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Post by kris on Feb 26, 2021 17:24:40 GMT
Doh my bad, I haven’t put power to the relay. What should the fuse on the ground be rated at? Fuses are not needed on ground wires, only on wires connected to battery positive. It shows it fused on the wiring diagram for the bgb250 that I posted the other day.
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Post by Telemachus on Feb 26, 2021 18:26:01 GMT
Fuses are not needed on ground wires, only on wires connected to battery positive. It shows it fused on the wiring diagram for the bgb250 that I posted the other day. Yes you are quite right, sorry. It is unusual for a negative to be fused but in this case it is because the positive wires are very fat and taking a lot of current, the negative wire is thin and only taking that proportion of the current used to operate the device.
Anyway, looking at the spec it mentions an inrush current of 2.6A. Obviously the device normally only takes a few mA but I suspect that when it is switching it make take a few amps, just momentarily. I suggest that a 5A fuse would be fine.
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Post by kris on Feb 26, 2021 18:39:30 GMT
Yes I’ll do some more tmw. Thanks for the reply.
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Post by peterboat on Feb 26, 2021 20:25:55 GMT
Nick these are the same as Johns, Tony and mine and full charge is 14.6 and float is the same so float should be ok at 13.6 its what I have settled at, the cutoff is the one I worry about as I ran mine down last year and 12.8 downward is very fast this is the second time I have done it with the same results it like falling off a cliff
I think you should check the spec sheet. The one I’m looking at says 14.6 maximum charge voltage, and 13.8 float. This means that Kris’ initial settings are within limits, however the point is that this value of float voltage holds them at very close to 100% SoC. In order to maximise life I suggest that it would be better to back the voltage off slightly such that they are held at perhaps 97% SoC, especially as with solar this voltage will be held for long periods every day in summer. Minimum discharge voltage on the spec sheet is 10v which is clearly right at the bottom of the knee. As you say there isn’t a huge amount of charge left below a static voltage of 12.8 but you have to bear in mind that the voltage dips under heavy load. If you set the relay to open at 12.8v it will open under heavy load when there is still quite a bit of charge left. The point of this relay is not to set the point where the batteries are routinely taken down to, but to set an emergency cut off limit. So one could in theory set it to 10v but that seems a bit drastic and assumes the cells are well bottom-balanced. So as a compromise I’d set it to 12v. In my system with per-cell monitoring im going to set the shut off at 2.6v on the lowest cell (2.5 being the minimum) as I don’t want the thing tripping off when I put the kettle on. You are right Nick I am having a decorating day so mind isnt on it, I have to say that 10 volts brings on the red flashing BMS! which unless immediately recharged equals red flashing light forever I have put in link to the spec sheet which shows 13.8 to 12.8 volts is the usable power for the batteries. As you know I have no cutoff for my system I have a board to do upper and lower voltages aroundtuit keeps on getting in the way, so Kris have a look at the nice graphs and it shows where the power falls off a cliff
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Post by kris on Feb 26, 2021 20:49:04 GMT
I think you should check the spec sheet. The one I’m looking at says 14.6 maximum charge voltage, and 13.8 float. This means that Kris’ initial settings are within limits, however the point is that this value of float voltage holds them at very close to 100% SoC. In order to maximise life I suggest that it would be better to back the voltage off slightly such that they are held at perhaps 97% SoC, especially as with solar this voltage will be held for long periods every day in summer. Minimum discharge voltage on the spec sheet is 10v which is clearly right at the bottom of the knee. As you say there isn’t a huge amount of charge left below a static voltage of 12.8 but you have to bear in mind that the voltage dips under heavy load. If you set the relay to open at 12.8v it will open under heavy load when there is still quite a bit of charge left. The point of this relay is not to set the point where the batteries are routinely taken down to, but to set an emergency cut off limit. So one could in theory set it to 10v but that seems a bit drastic and assumes the cells are well bottom-balanced. So as a compromise I’d set it to 12v. In my system with per-cell monitoring im going to set the shut off at 2.6v on the lowest cell (2.5 being the minimum) as I don’t want the thing tripping off when I put the kettle on. You are right Nick I am having a decorating day so mind isnt on it, I have to say that 10 volts brings on the red flashing BMS! which unless immediately recharged equals red flashing light forever I have put in link to the spec sheet which shows 13.8 to 12.8 volts is the usable power for the batteries. As you know I have no cutoff for my system I have a board to do upper and lower voltages aroundtuit keeps on getting in the way, so Kris have a look at the nice graphs and it shows where the power falls off a cliff
It’s difficult to know exactly what to set the low voltage cutout at 12.6-12.7v I reckon. I’ve got to reset it on my inverter as well.
