Battery Capacity Test Results

andyrud

Last week I posted for some help figuring out the capacity of my 1110 amphour 2 volt batteries because I thought they were going bad. I just got back from the ranch and thought I would share some information with you guys.

I fully charged them and after waiting for 1 ½ hours the voltage was 12.9V.
I then put a 125 amp draw on them for 1 minute and the voltage dropped to 12.6Volts.
I then put a 32 amp draw for 30 minutes checking the voltage every 10 minutes and it stayed between 12.1 and 12.2V. After the 30 minute test the voltage was 12.55Volts.
I then turned on my drill press and some other tools and started pulling 72 to 82 amps for 30 minutes. The voltage dropped to 11.6 volts during the test. After the test the voltage was 12.5 Volts.

I the turned on a 1500 watt heater as recommended by one of the members and the voltage dropped to 10.3 volts. I left it on for about 10 minutes and the after the test the voltage was about 12.4 volts.

The results seem to tell me that the batteries are probably OK. What do you think?

Question: What makes the battery voltage go so low when drawing over 100 amps? Is it the internal battery resistance? ( I don’t usually pull more then 30 to 40 amps)
I am using 2AWG size battery cables and 5 feet to the inverter. Should I be using larger cables?

Thanks in advance,
Andy

BB.

Re: Battery Capacity Test Results

andyrud wrote: »

• I then put a 125 amp draw on them for 1 minute and the voltage dropped to 12.6Volts. [125 amps * 12 volts * 1min/60minperhr=25WH]

• I then put a 32 amp draw for 30 minutes checking the voltage every 10 minutes and it stayed between 12.1 and 12.2V. After the 30 minute test the voltage was 12.55Volts.[32A*12v*30min/60minperhr=192WH

• I then turned on my drill press and some other tools and started pulling 72 to 82 amps for 30 minutes. The voltage dropped to 11.6 volts during the test. After the test the voltage was 12.5 Volts.[80A*12V*30/60=480WH]

• I the turned on a 1500 watt heater as recommended by one of the members and the voltage dropped to 10.3 volts. I left it on for about 10 minutes and the after the test the voltage was about 12.4 volts.[1,500W*10/60=250WH]

• 25WH + 192WH + 480WH + 250WH = 957 WH

From the Battery FAQ, lets assume your 12.4 volts is after several hour of rest and the batteries were around 77F... 12.4v =>~75% state of charge.

And the amount of energy a "Good" 1,110 AH 12 volt bank should be able to supply is:

• 1,110 AH * 12 volts * 0.25 charge used = 3,300 Watt*Hours

So--something may not be quite right--Either you used more power than you thought (I ignored the 85% efficient inverter in the heater calc).

Or your batteries were not fully charged. Or you did not let the battery sit for ~3 hours before measuring the 12.4 volts. Or the battery was not 77F, the meter was not accurate, etc....

Question: What makes the battery voltage go so low when drawing over 100 amps? Is it the internal battery resistance? ( I don’t usually pull more then 30 to 40 amps)

Did you measure the battery voltage right on the battery posts--or somewhere else in the wiring?

Normally, we should expect those cells to output at ~13% rate of discharge for ~6 hours (1,110 AH * 0.13 = 144 amps continous load or 144a*12v=1,728watts).

I am using 2AWG size battery cables and 5 feet to the inverter. Should I be using larger cables?

I don't remember the size of your inverter... Lets assume 2,000 watt:

• 2,000 watts * 1/10.5 volt batt cutoff * 1/0.85 inverter eff * 1.25 NEC safety factor = 280 amp minimum wire/breaker circuit

Using a voltage drop calculator for 224 amps (without 1.25 NEC derating) and 5' of 2 awg cable, we get:

• 0.4 volt drop

Which is about the maximum drop I would like to see on a 12 volt system.

Depending on whose wire gauge / capacity table you like to use--a 280 Amp circuit should be from OO to 400 CM (not small cable).

Do you have battery problems? Perhaps. The raw numbers suggest you are (sulphation?).

If you are looking to power large loads (2kW inverter)--you may want to look at a 24 or 48 volt battery bank system next time...

My 2 cents view of what you posted--I am certainly not 100% clear on your testing and voltage readings and how you did them. And I am no expert.

Review your battery bank state of charge, check voltage drop (and rise) on each cell when under heavy load (and charge) to see if you have "weak cell" and/or bad wiring somewhere.

Review your charging system and make sure that you are getting all of the charge current into the battery bank you can.

And--if you can setup your signature with your system basics (XXX watts of solar, XYZ 60 amp charge controller, XXXX watt inverter, XXXX AH of YY Volt AGM/Flooded Cell/ABC brand battery bank) or similar.

-Bill

andyrud

Re: Battery Capacity Test Results

Thanks for the reply Bill.

You are right, the batteries were not fully charged when I thought they were. After what was what I thought was a full charge as shown as in the float mode and 100% on the Tri Metric meter, the SG was in the 1.250 range. There was some variances in the cells from 1.235 to 1.250. Trojan Battery says one of these fully charged battery's SG should be 1.265. As marc has pointed out in some posts the absorb mode on large batteries has to be substantial. I think mine is only 2 hours so I am going to raise it to 4 hours. Do you think it would make much of a difference if I changed the 2AWG cables to something bigger?

