Small Off Grid Setup Issue..

TheBackRoads

Hey guys,

I don't think I have formally introduced myself. My name is Jeff and I live in Minnesota. I'm 21 and love renewable energy technology. Now I have a small setup with the batteries being my weak link. I have 3 Kyocera KD135GX-LPU running in to a Xantrex Mppt charge controller. From there I used what I had laying around and am running 2 AGM Optima Yellow Top batteries (about 55AH each). For AC power I'm running a Exeltech XP1100. My system is 12v. I think that sums it up. Anyway, I'm having issues with my MidNite battery monitor, (http://www.solar-electric.com/mnbcm.html) so I emailed NAWS and MidNite for some possible help. They stated that I should setup a custom battery charge in my Mppt. So I changed that last night and today is a beautiful sunny day. Now at 11AM here, my charge controller was in Bulk mode and only dumping like 75W into the batteries. From there I unplugged my battery temp module and set the default battery temp to cold, thinking it would dump more into the "cold" batteries. No go. I then turned on some AC loads (big desktop computer and 40" LCD, 171watts AC) to see what would happen. Now my system is currently dumping 14.5V, 15.9A @ 230W. Now I guess I assumed the Bulk charge would be dumping in what ever the solar produced until the set voltage its at, and then roll over to the Absorption charge set parameters... Am I missing something about how the charge controller works, or perhaps I have a bad parameter? Here are a few parameters I have setup.. Force state Bulk, 180min absorb time, recharge at 12.9v, bulk 14.3, absorb 14.5, float 14.4 (only to see if the Midnite would change the battery status to green), -21mV/degC temperature compensation (manual states for AGM batteries). Now I have always had the recharge and absorb time the same, so the only new things are the charging voltages.

Sorry for the memory dump of information.. I just cannot figure out why it was putting in so little charge in bulk mode, perhaps why the MidNite says my batteries are not being fully charged week to week.

Regards,
Jeff :cool:

BB.

Re: Small Off Grid Setup Issue..

[everyone]Hi Jeff! [/everyone]

TheBackRoads wrote: »

Now I have a small setup with the batteries being my weak link. I have 3 Kyocera KD135GX-LPU running in to a Xantrex Mppt charge controller. From there I used what I had laying around and am running 2 AGM Optima Yellow Top batteries (about 55AH each). For AC power I'm running a Exeltech XP1100. My system is 12v.

AGM batteries are really good at surge current--The proper AGM in a UPS can output C*4... Or in your case, possibly surge current upwards of:

• 2*55AH * 4 = 440 Amps

But, in reality, most people are not looking for 4,000 watts of AC power for 15 minutes before destroying the battery bank.

For most applications, they are looking for at least 5-10 hours of battery powered operation. And we recommend for daily usage (such as off-grid), that you design your system that you only discharge the batteries to 50% capacity for longer life.

If you have an application where you use power weekly or less (emergency power, camping on weekends, etc.), you can draw down batteries to 20% state of charge--as long as you get them quickly recharged above 75% state of charge again (batteries that set below 75% state of charge for days/weeks/months will "sulphate" and permanently loose capacity--leading to an early death.

Try to avoid drawing below 20% of capacity as this can kill a battery in days.

So, for a 110 AH 12 volt battery bank, you should plan on a C/10 or C/20 discharge rate of:

• 0.50 max discharge * 110 AH / 10 = 5.5 amps for 5 hours
• 0.50 max discharge * 110 AH / 20 =2.75 amps for 10 hours

I think that sums it up. Anyway, I'm having issues with my MidNite battery monitor, (http://www.solar-electric.com/mnbcm.html) so I emailed NAWS and MidNite for some possible help. They stated that I should setup a custom battery charge in my Mppt. So I changed that last night and today is a beautiful sunny day. Now at 11AM here, my charge controller was in Bulk mode and only dumping like 75W into the batteries. From there I unplugged my battery temp module and set the default battery temp to cold, thinking it would dump more into the "cold" batteries.

What was the charging voltage at the battery bank and right on the charge controller's battery output terminals (i.e., is there a voltage drop problem confusing the charger).

No go. I then turned on some AC loads (big desktop computer and 40" LCD, 171watts AC) to see what would happen. Now my system is currently dumping 14.5V, 15.9A @ 230W. Now I guess I assumed the Bulk charge would be dumping in what ever the solar produced until the set voltage its at, and then roll over to the Absorption charge set parameters... Am I missing something about how the charge controller works, or perhaps I have a bad parameter? Here are a few parameters I have setup..

14.5 volts at the battery bank is a good voltage.

Batteries will also self regulate the maximum current when charging at a set voltage... For example at 14.5 volts and the batteries 50% state of charge, they may accept 20-40 amps (or more). This is the "Bulk" phase charging where the charger is simply supplying as much current as it can.

However, when the battery reaches around 80-90%, the charger will hit the 14.5 volt set point and you may see 10-20 amps maximum tapering down to perhaps 2 amps or less when the batteries are 100% full. This is the Absorb phase where the charger is at constant voltage charging and the battery is naturally reducing charging current to near zero amps when the battery is fully charged. The 14.5 volts (typical) is the "transition voltage" between "Bulk" and "Absorb" charging state.

