Ok, I am new to this, so please be gentle. We just had eighteen Sharp 208 watt panels installed, tied to a Xantrex XW MPPT60-150 Charge Controller, powering a XW6048 inverter. We tied in four Deka 8A4D, 12 volt, 200 amp-hour, sealed batteries in series. So it looks like a 6 kW inverter with about 3 kW of solar input, using a 200 amp hour battery bank. We are thinking about adding another array next year, depending on rebates, Colorado REA doesn’t give much (a maximum of $9,000 on a 3 kw system or larger).

I’ve been reading information on battery bank size on the web, and came across a posting by Solar Guppy saying that 400 amp hours of batteries was minimum for a XW6048 inverter, and that using a smaller bank causes oscillations and non optimal tracking. I do notice the battery voltage and current fluctuating on our system, as well as seeing the power output given on the inverter display always oscillating up and down every few seconds. Is this a “fault” you described that is caused by the small battery bank?

Everyone has reasons for solar. Personally I do want to decrease my carbon footprint, but our biggest reason is a bit different. My wife is on oxygen full time, and uses an ac-powered concentrator when at home. We have a generator, as well as backup oxygen cylinders, which can be used during a power failure, IF IF IF we know one occurs. Obviously alarms can be used to warn of power failures at night, but they require batteries. So, we decided a solar system with enough battery backup to last one night would be the best solution. It seems to me that fifty percent of a 200 ah, 48-volt system is more than enough for that purpose. Although the grid power here is very reliable, it would only take one occurrence to produce a possible tragedy.

Having said all that, I really would like our batteries to last as long as possible. If the XW6048 inverter will damage the 200 ah bank, but not a 400 ah bank, I’ll buy four more Dekas.

The system does seem to be operating well. In fact it has supplied over half our electric usage for the past two months, and these have been unusually rainy months for the Colorado high country.

Thank you in advance for your time and trouble.

i'll throw my 2 cents into this and you decide what's best for you. as you know doubling your present battery bank is on the agenda here minimally. now that 42 amps of pv (good guess) at 48v would represent a strong charge rate to the battery bank. 42a/200ah=21.4%. i believe the batteries can take the high charge rate, but you need a 400ah minimum as you've heard to allow good operations to take place for the system so at 400ah of batteries at 48v this charge rate is still a nice 10.7%. you could stop here, but you say you'll add another 3kw to match the one you have so this would bring the charge percentage back to 21.4% on a 400ah 48v battery bank. the way i see it you should go 4x your present battery bank and have 800ah. the charge percentage is still nice at 10.7% and you will have 4 days of power if the 200ah is 1 day of power. now if you planned on draining the batteries dead daily this is not good as the battery life will be greatly shortened so they should not go below 50% of their capacity meaning that up to half of the battery capacity is usable. now you're at a true 1 day of use for a 400ah battery bank and 2 days for an 800ah bank. imho, get the bank to 800ah now and expand on the pvs later or simultaneously. the charge rate with your present 3kw of pvs will still charge the 800ah bank at about 5.35% and is acceptable and is expandable for more pvs like you plan to do. batteries should not be mixed with different types, capacity, or age, but if you get the batteries now i think you are covered.
again, this is up to you, but that extra capacity is insurance for your wife's equipment to function. gasoline isn't always available for generators or easily obtained during emergency conditions and it doesn't store well.
if i've forgotten anything i'm sure sg will let you know.

What is your sell voltage setting on the XW-6048? .... for best performance , use 52V , anything higher and the XW can have some swings in the selling

Also, small battery banks make this issue worse and can effect the Mppt tracking ... 400ah is for a 6kW sell, so for 3kW you can get by with 200ah but your not going to have much backup time for big loads

Thanks to both of you for your inputs!

I'm glad I can put off adding batteries until I get more panels, thanks Solar Guppy.

My Grid Supp Volts was set to 59 per a suggestion from Xantrex Customer Support.

I lowered it to 52 volts, but then the system stayed in the sell mode even after the Charge Controller went to "low light". I don't want to use my batteries to power the grid, so .....

