Battery Charging with SW5548

System · August 2007

I have a dual Xantrex SW5548 power panel. I just replaced a very old bank of batteries with 16 new Rolls S-530 batteries wired in two parallel sets of 8 for 48V. Based on the recommended rate of C/20 for charging, I calculate that these 400ah batteries (800ah total battery bank) should be charged at about 800/20 = 40 amps.

The Xantrex SW5548's allow you to set the MAX CHARGE AMPS AC, with the default being 35. This is far too high it would seem. I set each inverter at 8 amps, which would be 8 x 120 = 960W. That implies a DC output of 960 / 48 = 20 amps (assuming 100% efficiency) on each inverter, giving me the 40 amps I need.

The problem is that the batteries will not charge above 52V temp comp volts (51V actual). I have set the bulk charge voltage to 57.6 (default). Unless I crank the MAX CHARGE AMPS AC up to 30 on each inverter, I can't get the batteries up to the 57.6V bulk charge voltage.

If I boost it to 35 amps, I can get it to start going higher (in equalize mode).

I am afraid I am cooking my batteries by doing this. But no matter how long I charge with the SW5548's set on 8 amps, the voltage never rises.

Am I missing something with the MAX CHARGE AMPS AC setting? Did I calculate something wrong? Any ideas?

Note: I have a 30kw generator providing AC in, so input power is not an issue.

Incidentally, my MX-60 solar charge controller puts out about 1600W (about 30A) all day and same thing - nothing above 52V is attainable.

The batteries were at 6.25 volts each when I received them.

Windsun · August 2007

Re: Battery Charging with SW5548

As I recall, the charger side of those is about 60% efficient, so should probably actually set it for around 12 amps.

mike95490 · August 2007

Re: Battery Charging with SW5548

Till the batteries actually start warming up, you aren't really damaging them, AFIK

They should take a really heavy charge when low, and after half hour, that should start tapering off, till they are full, and start really bubbleing. I'd say it's better to overcharge a bit, and add water, than to leave undercharged, and sulphated.

mike95490 · August 2007

Re: Battery Charging with SW5548

solarcarib wrote: »

I calculate that these 400ah batteries (800ah total battery bank) should be charged at about 800/20 = 40 amps.

The Xantrex SW5548's allow you to set the MAX CHARGE AMPS AC, with the default being 35. This is far too high it would seem. I set each inverter at 8 amps,

I'm not following this well, but don't Inverters take DC to AC ?

If the battery bank spec is 40A, why not feed it the 40A ? How many chargers do you have - are you trying to limit the charge current? I don't have a SW5548, so maybe I don't know what the heck I'm talking about.

halfcrazy · August 2007

Re: Battery Charging with SW5548

800 ah hours of surrettes should be charging at 80 amps or better looking for 10% of the amp hour not 5%

niel · August 2007

Re: Battery Charging with SW5548

it does sound like they aren't getting enough of a charge and you should be observing the dc amps going into the batteries. if you adjust the dc amps by the ac amps then fine, but i am not familiar with it so be sure to read your manual on it. also be sure that all is fine with the generator. we don't know what it is you have coming from the mx60 into the batteries either so maybe you can enlighten us on that too. if you have loads on the batteries while trying to give it a 5% charge rate, that would be a problem as the load current would be subtracting from any charge current. you can feed those batteries a net 10% rate of charge without problem. that would be 40amps dc from each inverter's charger to the batteries without loads connected. be sure of the voltages being measured are being accurate too.

System2 · August 2007

Re: Battery Charging with SW5548

Excellent feedback. Thank you all. I am surprised at the low 60% efficiency rating... I also don't remember hearing that the Rolls S-530 should use C/10 instead of C/20. Please confirm.

The real problem is that the meters on the SW5548 report the incoming amp draw from the AC source, but they do not report how much net DC amps are being put into the batteries. I guess I could get an external meter... any suggestions? Where and what to buy

Based on what I am reading here, is seems prudent to use about

80 x 48 / 120 / 0.6 = 53 total AC input amps

I will set the SW5548 chargers to 27 amps each for SET MAX AC AMPS and see if they can reach 57.6 volts. I realize that is is most important to reach this magic number each day rather than allowing batteries to remain undercharged.

Regarding the MX60, the generator typically only runs in darkness, so I am not considering the input from the solar panels in the calculation.

Next question:

I want to set the low battery cutout (LBCO) so that my battery bank never goes below 50% (so I do not have to monitor it constantly). I am in a warm climate, and the difference between the SW5548 actual and temp compensated voltage is about 0.8 volts. I know 50% would be about 48.24V temp compensated, implying about 47.4 actual. The LBCO is based on actual voltage on the SW5548 (does not seem very smart). It is disconcerting setting the LBCO so low, however. Should I be concerned?

Solar Guppy · August 2007

Re: Battery Charging with SW5548

The Lower value for cut off is so a turn on surge doesn't trip the invert offline

System2 · August 2007

Re: Battery Charging with SW5548

Solar Guppy wrote: »

The Lower value for cut off is so a turn on surge doesn't trip the invert offline

That does not really apply to my question. The LBCO voltage is delayed on the SW5548 (15 minutes default) to prevent shutdowns from momentary voltage drops.

