# Understanding Battery Bank Charging

Registered Users Posts: 102 ✭✭✭
edited June 2017 #1
Hi,
I have posted several questions about my system and t=my battery bank.. Needless to say I still need some help.

Deep Cycle Battery FAQ.
https://www.solar-electric.com/learning-center/batteries-and-charging/deep-cycle-battery-faq.html#Industrial deep cycle batteries

I am trying to grasp the basics of battery charging.  When a bank is in Float charge should there be a constant applied voltage to keep the battery bank from self discharge.  For example I have a 48v , 1.2 cell NiFe battery bank, where I can set the charging voltages.  When I set the Float charge to 1.47v no voltage is applied to thre float charge and in about 2 days the SOC drops from 90'ish to 70% SOC and the SMA inverter goes into Boost charge at 1.65v (never reaches 100% SOC after Boost Charge).  There are no loads on the battery bank.

Should I be setting my Float charge to something other than the 1.47v to keep the cell voltage up and to keep the bank from self charging?

Thanks.

«1

• Registered Users Posts: 384 ✭✭✭
If the cell float voltage is 1.47 Volts, then should the Battery Float Voltage be 58.8 Volts = 40 x 1.47 volts
• Registered Users Posts: 102 ✭✭✭
mvas said:
If the cell float voltage is 1.47 Volts, then should the Battery Float Voltage be 58.8 Volts = 40 x 1.47 volts
Sorry about not proving enough information.  With the SMA Sunny Island inverter I had to drop 2 cells to accommodate the maxim charging limit of the Sunny Island of 62.7v .  I have 38 cells on the battery bank.

What I'm asking is in float charge should there always be some kind od charging voltage to keep the battery bank from self-discharging?
Will keeping the voltage up on the bank like this keep the usable capacity (amp hrs) down on the bank?

I'm trying to understand the battery bank charging,  etc......   .

• Registered Users Posts: 384 ✭✭✭
edited June 2017 #4
Yes, in Float Mode your Charge Controller should be in Constant Voltage mode and very low amps.

Float:  55.1 Volts  = 38 cells x 1.45 volts per cell
Bulk:   58.9 Volts  = 38 cells x 1.55 volts per cell
Equa:  62.7 Volts  = 38 cells x 1.65 volts per cell

http://www.bimblesolar.com/batteries/nifebatteries

Sunny Island - Silent Mode ...
In silent mode, Sunny Island switches to standby and thus saves energy.
If the set time for the float charge (parameter SilentTmFlo) has expired in systems with electricity grid operation,
battery management switches to silent mode and any connected loads are supplied exclusively from the electricity grid.
The Sunny Island leaves silent mode at definable intervals (parameter SilentTmMax) or
whenever the battery voltage per cell drops by 0.14 V.

Do you have Silent Mode enabled?
• Registered Users Posts: 102 ✭✭✭
mvas said:
Yes, in Float Mode your Charge Controller should be in Constant Voltage mode and very low amps.

Float:  55.1 Volts  = 38 cells x 1.45 volts per cell
Bulk:   58.9 Volts  = 38 cells x 1.55 volts per cell
Equa:  62.7 Volts  = 38 cells x 1.65 volts per cell

http://www.bimblesolar.com/batteries/nifebatteries

Sunny Island - Silent Mode ...
In silent mode, Sunny Island switches to standby and thus saves energy.
If the set time for the float charge (parameter SilentTmFlo) has expired in systems with electricity grid operation,
battery management switches to silent mode and any connected loads are supplied exclusively from the electricity grid.
The Sunny Island leaves silent mode at definable intervals (parameter SilentTmMax) or
whenever the battery voltage per cell drops by 0.14 V.

Do you have Silent Mode enabled?
I have parameter [ SilentEna ]   disabled.  Should I enable this parameter?

My backup system is grid-tied.
• Registered Users Posts: 384 ✭✭✭
edited June 2017 #6
I think you have Silent Mode set correctly.
If [ SilentEna ] was ON, then Sunny Island could turn off Float Mode.
And we don't want that to happen - so OK.

So, what actually happens when you charge your battery?
Something like this ...
Step 1) Start with Constant Current / Bulk Mode = Ramps up to 58.9 Volts
Step 2) Switch to Constant Voltage / Absorb Mode = Hold at 58.9 Volts
2a) Boost Phase
2b) Full Phase
2c) Equaliize Phase
Step 3) Switch to Constant Voltage / Float Mode = Hold at 55.1 Volts

In Absorb Mode do you see: 2a) Boost Phase, 2b) Full Phase or 2c) Equalize Phase?