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Post by Telemachus on Feb 26, 2021 20:51:04 GMT
I think you should check the spec sheet. The one I’m looking at says 14.6 maximum charge voltage, and 13.8 float. This means that Kris’ initial settings are within limits, however the point is that this value of float voltage holds them at very close to 100% SoC. In order to maximise life I suggest that it would be better to back the voltage off slightly such that they are held at perhaps 97% SoC, especially as with solar this voltage will be held for long periods every day in summer. Minimum discharge voltage on the spec sheet is 10v which is clearly right at the bottom of the knee. As you say there isn’t a huge amount of charge left below a static voltage of 12.8 but you have to bear in mind that the voltage dips under heavy load. If you set the relay to open at 12.8v it will open under heavy load when there is still quite a bit of charge left. The point of this relay is not to set the point where the batteries are routinely taken down to, but to set an emergency cut off limit. So one could in theory set it to 10v but that seems a bit drastic and assumes the cells are well bottom-balanced. So as a compromise I’d set it to 12v. In my system with per-cell monitoring im going to set the shut off at 2.6v on the lowest cell (2.5 being the minimum) as I don’t want the thing tripping off when I put the kettle on. You are right Nick I am having a decorating day so mind isnt on it, I have to say that 10 volts brings on the red flashing BMS! which unless immediately recharged equals red flashing light forever I have put in link to the spec sheet which shows 13.8 to 12.8 volts is the usable power for the batteries. As you know I have no cutoff for my system I have a board to do upper and lower voltages aroundtuit keeps on getting in the way, so Kris have a look at the nice graphs and it shows where the power falls off a cliff
I think it is important to distinguish between the lowest state of charge one normally want to operate down to, vs the emergency disconnect lower limit. Also, voltage is only a good guide to SoC if the current is near zero. Under heavy load, the voltage can dip significantly. But the risk of damage to the batteries is about their SoC, not about the voltage under heavy load.
It is worth noting that the voltage dip due to load increases at low SoC. So mid SoC, the split between the C/8 and 2C is around 0.8v. At 10% SoC this increases to about 1.2v.
Anyway, the graph also shows that at C/8 (about 125A for kris) the voltage hits 12.8v at 20-25% SoC. Remember that as far as the Li batteries are concerned, they are no more stressed at 20% SoC than they are at 80% SoC, which is not a lot. Thus my case is that 12.8v is much too high a voltage for an emergency disconnect system. There is no need to put the lights out with 25% of capacity remaining under heavy load, or perhaps 15% remaining at light loads.
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Post by Telemachus on Feb 26, 2021 20:56:07 GMT
You are right Nick I am having a decorating day so mind isnt on it, I have to say that 10 volts brings on the red flashing BMS! which unless immediately recharged equals red flashing light forever I have put in link to the spec sheet which shows 13.8 to 12.8 volts is the usable power for the batteries. As you know I have no cutoff for my system I have a board to do upper and lower voltages aroundtuit keeps on getting in the way, so Kris have a look at the nice graphs and it shows where the power falls off a cliff
It’s difficult to know exactly what to set the low voltage cutout at 12.6-12.7v I reckon. I’ve got to reset it on my inverter as well. Obviously setting the low voltage cutout too high isn't "dangerous" but it does make a chunk of available capacity, unavailable. I think the answer is quite clear from the graph Peter posted. Remember that approaching 0% SoC is no worse for the batteries than approaching 100% SoC. I am aware that you are talking about cooking with elec, so you will need to be set up for discharges of a few hundred amps, so the C/2 line of the graph is probably relevant as a worst case.
The batteries will not actually be significantly damaged until the cell voltage goes below 2.5v (10v total). It is just that years of indoctrination with LA batteries is hard to shrug off!
