Kid dropping to Absorb voltage

[Aussierob]

Question about absorb voltage display that I can't find an answer to anywhere else. As my batteries come to full charge in BulkMPPT mode, the battery voltage displayed is 15.0 or 15.1 volts. The Kid then goes into absorb mode, but the voltage is still displayed as 15.0v. I have the absorb voltage set to 14.7 volts. I would assume the display would show this voltage? When the Kid goes to Float mode it shows 13.4 which is what I have set it to.

Appreciate any help 

Thanks, Rob

[Vic]

Hi Rob,

Am not quite certain just what you are saying.

Bulk stage usually charges Flooded batteries to about 80 - 85% of full charge.

You are correct that at the very end of Bulk,  the battery voltage will be very close to or at the Absorb voltage.  It is this Absorb voltage setting that triggers the transition to Absorb,  which finishes the last part of fully recharging batteries.

Even though Bulk ends at the Absorb voltage setting,  most batteries are not fully charged.

We do not know the type of batteries that you are using,  but even AGM batteries need to be Absorbed,  and some AGMs need a  L O  N    G  Absorb.

Hope that you are using the Battery Temp Sensor with the KID.  Perhaps that is why your Bulk ends at 15-ish V,  as you probably think that it IS Winter there,  cool batteries = higher charge V.

What type batteries are you using?   Thanks,   Vic

[Aussierob]

Hi Vic,
I have two 6v 400Ah AGM's in series. The absorb is set to 14.7 so my expectation is when the battery voltage gets to that, or maybe 15v, the charger will switch from bulk to absorb and reduce the charging voltage to 14.7. It should hold that for 120 minutes then switch to float.
Batteries are http://usbattery.com/products/us-agm-batteries/us-agm-l16/

Thanks, Rob

[Vic]

Hi Rob,

Thanks for the info on the AGM batteries.

OK,  so,  you are PROBABLY using the Battery Temp Sensor ?

If you set the Absorb voltage to 14.7,  and the batteries are cool,  the BTS  would raise the Absorb voltage above what you set.  So,  you would see that you could be at a battery voltage of 15 or so volts in Bulk.

When the temperature-compensated voltage in Bulk reaches the COMPENSATED Absorb voltage,  Absorb begins.  Absorb will continue at that increased voltage,  resulting from cooler than reference battery temperature.

During charging,  especially in Absorb (even for AGMs) the battery temperature can rise,  and you might notice that the Absorb voltage might be decreased a bit compared to the beginning of Absorb due to an increase in battery temperature.

On the KID,  and many Charge Controllers  (CCs),  there is NO voltage setting for Bulk stage.

The BTS is a very good thing to have on a CC

Just guessing on what you may be seeing.   FWIW,   Vic

[boomadge]

Just throwing this out there. Have you checked the battery voltage with a multimeter ? If readings between the batteries and kid are off from each other you will have to go into rhe kids càlibration menu and adjust them.

[Aussierob]

Thanks for the tip on the BTS. I have one connected and will have a play with it to see if that is causing what I am seeing. I have also calibrated the Kid so that should be OK. I should note that my question relates to what the Kid is displaying compared to what I have programmed. I'll let you know what I find.

Rob

[Vic]

Hi Rob,

On the main Status display of the KID,  the voltage shown,  is the actual battery voltage at that moment.  If the charge Stage is Absorb,  then the KID has reached its Temperature-Compensated charge voltage.  This will be the Vabs that you have set,  plus or minus the value of compensation determined by what the BTS reports to the KID as the battery temperature.

It may not be easy for you to unplug the BTS while the KID is charging in Absorb,  for example,  but,  if you could,  and there is some compensation occurring,  you would notice that the displayed battery V would change,  when the BTS is unplugged.

Also if the KID was in Float,  and the batteries were warmer,  or cooler than the Ref temperature,  you would also see this change in Vfloat when the BTS was unplugged,  and,  then back in again.

