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Thread: Inverter Setting Generator Battery Charging rate

  1. #1
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    Default Inverter Setting Generator Battery Charging rate

    Reviewing my settings and have this question.

    48 volt system, 64 390 AH Trojan flooded cell batteries, two banks of 32 batteries set up in 4 strings of 8 in series.

    Four 5500 watt SW plus inverters stacked, two per bank.

    What would you recommend the inverter generator charging rate be set at? Presently, we have set the inverter charging level at 30 amps and run time 2 hours when the genny is fired to bulk up the banks during low solar days.

    Is this too low or about right? I believe the stand alone max for each battery would be 37 amps or less.
    Off Grid 4x5500SW Plus Xantrex SinePlus Inverters/48V DC System/44x185W Sharp Panels/64xTrojan L16-REB 6v/4x"Ornamental" Desulfators/2xOutback FlexMax 80s'/Water Misers/Inverter Managed Air Extractor System/Output to 2x100Amp AC Service Panels/Two 2.5 Ton Central A/C Units/Generator CumminsOnan RS20000/RASTRA Construction

  2. #2
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    Default Re: Inverter Setting Generator Battery Charging rate

    Is this 30 amps per series string?

    Next question is what is your generator's rating (both kW and KVA) and the SW's (or other charger's) Power Factor?

    Load wise, if I recall correctly, your genset was a diesel and you would want the watts to be ~60-75% of rated load for best fuel efficiency and long life for the engine.

    The other limit is the VA rating of the battery charger... I don't think the SW are power factor corrected--so you may be dealing with a PF=0.60 or so...

    If PF=0.60--Then your maximum would be 60% of rated power from the genset would result in 100% VA rated load (assuming VA=Watts load rating for your genset).

    -Bill
    Last edited by BB.; June 5th, 2010 at 8:38 PDT.
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  3. #3

    Default Re: Inverter Setting Generator Battery Charging rate

    Are these banks totally separate or paralleled? I don't see how you can have "two per bank" on the inveters if the batteries are all connected. I'm probably misunderstanding something. It's early and the 'r' key is refusing to work properly!

    Anyway ...

    You either have two banks of 390 Amp/hrs, in which case the charging rate would be 19.5 to 50.7 Amps.
    Or you have 780 Amp/hrs, in which case the charging rate would be 39 to 101.4 Amps.
    Keep in mind that 10% of the Amp/hr rate is a good target because loads will detract from the actual charge rate.
    Also, since these are the 'tall case' batteries you want the current higher because this type of battery has a tendency towards stratification. 30 Amps is probably a bit low.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  4. #4
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    Default Re: Inverter Setting Generator Battery Charging rate

    I have two SW5548+ connected in series stacked.

    The current setting is based on AC input current. SW's have no ability to measure DC current to or from battery.

    The way you describe your system, I take it as two separate systems of series stacked (120/240vac) inverters each on their own 1560 A-hr battery array.

    Maximum DC charging output is about 75 amps on SW5548+ so with two inverter putting out their max would be 150 amps but you will typically not get this with two SW's connected on same battery.

    Because of slight tolerance variation the two series inverters will 'thrash' each other that will consume some of the charging current. The temp sensors will aggrevate this further because the compensation are not continous versus temp but discrete steps of about 6 degs C.

    I can get about 120 amps DC max from my two series connected SW5548+'s

    The best way to figure output DC current is by input power = output power * inverter efficiency.

    For example: AC charging setting = 30 amps

    Input power = 120vac * 30 amps = 3600 watts.

    Inverter efficiency = 91%

    Output power = 3600 watts * 0.91 = 3276 watts.

    If batteries are at 50.0 vdc, then charge current will be 3276watts / 50 = 65.5 amps

    Note: charge rate on SW's are regulated by AC input current so, for a given setting, the DC output current will drop as battery voltage rises during charging.

    For two series connected the sum would theoretically be 2*65.5 amps = 131 amps.

    You will likely not get this full summation due to the thrashing between the two units.

    It is very important the absorb and float voltage settings be the same between the two units. Variation will increase current thrashing between units a great deal.

    With a 1560 amp-hr battery, they will take the maximum charge capabillity of the two SW5548+'s . You could feed 200 amps at batteries but two SW's cannot generate that much.

