XW-MPPT60 Undercharging Batteries

marsofold · April 2012

My XW-MPPT60 on the default settings was resulting in my battery bank being undercharged. Called out at 90% by my Midnite Monitor and verified with a digital voltmeter. I generally reserve my battery bank only for use with catastrophic situations, thus keeping it full all of the time. However, this state makes the XW-MPPT60 almost immediately go into Float voltage, which eventually results in an undercharged battery bank. I have been dealing with this by running an IOTA boat charger on it every two weeks, but I'd rather fix the situation by tweaking the XW-MPPT60 settings. So I made the ultimate sacrifice and broke out the XW-MPPT60 manual. I tried bumping up the Bulk voltage setting up to 14.8 volts (since 0.35 volts is dropped on the way to the batteries during 60 amp charging), but left the Absorb voltage setting on the default 14.4 volts. Then to test the change I put a 1600 watt load on the system for a few minutes to get the Bulk charging to initiate. It did initiate, but after minutes went into Absorb, then a few minutes later went into Float. So what I learned was: the XW-MPPT60 program will only maintain the Bulk voltage until the battery bank's voltage reaches the Absorb setting voltage. Which for an almost full battery bank is only a matter of minutes. The Absorb cycle only maintains its voltage until the charging current drops to 2% of the battery bank Amp-Hour capacity. Also minutes. Then the charger goes into float without the batteries having spent any significant time at 14.4 volts. Which leaves them undercharged. So I am left with three options to deal with my undercharged battery situation:

#1 use my IOTA charger every week or two to force the battery bank into being 100% charged...

#2 Once every week or two put a medium load (600 watts) on the battery bank for two hours at night to deliberately drain the batteries down to a SOC of 85%, enough to get the XW-MPPT60 to initate a full charging cycle the next morning...

#3 program the XW-MPPT60 with a selected battery Amp-Hour capacity of zero, which forces the daily Absorb cycle to be 3 hours long regardless of battery charge state (hard on the batteries?).

So I have two questions:

Which of the three above options will result in the best battery bank lifespan?

And since I appear to be dropping 0.35 volts between my controller and the battery bank when at 60 amps charging, should I just bump up both the Bulk and the Absorb voltage to 14.8 volts to compensate? Would doing so hurt my battery bank lifespan?

Thanks for all of the wise advice here. Part of the reason I buy most of my solar supplies from our sponsor.

waynefromnscanada · April 2012

Re: XW-MPPT60 Undercharging Batteries

Welcome to the form Marsfold.
From everything I've ever read on this form, from folks who know far more about batteries than I - - - - batteries "like" to be cycled to some extent on a regular bases, otherwise their lives will be cut short.
Now we'll wait for those folks to favor us with their battery wisdom.

vtmaps · April 2012

Re: XW-MPPT60 Undercharging Batteries

From your post, I have no idea whether or not your batteries are undercharged. Battery monitors are not reliable unless they are calibrated to the battery. Your digital voltmeter isn't much better at gauging the state of the batteries. Are your batteries flooded? If so, then measure the specific gravity. If not, tell us more about your batteries: brand, model, and wiring configuration. Also tell us what your digital voltmeter shows and the conditions under which you are taking the readings.

--vtMaps

mike95490 · April 2012

Re: XW-MPPT60 Undercharging Batteries

If you are loosing .3V in your wires, that means they are WAY TOO SMALL.

What does a voltmeter read, when the batteries have been idle for 3 hours ?? You cannot take reliable voltage measurements when batteries are in use.

Cariboocoot · April 2012

Re: XW-MPPT60 Undercharging Batteries

As vtMaps said, we need to know what the batteries are. The nice thing about FLA's is; you can read the SG and know if the battery actually is charged, regardless of what Voltage readings you get. Even so the batteries should be left sitting with no current in or out for an hour to get accurate SG readings. If they're sealed, you're out of luck here.

And Mike's right about the V-drop; that's about 3% and you don't want that. Bigger wires will help keep the Voltage difference between charge source and battery to a minimum. This is a big problem with 12 Volt systems. He's also right about the Voltage readings being unreliable while in use.

Otherwise I'd suggest putting the programming back the way it was on the controller and cycling the batteries once a week. Over-all that uses the least amount of power (since recharge comes from the sun not the gen or utility) and will result in the longest life given the circumstances. Possibly you could use a timer to switch power to a load automatically?

Joe_B · April 2012

Re: XW-MPPT60 Undercharging Batteries

Morsofold,

Assuming that you have FLA batteries, then you can put the SCC into a mode called boost charging. Boost mode can be enabled from the custom battery sub menu. By setting the bulk voltage higher than the absorb voltage, the algorithm will use the higher bulk voltage for the first hour of the absorption stage. Your batteries will gas a little more, use a bit more water but should benefit from the boosted charge voltage. Again, do this only if y ou have flooded lead acid batteries.

