A question for the brains trust / battery specialists.

I have 560 Ah of lithium batteries & 1200w of solar

As can be seen on the picture, overnight the voltage drops to 13.2v with the Pi4 etc drawing 1.2amps & then down to 13v when the fridge kicks in drawing a total of 5 amps.
When the sun comes out the batteries become fully charged again & sit around 13.6v.

This is for no-one on the boat on it's mooring - very different with people aboard.

Is this normal??
It seems to me that the voltage is dropping too much under load for this size battery bank!!

Did your battery come with a charge/voltage graph? As you may know, the lithium chemistry is not like any lead-acid batteries - a quick decline in voltage for the first 5-10% of capacity followed by a long period at a relatively steady voltage is what I would expect.

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Woylie said..
Did your battery come with a charge/voltage graph? As you may know, the lithium chemistry is not like any lead-acid batteries - a quick decline in voltage for the first 5-10% of capacity followed by a long period at a relatively steady voltage is what I would expect.

Yeah, I have the tech info for the batteries but doesn't mean much to me!!
Also the C-Rate curve.

Sometimes the voltage will drop way down to 12v but that is with the inverter under load - it jumps back up to 13.2 or so when the inverter is off though.

Probably all good, I just want these to last!!

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Sometimes the voltage will drop way down to 12v but that is with the inverter under load - it jumps back up to 13.2 or so when the inverter is off though.

OK, I'm not an electrical engineer, so I'm going to ask the dumb question. Why are you worried about it dropping "way down" to 12 Volts? If you have a 12 Volt system, doesn't 12 Volts means that it is working as should? Isn't anything between 12 and 14 Volts a bonus?

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Achernar said..
OK, I'm not an electrical engineer, so I'm going to ask the dumb question. Why are you worried about it dropping "way down" to 12 Volts? If you have a 12 Volt system, doesn't 12 Volts means that it is working as should? Isn't anything between 12 and 14 Volts a bonus?

Lithium is very different to AGMs or lead acid.

As on the C-Rate Curve pic when the batteries get down to around 12.6v or so they drop off very rapidly & are flat.

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Achernar said..

OK, I'm not an electrical engineer, so I'm going to ask the dumb question. Why are you worried about it dropping "way down" to 12 Volts? If you have a 12 Volt system, doesn't 12 Volts means that it is working as should? Isn't anything between 12 and 14 Volts a bonus?

Twelve volts is what is known as the "nominal" voltage - just as your house voltage is 240 V +10%/-6%, so a 12 V system is around that (most components designed to work from 11-15 V).

Gday Laz

Should you be measuring the voltage as a battery instead of the individual Eve's? From the little I have read about Eve cells they can have slightly different voltages. Maybe the BMS and the bus bars are causing a slight drop in voltage. Maybe the cells need a top balance again?

My point is that maybe the cells are doing fine but a combination of resistance in bus bars, slight cell differences, and slight resistance in the switch panel could cause a drop in voltage to the sensor (I guess the sensor could be a little off too - most people don't care too much about 0.2 of a volt). I once had a switch on my fridge circuit that was causing a 0.6 volt drop. I got rid of the switch and the fridge worked much better at start up. I would go along each join in all of the circuit and check that there is no voltage drop across the joins. You may have already done this.

I still like the way yout boat shows the Signal K continuous data. I will be getting lithiums sometime but my Trojans haven't died yet.

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Kankama said..
Gday Laz

Should you be measuring the voltage as a battery instead of the individual Eve's? From the little I have read about Eve cells they can have slightly different voltages. Maybe the BMS and the bus bars are causing a slight drop in voltage. Maybe the cells need a top balance again?

My point is that maybe the cells are doing fine but a combination of resistance in bus bars, slight cell differences, and slight resistance in the switch panel could cause a drop in voltage to the sensor (I guess the sensor could be a little off too - most people don't care too much about 0.2 of a volt). I once had a switch on my fridge circuit that was causing a 0.6 volt drop. I got rid of the switch and the fridge worked much better at start up. I would go along each join in all of the circuit and check that there is no voltage drop across the joins. You may have already done this.

I still like the way yout boat shows the Signal K continuous data. I will be getting lithiums sometime but my Trojans haven't died yet.

Yeah, thanks mate. The BMS has a bluetooth app that I can check the individual cells - I'll have a look at that again on my phone (I haven't bothered to hook it up to the Pi4).

I'll get to work with the multi meter & do some checking.

Signal K is great - can never have too much information!!

You shouldn't worry about dropping the voltage. Don't forget that lithium batteries must be discharged every 30 days and recharge again. Ones a month jump into your boat, before you start the engine, put your inverter on, make your self a cup of coffee and then start the engine. Batteries will not be too healthy if constantly in flow state.
cheers