Optimal charging current

kevinjones · February 2017

Hello,

I am setting up a charge controller which charges a bank of 18 6V Trojan L16 batteries, ~400Ah each. I am calculating this to be 1700Ah 3600Ah total, or 43,200 Wh. It is set up for 12 Volt. (Please spare the recommendation for not having so many parallel connections, it is what I am stuck with right now). I am not sure what the specs on the solar array is.

I observe that when the sun is good and the controller is calling for bulk charge, it will pass 150 Amps to the bank. I observe that in this condition, the batteries "boil" quite violently and the battery room smells pretty strong of hydrogen sulfide, albeit being vented fairly well.

My question is, is there a maximum rate at which batteries should be charged? eg, say a battery is 400Ah, is there an X amount of charge amps that it would be good not to exceed? I am wondering if I am doing damage to the batteries charging them at this rate. They are maintained regularly and never get too low on water, however, I just wonder if the violent action itself is hard on the batteries?

Since there are 9 pairs of 6V batteries wired in series, theoretically each 400Ah battery has 150/9 ~= 17 Amps charging it until it reaches bulk charge voltage.

Thanks,
Kevin

westbranch · February 2017

WHOA! you have 9 12V batteries at 220A = 9 x 225 ah = 2025Ah...Solar uses the 20 or24 HR rate for calculations such as this
STOP! you are killing those batteries.

more later

http://www.trojanbattery.com/pdf/TRJN0111_ProdSpecGuide.pdf

kevinjones · February 2017

The total amps to the entire bank is 150 Amps. IIUC, that would be 17 Amps per 12V battery (9 12V batteries in parallel).

I am not following the numbers you posted.

kevinjones · February 2017

I edited the OP as I had incorrectly calculated the total Ah of the bank.

BB. · February 2017

Minimum of 5% rate of charge with 10-13% being really nice for full time off grid use.

Using the 20 hour rate for battery capacity, then a 3,600 ah battery bank would like;

3,600 ah x 0.05 = 180 amp minimum
3,600 ah x 0.10 = 360 amp nominal
3,600 ah x 0.13 = 468 amp typical maximum

-Bill

kevinjones · February 2017

So then the batteries "boiling" like what seems crazy to me, is okay?

westbranch · February 2017

You are not crazy. There is something not correct in what you are trying to do..
Please post a description of what you you equipment are using and the settings of all components, pics are good too.
In BULK the battery temp should rise but not anywhere near the point that the electrolyte boils, that is the sign that ''something is wrong'' now we just have to figure out what...

mike95490 · February 2017

Each battery should have no more than 40A going into it. (10% of rated capacity, 400Ah = 40A charging max)
so 2 in series, for 12V should still have only 40A
a pair of strings [ 2S2P ] should only see 80A ( 40 into each string )

Wet batteries should only be gassed for no more than an hour. it might take several hours to charge up a low bank, but
once charged, and gassing well, you should be prepared to stop.
It sounds like you have not been depleting the bank much, and the controller is at default settings for a deeply cycled bank, and you are cooking them pretty well.

mcgivor · February 2017

You've not stated the voltage at 150A, the charge voltage applied to the batteries, bulk absorbtion and float.

westbranch · February 2017

At 24 volts it could do that, but should not at 12v.... we need to know about the charge controller settings...

kevinjones · February 2017

Heliotrope CC 120 PWM charge controller, upgraded to 240 Amps.
Bulk charge voltage = 14.7 14.4 volts
Float charge voltage = 13.7 volts
The 150 amps current I am seeing is when switching from float charge to bulk charge, where the battery voltage is still below 14 volts.

@westbranch: I used the term "boiling" figuratively. I don't think the batteries are getting hot, just that they are gassing at a very rapid rate.
@mike95490: Yes, I don't think the bank gets depleted all that much.

kevinjones · February 2017

edited typo on last post - bulk charge voltage is 14.4 volts.

kevinjones · February 2017

@mike95490:

I have used a Morningstar ProStar 30 in the past. It had an automatic daily charge cycle which would do a bulk charge once every day, to 14.4 volts. The float charge was 13.7 volts. The stated reason for the daily bulk charge was to provide a degree of equalization. I have been operating off of that paradigm.

It sounds like you are saying that if the bank is not getting sufficiently depleted, then a daily bulk charge may not be a good idea?

kevinjones · February 2017

@mike95490:

It looks like then, as far as raw amperage goes, I am okay. As I said, I calculate about 17 amps (avg) flowing through each battery.

mcgivor · February 2017

Calculations are theoretical, measurements are reality, trying to ballance the current equally with that number of strings, would be like trying to make love to a vampire with a monkey on your knee. A DC clamp on ammeter is needed to verify each string current during charge, they need to all be equal, or very close at least.

westbranch · February 2017

Cycling above 90% in most cases is not good for longevity of a battery, if you are full time using the system, an 'every 2 day --full charge ' regimen is better.... all batteries have X number of charge cycles in them.