I raised the Grid Supp Volts to 53 volts. That caused it to stay in the Bulk charge mode all day. Note Inverter charger is set for two stage, Charge Controller to three stage. Bulk = Absorption = 58 volt setting, float = 54.6 volt setting.

This doesn't seem good to me either. The Inverter manual, on page 3-17 says "If battery voltage is above the Grid Supp Volts setting, the XW Inverter/Charger uses available DC power to support the loads." It implies this happens AFTER Absorption charging is completed, but mine does not work that way.

For tomorrow I'm going to try a Grid Supp Volts of 55, and see what happens. Do you think my system is defective? It has only been in a few months.

Thanks for all inputs.

go back to 52V, turn off the XW-6048's charger, its not needed for anything and let the system do its thing ( this assumes your doing gridtie only and no offgrid loads at night

The reason it sold for a bit is you have been OVER charging your battery like crazy ... 59 volts is nuts, that almost a constant EQ level

in a day or two, it will work exactly as it should. Battery's only need to be maintained slightly above the full charge reset voltage which is ~12.7V per battery. I have run for years at 52V without any issues.

There is nothing wrong with your system expect the setting its be commanded to operate at ...

Hmmm.

What batteries do you use Solar Guppy?

The 59 volts was the Grid Supply Volts, not battery volts. I think it makes sure the batteries are NEVER used to send power back out to the grid, as it is a higher voltage than the batteries ever hit. Until my Grid Supply Voltage exceeds 55 volts, the Charge Controller remains in Bulk Charge all day. Is this what I want? Seems like batteries never get fully charged that way.

Inverter charger is set for two stage in case grid power lost during night for a bit, system uses batteries, then grid power returns. If batteries are partially discharged, grid will be used to top them up in case of another grid failure. Seems to me Inverter charger will never be used unless above occurs.

Note the DEFAULT settings in the Xantrex for a 48 volt AGM battery is 57.2 volts charge, 53.6 volts float. My batteries are Deka 8A4D's, and the data sheet recommends charging between 57.6 and 58.32 volts. Deka data sheet recommends float be 54 to 55.2 volts.

Thus I set my system for charge of 58 volts, float of 54.6 volts. Deka data sheet warns NOT to use lower voltages as it will compromize amp hour output capability.

Am I missing something here? Sorry to be so dense. I think you are using different batteries?

Again I appreciate your help, and forebearance with my questions.

a couple of things here ...

If you have your sell voltage ( XW-6048->Advance Settings->Grid support ->Grid Supp Volt to 59V, your going to send to the grid energy only in excess of a Bulk charge voltage, in other words you will be cooking the batterys day after day excessively. Battery don't want to be continually over charged, you charge them when they have been discharged ( rebulk typically 12.5V )

If you setup is primarily Gridtie, you want all the excess to be sold, if the battery's are fully charged ( 12.7V per battery for nominal 12V wet cells ) anything over 50.8V is going to start charging the battery. Now A bulk charge will refill faster than a lower voltage, but both will bring battery back to a full charge eventually

59V is just flat wrong, I have no clue why that would be recommended, outback inverters use 51-52V as the default sell ( GTFX-3048 for example ) and my XW-6048 is set to 52V to elimate all signs for hunting when selling.

I use AGM's, but its the same concept. the default from the factory is 54V by the way

Also, since you have solar, there is no reason to have the XW-6048 charger enabled, it will just waste grid power to recharge when solar the next day will do it for free. Turn off the charger on the XW-6048

Well I have two stage charging enabled on inverter in case grid goes down for say 1 hour at nite. I would like batteries recharged as soon as possible in case ANOTHER grid failure occurs during same nite. Yes, that is fairly common here in heavy snows. In fact we could get a few overcast days where panels would not recharge batteries even during daytime.

If I set my Grid Supp Volts to 52, any current trying to raise battery voltage higher will be used to create power for the grid. Thus my batteries will NEVER get fully charged. In fact my system stayed in Bulk charge the entire summy day when I tried it. Deka says this will eventually lead to reduced amp-hour capacity of batteries.

I asked Deka, the manufacturer of my AGM batteries, what they suggest. Here's THEIR numbers.