BB. · August 2007

Re: Battery Charging with SW5548

Here is a link to Wind-Sun's battery meters.

-Bill

Ralph Day · August 2007

Re: Battery Charging with SW5548

Solarcarib

I'd suggest visiting the Rolls/Surrette website and look up the bullitens regarding charge delivered to s530's. Jamie Surrette says to charge aggressively, good enough for me. As Mike said, until they heat up, batteries are not in distress...yes they bubble, but that's NOT boiling.

My Surrette's are charged by genset seldom except in winter, but i've measured the change in electrolyte temperature during a long heavy charge (up to 70amps dc for 4 hours) and the temp only rose about 3 deg C. On investigating i was told by Surrette that you can charge with up to 25% of the 20 hour rate. I have cs type with 820 ah 20 hour rate...max charge 205 amps dc 48 volts. I genset charge at a max of about 100 amps because of generator limitations (10kw).

If you're solar input is not bringing the voltage up to absorb level, get a good hydrometer and check the specific gravity of the batteries! They may be at a low state of charge and need hours, several to many, of generator charge. Fuel is cheaper than new batteries.

ralph

BB. · August 2007

Re: Battery Charging with SW5548

Ralph,

Just to be clear, I think you are charging at 25% rate of the 20 Hour capacity. Or 25% of 820 AmpHour capacity or 205 amps... or at a 1/4*C rate.

The 20 hour rate would be 41 amps for 20 hours (=820 AH)...

-Bill

halfcrazy · August 2007

Re: Battery Charging with SW5548

BB i think ralph was saying he charges at 25% of the 20 hour rate i dont think he ment he charges at the c20

BB. · August 2007

Re: Battery Charging with SW5548

No,... I think I was correct in my statement... I was trying to clarify terms, which at the best of times, can be confusing.

The 820 AH battery capacity rating is defined at a 20 hour discharge rate... He said that the manufacturer would support a 25% charge rate (c/4) or 205 amps of 820 AH capacity--which would be 25% of the 820 AH capacity (capacity as measured using the 20 hour rate).

25% of the 20 Hour Rate would be c/(4*20)=C/80 which is probably closer to a maintenance charge rate--or even less.

The numbers Ralph quoted seem to be fine--it was just the statement of "...you can charge with up to 25% of the 20 hour rate..." that I was trying to clarify (basically, a very "healthy" C/4 charge rate).

It is not a big deal--the numbers are correct (I believe). It was just a confusion in terms.

-Bill

halfcrazy · August 2007

Re: Battery Charging with SW5548

wouldnt the 20 hour rate be 840 ah?

BB. · August 2007

Re: Battery Charging with SW5548

Hmmm...

820 AH is the capacity rating under the conditions of a 20 hour discharge.

The battery should be able to supply XX current from a 820 Amp*Hour capacity battery bank, at a 20 hour rate of:

Rated Current = 820 Amp*Hour / 20 Hours = 41 Amps (for 20 hours).

Otherwise, 820 Amp*Hours * 20 Hours = 16,400 Amp*Hours^2 (squared)--not quite sure what units that would be... I don't think that these batteries would be able to supply 820 amps (continuous) for 20 hours.

And a 16,000 amp*hour battery (at 48 VDC in this case) bank is pretty large.

Maybe I am miss-understanding you... What do you mean by the statement of "wouldn't the 20 hour rate be 840 ah?"... xx amps for yy hours equals zzz capacity or what?

-Bill

mike95490 · August 2007

Re: Battery Charging with SW5548

Don't be a wuss with these batteries - if you can discharge them at 40A, then you *can* recharge them at 40A. (for 20 hours)

You have a honking big battery bank, and you need a honking big charger for it.

Better if you recharge them, than let them sulphate. They may be so low now, it will be tough to recover.

""manufacturer would support a 25% charge rate (c/4) or 205 amps of 820 AH capacity""

You should be looking at at least a 100A charge rate.

""The batteries were at 6.25 volts each when I received them.""
What are they now ??

Ralph Day · August 2007

Re: Battery Charging with SW5548

Sorry for not replying/clarifying, on vacation.

terms as corrected are correct. I charge maxed out at about 100 amps dc. Oops for the confusion.

Bulletin 614 from Surrette is what is pertinant to this thread.

ralph

crewzer · August 2007

Re: Battery Charging with SW5548

Based on the recommended rate of C/20 for charging...

I’m curious about the source of this “recommendation”. Trojan Battery recommends 10% to 13 % (C/10 to ~C/8 ) for their flooded-cell lead-acid deep-cycle batteries, and here’s some pertinent info from Rolls / Surrette’s bulletin #507:

“On a manual charging system the charging current at less than 75% charged can be 25% of the 20 hour rate. The rate can even be higher below 50% charged. Once the battery approaches 75% charged reduce to 10% of the 20 hour rate. At 85% reduce the rate to 3% of the 20 hour rate.”