Force state Bulk, 180min absorb time, recharge at 12.9v, bulk 14.3, absorb 14.5, float 14.4 (only to see if the Midnite would change the battery status to green),

I am certainly no expert, but from what you describe, the Battery Monitor will reset to 100% full when it measures Vbatt>14.3 volts for 3 hours... That absorb time is usually between 1-4 hours depending on lots of things. You may choose to drop that to 120min/2hours as your 100% full reset point.

You also need to check how your charger behaves... Some control using an Absorb timer (charger set for 2 hour time and Battery Monitor set for 3 hour reset--Batt Mon will never reset to 100% full).

Other chargers (or other config options) may look for battery charging current falling to 2% rate of charge (battery not accepting much current @ 14.5 volts--therefore full).

-21mV/degC temperature compensation (manual states for AGM batteries). Now I have always had the recharge and absorb time the same, so the only new things are the charging voltages.

Batteries require lower charging voltage as they get hot--TC is normal and works very well when you have temperature extremes (freezing or 100F weather).

Sorry for the memory dump of information.. I just cannot figure out why it was putting in so little charge in bulk mode, perhaps why the MidNite says my batteries are not being fully charged week to week.

You have hit the major weakness of battery monitors... They "infer" from battery parameters when the batteries are fully charged.

It is equivalent to measuring the quanity of fuel in your car's gas tank by measuring the fuel gallons in/out through the fuel line. Works well, but errors slowly build up over time.

They use the "Reset" parameters to "recalibrate" back to 100% full...

Another issue is that battery capacity slowly changes over time... From new, capacity will increase by 10-20% over ~50 cycles... Then as the battery cycles/ages, it slowly looses capacity.

Eventually, the battery will die even-though the Battery Monitor shows it at 50% state of charge. At that point, you would replace the battery bank.

Deep Cycle Battery FAQ
www.batteryfaq.org

Does that help?

-Bill

TheBackRoads

Re: Small Off Grid Setup Issue..

Bill,

I sure appreciate the in depth response (and welcoming)! The meter has reset, as it needs 14.25V for 2 hours I believe it is. I guess I was just not properly charging the batteries. I know however that they have been abused and are not 100% previous to my solar application.

Will a charge controller skip absorb for the 180 min like mine is set for if it senses the batteries are full and go right into float from bulk? Only reason I ask is because I observed the screen every hour or so today and noticed that it went from bulk to float, as I sure would have seen it in absorb charging today, and I did not.


This is a great forum with lots of knowledge, I appreciate any and all input!

Regards,
TBR

dwh

Re: Small Off Grid Setup Issue..

TheBackRoads wrote: »

Will a charge controller skip absorb for the 180 min like mine is set for if it senses the batteries are full and go right into float from bulk? Only reason I ask is because I observed the screen every hour or so today and noticed that it went from bulk to float, as I sure would have seen it in absorb charging today, and I did not.

Do you still have your float set higher than your bulk?

That charge profile in your OP seems all wrong. 14.3v bulk, 14.5v absorb and 14.4v float...

The Optima Yellow Top charging specs don't specifically state the set points for a 3 stage, but from what I can see, it looks like it should probably be something like 15.6v bulk (rapid recharge), 14.7v absorb for one hour and 13.2v-13.8v float (13.2v for long term float).

http://www.optimabatteries.com/product_support/charging.php


EDIT: Or, it could be like 14.7v bulk, 13.2v-13.8v float and "something in between bulk and float for some amount of time" as absorb.

You should contact Optima and ask them exactly what settings to program into your charge controller.



(Others note, the Optima is a spiral cell AGM battery and so it's a bit different than other batteries.)

TheBackRoads

Re: Small Off Grid Setup Issue..

I really appreciate the reply. I have changed the float lower and seems to be working alright. Although, I am not home during the day to see how long it actually is in absorb charging, and a cloudy weekend not looking good, it might be awhile before I can observe this. I have looked at that Optima page before and assumed/figured that if I set Bulk to 14.2, absorb 14.6, 13.5 float would suffice.


Thanks, TBR

TheBackRoads

Re: Small Off Grid Setup Issue..

Just wanted to add an update! Today once I hit float (around 12:30pm) I started plugging in some loads around the house due to it being Saturday and no clouds! I ended up running our very old, energy-hog chest freezer, ((the compressor runs continually 24/7 and keeps food just barely cold enough) my dad cant part with it yet) dsl modem and natural gas hot water heater. Minimal loads but sustained 97-100W continually. Charge controller stated 106.2Ah or 1.46kWh... the most I have ever produced to this date. Now I know this is nothing for some of you but I was happy with what I managed to produce today.:D Thanks again for the help in setting my charge voltages, everything is running great.

Dave Angelini

Re: Small Off Grid Setup Issue..

The other thing you can do is set your capacity on the CC to zero or the smallest number it will allow. This will force the CC to stay in absorb for the full time that you program. Small banks of batteries can be helped by doing this.

BB.

Re: Small Off Grid Setup Issue..

Regarding your father's old chest freezer--Get a Kill-a-Watt meter (or equivalent) and through it on the old boy (the freezer).

You will probably find it is running around 120 watts 24x7:

• 120 watts * 24 hours per day * 365.25 days per year = 1,051,920 Wh = 1,052 kWH per year
• 1,052 kWH per year * $0.12 per kWH (or your power rate) = $126 per year

A typical freezer is probably half that power usage (less than 400 kWH per year)... Or save $60-$70 per year.

A new freezer would pay itself off in 5 years or so... More if your power is more expensive or the price of electricity goes up.

-Bill