The remaining charging time (display value "120.04 AptTmRmg") of this Phase and
the actual process (display value "120.05 BatChrgOp") can be read on the display.

What MODE is Sunny Island in, when you think it should be in FLOAT Mode?

[ BatChrgCurMax ] ?
[ GnCurNom ] = ?
[ GdCurNom ] = ?
[ InvChrgCurMax ] = ?

[ ChrgVtgBoost ] = ?
[ ChrgVtgFul ] = ?
[ ChrgVtgEqu ] = ?
[ ChrgVtgFlo ] = ?

[ AptTmBoost ] = ?
[ AptTmFul ] ?
[ AptTmEqu ] ?

• Registered Users Posts: 102 ✭✭✭
edited June 2017 #7
mvas said:

[ BatChrgCurMax ] ?
[ GnCurNom ] = ?
[ GdCurNom ] = ?
[ InvChrgCurMax ] = ?

[ ChrgVtgBoost ] = ?
[ ChrgVtgFul ] = ?
[ ChrgVtgEqu ] = ?
[ ChrgVtgFlo ] = ?

[ AptTmBoost ] = ?
[ AptTmFul ] ?
[ AptTmEqu ] ?

[ BatChrgCurMax ] = 110 A
[ GnCurNom ] = 30 A
[ GdCurNom ] = 30 A
[ InvChrgCurMax ] = 48 A

[ ChrgVtgBoost ] = 1.65 V
[ ChrgVtgFul ] = 1.65 V
[ ChrgVtgEqu ] = 1.65 V
[ ChrgVtgFlo ] = 1.47 V  (SI reading = 55.8 V )

[ AptTmBoost ] = 300 minutes
[ AptTmFul ] 7 hrs
[ AptTmEqu ]  8Hrs

When the SI is in Float mode, that's where I think it should be but there is no voltage charge being applied to maintain the batterybank  from self-discharging.  If I leave the float charge at 1.47 the battery bank drops 15 - 20% in a day-and-a-half to 70% and the Sunny Island then goes into Boost charge (I have it set at 1.65v, maxim the inverter can do).

The charging boost cycle starts when the battery bank's SOC reaches 70%.  When the SI goes into Boost mode it does show that on the SI Read out. Then switches over to Float mode when the SOC reaches 85 - 90% during charging.  I never see Absorb or Equalize displayed with the SI.  All I see is Boost and Float.
The battery bank is 200 Ah @ 1.2 v per cell with 38 total.

I wonder about using the SOC 70% before starting a charging cycle. That makes sense with Lead Acid Batteries, but I am not sure that it does for NiFe batteries.

Do they have a "shorter life" if shallow cycled?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 384 ✭✭✭
edited June 2017 #9
@BB.
The problem here is ... OP states, no Voltage is output during FLOAT MODE, should be 57 Volts = 1.5v x 38 cells
• Registered Users Posts: 102 ✭✭✭
edited June 2017 #10
Here are a few screen shots.  The last picture shows the SI getting reading to go into Boost charge.  The Battery bank  is currently  at 70.4% SOC.

Float Charge @ 1.50v (notice the SOC slowly keeps dropping off)

Continuation of above Graph

Continuation of above graph but now in Boost Charge

Boost Charge off around 2:30PM  and is now in float charge @ 1.50V

• Registered Users Posts: 102 ✭✭✭
Seems if I change the Float charge to 1.51v there is a steady voltage being applied to the battery bank (see pic).  Is this what is ideal - to have a low volatage being applied to the battery bank during float charge?

• Registered Users Posts: 384 ✭✭✭
edited June 2017 #12
Yes, FLOAT Mode is Constant Voltage.
But Float at 57.3 V ( 38 cells x 1.51 volts per cell ) is on the "High End".
Contact battery mfr to see if OK.
They say, no harm if over-charged, but check for Warm Electrolyte.
"... If leaving unattended for long periods of time and worried about water loss you can reduce the Float to 1.44v per cell.  If wanting to get more power stored and you can keep the electrolyte topped up then you can increase the Float up to 1.5v per cell ..."

Is there a setting between 1.47v & 1.51v, like 1.49v ?
What Battery Type is selected?  NiCd ?
So, then the valid range for [ ChrgVtgFlo ] is 1.4 to 1.6 v, right?

