have chosen to use to extend the life of my battery bank and to keep me from getting close to the magic 50%. With my usage and amp hour capacity, I can easily go 3 to 4 days with no charge source before I get to my 70% number. YMMV based on usage and battery bank size/capacity.

Jack Mayer · April 10, 2015

Either do a Chad suggests, or use the number of amphours drawn from the bank to calculate in your head "where you are" as regards SOC.

If you have 600 Ah bank and know you used 150 overnight then you can draw some conclusions from that. You will count that down to zero during the day as you charge with solar. I like to be at zero each day. OR, simply use the percent full function.

Chad Heiser · April 10, 2015

Either do a Chad suggests, or use the number of amphours drawn from the bank to calculate in your head "where you are" as regards SOC.

If you have 600 Ah bank and know you used 150 overnight then you can draw some conclusions from that. You will count that down to zero during the day as you charge with solar. I like to be at zero each day. OR, simply use the percent full function.

I use Jack's method of counting amp hours in and out as well just for getting a feel for how much power I am using (and replenishing), but I typically leave my display on SOC so I can get a quick idea of where I am at at a glance without having to think about it.

Yarome · April 10, 2015

Switch your BMK display over to state of charge and get rid of the headaches of trying to calculate a resting volts DC reading.

X2. Why make it more complicated than it needs to be. You invested in the right equipment... trust it and go play.

Jack Mayer · April 10, 2015

I use Jack's method of counting amp hours in and out as well just for getting a feel for how much power I am using (and replenishing), but I typically leave my display on SOC so I can get a quick idea of where I am at at a glance without having to think about it.

Chad, mine is sitting at % SOC right now, and has been for the last couple of months. I only use the count now when I'm trying out new things. Also, with such a large battery bank counting is not as "interesting" as it once was. All I really need to know is if I can restore what I used....

Yarome · April 10, 2015

Chad, mine is sitting at % SOC right now, and has been for the last couple of months.

I gotcha beat My BMK hasn't been off % SOC for probably 8 months or so. I DO watch my tristar panel for in n' out, but that's about the extent of daily monitoring.

Smitty77_7 · April 10, 2015

Yep, I use the SOC, but have questioned it before. And yep, it is new residential fridge and other electronics that I'm trying to sort out the changes to the battery bank (800AH Lifeline's). Lifeline has the Resting value in relation to SOC, and that is what I was wondering if a rule of thumb existed.

I'll do a few nights of checking out the AH usage in relation the bank size, and compare that to SOC.

I did add the WhizBang Jr to the MidNite Classic controller, but have not updated the Classic's software yet to activate it. (Like to do upgrades while parked at home, and just ran out of time before we headed out.) This will replace the Magnum BMK meter reading...

Thanks gang,

Smitty

Chad Heiser · April 11, 2015

Chad, mine is sitting at % SOC right now, and has been for the last couple of months. I only use the count now when I'm trying out new things. Also, with such a large battery bank counting is not as "interesting" as it once was. All I really need to know is if I can restore what I used....

I'm right there with you. I pretty much always leave it on %SOC. I only check amp hours in and out when I want to see what a particular item (light, appliance, etc.) draws. Since I am not full time and I haven't added anything new to my trailer for a while, I haven done this in a while.

I recently put a Trimetric in a buddy of mine's Motorhome. He couldn't stop playing with the amp hours in/out. He was going around his rig turning things on and off to see how many amps each one pulled. He was like a kid with a new toy.

Yarome · April 11, 2015

He couldn't stop playing with the amp hours in/out. He was going around his rig turning things on and off to see how many amps each one pulled. He was like a kid with a new toy.

I dare say we've all been there. After my solar went "live" I seem to remember turning on the microwave a few times... just because I could.

Smitty77_7 · April 11, 2015

LOL! To being in the 'Let's play and see what the AH's are with different items!' club:)!

You can say I'm doing that now, due to all of the recent electronic consumption mod's.

So, from 8:00PM to just before 6:00AM, we consumed -136 AH's, and the SOC reflected that at 83%. This was with everything powered off that cold be, including unplugging of wall warts, Dyson vacuum charger, no lights on, no hydro hot heater on. Call this 'idling' as far as AH's usage. About a -6 to -8 AH's usage in this state, for all the various coach systems that draw, as well as electronic components still plugged in and parasitic drawing too. For that 10 hour period, using -8 AH for idle, that would be about -80 AH's. I assume the difference -56 AH's is the Samsung fridge doing it's thing. (Ice maker off, and in Energy Savings mode. I left the temps at normal level.) So about 5.6 AH's average over that 10 period.

I'll keep playing around, and I've started to write down the SOC and OCV readings. More to just get a feel for how the Non Resting OCV values may correlate to SOC, to have another reference point. (Yeah, it's 'Hmmmm' information. You know, the kind the bosses would as you to get, requiring several PITA hours (PA's) to obtain - just for them to take a minute to look at then go 'Hmmmm'!)

Excuse me now, I'm going to go read the draw difference as the DW plugs in her Curling Iron!!!

Smitty