AGM Battery Balance

Horsefly · June 2017

The off-grid cabin install that I started paper designing last fall - and got lots of great help from the fine folks on this board - is coming together. I'm at the cabin now. The PV array will still be a few weeks off, as the ground mount is taking a while to get done.

I've got the Inverter / Charger installed and wired into both generator and the breaker panel for the cabin. The battery configuration is 2 parallel strings of 2 12V AGM batteries. The four batteries are 155AH each.

Just before I hooked up the #4/0 cables to all the batteries, I checked the voltages. One string has both batteries at 13.09V. The other string has one battery at 12.94V, and the other at !3.15V. I went ahead and hooked them up, and have used them through a couple of shallow cycles (using the generator to charge, since I've got no solar yet). When the charge transitions to float I've turned off the generator, and after about 30 minutes the bank settles at between 26.1V and 26.16V. With only two cycles I don't have much of a history to work from.

Is it safe to assume these will balance out eventually? I don't have a smart 12V charger that I could use with these AGMs, so if I need to do something to individual batteries I'll need to wait for the next trip up here. Anything I can or should do to make sure they eventually balance out?

mvas · June 2017

Balancing AGM's is not as easy as flooded.
Balancing the batteries individually is great idea.
Do you have "equal length" battery cables ?
Are the Charger Cables connected to opposite diagonals ?

Typically, it is not safe to assume that batteries will eventually balance out.
What are you doing that will cause the batteries to equalize?
Are you doing an Equalize Charge?
Does the manufacturer provide Equalize Mode parameters?

Estragon · June 2017

I wouldn't worry too much about 0.1v. You could charge the 12.94v battery by itself for a bit, but they're probably close enough. With 4/0 cables, they'll likely settle on their own just fine.

Horsefly · June 2017

mvas said:

Balancing AGM's is not as easy as flooded.
Balancing the batteries individually is great idea.
Do have "equal length" battery cables ?
Are the Charger Cables connected to opposite diagonals ?

Typically, it is not safe to assume that batteries will eventually balance out.
What are you doing that will cause the batteries to equalize?
Are you doing an Equalize Charge?
Does the manufacturer provide Equalize Mode parameters?

Lots of stuff there. First, I did ensure I have the same length to everything. I have a diagram I used somewhere, but I can't find it. I have four 13" cables. Two go between the + and - tying the two batteries in each string. I then have the other two 13" going between the +'s on one string and between the -'s on the other string. I have two 5' #4/0 cables with one going to the positive post on the first string, and the second going to the negative post on the second string. So all electrons have to travel the same distance: Through two 5' cables and two 13" cables. I'm pretty confident I got that part right.

No equalize charge, as these are AGMs.

Sounds like you are saying I should get a 12V smart charger and do them all individually?

Horsefly · June 2017

Estragon said:

I wouldn't worry too much about 0.1v. You could charge the 12.94v battery by itself for a bit, but they're probably close enough. With 4/0 cables, they'll likely settle on their own just fine.

Well then. Looks like I got opposite answers from two guys I've grown to respect on this board.

Either way, I appreciate the input. I'll consider getting a 12V smart charger that I can run off the generator here. Maybe what I should do before I buy one is to go a couple more shallow cycles and then disconnect them all, let them settle out, and check to see what they are at now.

Estragon · June 2017

That's the thing about a forum like this. You'll get sometimes conflicting views on things that we can kick around. That's kind of the point of the exercise.

Ckecking the individual resting voltage of all the batteries is about the best you can do. If one looks low, charging it by itself may help.

mvas · June 2017

Horsefly said:

Lots of stuff there. First, I did ensure I have the same length to everything. I have a diagram I used somewhere, but I can't find it. I have four 13" cables. Two go between the + and - tying the two batteries in each string. I then have the other two 13" going between the +'s on one string and between the -'s on the other string. I have two 5' #4/0 cables with one going to the positive post on the first string, and the second going to the negative post on the second string. So all electrons have to travel the same distance: Through two 5' cables and two 13" cables. I'm pretty confident I got that part right.

No equalize charge, as these are AGMs.