I don't understand why ...
a) [ ChrgVtgFlo ] = 1.47v Turns OFF Float Voltage Mode
b) [ ChrgVtgFlo ] = 1.51v Turns ON Float Voltage Mode

http://www.bimblesolar.com/batteries/nifebatteries
Float:  55.1 Volts  = 38 cells x 1.45 volts per cell
Bulk:   58.9 Volts  = 38 cells x 1.55 volts per cell
Equa:  62.7 Volts  = 38 cells x 1.65 volts per cell

Is this the correct User Manual = SUNNY ISLAND 4548-US / 6048-US ?

Did you see the voltage drop from 58.9 Volts at the end of "Absorb Mode - Boost Phase" to 57.3 volts Float Mode?
Have you ever seen "I-Phase Bulk" charge mode displayed on the SI panel, yet?

NOMINAL BATTERY VOLTAGE SETTING
================================
The Nominal voltage of the battery ( 43.2 V to 48 V ) is adjustable in 1.2 V steps
Default Setting = 45.6 V for NiCd/NiFe = 38 x 1.2  <<< not 48 volts, right?
• Registered Users Posts: 102 ✭✭✭
Graph is lasted boost charge (1.65v) then inverter switches to Float charge (1.51v).  see the downward trend after float charge is on.

• Registered Users Posts: 384 ✭✭✭
edited June 2017 #14
Yes, I see the downward trend in "Bat SOC %"  in the chart.
Do you install a "Battery Current Sensor" and did you commission it?
• Registered Users Posts: 102 ✭✭✭
edited June 2017 #15
mvas said:
Yes, FLOAT Mode is Constant Voltage.
But Float at 57.3 V ( 38 cells x 1.51 volts per cell ) is on the "High End".
Contact battery mfr to see if OK.  Manufacturer said the NiFe batteries like to be charged between 1.65v to 1,7 V.  1.7 V is better.
They say, no harm if over-charged, but check for Warm Electrolyte.
"... If leaving unattended for long periods of time and worried about water loss you can reduce the Float to 1.44v per cell.  If wanting to get more power stored and you can keep the electrolyte topped up then you can increase the Float up to 1.5v per cell ..."

Is there a setting between 1.47v & 1.51v, like 1.49v ?  I can do 0.1 increments, so yes.
What Battery Type is selected?  NiCd ?  Yes, NiCd
So, then the valid range for [ ChrgVtgFlo ] is 1.4 to 1.6 v, right?  Correct.

I don't understand why ...
a) [ ChrgVtgFlo ] = 1.47v Turns OFF Float Voltage Mode
b) [ ChrgVtgFlo ] = 1.51v Turns ON Float Voltage Mode
I have been trying to figure this out myself.

http://www.bimblesolar.com/batteries/nifebatteries
Float:  55.1 Volts  = 38 cells x 1.45 volts per cell
Bulk:   58.9 Volts  = 38 cells x 1.55 volts per cell
Equa:  62.7 Volts  = 38 cells x 1.65 volts per cell

Is this the correct User Manual = SUNNY ISLAND 4548-US / 6048-US ?  Yes

Did you see the voltage drop from 58.9 Volts at the end of "Absorb Mode - Boost Phase" to 57.3 volts Float Mode?  Yes, as soon as the Boost mode turns off the volts drop.  It does take a minute to settle.
Have you ever seen "I-Phase Bulk" charge mode displayed on the SI panel, yet?  No, I have never seen that mode displayed.

NOMINAL BATTERY VOLTAGE SETTING
================================
The Nominal voltage of the battery ( 43.2 V to 48 V ) is adjustable in 1.2 V steps
Default Setting = 45.6 V for NiCd/NiFe = 38 x 1.2  <<< not 48 volts, right?  Yes, NiCd default is 45.6V.  I can not change the default voltage if I select NiCd as the battery type.
I replied after your questions above
• Registered Users Posts: 102 ✭✭✭
edited June 2017 #16
It has been about 18 hrs since Boost mode turned off and you cab see from the graph the SOC downward trend continues.  The downward trend will continue for about another day before the SOC reaches 70% and the Boost charge mode will kick on.  There are no loads on the battery bank.  The system is in Grid mode.

