Battery charging, quite strange happenings!

[Daiboy]

I cannot get my head around all this charging business. Today; battery monitor (Nasa BM2) reads soc at 59%. Went for a jolly to Uxbridge boat centre to pick up my new 3 way fridge and back. Total time around 4 hours give or take. Moored up and soc shows 75%, ok that's great I thought and left the engine running for a few more hours. BM2 now shows 80% at 13.5 volts and 28 amps.

 

All engine stop and have a drink. Sit at desk and use computer and (because it's new) have a sideways glance at BM2. Now showing 90% soc at 12.8 volts and only pulling 0.5 amp due to a light above my computer.

 

How is it still charging when I have no engine or generator running?

 

I think I understand that SOC means "state of charge" meaning how much there is left in the battery bank. Maybe not!

 

New battery bank of 4 x 110 amphour

 

HELP

 

Daiboy

[by'eck]

The NASA BM2 uses an algorithm based on amp hour counting from its shunt and battery volts to determine the batteries state-of-charge (SOC). Whilst the charge amps going in to batteries are indisputable, maybe abnormal voltage fluctuations gave rise to this apparently strange indication. Although NASA claim the SOC display doesn't need calibration, I'm sure the reading and batteries themselves would benefit to being taking to 100% SOC, maybe via a mains charger.

 

In any case four hours engine running should have been adequate to re-charge your 440 Ah battery bank and I would have expected a charge voltage over 14 volts by then, so maybe the efficiency of your engine charging system is in doubt and needs to be investigated. What's the alternators max output current?

[ditchcrawler]

The NASA BM2 uses an algorithm based on amp hour counting from its shunt and battery volts to determine the batteries state-of-charge (SOC). Whilst the charge amps going in to batteries are indisputable, maybe abnormal voltage fluctuations gave rise to this apparently strange indication. Although NASA claim the SOC display doesn't need calibration, I'm sure the reading and batteries themselves would benefit to being taking to 100% SOC, maybe via a mains charger.

 

In any case four hours engine running should have been adequate to re-charge your 440 Ah battery bank and I would have expected a charge voltage over 14 volts by then, so maybe the efficiency of your engine charging system is in doubt and needs to be investigated. What's the alternators max output current?

I expected the current to be a bit lower as well, 28 Amp is still a good charge rate.

[Richard10002]

The SOC reading on the NASA seems to be extremely variable, and is definitely not accurate. It gives a flavour, but that's all.

 

I use my NASA for reading amps at any moment, and cumulative amps in or out, and treat the soc reading as a guide, but not definitive.

[Phil Ambrose]

I expected the current to be a bit lower as well, 28 Amp is still a good charge rate.

I thought this too but then considered the 4 hours boating and wondered at what speed the alternator was running, if at tick over not a lot of output, a reasonable RPM is needed to stuff amps back in battery bank.

Phil

[Richard10002]

Assuming your alternator is regulated to 14.2V or so, at 13.5V, it is still rising, So the batteries aren't close to full, as suggested by the amps being drawn and, funnily enough, the soc indicated.

 

At 59%, you have nominally used about 189Ah. My 55a alternator produces 35A at best, and much less as the soc increases.

 

What max amps is your alternator?

[Keeping Up]

The BM2 appears to reset its SOC calculations whenever it first sees a discharge, and thereafter as said above it relies on amp hour counting and voltage readings. It doesn't seem to know how to deal with the surface charge that increases the voltage during and immediately after charging so it gives odd results when you have charged the batteries fully. After that bad starting point it then gets worse if your batteries don't actually have the capacity that you have set up in the BM2's menu, and if they are more than a couple of weeks old their capacity is probably less than you think. I too expect the SOC figure to be no more than a guide, the voltage and current readings are more useful.

 

Having said that, if you were putting in 28 amps your batteries were certainly not fully charged, I don't regard my 440 amp-hour bank as charged until it is taking less than 1 amp at 14.5 volts.

[Ex- Member]

Total time around 4 hours give or take. Moored up and soc shows 75%, ok that's great I thought and left the engine running for a few more hours.