Sounds like you are saying I should get a 12V smart charger and do them all individually?

Even the 6 cells inside a single 12 Volt battery will continuously deviate, which is why I equalize my FLA.
You have 12 cells in series and they are already out of balance.
So when are you going to do something about that weak cell?
Are going to wait until the sulfate has hardened, causing a permanent loss of capacity?

If you do nothing today then at least measure the battery voltages periodically,
if the difference continues to grow then you will have confirmed that you should have done something.

No equalize charge?
That is unfortunate.
Lifeline makes AGM batteries and they specify a "Conditioning/Equalizing" Charge.
That is a benefit.

Raj174 · June 2017

You might try disconnecting the bank from the system and rewiring all the batteries in parallel for about 8 hours. Disconnect the batteries and measure voltage. They should be very close to equal.

Rick

Marc Kurth · June 2017

My normal method is to charge each battery fully, then float all of them over night before shipping. I wouldn't worry about cell balance until the batteries have settled in and have seen at least some more shallow cycles.

As I recall, your system is going to spend a lot of hours on float. It is my experience that lots of float time is a good thing.

Marc

Horsefly · June 2017

Yes, the cabin will have no one here for 5-6 months every year. That means two things: lots of time in float, and no one to service the batteries, hence we couldn't go with FLA.

I like Rick's idea, although I don't have enough cables to make that happen. I can get 3 in parallel with my four 13" cables.

Horsefly · June 2017

I have what may be a stupid question. Can I make careful use of a "dumb" car battery charger to get these to at least better balance? I'm thinking I would watch the current closely, and go only until the current gets down to about 9 amps or so (about 0.06C on these batteries). Is that safe?

I have access to a dumb charger. I will be going home for a week or two on Sunday, so if I need a smart charger it will have to wait until my next trip up here.

Marc Kurth · June 2017

I would have no problem using a standard auto battery charger for this purpose - if you are monitoring the voltage and the charge rate is range.

AS long as the charging voltage is the specified range, keep charging until the battery is drawing about 2 amps per battery or less. (Unless you have a reason not to?)

Marc

Horsefly · June 2017

Awesome. Thanks Marc! (again)

Estragon · June 2017

Do watch the voltage though. I suspect most "dumb" automotive chargers work okay for AGMs as there are lots in cars. Still, you wouldn't want your shiney new batteries venting on you from overvoltage, so it's worth watching.

Horsefly · June 2017

For now, I'm going to try the idea from Rick / @Raj174 I found some jumper cables in the tool shed, so with my existing 13" 4/0 cables I've got all four tied in parallel. I charged them all (in my 2x2 24V config) with the generator before I pulled them. They've been connected for a couple of hours now, and I'm going to leave them that way until morning (around 14 hours total). Then I'll disconnect them and let them settle for a couple of hours. If there is still much of an imbalance, I may try the cheapo charger for a bit on the low ones. I'll stand right there with them and my meter. It's a clamp meter, so I can watch the voltage with my probes and the current with the clamp.

Horsefly · June 2017

I will say, the charging voltage on these batteries (Vmax XTR12-155) is supposed to be between 14.4V and 14.9V. That seems high compared to what I see for other AGMs, but OK. I set my CC and Inverter to the mid-point for my 24V bank, or 29.3V.

westbranch · June 2017

I would start at the minimum level and observe, then increase based on my battery makers recommendations... here is a good reference doc from them.... http://www.cdtechno.com/pdf/ref/41_2128_0212.pdf
High Volts are used in dire situations....

mvas · June 2017

Horsefly said:

I will say, the charging voltage on these batteries (Vmax XTR12-155) is supposed to be between 14.4V and 14.9V. That seems high compared to what I see for other AGMs, but OK. I set my CC and Inverter to the mid-point for my 24V bank, or 29.3V.

The Charging Voltage will be Temperature dependent, not a constant value.

A 0.10 Volt difference in Voc is about a 10% difference in SOC (see typical FLA SOC chart below)
You have batteries that are at three different SOC levels.

VMAX AGM 100% SOC at 25°C / 75°F = 13.05 Volts, after a 3 hour rest.