It is not clear what version of FirmWare that you have,   but the KID may have a readout of what it reads as the battery temperature.

If you had a thermometer that has a temp probe,  or,  perhaps your DMM (etc)  you could place the probe under some Styrofoam )Expanded Polystyrene plastic)  and get a fairly good idea of what is the actual battery temperature.

All FWIW,   Vic

[Aussierob]

I have the newest firmware (1797) so have the temp readout. Once my system gets back to float I will use some ice water or heat source to fool the BTS and see what it does to the voltage displayed and measured by my meter.

[Westbranch]

No need to wait for float, if the temp sensor is working it will show as on page #24 of the manual... once you have clasped it in your hands or dunked it in ice water.  a lower temp will increase the voltage to the battery a small amount too, check the temp comp you have set in the KID...

hth

[Vic]

I have the newest firmware (1797) so have the temp readout. Once my system gets back to float I will use some ice water or heat source to fool the BTS and see what it does to the voltage displayed and measured by my meter.

OK  Rob,  but you need not do this ...   just look at the reported battery temp from the BTS to the KID.   If it is different from  the Reference temperature that you set in the KID (usually 25 C),  the Absorb,  Float (or even the EQ)  voltage  will be changed  by the number of degrees different from your Ref temperature multiplied by the Temp Compensation Coefficient that you also set in the KID  (EDIT: and then multiplied by the number of Cells in the battery bank  --  six in your case)<.   For battery temps that are lower than the ref temp,  the charge voltage will be increased,  and for batt temps that are higher than the ref temp,  the charge voltage will be lower.

The above only applies to charge states that DO NOT end in MPPT.   That MPPT suffix on the charge state indicate that the compensated voltage of that state has not been reached due to inadequate PV power availability verses tha amount of load on the system.

Placing the BTS in ice water will make the compensation much more evident,  but,  seems to me that what you are seeing is due to your batteries being cool relative to the Reference temp,  and thus causing the KID to raise the Absorb and/or Float voltage,  and seems that this is the question that you were asking about,  IMO.

Just my opinions,    Vic

[Aussierob]

Hi Vic,
So the ref temperature is set to 25v and the batteries are 24-26 so based on that I would expect to see the Float and Absorb voltages to be at the set value. They are both high. Float 13.7 vs 13.4 and absorb 15.0 vs 14.7. I did try putting the sensor in cold water and the float voltage went up to 14.3 and then went down after I took it out. I think the best way to compensate is to set the voltages lower so when in float/absorb it is actually at the value I want.

The other possibility is the voltage is high after charging is complete and is staying high (no load on the system) so the Kid is reporting what it is seeing and not applying any power to the batteries. There is very little current flow and only a few watts reported from the panels.

Rob

[Westbranch]

Rob, I suspect  those 2 hi temp 'adjusted' voltages are right after / at the end of the phase....  Do they then settle down to the lower value a few hours later? (for Absorb, only if the charge ended there)

[Vic]

Hi Rob,  Thanks for the update.

YES,  would expect that with a DIAPLAYED battery temperature,  on the KID page for battery temp of 24 - 26 C that any temp compensation would be quite small,  and this is probably not what is causing the difference in the voltage displayed ON the KID.   Of course,  if there was some time difference in noting the displayed voltage difference and the check of battery temp,  this might explain some of the divergence.

With very,  very light loads on the batteries,  during Absorb and in Float,  AGM batteries can evidence some higher-than-setpoint conditions.  This usually is reported when the batteries are mostly,  or fully charged.  It has been attributed to Surface Charge effects.   Have never run AGMs,  so cannot report from direct experience.

If you were able to apply some loads on the system,  either direct DC loads,  or via an inverter,  it MIGHT change this.

I dunno.   AGMs are sensitive to overcharging,  so am sure that this is why you are concerned,  but,  still this voltage divergence is not large.

If these batteries were mine, would probably also set the Absorb & Float setpoints lower,   as well.

FWIW,  Good Luck,   Vic