Bulk current 30 A per 100 ah capacity, voltage 57.6 to 58.32, constant current.
Absorption voltage 57.6 to 58.32, constant voltage
Float 54 volts, constant voltage.

Per Xantrex manual, Bulk is constant current up to float voltage, then Absorption is constant current until voltage limit set, then it becomes constant voltage. Thus it appears to me my 58 volt setting for Absorption is perfect, 54.6 I had should be 54 for Float.

Only surprise to me in their answer was the recommended temperature compensation of .005 V/cell per degreee C. This works out to be 120 mv/C on the Xantrex for a 48 volt system. Default is -84mV/C on XW-MPPT60-150 Charge Controller.

Interesting to play with. I'm considering trying another set of batteries, to increase my ah's to 400, before I add the other 3 kW of panels, just so I can see if the power variations decrease. It would be an interesting experiment, but that is for next year.

Have fun out there,
Chuck

When your setup to sell to the grid , there is no reason to put the battery's thru a charge cycle as they ARE fully charged already.

If your not discharging the battery's, there is no reason to ever go above a float value, ever. AGM's can go for months without any charge at all, not even a float charge.

I've done my best to explain how this is intended to work, if you want good sell to grid performance, you have to use a lower sell voltage. You indicated you were not happy with the performance, setting your sell to 59V and I explained why that is not the proper setting.

Just so you know, this is what I do for a living, design & test RE electronics, I was part of the team that developed the XW charger and know first hand the XW inverter and who it is intended to be used.

I appreciate your sharing your solar background, it sounds like a fun job. The sell voltage was set to 59 per the instructions of Charles Miller from Xantrex Customer Support, thus you can appreciate my confusion. Hopefully I’ll get it all straight in my mind with your kind assistance.

Note it appears that my 3 kW PV system is selling all excess power to the electric company; in fact it is supplying over half our electric usage, and has been doing so since its installation a few months ago. It was my understanding, possibly erroneously, that setting the “Sell Supp Volts” to 59 just told the Xantrex system not to use any of the BATTERIES capability to send power to the grid.

I got up before dawn (groan) and checked the battery voltage. Everything was connected, but there had been no grid failures, and the Charge Controller was still in the Low Light condition. The battery voltage was 10.9 volts. I think this must be close to the “resting” voltage. Once daylight occurred, the Charge Controller starting putting out power, the SCC charger went to Bulk mode, and very quickly the voltage exceeded the 54-volt level I have currently set for Float, so the charger went to Absorption. It stayed there until just after 09:00 when the charger went to Float mode, at 58 volts, where it remained for the rest of the day. Obviously this is with the Sell voltage set to 59, Bulk = Absorption = 58, Float = 54.

I assume you do not want the SCC charger turned off completely, as then how would the batteries ever get recharged should a grid failure occur?

If I set the sell voltage to 52, the SCC stays in Bulk mode all day. Is this what you are suggesting is correct?

If I set the sell voltage to 55, the SCC stays in Absorption mode all day. Is this better?

I understand about not needing to charge the batteries every day, I’m just not sure how to accomplish that feat. I don’t want to rely on my having to manually start the charger if a grid failure discharges the batteries. It would be ok if I was home, but if my handicapped wife were home alone, it would be difficult for her. Worse still might be if nobody was home, for example if we were on vacation. I don’t think it would be good for the batteries to sit discharged for weeks, and I doubt that a 52 volt application would charge the batteries very quickly, if at all.

The other alternative might be to turn the SCC charger off and rely on the Inverter charger to recharge the batteries should a grid failure occur. This seems like a good idea at first, except we actually did get a long daytime grid outage recently, lasting almost all day. We made sure to really minimize our electric usage, and I watched the battery voltage just to see if I needed to start the generator to power my wife’s oxygen support. To my delight the SCC charger kicked in and maintained the batteries, as well as supplying all the Inverter power we needed. Had the SCC charger been turned off, I’m not sure exactly what would happen, but I doubt the batteries would have been recharged, would they?

So I guess I’m really not sure what you suggest I change other than the Sell voltage, unless having the SCC in the Bulk mode every day is correct. Sorry I’m so confused, and thank you for your forbearance with all my questions.

Best Regards,
Chuck.