 

 

Hi Daiboy

 

Surely a perfect example of where just a small 100w solar would be saving you a fortune in diesel and charging your batteries fully at very little cost.

Edited August 2, 2014 by Julynian

[Paul C]

I have a similar battery monitor but I don't rely on one variable (eg SoC). I look at various different variables, depending on what the time of day is, what I want to know, or whether cruising or not. For example, if its early in the morning I'm primarily interested in last night's discharge in Ah. If the engine's running, I'll look at Amps coming in - in the morning it might be high (say 25A or more) but hopefully by the afternoon, its at a 'trickle' of 2-3A. If its not, and its towards the end of the day's cruising for example we've moored at a chosen spot, I might leave the engine running; or more efficiently, say if moored at 5pm, watch telly and internet for a bit, then run the engine at 7-8pm (the logic being, between 5-6pm it might put in charge at a rate of 3A, if I've used a bit then at 7-8pm it will put in at 10A - more electricity for the same/similar diesel use). Or if its sunny, then I'll monitor the solar controller too, to check its putting something in. And during the evening, I'll keep a general eye on amps consumed, for example with the telly and some lights on, it might be 7-10A.

 

Sometimes I'll look at voltage but it doesn't mean much. I'll occasionally look at the history for lowest and highest voltages, and check the voltage at first turning the engine on (the battery monitor also monitors engine/start battery).

 

If I leave the boat for a while then come back to it I'll look in the history for deepest discharge, and check the SoC reading which will hopefully be 100% (we have solar).

Edited August 2, 2014 by Paul C

[Ex- Member]

I have a similar battery monitor but I don't rely on one variable (eg SoC). I look at various different variables, depending on what the time of day is, what I want to know, or whether cruising or not. For example, if its early in the morning I'm primarily interested in last night's discharge in Ah. If the engine's running, I'll look at Amps coming in - in the morning it might be high (say 25A or more) but hopefully by the afternoon, its at a 'trickle' of 2-3A. If its not, and its towards the end of the day's cruising for example we've moored at a chosen spot, I might leave the engine running; or more efficiently, say if moored at 5pm, watch telly and internet for a bit, then run the engine at 7-8pm (the logic being, between 5-6pm it might put in charge at a rate of 3A, if I've used a bit then at 7-8pm it will put in at 10A - more electricity for the same/similar diesel use). Or if its sunny, then I'll monitor the solar controller too, to check its putting something in. And during the evening, I'll keep a general eye on amps consumed, for example with the telly and some lights on, it might be 7-10A.

 

Sometimes I'll look at voltage but it doesn't mean much. I'll occasionally look at the history for lowest and highest voltages, and check the voltage at first turning the engine on (the battery monitor also monitors engine/start battery).

 

If I leave the boat for a while then come back to it I'll look in the history for deepest discharge, and check the SoC reading which will hopefully be 100% (we have solar).

 

We hardly bother or take much notice of any of the gauges at all now, the weather has been consistently good since March, we certainly won't run out or power if it stays much the same till end Sept that's for sure. If the weather gets gloomy/overcast we'll keep an eye on the batteries level gauge and take some amp readings if we think necessary, not happened yet though this year. Hopefully it'll stay like that till mid Oct like last year.

 

At the end of the day we use between 150 and 200ah per day which includes running a small freezer. Our 740w of solar will produce that on a non sunny summer day.

 

I'll take the Outback readings from March as I believe we've reached a full SOC every day since then. So the AH's the Outback supplies each day is basically our usage.

 

The freezer will go off in Sept and we're planning on adding 2 or 3 100w flexi panels, we'll hopefully get through winter with minimal engine or generator running at 1 hour per day.

 

If planning to get solar I would advise to get more than you need based on your power audit, then you can use energy just like being on shore line when you're off grid. It's just brilliant!

 

Also I'll hopefully get this solar dump load sorted so we'll not only have plenty of leccy, but some warm water too, all courtesy of the current bun, luverly jubilee

Edited August 2, 2014 by Julynian