Input current begins to climb toward end of charge cycle

[Powerplay]

My absorb phase is terminated by wzbjr and I set it at .01 C.  There is also a long timer that will cut things off but it has been set to 5 hrs in the winter.  Lately, I've noticed the current being accepted by the batteries has been climbing rather than falling toward the end of the absorb phase.  Is this generally a sign the T-comped absorb set pt is too high or too low?  It is a 48V 255 Ah bank and at the moment the set pt is 59.7 V with the batteries at 14 C.  The charge flowing to the Batts has slowly climbed to 3.7A from about 3.3A 1 hour ago.  I expect it to go the other way to about 2.4A before going to float and it has been working that way until recently.  Sunny day so things should be great.  I wonder what has changed, though we've been running a 1200 Watt heater during the day in winter from the solar power.  Thanks for opinions.  These are AGMs.

[Resthome]

When I dump the WBjr data (2sec readings) from the Classic I usually see .5-1.0 Amp of noise on those readings. Not sure if you are looking on the Classic MNGP or not. BUt that doesn't sound like it is an issue with that small of a change.

[boB]

So, you are saying that the voltage is constant but at the end of the the absorb cycle the amps is rising slightly ?

If this is repeatable, then it is kind of weird, yes.

What is the battery temperature during this time period ?  I wonder if that is changing something maybe ?

No loads that might be going up at that time I don't suppose ?

John may be right here that it is just noise.  Hard to tell unless I see it myself and even then....

Maybe some battery characteristic ?  What are they ?

boB

[Powerplay]

So, you are saying that the voltage is constant but at the end of the the absorb cycle the amps is rising slightly ?

So I thought, but your battery temp question makes me wonder a bit.

What is the battery temperature during this time period ?  I wonder if that is changing something maybe ?

I think the battery temp might have been rising up to 7 C during the absorb phase (unusual amount AGMs).  In this case going from 7C to 14C drops the T-comped set pt from 60.5V down to 59.7V.  So the overall set pt is dropping as the temperature rises.  Also, the heater is on a thermostat, and I need to check it, but I think the absorb V drops at least .1V when the heater kicks in.  This could be cycling in & out during a temp rise.

Possible theory:  As the temp rises the capacity of the bank increases.  This increases the current without necessarily changing the voltage and heats up the batts a bit more.  This might start a heat/charge spiral.

When I dump the WBjr data (2sec readings) from the Classic I usually see .5-1.0 Amp of noise on those readings.

Yes it could.

Next sunny day when I can watch the system closely, I'm going to go all scientific on this and maybe I'll learn something.  First, I'll try to keep everything else as constant as possible and just change out one variable.  Maybe we can turn off the heater during the absorb phase to keep the loads more steady. etc.

Thank you both for the input it gives me some avenues to think about and help fine tune the system.  These anomalies seem to be stretching out the absorb phase quite a lot.

[Resthome]

If you have your WBjr connected to the Shunt and only ONE cable going to the Battery Negative and the Classic set to monitor Shunt. Then loads should not effect your WBjr system readings, unless the load is dropping the voltage from the set point. Temp Compensation could be adjusting the set point, but it would be very gradual unless there is some rapid change in Battery Temp including ambient .

[Vic]

Powerplay,

Think that you are on to something,    regarding voltage drop in cables and circuit breaker between the Classic and the battery.

With a medium sized load,  like 1200 W to the heater,  the Classic will keep the Vabs constant  ( EDIT: as long the Classic remains in Absorb<),   but the battery knows different.   Even a small voltage drop between the Cl and the battery will result in a fairly significant change in battery Acceptance.  When the heater thermostat starts the heater,   the battery supplies some of the current to run the heater,  due to the voltage drop between the Classic and the battery.   When the heater goes off,  the Classic recharges the battery for that small-ish discharge,  due to the battery voltage rising a bit,   when that load switched off.    This recharge represents an increase in current measured by the WBjr.

Based on the absolute numbers you noted for temp comp verses voltage,   it would appear that the Temperature Comp value is about -- 4.75 mV/Cell/C.   There is some error in this,   due to (at least)  voltage and temperature resolution in the readings.    This seems a bit high for most AGMs.

The other initial thought was that the Temp Comp  value was set to too low a value for the batteries.   And with some temp increase during Absorb,   that the Vabs was not lowered enough for the needs of these AGMs.

It could be possible that each battery in this single string of AGMs is not well-balanced,   and some batteries in the string have different temperatures,   and the one with the BTS is somewhat different in temperature than that of the average battery ...

Are the batteries all the same age?

Some day,   many of us do hope that MN CCs will be able to measure the actual battery voltage,   which will be a big help with intermittent lagre-ish,  loads,   and the variable voltage drops that these cause,  primarily during Absorb.

FWIW,   Vic

[Powerplay]

Based on the absolute numbers you noted for temp comp verses voltage,   it would appear that the Temperature Comp value is about -- 4.75 mV/Cell/C.   There is some error in this,   due to (at least)  voltage and temperature resolution in the readings.    This seems a bit high for most AGMs.

Yeah the -5 (ish) mV/Cell/C always seemed a bit high to me for AGM but it is info straight from the manufacturer.  I ran it that way all last summer as a 24V system and didn't see many issues.  When I reconfigured to 48V last fall the temps were moderate for a while and things have been fairly steady.  After a very overcast January, now I'm starting to see this behavior, so the parameters need some tweaking.  The batteries are all about 1 1/2 years old.  I'm calling them 92% efficient but might need to adjust that.