Did you take any starting electrolyte SGs. that is the only way you can determine the SoC for those cells.... As you put it the monkey on your knee is the total number of cells.... best keep a good frequent tally on the SGs 18 x 6 cells = 108 readings..

kevinjones · February 2017

@westbranch:

Thanks for the advice, I'll do that. I didn't realize it was important to let the batteries get depleted.

For automation sake, what would be a good rule-of-thumb voltage to allow the batteries to get down to before charging?

No, I haven't taken any electrolyte readings. Can you please define:

SG
SoC

Thanks....

kevinjones · February 2017

Also, does cycling apply to float charge? I thought that was meant to be on them all the time.

westbranch · February 2017

sg = specific gravity of the electrolyte, you need a hydrometer
SoC is the State of Charge and you can only tell this exactly,only by using the hydrometer.
Trojan puts out a lot of literature you need to read this one , it explains a lot...
http://www.trojanbattery.com/tech-support/battery-maintenance/
and this one about charging
http://www.trojanbattery.com/Tech-Support/FAQ/Charging.aspx

if you have to to get your first reading, contact your vendor, he must have a few on site..

good luck

kevinjones · February 2017

Thanks :-)

westbranch · February 2017

and this one made me think about watering and you might wantt to look into one of their watering systems with that many cells to fill
http://www.trojanbattery.com/pdf/TRJN0111_ProdSpecGuide.pdf

Estragon · February 2017

@kevinjones -
Are all the cells gassing vigorously or just some?

It could be that differences in resistance between strings resulted in some being fully charged while some are not. The controller is only seeing overall voltage so could be overcharging the full ones and undercharging others.

If so, the undercharged ones will build up sulfate, compounding the problem over time.

I would get a hydrometer asap and establish state of charge for each cell definitively, then disconnect the parallel strings and charge in pairs of batteries with similar SOC, equalizing if necessary, so that all cells reach the same fully charged state. You will likely have to do this regularly to keep the bank balanced, but you really need to take SG readings to know how often.

You could start this process now using resting voltage of individual batteries or strings if it will take time to get a hydrometer (remote location or whatever).

Also, how are the parallel connections wired? I don't have the link handy but I think it's smartguage.co.uk that has a good article on this.

kevinjones · February 2017

@Estragon Thanks, that sounds like a good idea. I think there is a hydrometer around here somewhere.

smartguage.co.uk gives page not found. My practice with parallel strings has been to connect positive and negative on opposite ends of the bank.

kevinjones · February 2017

"You could start this process now using resting voltage of individual batteries or strings"

How long does a battery need to rest before getting an accurate reading?

mike95490 · February 2017

kevinjones said:

"You could start this process now using resting voltage of individual batteries or strings"
How long does a battery need to rest before getting an accurate reading?

At least 2 hours, no charge or discharge, Some say 3 or 4 hours. What happens is the local area in battery plates gets depleted/built up from discharge / charge and takes some time to stabilize. Right when you pull a battery off a charger, it may read 13.8V. but as time goes by, it settles at 12.7V which takes a couple hours. Or right after a heavy discharge, it may read 11.8V but an hour later, it's at 12.2V (Like when you call for a tow truck to jump start your car, and it shows up an hour later, and your car cranks right up, the battery sitting, recovered from the last episode of heavy cranking)

kevinjones · February 2017

Thank you.

Estragon · February 2017

Try
Http://smartguage.co.uk/batt_con.html

The idea is to get string resistances as equal as possible, for example, by wiring each string to a common connection or buss using equal lengths of cable.

It will take several hours with no charge sources or loads to get resting voltage. You could do it without resting just to get relative states of charge if you check the strings at about the same time to see if some are low relative to others. Resting is still needed to get a decent measure of absolute SOC.

If you can check SG though, that is the better method.

kevinjones · February 2017

This gives an idea of what the setup appears like. I couldn't get all of them in the photo, but it continues to the right in the same fashion. You can see two batteries to the left which are out of service. The interconnecting cable lengths are about as consistently equal as is reasonably possible.

Image: https://us.v-cdn.net/6024911/uploads/editor/q6/jp5dmc1wpkw8.jpg

kevinjones · February 2017

State of charge - numbers correspond to position of cells as shown in the photo:

1235    1275    1250    1285    1245    1250    1275    1260    1280
1280    1260    1250    dead    1260    1250    1285    1260    1260
1290    1100    1250    1275    1250    1250    1275    1260    1275
------     ------     ------     ------     ------    ------     ------     ------     ------
1180    1260    1250    1275    1255    1250    1270    1270    1250
1255    1250    1255    1275    1260    1255    1285    1255    dead
1300    1280    1255    1275    1245    1250    dead    1250    1265

kevinjones · February 2017

@Estragon:

"Are all the cells gassing vigorously or just some? "

Yes, it is really only three cells which we gassing vigorously. The rest did not appear to be anything alarmed about. Right now I am doing an equalize charge at 15.3 volts for about an hour and a half.

westbranch · February 2017

Are the 2 , each with a dead cell, the ones out of Service?
You also should remove top row #2 from left and bottom row #1 from left and charge them separately till they come into sync with the whole bank..... or put together and really monitor them closely, VERY closely... they have a good chance of failing if they don't balance out...

Optimal charging current

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