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Post by peterboat on Feb 26, 2021 21:12:59 GMT
You are right Nick I am having a decorating day so mind isnt on it, I have to say that 10 volts brings on the red flashing BMS! which unless immediately recharged equals red flashing light forever I have put in link to the spec sheet which shows 13.8 to 12.8 volts is the usable power for the batteries. As you know I have no cutoff for my system I have a board to do upper and lower voltages aroundtuit keeps on getting in the way, so Kris have a look at the nice graphs and it shows where the power falls off a cliff
I think it is important to distinguish between the lowest state of charge one normally want to operate down to, vs the emergency disconnect lower limit. Also, voltage is only a good guide to SoC if the current is near zero. Under heavy load, the voltage can dip significantly. But the risk of damage to the batteries is about their SoC, not about the voltage under heavy load.
It is worth noting that the voltage dip due to load increases at low SoC. So mid SoC, the split between the C/8 and 2C is around 0.8v. At 10% SoC this increases to about 1.2v.
Anyway, the graph also shows that at C/8 (about 125A for kris) the voltage hits 12.8v at 20-25% SoC. Remember that as far as the Li batteries are concerned, they are no more stressed at 20% SoC than they are at 80% SoC, which is not a lot. Thus my case is that 12.8v is much too high a voltage for an emergency disconnect system. There is no need to put the lights out with 25% of capacity remaining under heavy load, or perhaps 15% remaining at light loads.
I get you Nick, but John and I now have flashing red lights on our BMS on a couple of the batteries, because we let the batteries go below 10 volts and did not know about it, now Kris spent a lot of money on his batteries and I dont think he wants flashing red lights not that it matters really. I can tell you from experience that once at 12.8 in winter with light loads they really do fall off a cliff in a very short time, your 12.2 volts would be the lowest I would go. I didnt know Kris was going to cook using his batteries but I never normally have my batteries below 13.1 volts even in winter, in summer its inbetween 13.8 and 13.3 volts. This is just my experience of valance batteries
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Post by kris on Feb 26, 2021 21:23:02 GMT
The cooking is only a recent idea, it was James who was talking about it to me. For me I just want to get a low voltage cutout set and the high voltage cutout set.
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Post by Telemachus on Feb 26, 2021 21:32:07 GMT
The cooking is only a recent idea, it was James who was talking about it to me. For me I just want to get a low voltage cutout set and the high voltage cutout set. Peter and I have presented our cases. You will have to make your own mind up! TBH the necessary information is all in the graph Peter linked to.
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Post by kris on Feb 26, 2021 21:39:32 GMT
Yes I know thanks nick.
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Post by peterboat on Feb 27, 2021 9:49:30 GMT
The cooking is only a recent idea, it was James who was talking about it to me. For me I just want to get a low voltage cutout set and the high voltage cutout set. James has cooked on electric ever since I have known him, I have thought of putting in an induction hob myself as in the summer I have more electric than I know what to do with, winter the Rayburn does it all. As Nick says the graphs shows you what you need to know, James, John and myself all work on the 20 -80% mantra and its really easy to stay in those parameters as you will discover with these batteries
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Post by kris on Feb 27, 2021 13:55:20 GMT
So finally got the bmv and relay wired in and working, need to test the relay is fully working and I’ll tidy up the wiring when the short network cable I’ve ordered turns up. I think I’ll leave the low voltage cutout at 12.6v and high voltage cutout at 14.1v
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Post by JohnV on Feb 27, 2021 15:02:58 GMT
Just got in from working and caught up with this thread. My charge settings are marginally lower than Peters but only by a gnats and my batteries aren't stressed and neither am I. I am a heavy electric user cooking by electric most of the time (Rayburn some of the winter ..... but not a lot ..... mostly electric) also other heavy use items, washing machine, tumble dryer, dishwasher....... but I run 24v which does make things a little easier going on the amperage !!! (by the way I have 6 Valence batteries) My Inverter doesn't like below about 21.5 v and has quite a hefty warning sounder before it switches off (needs manual re-setting) this functions as my low voltage cut off (and so far hasn't been needed)
Fairly effective as everything except the fresh water pump and the toilet are 240v
Not suggesting this as the system, just as an alternative that happens to suit me and my lifestyle
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Post by kris on Feb 27, 2021 16:44:12 GMT
Well that’s it I’ve made the switch to lithium world. The mppt is in and the inverter, the battery charger isn’t wired yet and it needs tidying but I’m away.
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