So, I'll try turning off the heater tomorrow and shortening the absorb timer to 3 1/2 hours then play with the T-comp value.  Which way should I go first?  Maybe actual -4.75 set?  Unfortunately, there are a lot more variables it looks like a slightly warmer day.  We'll see if that terminal acceptance value can be nudged toward .01 Capacity.  Today it bottomed out at 3.2A and then started climbing.

[Vic]

Yeah the -5 (ish) mV/Cell/C always seemed a bit high to me for AGM but it is info straight from the manufacturer   ...

So, I'll try turning off the heater tomorrow and shortening the absorb timer to 3 1/2 hours then play with the T-comp value.  Which way should I go first?  Maybe actual -4.75 set?    ...   

To me,  if the rise in battery current near the end of Absorb IS caused from heating of the battery (on which the BTS is placed),   then it appears that MORE compensation is required (more compensation would further reduce the Absorb voltage).    So you might try to increase the compensation somewhat.

One further suggestion,   would be to confirm that the BTS is mounted about half way down the case of one battery in the center of the battery bank,   and is well-attached.   Usually,  I like to place some Styrofoam (c) over the BTS,   taping it in place  with good tape.   This insulation can help to remove the effects of ambient temperature on the BTS reading.

Is there a significant temperature rise in the area/room where the batteries are located,   during Absorb?

FWIW,    Vic

[Vic]

Powerplay,

Also,  you might want to measure the voltage of each battery during a relatively high current charge,  or discharge,   looking for the voltage of each battery to be very close to the others.

If you did this during early Absorb,   at least the voltage from the Classic will be fairly constant.

Voltage readings that are very close to all other batteries in the string indicate relatively good balance among all of the batteries.

Vic

[Powerplay]

To me,  if the rise in battery current near the end of Absorb IS caused from heating of the battery (on which the BTS is placed),   then it appears that MORE compensation is required (more compensation would further reduce the Absorb voltage).    So you might try to increase the compensation somewhat.

Yeah, there is good reason for going that direction first in small steps.  Today the battery temperature had only risen about 3 C by the time the acceptance current had bottomed out during absorb phase.  This time it bottomed to 2.9A and was hanging around that level until the timer cut the cycle off (an hour later).  So battery temperature was higher but not rising as much, and absorb phase bottoms out lower & closer to terminal acceptance current but still not quite getting there.

BTW I agree 'more compensation would further reduce the Absorb voltage" but only as the battery temperature rises. It would also spike the absorb voltage up higher initially, however.  That could well be what it needs.


I plotted the two day curves for acceptance current during absorb phase I have so far.  It is unfortunate we can't get duplicate days but it is looking like the trouble might be occurring when the rising temperature crosses a threshold and the Classic drops the voltage during Absorb.

I'm going to perturb the system for a few days and plot the results.  It really looks temperature related just between today and yesterday.

I'll post the conclusion if you guys want to see the result.

I popped open the battery bank and those terminals are clean as a whistle.  LUVs them AGMs for uncorroded terminals.

[Westbranch]

PPlay, each EQ can do a fair bit of cleanup of the sulphite buildup on the plates... so as Vic recommends never use the auto EQ,  as you have seen the exact results on day 1 after EQ, may not be seen again.... and there is the possibility of Negative results, overheating etc... ie THERMAL RUNAWAY

hth

[Powerplay]

PPlay, each EQ can do a fair bit of cleanup of the sulphite buildup on the plates... so as Vic recommends never use the auto EQ,  as you have seen the exact results on day 1 after EQ, may not be seen again.... and there is the possibility of Negative results, overheating etc... ie THERMAL RUNAWAY

Engineering type friend suggests the cobwebs need blown out of the top of the batteries.     Translation: There is lead build up from too shallow cycling.

I just went through a system search for leaks & tweaks and it is a fairly tight ship out there.  So on a warmer night when not many loads are running the batteries usually only cycle down to about 93% of capacity.  I think lead build up is border line possible but rather than cycling them down to about 50% on purpose, I could switch the re-bulk to every other day until cooling season starts.  We should be recharging 82-85% batts every other day then, unless conditions change.  I could just try that for a while & wouldn't hurt anything.

Thank you for the input.

P.S. I also discovered the float voltages were set too high by .5V as well but it hasn't been in float for that long lately anyway.  I can't find good temp curves for float except it has to be reduced at crossing a certain threshold above STP (25 C) usually about 29 C.  So it isn't part of this equation but I like to have things correct anyway.   

[Vic]

Hi Powerplay,

I have not ever run AGMs.   But,  most Deep-Cycle Lead Acid batteries really need to be cycled below 90% SOC on each cycle,   otherwise,  generally Lead Dioxide can build up on the Positive plate.   This is not good for the battery.

On FLAs  this shallow-cycling can cause some cells to exhibit relatively high SGs,   and can be cause swelling of plates,   which can cause mechanical damage to the battery,  and other problems.   Am uncertain of the effects of this on AGMs.

Cycling your batteries to about 80 % SOC,  or lower on occasion should help alleviate many/most of any negative effects of shallow-cycling on your batteries.

Know that,   at least some AGM batteries do have different Temp Comp values for cyclic use,   verses those for Float service.

Many SLA batteries do have Limits on the range of temperatures for which temperature compensation should be applied.

Am a Flooded type person,  so,  many of these details are not well known by me.

FWIW,   Vic