See: http://www.trojanbattery.com/Tech-Support/BatteryMaintenance/ChargerSelection.aspx
And: http://www.rollsbattery.com/Bulletins/507.htm
And: http://www.rollsbattery.com/supportsol.htm

But no matter how long I charge with the SW5548's set on 8 amps, the voltage never rises.

Assuming 60% charge efficiency (YUCK!!), 8 AAC into each 5548 will deliver ~19 A to the 800 Ah battery bank, or ~2.4% of capacity. This is pretty low for flooded-cell batteries.

I would think that you could set your inverter/chargers to deliver 80 ADC (10% of capcitry) in bulk charge mode to your battery bank. The inverter/chargers will automatically reduce/limit the charge current in absorb-, float-, and eq modes in order to maintain target voltage(s). Allowing for charger inefficiency and power factor issues, you could probably leave the inverter/chargers set at their 35 A AC defaults.

Incidentally, my MX-60 solar charge controller puts out about 1600W (about 30A) all day and same thing - nothing above 52V is attainable.

1,600 W at 58 V is about 27.6 A, or about 3.4% of capacity. This should slowly charge the bank assuming no significant downstream loads.

HTH,
Jim / crewzer

RCinFLA · August 2007

Re: Battery Charging with SW5548

I have two SW5548 plus's and two SW4048's both pairs as series 240vac connect. The 5548's are only used for central A/C, hot water heater, and range.

The SW units have a problem with charging when connected as series but this relates to getting maximum output current. In your case, you are not needing the max charge of about 70 amps dc. With my two 5548's, I get max current of about 110 amps during bulk, not the 140 amps dc expected. They taper off constant current phase prematurely but do eventual raise the battery to bulk setting.

The two units will fake each other out, thinking the voltage on battery is higher then it actually is. I get slightly better results by running individual battery lines back to batteries then parallelling battery connection at units.

You can think of it as if you have a solar charge controller also feeding the batteries. The SW's will push to AC inputs if the battery voltage rises above set point voltage. The series connected case (probably same if AC parallelled), one unit will push some of the battery charge current to its AC input. Battery may be getting 15 amps, one unit is putting out 25 amps dc while the other unit is sucking up 10 amps dc. and pushing to grid (or load). This depends on the L1 and L2 AC line voltages as to which unit will do the pushing and which one is pulling.

First thing is get rid of the temp comp sensors. The comp'ing is done in increments and when the two units are on either edge of the voltage comp change there is large currents that flow back and forth between the units.

Make sure the Max AC input current is not limiting as it over-rules charge setting.

Suggest you get a DC amp measurement, at least a clip-on hand held. Make sure it does D.C.

RCinFLA · August 2007

Re: Battery Charging with SW5548

solarcarib wrote: »

Excellent feedback. Thank you all. I am surprised at the low 60% efficiency rating... I also don't remember hearing that the Rolls S-530 should use C/10 instead of C/20. Please confirm.

The real problem is that the meters on the SW5548 report the incoming amp draw from the AC source, but they do not report how much net DC amps are being put into the batteries. I guess I could get an external meter... any suggestions? Where and what to buy

Based on what I am reading here, is seems prudent to use about

80 x 48 / 120 / 0.6 = 53 total AC input amps

I will set the SW5548 chargers to 27 amps each for SET MAX AC AMPS and see if they can reach 57.6 volts. I realize that is is most important to reach this magic number each day rather than allowing batteries to remain undercharged.

Regarding the MX60, the generator typically only runs in darkness, so I am not considering the input from the solar panels in the calculation.

Next question:

I want to set the low battery cutout (LBCO) so that my battery bank never goes below 50% (so I do not have to monitor it constantly). I am in a warm climate, and the difference between the SW5548 actual and temp compensated voltage is about 0.8 volts. I know 50% would be about 48.24V temp compensated, implying about 47.4 actual. The LBCO is based on actual voltage on the SW5548 (does not seem very smart). It is disconcerting setting the LBCO so low, however. Should I be concerned?

SW5548 charging efficiency is same as it is as an inverter. They have about 34 watts of overhead for general electronics board that run the switching FET's. At bulk rate currents it will be in 92-95% overall efficiency range. You can use the charts in the back of user manual. Once charged, with a float current average of less then an amp dc, the efficiency is poor since dc output is only about 20-50 watts and there is the 34 watts overhead, or about 50-85 watts of AC input power.

As you stated, SW series do not have measurement of battery line current. The measurements are AC current and voltage on the three AC ports, AC1, AC2, and Inv output and battery dc voltage. The actual DC current to the battery depends on AC line voltage and battery voltage. You can get a fairly accurate prediction by calculating AC input power, multiply (loss) by efficiency at that power (about 0.92), then divide by battery voltage.

The charger set point tries to limit the input AC current to what you have set. Because of the limited selection of transformer voltages it will pump above and below this AC set point. When the battery is fully charged and leveled out at float voltage you will see swings of positive and negative 1 to 5 amps of battery dc current. I actual believe this 'breathing' action is good for the battery.

Battery Charging with SW5548

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