Fluctuating amps during battery charging

System

Hi folks,

I have searched this forum already and have learned a few things, but I was hoping to pick your collective brains on what may or may not be a problem with our off-grid system.

I am using an 8KW diesel genset to charge a 24 piece, 24V 1350AH Trojan T-105plus battery bank. Our renewable sources (solar and wind) need a little help at this time of year especially.

We are using a Trace Engineering (Xantrex) SW4024 inverter/charger which is currently set to 30A max charge for the batteries. This is 30A from the 120 VAC genset, so for the 24VDC system, I guess that is a max charging current of 150A. This is a little higher, I now realize, than the C/10 formula I keep seeing posted in the forum for optimum battery charging.

So... the problem is that when I start up the genset when the batteries are at, say 800-900AH according to the e-meter hooked up to the battery bank, the genset is putting out about 34A-35A@120VAC which is 30A going to the batteries and 4A-5A@120VAC going to the AC load (or 30A plus whatever the current AC load is). After a while, say 20-40 mins, the charging current begins to fluctuate. It will peak at 30A and drop down, sometimes by 5A-10A, sometimes to 0A. These fluctuations come in waves, with a peak and a trough about once a minute. Is this normal?!! Is this because the batteries simply cannot withstand a continued charge at this rate? I read else where that as the battery bank voltage increases, the charge in amps will decrease. Is this amount of fluctuation normal, however?

It takes about 7 or 8 hours to charge the bank fully if it has dropped down to 50% of the bank capacity, which again, reading threads on this forum, seems about right. But the fluctuations in the charging current are a concern to me. Should they be?! I have checked all parts of the system including connections and everything seems fine.

If any one has any thoughts on this, I would love to hear them. Thank you.

BB.

Re: Fluctuating amps during battery charging

Is the generator frequency / RPM / voltage stable through all of this?

-Bill

Cariboocoot

Re: Fluctuating amps during battery charging

This may be asking the obvious but are your AC loads fixed during this charging? In other words, if the AC loads go up power has to be diverted from charging batteries to make up the difference.
Your 150 Amp charge rate is within acceptable parameters for a 1350 Amp/hr bank. One other thing that might be causing it is resistance changes in wiring due to heating from the heavy current load. This is especially true if it didn't happen when everything was new, but is occurring now as wires/connections age and resistance goes up anyway.

System2

Re: Fluctuating amps during battery charging

The genset frequency, voltage and RPM are all stable throughout the charging period.

As for the AC loads, they also vary in accordance with the power consumption of the residents, but not at the same rate that output from the genset does. The genset can put out more than we ask of it - it is rated at about 63A and the inverter is set to allow it to potentially pull enough to cover the battery charging and cover any loads up to 20A-25A@120VAC, which we very rarely encounter unless a bunch of power tools are on. So, basically, there is not a direct correlation between the AC load and the fluctuating charging current.

If it is the wiring, beyond replacing all the connections, are there are any tests to establish whether this could be the culprit?

Windsun

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?

Cariboocoot

Re: Fluctuating amps during battery charging

Let's hope it's not another venerable old SW 4024 about to die.

As for checking connection resistance, you need to check for potential Voltage along the circuit. This means to connect a DVM across the wire: one lead at one end and the other lead at the other. With the system running, it should read zero (Voltage equal at both ends of the wire). Caveat: I may not have explained that in an understandable manner. Hopefully others can clarify.

There's also the possibility that the battery bank is getting old? In which case it may be "quick charging" - gaining surface charge via the high current rate which 'fools' the SW into cutting back. Have you watched what the battery bank Voltage is doing while the current fluctuates?

RCinFLA

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.

BB.

Re: Fluctuating amps during battery charging

My other questions would be along the lines--Cooling Fans running OK. No papers / etc. blocking vent holes. Nothing "smells hot" (heat sink screw failed and transistor/FET over heating), etc...

-Bill

System2

Re: Fluctuating amps during battery charging

Windsun - we have the voltage parameters for charging set wider than the factory defaults, so I don't think this is the issue.

Cariboocoot - I will check the wiring when I am on site, thank you for the how-to. The battery bank is only a year old, although the first 9 months we were with out a functional genset, so they never received an EQ charge and were rarely fully charged for that whole time. I'm not sure if they would have even reached the float charge cycle, as the bulk charge never completed. For the last 3 months, I have been fully charging the bank every 3 days or so and running EQ charges once a month. I have only recently learned about hydrometers, so the SG is not known at this point as I have yet to test the batteries.

The battery bank voltage varies in accordance with the charging current, so it does not stay at the bulk setting (which I think I have set at 29.2V), but drops down to 25,26,27V depending on how much the charge current drops by.

RCinFLA - thank you for a lot of information! The fluctuations you are talking about could vary at most from about 20A-30A for the charging current, if I read it correctly? The current drops sometimes between these, and sometimes all the way to 10A or 0A. It is these dramatic fluctuations which are the real concern.

System2

Re: Fluctuating amps during battery charging

BB. - the fans are running on the inverter and there is no smell or blockages. It certainly does warm up a lot during charging, but this is to be expected, right?

BB.

Re: Fluctuating amps during battery charging

Yes, it will get warm from charging... The charger is probably around 80-90% efficient--so 10-20% of that energy is going into heat.

Assuming the room/space around the inverter is well ventilated (SW is not heating up a small closet)--then all should be OK.

-Bill

Cariboocoot

Re: Fluctuating amps during battery charging

RCinFLA has pointed out that this sort of current fluctuation in charging is normal for an SW. I think we can certainly trust his word!

Somehow we all got the idea that this was representing a change in performance, which is always bad news. It sounds to me as though the OP only recently 'inherited' this system and can't say if this is the way it's always behaved.

So I wonder if these current fluctuations should be this extreme? We tend to forget about the older inverters' behaviour characteristics.

Also there is concern about the batteries having been without proper charging for some period of time. They should at least be checked to see what state they're in now.

peterako

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

trkarl

Re: Fluctuating amps during battery charging

Cariboocoot wrote: »

RCinFLA has pointed out that this sort of current fluctuation in charging is normal for an SW. I think we can certainly trust his word!

Yep. It must be in the charging algorithm in the SW inverters. My SW4048 did the same thing when charging. A friend of mine has a Trace SW4024 and it does the same thing. Even when floating it will be stable for a while and then start the fluctuating and then stop and be stable for a while. This is all while connected to the grid not a generator.

Ralph Day

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

RCinFLA

Re: Fluctuating amps during battery charging

I re-read your original message.

If the 'waves' are occurring at 1 minute intervals I would suspect the generator. There is nothing in the SW that would have this interval. The pumping I was talking about in earlier post occurs in less then a second intervals.

Some generators have more fluxuation at very light loads. As the battery comes up to absorb setting voltage the SW will fluxuate load on generator. This load variation at low load levels on the generator might cause a voltage or freq variation. The SW will release if it goes out of range.

You might try plugging something like a hair drier or toaster into generator when you see these fluxuation to see if they smooth out. Also look at the gen AC voltage and freq during the fluxuations.

At 30 amps AC input charging you are operating at near the max power capability of the inverter and it will get warm. You should also hear the internal fan go to full speed.

Overall, I would not be too concerned about the variations.