24v Lithium resting voltage

FrozenToast said:
Sorry I should have said that 27v reading is when I connected the batteries to the inverter with no input from Eskom or PV to provide float. I just went to check now and the inverter is reporting a battery voltage of 27.5v, with a multimeter both batteries are reporting 13.5v and the total voltage is 27.1v.
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Ok, so there is a 0.4V (no load) difference between battery and inverter? How long and thick is the battery cable?

What I think is happening is because of the voltage drop, the battery wasn't absorbing enough so you weren't getting a full charge (maybe bigger voltage drop under load) so resting voltage (SOC) would drop over time or become inconsistent, and now that you have float set to 27.6, this is giving the battery more time to absorb.

Can you set bulk back 28.8V, but instead measure the battery terminals and use that as your reference and increase inverter setting to reach that number, then turn off float again.

Then consider upgrading your battery cable. 3kW @ 24V = 125A so 50mm² cable @ 1m.

DC_wire_selection_chartlg.jpg
Choose_the_Fuse_Amperage.jpg
 
Like wingnut says if you measure the terminals you could adjust the voltage to have the right voltage at the terminal

3.65*8=29.2 so can push to there if needed

I would not go past that
As in CV phase the voltage drop can disappear and then return post charging that is what i have noticed anyway on my side i have just under 0.2v drop on a 48v system


Something to also consider

Some inverters has a time aspect to the charge algorithm

ie they would do CC and then when it hits voltage it does CV till the amps tapers off

But some has a time linked to this portion and could also then cut short

Especially with a bit of a voltage drop

Since you charge cc to lets say 28.3 while the inverter thinks it is 28.8 ,

By lifting the voltage slightly you extend the CC phase thus reducing the time needed in cv

And then don't need the higher float to compensate
 
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leon.davibe said:
Like wingnut says if you measure the terminals you could adjust the voltage to have the right voltage at the terminal

3.65*8=29.2 so can push to there if needed

I would not go past that
As in CV phase the voltage drop can disappear and then return post charging that is what i have noticed anyway on my side i have just under 0.2v drop on a 48v system


Something to also consider

Some inverters has a time aspect to the charge algorithm

ie they would do CC and then when it hits voltage it does CV till the amps tapers off

But some has a time linked to this portion and could also then cut short

Especially with a bit of a voltage drop

Since you charge cc to lets say 28.3 while the inverter thinks it is 28.8 ,

By lifting the voltage slightly you extend the CC phase thus reducing the time needed in cv

And then don't need the higher float to compensate
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I would stick to the original 28.8V. No need to go to 29.2.

and definitely upgrade the cables.
 
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Final resting voltage at 11pm (38 hours) before bed was 13.58V. Disclaimer: There is a volts display and the charger (unplugged from wall socket) connected with ring lugs, so a tiny load of 2 leds on charger and the volts display.

Maybe one day I can leave it to rest for a week or something to see what it ends up on but for now I would say 13.6V is resting voltage on a fully absorbed LFP battery which concurs with this:
LiFePO4-Battery-Voltage-Charts-Image-8.jpg

So I would set float to 13.4V if you have to enter a value.
 
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@wingnut771 To answer your initial question i would the say the cables are quite thick, somewhere between your pinky finger and ring finger. Thats possibly not he best measuring technique but I couldn't measure this morning. Length wise is probably about half a metre.

@leon.davibe Thank for the insight cause sometimes it does feel like the inverter stops charging to soon based off the voltages, this was of course prior to the changes I made as suggested by Wingnut on another forum post.

See below the voltage drop when going from resting to load cause of LS, so it will sit at 26.5/6v then drop down to 25.8v and come back up etc. Funnily enough the voltage will just hover in that range for a long time hence why I say performance wise the batteries are working.

