Conext XW6848 Charger Settings Questiond

ELYNN4 · October 2021

My system includes the XW6848, BatMon, AGS and SCP. I have 48 Trojan RE 2v batteries (2200 AH). I have 4500 watts of PV and a 9kw generator.

During the summer the PV maintains the batteries perfectly, achieving a SG of 1.277 on sunny days.

I’m located in Alaska so this is the time of year that my system is more and more dependent on the generator. When charging with the generator I cannot get the SG above 1.25, even though I have the XW6848 charger settings set with Absorb Voltage set to 60v, the Bulk/Boost voltage set to 61v and the Absorb Time set to 6 Hours. When the generator first starts the XW6848 display shows that 90amps are going into the system. The BatMon only shows 75 amps going into the batteries.

My questions center on the XW 6848 charger settings:

I have a BTS - my battery temperatures in the winter range from 40 to 50 degrees F. What is the proper temp compensation setting to use?

The battery voltage readings on the BatMon are .3 volts lower than on the XW6848. Which volt reading is the XW6848 using to set the charge voltage?

If the XW6848 is using its own voltmeter to set the charger Absorb Voltage should I bump up the voltage .3 volts to match the BatMon voltage? My handheld VM agrees with the BatMon readings. Seems like I need a higher charging voltage in order to get my SG up.

Thanks in advance for any thoughts/guidance folks might have.

Dave Angelini · October 2021

This really is a battery charge question. You use the voltage to get the correct SG. It does not matter what the equipment voltage is telling you. The SG needs to be right in winter and well as summer!

Ouch on your battery! 144 cells to maintain, I bet you will not choose that one next time!

ELYNN4 · October 2021

Thanks Dave, I am using Hydrocaps so only need to water the batteries twice per year, but having a third as many caps to deal with would have been nice…. Batteries are four years old now - seem to be holding up so far. We’ll see what the best available/affordable technology is when it comes time to replace them.

wellbuilt · October 2021

I’m no expert for sure but my out back inverter charger dosent use temp compensation for EQ so I have to crank it up
Manually
I set my absorb voltage up from 58.5 to 59.5 in winter .

1.25 is low
Check the battery voltage against the inverter / charger readings mine was off a little and I adjusted it in the mate .
By the end of winter my battery SG seams to slip a little .

But then the sun comes out and I’m all good by may .

ELYNN4 · October 2021

Trojan calls for a temperature compensation of .005mv/C. In my case that would be .005 x 72cells which equals .036, which I take to mean 36 mv. The XW6848 only accepts negative numbers, so I believe that I need to program in -36mv. What is confusing is the example they give in the XW manual calls for 108mv/C.

Any thoughts? Is my math correct?

wellbuilt · October 2021

I don’t know about the math but at 45 0 f my controller seams to take my 59.5 absorb voltage up over 62v.
It’s hard to say because it’s always adjusting but when I’m cycling my battery’s in the dead of winter they stay around 45o
but your readings are very low and you are in absorb for6 hours so all you can do is raise the voltage .
I get all twitchy if I get some cells in the 1.26 range .

ELYNN4 · October 2021

Thanks Wellbuilt. Good to hear you’re system is cranking the voltage up to 62v. I’m going to start cranking my voltage up. It’s amazing how much better my PV charges the batteries vs. the generator. Must be the number of charge hours we have in a day thru the summer.

I have one battery with cell readings that are .01 to .015 below all the other cells. But this only happens when using the generator. Should I use this battery to determine the proper charge voltage and absorb time?

thanks again for the response.

wellbuilt · October 2021

you need to get your bank up to full charge first then check to see how far off the low battery reads ,
then you can eq the bank
you have a large bank if you have one low battery you could try running with out it until the bank drops it voltage .
When the voltage is about the same wire the low 2 volt battery back in and see if it matches the rest of the bank in 48 hours or so , one low battery .15 is not so bad .
I have one low battery that seams to sit at 1.265 or a little more when the bank is 1.275 + this shows up at Christmas’s
I have a 430 ah battery and I watch my watt going in when absorbing when the battery’s are excepting 400 watts I call it done .
Your battery bank is 5 times the size so I would think 2000watts would be close to the end of absorb .
It seams like your temp compensation is off all so , I know I set absorb at 59.5 in the winter and see 61.5 or so on my controller .

My system voltage was off at first my charge controler V was off so I adjusted it to match then I adjusted the mate to match as close as I can .
Read the bank that is really the only real number that counts .
I watch my battery temp with a hand held inferred thermometer .

ELYNN4 · October 2021

Thanks Wellbuilt, I’ll give those suggestions a try. I do have midpoint voltage sensors on the battery bank. They almost always read the same, occasionally one bank will be a .1 of a volt lower but only for a very short period of time.

I have my absorb voltage set at 60.3. With the temp compensation it bumps up to 61.3, which is still lower than yours. I get frustrated with the long absorb times where the generator just loafs along throwing 10 to 15 amps at the batteries. Bumping up the voltage should help with reducing the lengthy absorb times as well.