• Registered Users Posts: 102 ✭✭✭
mvas said:
Yes, I see the downward trend in "Bat SOC %"  in the chart.
Do you install a "Battery Current Sensor" and did you commission it?
I did not install a battery current sensor but I can do so.  I haven't install the battery current sensor because I don't have a generator connected to my system right now.
• Registered Users Posts: 384 ✭✭✭
edited June 2017 #18
midijeep said:
Yes, FLOAT Mode is Constant Voltage.
But Float at 57.3 V ( 38 cells x 1.51 volts per cell ) is on the "High End".
Contact battery mfr to see if OK
Manufacturer said the NiFe batteries like to be charged between 1.65v to 1,7 V.  1.7 V is better.
change this if I select NiCd as the battery type.

Correct the battery should be charged with (approx) 1.55 Volts / cell during:
a) Bulk Mode = ramp up to 1.65 Volts / cell
b) Absorb Mode ( Boost, Full, Equalize Phase <= SI's terms) = Hold at 1.65 Volts / cell

But after charging with both Bulk Mode and then Absorb Node the battery should be very near 100% SOC.
Float mode should be a lower voltage (1.45V / cell) to STOP CHARGING, which reduces water usage and reduces heat.
Float mode is just to hold the battery at 100% SOC.
• Registered Users Posts: 102 ✭✭✭
I measured each battery cell and they correspond to the voltage that is being applied to the battery bank i.e. Boost, float charging.  The downward SOC trend seems to look like something is drawing off the battery bank or the SI's SOC calculation is not able to do the correct calculation for NiFe batteries.  For example right now the SI is saying the SOC is 75.24%; Battery bank voltage = 55.8 v , where as the Victron 704 is reading 100% SOC; Battery bank voltage = 55.72 v.

The other thing with the  Victron 704 is it always shows 100% when the battery bank is not under load (Full).  The Victron's SOC reading does move downward when the battery bank is under load and moves up as the battery bank is being charged and ends up at 100% SOC after the SI charging cycles is completed.  However the SI reading never gets to 100% SOC no matter what the charging voltage is for the Boost charge.   The SI SOC always starts dropping after a charge to 70% SOC and the Victron stays at 100%.

If the Victron is correct that doesn't do me any good because the SI's SOC is wanting to charge the battery bank every day-and-a-half.

I think I may turn off the SI for a day and turn it back on to see if the SI recalculates the SOC higher than what is was before.
• Registered Users Posts: 384 ✭✭✭
edited June 2017 #20
"... When the full charge states are reached, the battery state of charge is reset one of these values: 90%, 95% or 100%,
depending on how full the battery was actually charged ( in Absorb Phase ) .
If default settings are not changed then,
SOC is set to   90%, after Absorb Phase  - Boost Charge <<< This is what you said is happening
SOC is set to   95%, after Absorb Phase  - Full Charge
SOC is set to 100%,  after Absorb Phase - Equalization Charge
is completed. ...
"
In Section 13.4 State of Charge (SOC) and State of Health (SOH) of the manual, read about ...
State of Charge (SOC)
State of Health (SOH)
Recalibration of the State of Charge (SOC)

Default Settings:
Absorb Phase - Boost Charge       = happens daily
Absorb Phase - Full Charge          = happens once every   14 days or after   8 Boost Charges
Absorb Phase - Equalize Charge  = happens once every 180 days or after 30 Boost Charges

Charging routine
Bulk => Absorb => Float

NOTE: The TYPE of Absorb Phase changes periodically...
a) Boost      = 90% SOC
b) Full         = 95% SOC
c) Equalize = 100% SOC
• Solar Expert Posts: 9,361 ✭✭✭✭✭
The battery meter needs to be loaded with NiFe correction factors, like the Full & Empty voltages, Charge Efficiency Curve (based around 65-70%, not 85% like lead acid)
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 384 ✭✭✭
edited June 2017 #22
It is not configured for Lead Acid, it is configured for NiCd.
I do not see any parameter for "Full & Empty voltages, Charge Efficiency Curve" in the Battery Settings menu.
Can you be more specific as to which Menu and which Parameter?
• Registered Users Posts: 102 ✭✭✭