Also, you should get a hydrometer and measure the specific gravity of the electrolyte to determine state of charge.

Hope this helps.

boB · April 2012

Re: XW-MPPT60 Undercharging Batteries

It sounds like the XW-MPPT controller Absorbs for a time proportional to the time it spends in Bulk.

It also looks like they missed on important feature when they copied the MX60... A "minimum" Absorb
time adjustment. It may be that the adjustment actually IS in the menus somewhere and it just needs
to be found.

The short time in Absorb, is exactly what the MidNite monitor is telling you is a problem. This can work great
as long as the jumpers are set to the proper battery type and voltage. I would also check SG with a hydrometer
as cariboocoot mentioned.

One thing you might try is to lower the battery charging current limit OR turn on more loads so that it
doesn't go to Absorb so quickly.

boB

SolaRevolution · April 2012

Re: XW-MPPT60 Undercharging Batteries

marsofold,

It seems to me that if you do as you have described you may end up overcharging your batteries. (assuming they are sealed, if not you should probably ignore a lot of this) :roll:

You may be able to program a battery Ah rating into the XW which would "fake" the CC into giving the batteries a lower finish amps setting which would be more appropriate than 2%. It sounds like you may have already looked into this and I am guessing that your battery is too small.

You have mentioned that this is a "stand by" system. If so, the batteries are "floating" most of the time and are normally rarely cycled. I suggest you make the "ultimate scrafice" again. This time with the Midnite Battery Monitor manual. It is not a fully comprehensive monitor and as such, follows certian assumptions.

From the manual:
1- The top GREEN LED will light up when the batteries have received a “Full Charge” .
2- The center AMBER LED will come on if the batteries have not been fully charged for 1 week.
3- The bottom RED LED shows that the batteries have not been charged for 2 weeks or more.

It seems to me it was designed to be a cost effective way of tracking batteries which were being cycled fairly regularly. A constantly floating battery may not get up to the MBM's "reset" voltage every week and still remain fully charged.

Once again, you should make the "ultimate scrafice". This time with the battery manual.(or mfgr's tech support) Look up recommendations for prolonged float charging. They may have a specific voltage set-points for this. Keep in mind that with the MBM and CC connected, your batteries will not actually be at an "at rest" condition so the voltage may show slightly low. As a better test, you could try disconnecting (turn off circuit breakers, pull fuses) the MBM and the CC for a couple of hours and then test the voltage with your DVM.

With some CCs, you can set the EQ voltage low enough to be used as a "slightly higher than normal" absorb voltage setting. You would need to use the XW's custom settings. Although I have done this with Outbacks, I'm not sure if the XW MPPT custom EQ voltage settings can be set low enough for your needs. The XW does not have an automatic EQ setting (many other CCs do) so you would need to initiate a manual EQ every time you want to do this, but it would be easier than readjusting your bulk settings every week or so.

Alex Aragon

silvertop · April 2012

Re: XW-MPPT60 Undercharging Batteries

I would also check SG of batteries if flooded. I use a Tri-Metric battery monitor and you can program in the charging efficiency, Tri Metrics default is 94% efficient; meaning batteries take more in to charge back to 100%. that being said meters are just a guide a reference , SG of batteries will give you the truest state of charge. Maybe you are at full charge? When I ran 12V with my XW's I ran Bulk 14.8,Absorb14.8,Float13.4 check battery manufacturers recommendations,you will gas more with these settings so check water once a month The XW when it reaches 14.8 Absorb it should taper off Amps to 2% of battery cap. or the battery Absorb timer to exit to float whichever comes Ist. In the winter I set my timer to 5HR. summer 4HR. Hard to tell what you have happening , maybe too low Absorb maybe you ARE full charged .....

Joe_B · April 2012

Re: XW-MPPT60 Undercharging Batteries

boB wrote: »

It sounds like the XW-MPPT controller Absorbs for a time proportional to the time it spends in Bulk.

It also looks like they missed on important feature when they copied the MX60... A "minimum" Absorb
time adjustment. It may be that the adjustment actually IS in the menus somewhere and it just needs
to be found.

The short time in Absorb, is exactly what the MidNite monitor is telling you is a problem. This can work great
as long as the jumpers are set to the proper battery type and voltage. I would also check SG with a hydrometer
as cariboocoot mentioned.

One thing you might try is to lower the battery charging current limit OR turn on more loads so that it
doesn't go to Absorb so quickly.

boB

boB,

There is a setting in the charger menu for max. absorb time, it is defaulted to 180 minutes but can be adjusted up to 360 minutes. So tricking the charger by changing the battery bank capacity is about the only way I can see to fool it into working properly. Since the absorption stage is exited when the charge current falls below 2% of the battery capacity, he should be able to crank down the value of the bank size to increase absorb time.

Joe

XW-MPPT60 Undercharging Batteries

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