Fluctuating amps during battery charging

Re: Fluctuating amps during battery charging

Have you checked the charging voltage settings on the inverter? You might have them set too tight and it is dropping in and out of bulk?

Re: Fluctuating amps during battery charging

It is normal for the SW series current to vary.

I am assuming you are not hearing too much rpm variance from your generator.

The SW series trys to match the input AC with three transformers providing up to a maximum 27 discrete steps. The steps to create the sinewave are 12-18 v increments throughout the sinewave instantaneous voltage. It also has to make a best guess prediction on what the future few cycles of AC input voltage will be to set up the transformer stepping algorythm.

Since it is not able to perfectly track the sinewave it keeps track of overages and underages on the AC inputted to inverter during charging. There will be +/- pumping as the result of this. It trys to keep an average current over time.

Also, the SW has no battery DC current measurement capability so all the calculation is based on 120 vac current measurements. When you set 30 amps for max charging current the psuedo regulation is being done on the AC current. This means the battery DC current will drop off as the battery voltage rises.

The most accurate way to predict what the D.C. charge current is is to take the AC current setting times 120 vac to get input power. Derate this by efficiency of inverter at that power level. Then divide that output power by the battery voltage.

For 30 amp input AC current charge setting this would be 3600 watts input. At that power level the inverter is about 87% efficient so the output power would be about 3130 watts.

If battery is at 24.3 vdc then charge current would be 3130w/24.3v = 129 amps.

When battery gets to 27.0 vdc then charge current would be 116 amps.

At bulk point of 28.6 v the DC current drops to 109 amps. Because of the voltage fluxuation at the battery from the step inaccuracies the actual current is normally a bit lower then this. This also happens at float voltage setting. The above calculation are most accurate when the battery voltage is a few tenths below the battery voltage bulk and float set points so the battery voltage regulation is not effecting the AC current input charge current regulation.

With regard to charge rate, this dropping of charge current with battery voltage actually is good. The battery charge current can be much higher when the battery is below 85% state of charge. An initial 20-25% A-H rating charge current is okay below 85% SOC. At that initial charge rate, when the battery voltage rises to it bulk setting voltage it will be about 75-85% SOC.

I have two SW4048's series stacked. Things get really squirelly with the series stacking. When battery is near the bulk or float setpoint voltages, the two inverters throw high DC current 'spit balls' between themselves due to the step inaccuracies and variations in the neutral to L1/L2 voltages. The average current to battery works out though based on battery voltage measurements by the SW's. Another problem with the series stacking is the temp comp adjustments. They are done on discrete 2 deg C increments and when one unit is on one side and other unit on other side of discrete temp comp point they throw a continous high DC current between units.

Re: Fluctuating amps during battery charging

one more to check if the current is to high for the battery there is a lot off gassing.
All the gass bubles are dropping the charge current. this also means that you have to fill more water than normal. So try to look inside and if you see the cell boiling or is getting warm your current is to high.

if this is the problem lower the current because it is one of the reasons to distroy a battery. ( you can see this alot on forklift trucks they speed up the charging to be faster and after a few months the battery is in battery heaven.:D

The charging current is dropping as the battery is filling up but is not droping to zero and back that is a problem.

greetings from Greece

Re: Fluctuating amps during battery charging

I PM-ed the OP about my experience. A more concise version here.

My SW4048 would vary widely in charging performance, as described earlier in thread. Xantrex said it was normal, then blamed my genset and said if it wasn't charging properly it wouldn't invert properly...then the lights went out. No charge, no invert!

Unit was under warranty at the time, installer lent me another unit and sent the faulty one away for service. Turns out a/the 35amp relay was faulty. When returned, using all the same genset and loads it performs flawlessly. There is ''hunting" when charging, but the up and down of charge current is only 2-3 amps straddling the charge set point...30amp setpoint sees 29-32amp range.

My advice was service before catastrophic failure...or replace with an XW unit (my next move whenever)

Ralph