mike95490 said:
The battery meter needs to be loaded with NiFe correction factors, like the Full & Empty voltages, Charge Efficiency Curve (based around 65-70%, not 85% like lead acid)
With the SMA Sunny Island you can not change these parameters.  They are preset by the default battery type SMA has loaded into the inverter's software.  THe Sunny Island has the following default battery Types:
VRLA, FLA, NiCd, & Lithium ion with external battery management system.
• Registered Users Posts: 102 ✭✭✭
edited June 2017 #24
Digging up some stuff:
I contacted the manufacturer and they said to charge at the normal charging rate, which is 0.2 x C/5 capacity.  My batteries at C/5 capacity are 200Ah, so the charging rate should be 40amps (0.20 x 200Ah).  They also state the charging formula is : " C/5 capacity x 0.20 for 8 hours (maximum).  The battery is deemed to have been fully charged when an additional 2.5 to 3.0 hours of charging was completed after the charging voltage had risen to constant 1.65 - 1.70 V/cell during the charge".

So I think I need to change the inverter's charging amps (GdCurNom) to 40 A (0.2 x 200Ah = 40A) and change the Boost charge voltage to 1.65 (the highest I can go with 1.2 v per cell at 38 cells), and also change the boost charging time to 8 hrs instead of the 5 hrs default.  They state 8 hrs is the maxim charging time.
Does this sound correct?

They also stated that float charge is normally between 1.40 to 1.50 V per cell and the ampere is about 1/40 of the 5-hour rate current (1/200 x c5 capacity.  Gradual drop of capacity indicates the charging voltage is too low.  So how would this translate into the SOC% gradually dropping over the day-and-a-half before going into boost charging mode?  Are the batteries not fully charged in the first place because the charging amps are set to low?

They also gave me formulas to calculate different charge rate currents:
Capacity (C5 rate) x 0.20 of C5 rate) x 8 hours
Over charge: 0.25 x 12 hours
Fast Charge: 0.50 x 4 hours

So would the boost charge be the 0.50  of C5 x 4 hours , instead of the 0.2 x 8 hours?
• Solar Expert Posts: 9,361 ✭✭✭✭✭
All those #'s are for Grid use, with infinite long charge and float time
For solar, and our 5 hour charge window, with NiFe, set the voltage as high as you can, and as much amps as the battery will take without heating up. Heating the battery more than a couple degrees, is BAD.  You use the higher voltage to push more joules in than standard voltage will, and as many amps as it'll take w/o heating.  The only penalty is more distilled water needed.
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 102 ✭✭✭
edited June 2017 #26
Thanks Mike for the information.  I will try this tomorrow and watch the battery temp to figure out the best voltage and amps.

Also, I wanted to show this graph from the SI (see below).  You can see around 2pm the SI recalculated the SOC and it jumps.

• Registered Users Posts: 102 ✭✭✭
edited June 2017 #27
I found to keep the battery bank voltage (SOC) steady without a gradual voltage drop after a Boost charge (1.65v), I need to set the float voltage between 1.51v to 1.55v.  the voltage being applied is constant and is using about 100  - 200 watts for the float charging.  The battery temperature changed from 19.2C to 19.7C and the SOC stays stable.

Is it ok to keep this constant float charge on the battery bank in float mode?

When I set the Float voltage below 1.49v the voltage comes on periodically (off/on) but the SOC always goes into an 18 hr downward trend to 70% SOC , then the Sunny Island switches to boost charging.  Also, the battery temperature has risen 0.2C from 10.7C to 19.9C.
• Registered Users Posts: 102 ✭✭✭
edited June 2017 #28

Is it ok to keep this constant float charge on the battery bank in float mode, instead of the voltage being turned off and on as the inverter wants to do?

Also, this what happens when I turn off the grid (main outside panel) and the entire Solar system (inverters and all).  You can see that once the Sunny Island is brought back on line, it recalculates the SOC and it jumps up.  There was no kind of charge between 2pm  & 2:30pm

• Registered Users Posts: 102 ✭✭✭
edited June 2017 #29
Interesting how the SI charges the NiFe battery bank at different Boost charging voltages.
The first graph shows a "shallow upward slope with a 1.65v (62.7 overall voltage) as opposed to the second graph that shows a steeply upward slope when using a 1.55v (58.9 overall voltage).

I still don't understand why the SI wants to charge the batteries every day-and-a-half when there has been no load on the batteries?

1.65v

1.55v

• Registered Users Posts: 102 ✭✭✭
edited July 2017 #30
Finally figured out the Boost and float voltage settings for the Sunny Island.  Now the SOC stays stable and only a 0.5C increase in battery temperature.

• Registered Users Posts: 1
How are you producing those graphs?  Can my SI5048U do that?