    If you have a clip on DC amp meter it is a good idea to check each of the four strings to see if the current is being distributed similarly. I have ten strings, they tend to have about 20% variation in current but the string that starts out initially high usually drops down and initial lower current bank takes up higher current as charge cycle progresses.

  5. #5

    Default Re: Inverter Setting Generator Battery Charging rate

    Right. Read it wrong = did the math wrong = sorry.

    If that's 30 Amps DC there's no way it will charge 1560 Amp/hrs of battery.
    If it's as RCinFLA says, 65 Amps DC, it's still too low. Even at the SW's max 75 Amps it is lacking:
    1560 * 5% = 78 Amps.

    I take it the DC side of your dual inverters are entirely separate? And you feed the charging circuit of each from one leg of a 240VAC gen output? If that is the case then there should be no thrashing between inverters or temp discrepancy.

    Do you really need such a big battery bank? That's pretty huge power for off-grid!
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  6. #6
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    Default Re: Inverter Setting Generator Battery Charging rate

    He has four inverters. Two series connected (to get 240vac) on each 1560 AH battery bank. He will get nearly twice the output due to summation of two series connected inverters.

    The maximum settings (40 amps AC) will yield about 135-140 amps DC to batteries from the two inverters.

    The SW5548 plus also has a bulk (actually absorb phase) termination current setting. Set this to 4 amps AC (about 8 amps @ 58 vdc at battery)

    The thrashing is caused by the DC voltage regulation. As for a solar PV controller hooked to battery, PV set float level 0.2vdc higher then SW float setting, the SW will not allow the DC voltage to rise above its DC voltage set point. As the PV controller tries to push a higher voltage to the batteries, the SW will suck off all current that takes batteries above the SW charge voltage setting. This is the same effect with two inverters on same battery.

    If the two inverter have a slightly different DC charge voltage setting, one will push current to battery thinking it needs charging, the other will suck current from batteries thinking it has too much battery voltage.

    Just the calibration tolerance between the two unit will cause some offset. The worse offender is the temp comp system which changes the charge voltage setting based on 6 deg C increment steps. One of the change points is 25 deg C. If one inverter thinks it is at 24.9 deg C and the other thinks it is at 25.1 deg C there will be a 0.8 vdc battery regulation setting voltage difference between unit. This causes a really large push between units.

    I do not use the temp comp sensors on my units because of this. I live in S. Fla. so I don't have very large temp variation. I do manually tweek the charger voltage up a bit in winter.

    During the initial bulk (constant current phase) charging, there is no thrashing due to DC voltage variation settings (have not reached absorb or float voltage settings). The regulation in this first phase is only the AC current measurements by the SW inverters. There is some variance in battery charge current due to interaction in push currents by each inverter that effects the AC current measurements by the inverter. This prevents you from getting the full 2x current net to batteries. As mentioned prior, the DC current to batteries will drop off as battery voltage rises during charging because the bulk current regulation is based on input AC max charge current setting.

    As to generator setting, do 85% of generator rating or 40 amps setting on inverters, whichever is less. Max AC2 input current overrides Max chgr current if max input current is less then charging current setting. Max AC2 input can be 60 amps or less, Max chgr input can be 40 amps or less.

    I have a 15kW generator and push it at 85% of rated current.
    Last edited by RCinFLA; June 5th, 2010 at 14:29 PDT.

  7. #7
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    Default Re: Inverter Setting Generator Battery Charging rate

    Thanks RCinFLA and everyone. I always get a good sit down lesson on this great forum.

    You described my system better than I ever could.

    I had never heard of thrashing between inverters but I think I get it. I notice at the end of the two hour generator charging cycle, I hear the generator hunting a little i.e. rpm increasing and decreasing slightly. Could this be the thrashing effect you described between the inverter banks?

    The solar mechanical room is in a stone building below grade. The temperature surrounding the batteries, inverters and charge controllers never gets above 85 degrees or below 45 degrees or so. Batteries always seem to be within normal operating ranges.

    The 1500 rpm water cooled generator is an industrial 15 kw propane unit derated for 5,100 feet altitude (say 13 kw) with 60 amp circuit breakers.

    "The SW5548 plus also has a bulk (actually absorb phase) termination current setting.
    Set this to 4 amps AC (about 8 amps @ 58 vdc at battery)." Not sure I clearly understand this point.