Thanks again for sharing your info

BB. · October 2021

Be careful with the temperature offset number... You are mixing units a bit here. For typical Lead Acid batteries:

Typically -5 mV per degree C per Cell = -0.005 Volts/C/C (not -0.005 mV)

So the math should be (assuming you have 24 cells in series for 48 Volt bank, not 72 cells in series or 144 volt bank):

40F = 4.4C
"standard conditions" = 77F or 25C
-0.005 Volts/C/C * (4.4C - 25C) * 24 Cells = +2.472 volt offset for 48 volt bank @ 40F
59 Volts Charging (@ 77F) + 2.472 volts temperature offset = 61.472 Volts Absorb charging setpoint @ 40F

You have other things that can affect the "programmed voltage"... Voltage drop on wiring and sometimes a brand/model of charger will give a "temperature" corrected reading on the user display (i.e., say 59 volts charging--because that is what you programmed, but actually output 61.5 volts charging with temperature offset).

There is always having a good quality/calibrated DMM so you can measure actual terminal voltage at the battery bank (and each 2 volt battery in series). Ensuring you have a good battery in the DMM too (we here have had DMMs with bad batteries--And they gave consistently inaccurate voltage readings).

One good way to double check your meter... Check each 2 volt battery voltage, then add all them up and see that they are "equal" to the 48 volt battery bus voltage. That double checks the meter is at least consistent (the dead battery meter I talked about, the individual battery voltages did not add up to the battery bus voltage--But the readings were repeatable--just repeatably wrong).

Another issue for "our friends up north"... Many (older?) inverter designs had high battery bus cutoff voltages that shutdown the AC inverter at (for example) 60 VDC--Instead of >62 VDC to allow for cold temperature offsets and EQ charging voltages... It was not unusual for folks with "cold banks" needing to shutdown the inverter until EQ was completed.

-Bill

ELYNN4 · October 2021

Thanks Bill, I will do some double checking on my instruments. I am using a Refractometer to get my SG readings. In theory my readings should be temperature compensated. The bottom line is that when I’m charging with the generator I’m not getting the SG up to the recommended 1.277 yet. So I need to either keep cranking up the charging voltage or run the generator for extremely long periods of time, which I would rather not do.

The PV is able to hold the battery bank at 1.277 throughout the summer (8 months) so I’m pretty sure that all of the batteries are good at this point. The whole system is only four years old, so I don’t think that the Inverter is as old as what you described above. This under charging with the generator has been an issue since day one. But I only need to use the generator for four months out of the year. Then the sun climbs back up and everything and everyone is happier - the joys of living in Alaska!

Dave Angelini · November 2021

Bill is right BTW ! Even though that battery is a design nightmare of 3 parallel cells in a 2V battery, his math is right. You have 24 x 2v cells for your temp math with XW.

I have a bunch of clients up your way. The rule for any extended winter gen runs is a 2 stage charge to save fuel and once a week you run to get correct SG in a pilot cell. There is more but I have to save something for the people who pay me. Good Luck!

ELYNN4 · November 2021

Thanks Dave. 2x24 makes total sense, what was I thinking…😏

wellbuilt · November 2021

ELYNN4 said:

I get frustrated with the long absorb times where the generator just loafs along throwing 10 to 15 amps

This seams funny to me @ 15 amps 915 watts +- you should have reach your absorb voltage I would thing.
I would think once you get down to excepting 2000 watts you should be done charging I go by a % of my battery in watts to end absorb ? 430 ah I absorb down to 400watts .

Maybe bill/ Dave could speak on this .

wellbuilt · November 2021

Just thinking when I’m absorbing at 400watts my sq is very close or above 1.275 except my one lagging sell witch seams to stay around 1.260 /67 ?
Seams like some thing is fishy .

ELYNN4 · November 2021

If I stop charging when the return watts are 2000 my SG is at 1.25. Maybe if I keep bumping the voltage higher I will find the sweet spot? The XW switches from absorb to float at 2% return watts - I had to program in the battery bank to show 0 amp hours in order to extend the absorb time. Otherwise it was shutting off the charging when the SG was 1.20.

mike95490 · November 2021

Absorb is not supposed to get the batteries completely full. A proper float charge does the finishing off. Sadly, there are not usually enough solar hours in a day, to always be able to complete Bulk, Absorb, and then Float

ELYNN4 · November 2021

The beauty of being in Alaska is that we do have enough daylight hours to fully charge the batteries - as long as we have sunny days…

wellbuilt · November 2021

1.25 is low about 85% that could be ok for a few days but you really need to get up to 1.275 once a week
There is a manufactures max voltage charge rate to follow ,mine is 62v but with temp comp it go’s above that for eq

ELYNN4 · November 2021

Trojan calls for a max of 64.8v when Equalizing. With my temperature compensation it could go up another 1.5 to 2.0 volts. I’m well below those numbers but significantly higher (+/- 61) than the 59.28v that they call for when Bulk charging.

wellbuilt · November 2021

Are you drawing your battery down a lot over nite and using a lot of power during the day ?
Maybe you don’t have enough power to get to full charge ?