VMAX Initial Commissioning
=======================
Fully recharge your batteries ...
A float charge of 48 - 96 hours at room temperature is highly recommended ( 13.5v - 13.7v ).
Or you can fast charge your batteries ( 14.5v - 14.9v ) for 12 - 24 hours.
Check battery voltage with no load - it should be above 12.9 volts.

VMAX Charging
=============
"... The recommended current rating of the charger should be 10% - 15% ( 15 Amps - 23 Amps )
of the C20 AH rating of the VMAX 155AH.
It will take a 10% rated charger a period of 10 hours to fully charge a fully discharged VMAX.
Changing any of the variables can be pro-rated accordingly,
so increasing the charger rating by 10% will lower charging time by 10% and so on.
In the case of an urgent need for a fully charged VMAX,
higher rating chargers can be used provided that the VMAX voltage is continuously monitored and
the charger's rating is never above 30% ( 46 Amps ) of the C20 AH rating of the VMAX 155AH,
or more than 1 hour and then charging shall resume at the normal 10% rate ..."
(note: it will take longer that they estimate because during absorb mode the current will taper towards 0 amps)

VMAX Float Charging
==================
Recommended Voltage is 13.5 volts - 13.7 volts
VMAX Float Voltage (temperature compensated) Chart:
https://www.vmaxtanks.com/assets/images/technical%20info%20folder/float_charge_chart.pdf

VMAX CYCLES
=============
DOD Lab Cycles
------- ---------------
10% = 3,500
25% = 1,500
50% = 900
75% = 600
100% = 300

8 - 10 Years FLOAT Service

Horsefly · June 2017

Thanks @mvas, but I already have all that. I programmed in the temp coef of -30mV/degree C when I commissioned the CC and Inverter/Charger.

The SoC chart you provided is - as you said - not applicable to the Vmax AGMs. Vmax provides a different SoC chart with quite a bit different voltages.

I had printed out all of the appropriate data for these batteries and included it in my design document, which will stay here at the cabin.

Estragon · June 2017

I'm with westbranch on starting at the low end of absorb voltage, and increasing from there if need be (eg not getting fully charged with decent sun).

Also, I wouldn't leave them floating over the winter. AGMs tend to have low self-discharge, so I would just make sure they're fully charged in fall and turn everything off.

Marc Kurth · June 2017

While the Vmax labeling is more recent, Vision Battery has been around a long time and is one of the largest battery manufacturers in China. Their AGM's have roughly the same internals, performance and longevity as Universal Power Group (UPG) and Deka - who sell a "whole lotta" batteries daily.

Stay shallow on your cycles and follow their guidelines in order to maintain your 1 year warranty, but you should not have a problem because your particular application is so gentle on your batteries. NEVER (EVER) let them sit dead because that particular AGM design does not take kindly to equalization.

Relax and enjoy the cabin!

Marc

Horsefly · June 2017

Estragon said:

I'm with westbranch on starting at the low end of absorb voltage, and increasing from there if need be (eg not getting fully charged with decent sun).

Also, I wouldn't leave them floating over the winter. AGMs tend to have low self-discharge, so I would just make sure they're fully charged in fall and turn everything off.

I guess I'm a bit confused. I was assuming that turning the inverter off but having the charge controller keeping the batteries at float all winter was a good thing. What's the harm in floating them over the winter?

Estragon · June 2017

In my case, the problem is depending on late fall weather, the panels can get covered by a fairly thick layer of sticky snow. Once covered like that, the sun can't warm the panels, so they would stay snow covered until spring. The charge controller tare would discharge the batteries completely if I couldn't get there to manually clear the snow.

Aside from the snow problem, you would also be cycling the bank unnecessarily. My MN classics have the ability to skip absorb for 'X' days which reduces this issue. You could set absorb to a low voltage for winter to reduce it as well.

With flooded L16s, I was concerned that self discharge would leave the banks at too low SOC in spring. If I had AGMs, that would likely be less of a problem, so I would have just shut everything down for winter. With everything shut down, there's just not a lot to go wrong.

AGM Battery Balance

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