    Shall I set all the inverters charging rates at the same 35 AC amp rate? Currently, I have two inverters set at 30 amps AC and the other two inverters set at 40 AC amps. My reasoning which is pretty uninformed was because one battery bank works about 10%-15% harder than the other bank given differing air handler work loads (house layout). I reasoned rightly or wrongly, I could compensate by charging faster the lower battery bank voltage at a higher charging rate.
    Off Grid 4x5500SW Plus Xantrex SinePlus Inverters/48V DC System/44x185W Sharp Panels/64xTrojan L16-REB 6v/4x"Ornamental" Desulfators/2xOutback FlexMax 80s'/Water Misers/Inverter Managed Air Extractor System/Output to 2x100Amp AC Service Panels/Two 2.5 Ton Central A/C Units/Generator CumminsOnan RS20000/RASTRA Construction

  8. #8
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    Default Re: Inverter Setting Generator Battery Charging rate

    Two of the SW5548's can consume most of a 15kW generator so you have to back down a little for four units running all at once.

    Generator at 1500 rpm? You must have a belt driven alternator to get 60 Hz.

    The absorb phase current termination is just deciding a point where the absorb phase of charging terminates and switches to float voltage level. The regular SW (non-plus version) does not have this feature. They only have timed absorb termination.

    For current based absorb termination, it is usually set to about 10% of bulk mode current. Running from generator you might want to run absorb voltage up to 60 vdc and terminate with less absorb time. It saves a little generator fuel. Getting the last 10% of recharging capacity into battery is a very inefficient process and consumes a lot of fuel for what you get back. This is particularly true for a propane engine run at 1800 rpm (vs. 3600 rpm) as the engine has to be phyically larger since the horsepower for physical engine size is less at 1800 rpm's. A friend of mine has a 45 kW Generac backup generator. It uses a V-6 engine running at 1800 rpm's from a Ford Taurus and sucks up 1 gallon of propane per hour with no load on generator.

    If you have full 62 amps wiring from generator and want to run all four at once, you have to set inverters to 30 amps each. Reason I ask about 62 amps is this requires special direct wiring to generator as largest NEMA plug only carries 50 amps. This also assumes your generator is rugged enough to take 60 amp, 240 vac (14,400 watts) continuously without stressing the generator too much.

    The generator 'wobble' is probably not caused by the inverters thrashing as the current demain has dropped off by this point in charge cycle. Many generators rpm regulation wobbles when the load is very light or no-load condition. When the throttle plate in carb is closed down at light loads, the rpm change rate is touchy to slight opening/closing adjustments of throttle control. (Higher loop gain in the feedback control creating a little instability)

    This can cause a problem with inverters like SW and XW when they are trying to initially syncronize to generator. Although the wobble rate still results in AC freq within inverters tracking freq spec they have problem initially locking onto to a moving target. My gen / SW5548+'s have this problem once in a while. A 100 watt light bulb plugged directly into Gen outlet socket smooths it out allowing the inverters to lock on.
    Last edited by RCinFLA; June 5th, 2010 at 20:01 PDT.

  9. #9
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    Default Re: Inverter Setting Generator Battery Charging rate

    Got it.

    Mis keyed, the SG015 genny is an 1,800 rpm unit not 1,500. I do have the SW 5548 Plus inverters.

    The genny is hard wired to the inverters through metal conduit.

    I think your right about the lighter load wobble when the SWs' are trying to syncronize to the generator. It only seems to occur towards the end of the cycle when the batteries are charged 100%.

    "A 100 watt light bulb plugged directly into Gen outlet socket smooths it out allowing the inverters to lock on". Pretty clever. I'll try that if I've got the outlet.

    Thanks for putting this in terms easier for me to understand.
    Off Grid 4x5500SW Plus Xantrex SinePlus Inverters/48V DC System/44x185W Sharp Panels/64xTrojan L16-REB 6v/4x"Ornamental" Desulfators/2xOutback FlexMax 80s'/Water Misers/Inverter Managed Air Extractor System/Output to 2x100Amp AC Service Panels/Two 2.5 Ton Central A/C Units/Generator CumminsOnan RS20000/RASTRA Construction

  10. #10
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    Default Re: Inverter Setting Generator Battery Charging rate

    The generator rpm wobble is usually only a problem initially when inverter is still searching for syncronization (gen LED blinking), trying to align its AC phase to the generator.

    Once the phase lock is achieved the LED goes solid green, and you hear the 'clunk' of the relay that ties the generator output to the inverter.

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