If I draw my battery’s down 40% and try charging every 2.5 /3 days it takes me a long time to get the sg up to full again

ELYNN4 · November 2021

I use around 75 - 100 amp hours per day. The generator runs when I’m down +/- 400 AH or 20%. The batteries rarely get below 75% SOC.

When the generator is charging the batteries it’s putting 75 amps into the bank until the latter part of the absorb cycle (when there are 50 amp hours remaining to top it off according to the BattMon).

I’ve cranked up the Absorb voltage to 60.3. I also bumped the temperature compensation to 120 mv/C. With the batteries at 60F I was seeing 62.5v during the Boost, first hour, of the Absorb cycle. The battery bank seemed to be accepting more amps later into the Absorb cycle. I’ll have to let the system go through a few cycles to see how much the SG improves.

Appreciate your thoughts and interest with this.

wellbuilt · November 2021

So you are using 75/100 amp hours per day and charging every 4or 5 days ? Hmmmm
Your #s there look good ,but I was having charging issues letting my bank sit for day with no charge going in .
I use 10 /15% of my bank in 24 hours so I was going to the cabin on Thursday nite and charging Sunday befor I left for the week.

It sat 21/2 days before charging and by Christmas I was seeing low sg 1.260 and one battery was 1.245 .

It took a long time to bring the battery’s back up to 1,275+ .

The battery’s did not recover 100% until they where getting long days of solar in may of the following year.

Now I charge every day ( I’ve never heard any one else doing this )
What I’ve been doing is charging every morning , I wake up and start the coffee and turn the generator on and let it run for a hour it gives me 25ahs into the bank .

The next day it will run for 1.5 hours
on day 3 I’ll let it run for 2 hours
day 4 I go thru 2 hours of absorb .

I think 4 days is to long . Just my opinion but this keeps my sg up ina safe range

ELYNN4 · November 2021

Hmmm, very interesting- so many schools of thought on how to best treat FLA batteries. I’ll keep pecking away at this and see if I can find a recipe that works best for my system. I’ll let you know…thanks again for the excellent guidance.

wellbuilt · November 2021

I know bill speaks of just charging to 90% for a week or weeks then getting a full charge in to the battery’s every few weeks and it seams to work for me if I don’t go days with out any charge.

Some sun is allso a game changer for me , if I can get some sun power in it really helps .

Last year my panels where covered in snow for 10 weeks so the solar was off .

Charging every day seamed to work ,it’s kind of like having sun come and go ?

ELYNN4 · November 2021

One new development on my end. The SG of my batteries is higher in the morning after the generator has run than it is immediately after stopping the charge cycle. The only thing that I can think of is that the Hydrocaps are adding water to the top of the cells, resulting in some stratification of the acid in the cells. Then after sitting over night powering the house the acid gets mixed more thoroughly. At any rate the SG is .01 higher in the AM, or 1.26 instead of 1.25, which is much closer to where I need to be. Interesting…

wellbuilt · November 2021

That sounds interesting , I allways swish the fuel around in the battery’s when checking sg .

I draw in and out 3 times before checking .

It helps . 1.26 is still low but much better .

And you have been drawing power from the battery’s all so .
I allways draw fluid from below the top of the plates .

For me the hydrometer allways seams like a mystery I have a few different models and I get different readings some times . 🤷‍♂️

ELYNN4 · November 2021

I’m using a refractometer, which is temperature compensating. I just use a stirring stick, swish it around and place a drop on the lens. I had used a hygrometer to swish the acid around in the past, but hadn’t picked up on the difference. I’ll have to go back to that practice and see how things look…

wellbuilt · November 2021

Your battery’s must be tall so maybe the top is not mixed ?

Are you getting good fizz when you are in absorb , that should mix it up .

ELYNN4 · November 2021

The Trojan L16 is just under 18” tall. I would call the bubbling “lazy” when it is under full charge and at the highest voltage. I think that part of my problem is that I don’t have the capability of charging the bank at a high enough charge rate. My generator can only carry a 75 amp load. I think the recommendation is C10, which would put me up at 220 amps. I can’t even hit that on the sunniest day with my panels putting out their max of 80 amps and the generator putting out the 75 amps it’s capable of producing.

on the bright side - my batteries don’t get splattered by the off gassing 😏

mike95490 · November 2021

ELYNN4 said:

..... I would call the bubbling “lazy” when it is under full charge and at the highest voltage. I think that part of my problem is that I don’t have the capability of charging the bank at a high enough charge rate. My generator can only carry a 75 amp load. I think the recommendation is C10, which would put me up at 220 amps........

Then you are doomed to keep replacing batteries that you kill. You need more PV so you don't have to run the generator as much
Try starting the generator early morning, before sunrise, and run it all day while the solar is active, it may take 2 days of this to fully charge the batteries before you can start an EQ to see if they are salvageable.

Conext XW6848 Charger Settings Questiond

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