Andy

BB.

Re: Battery Capacity Test Results

There are two reasons for heavier cables... 1) to reduce voltage drop and power loss... 2) for safety--so cables do not overheat and can trip fuse/breaker without damage/fire.

I don't think changing the cables will change the inverter/battery performance much.

And, I do not know your "standard" loads / current you normally draw (I just guessed at the 2kW).

If you plan on drawing 2kW or more for long periods of time--it would be safer to have at least a "OO" cable.

Do you have any fuses/breakers in line?

Anyway, this is one of the reasons for going to a higher voltage battery bank next time--the size of copper wire needed to carry such heavy currents.

-Bill

andyrud

Re: Battery Capacity Test Results

Thanks Bill,

I am usually using about 300 watts for 5 hours during the day and about 50 watts for the rest of the time. That's about 2450 watts per day. I now know that I should have went with higher voltages. When I started 15 years ago setting up my system I was kind of learning as I went. Before I found this forum! I updated my signature as you requested.

Andy

BB.

Re: Battery Capacity Test Results

Hi Andy,

Not a problem--If we do it right--we learn our whole life.

How old are the batteries?

-Bill

PS: Sorry, I did not answer the extended charging question... But, back to basics, how are you setup now (absorb voltage settings and such).

andyrud

Re: Battery Capacity Test Results

Bill,

The batteries are only 9 months old. The Absorb voltage is 14.8 volts for 2 hours, Float is 13.2Volts. I don't think I am getting them to full charge. The factory is not much on Equalizing, but I think I need to do it as the SG vary from 1.235 to 1.250 in the cells. These 2 Volt batteries each have three cells per battery for a total of 18 cells.

andy

BB.

Re: Battery Capacity Test Results

The three cells are in parallel per battery (separate electrolyte space per 1/3rd cell).

Anyway--measure each 2 volt "brick" three ways:

1. Resting voltage (no load/discharge for ~2-3+ hours
2. Charging Voltage (heavy charging current)
3. Discharge Voltage (heavy discharge current)

#2 and #3 can be done with the battery at different states of charge.

What I am looking for is one or more cells that behave differently. One that seems to sag more under load, or one that has a higher voltage than the rest when charging. There will be some natural variation that may change over state of charge--just looking for bigger differences that may indicate one cell that has a problem and the rest are "Good".

You are measuring S.G. in all "18" cells--I presume. Equalizing a lot is not good for the batteries... Trojan recommends to equalize when you have a 0.030 SG difference between high and low charged cell.

Also, when nearly finished charging (and/or when equalizing)--look for bubbling in the cells. And see if there are any cells not bubbling at all or too much--again--trying to find differences (no bubbling may be low SG until cell is "equalized with others").

Setting a longer absorb time should be OK... If you want to be really careful--measure SG.... When SG stops moving up between measurements (30-60 minutes apart)--that should be when the batteries are "full".

You can also review the setup on the Trimetric Battery Monitor--Basically it resets to 100% based on reaching a certain voltage for a certain length of time (as I recall--I do not own a battery based PV system). You may also review the other setup information too and see it still makes sense (just guessing).

-Bill

andyrud

Re: Battery Capacity Test Results

Special thanks for the info Bill. I will do what you suggest.

Andy

westbranch

Re: Battery Capacity Test Results

Hi andy, how often do/have you used the gen set to charge you batteries?

the 125 A your inver/charger puts out should allow you to go through BULK charge and let your PV s do the finish charge.

Is your Classic set up as per the manufacturers specs for recharging?

An EQ may be needed but only once you are sure you have a full bulk/absorb/float charge.

Eric

andyrud

Re: Battery Capacity Test Results

Hi Eric,
I don't use the genset very often. The panels usually topoff the batteries by 1 o'clock.

I have the Classic set up for 2 hour absorb at 14.8 volts, but when I go back to the ranch I am going to set it to 14.9 volts for 4 hours. Float at 13.2 volts. Any suggestions?

Andy

BB.

Re: Battery Capacity Test Results

The Trojan Battery User Guide seems to not be really clear on charging (14.1 volts "forever?")...

I probably would take float to 13.6 to 13.8 volts... Gives a little charge to the battery bank. If you were on Grid Power with a battery charger--then I would look at a lower float voltage... But on solar, you only have X hours per day of sun--so a higher float voltage should not be a problem.

However--I will bow to anyone that has more experience than I (which is not much) on batteries.

14.9 volts is getting a little "hot" for a long absorb charge... Also, if you have other 12 volt devices (and some inverters), you may have problems with operation at that high of voltage, or may even voltage fault.

-Bill

stephendv

Re: Battery Capacity Test Results

I agree with Bill, sitting at such a high absorb voltage for 4 hours is not a good thing. The Trojan User guide is a bit tricky because that charge curve is not the curve supported by off-grid chargers. It's the same curve proposed by forklift batts for heavy cyclic applications, i.e. discharge to 20% SoC daily and then recharge overnight.

The different curves discussed briefly here: http://forum.solar-electric.com/showthread.php?t=11761

Soooo, I'd tend to follow the 'standard' RE charging curve: 2.4V for absorb for a time period to be determined experimentally while checking SG of a charging batt. As Bill said, a slightly higher float shouldn't be a problem because it's not higher than the gassing voltage and won't last that long anyway.