It seems like float stops "floating" in the middle of the night but the inverter says its providing float based on the image on the display. Weird.

chart.png
 
FrozenToast said:
@wingnut771 To answer your initial question i would the say the cables are quite thick, somewhere between your pinky finger and ring finger. Thats possibly not he best measuring technique but I couldn't measure this morning. Length wise is probably about half a metre.

@leon.davibe Thank for the insight cause sometimes it does feel like the inverter stops charging to soon based off the voltages, this was of course prior to the changes I made as suggested by Wingnut on another forum post.

See below the voltage drop when going from resting to load cause of LS, so it will sit at 26.5/6v then drop down to 25.8v and come back up etc. Funnily enough the voltage will just hover in that range for a long time hence why I say performance wise the batteries are working.

It seems like float stops "floating" in the middle of the night but the inverter says its providing float based on the image on the display. Weird.

View attachment 1491305
Click to expand...
Half a meter is great length (shorter the better) but there should be no voltage drop over such a short distance which tells me the cable is too thin. Upgrade cable to 50mm squared. It will solve all your voltage issues. Just check in the manual what cable thickness can fit in the inverter connections. 50mm might be too thick, but 35mm will also work.

It stops floating in the middle of the night because it has fully absorbed and is not accepting any more current, and now the battery is resting with some surface charge voltage, and will slowly drop to 13.6V.

If you change your setting so it bulks at 28.8V at the battery terminals and leave float setting unchanged, you will notice it doesn’t absorb anymore.

Moral of the story is your battery cables are underspecced.
 
wingnut771 said:
Half a meter is great length (shorter the better) but there should be no voltage drop over such a short distance which tells me the cable is too thin. Upgrade cable to 50mm squared. It will solve all your voltage issues. Just check in the manual what cable thickness can fit in the inverter connections. 50mm might be too thick, but 35mm will also work.

It stops floating in the middle of the night because it has fully absorbed and is not accepting any more current, and now the battery is resting with some surface charge voltage, and will slowly drop to 13.6V.

If you change your setting so it bulks at 28.8V at the battery terminals and leave float setting unchanged, you will notice it doesn’t absorb anymore.

Moral of the story is your battery cables are underspecced.
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Interesting that I paid an electrician quite a lot of money to setup everything and they put the wrong size cables in.

I will speak to the electrician to upgrade the cables.

Thank you for your time.
 
FrozenToast said:
@wingnut771 To answer your initial question i would the say the cables are quite thick, somewhere between your pinky finger and ring finger. Thats possibly not he best measuring technique but I couldn't measure this morning. Length wise is probably about half a metre.

@leon.davibe Thank for the insight cause sometimes it does feel like the inverter stops charging to soon based off the voltages, this was of course prior to the changes I made as suggested by Wingnut on another forum post.

See below the voltage drop when going from resting to load cause of LS, so it will sit at 26.5/6v then drop down to 25.8v and come back up etc. Funnily enough the voltage will just hover in that range for a long time hence why I say performance wise the batteries are working.

It seems like float stops "floating" in the middle of the night but the inverter says its providing float based on the image on the display. Weird.

View attachment 1491305
Click to expand...
it also dep[ends on inverter

ie as far as i understand it some inverters handle the USE settings as a lithium
thus it uses the lithium charge graph ie CC CV then no float as a lithium doesn't need it

and it does not actually supply
a float voltage

ie if it was on a lead acid setting it may use it

i know mine goes into a sleep setiing ie lithium style
and the float setting is just used as a trigger point for charging again

and my battery settles right down to resting volts quick 1.5hrs about

and the charging cycle only gets triggered again if it drops a certain amount under the float voltage
have not tried to pin point that voltage

setting the float higher you may find that resting voltage triggers the charge cycle again
and circumvent the time limitation as 2 charge cycles gets the batteries full if the CV linitation is a thing

before i upped my voltage when i noticed that i don't get fully charged
if i force a charge cycle it would get it full with the same voltage settings

hence why i was sure it is a time limitation on the charge cycle

in my case

the charging on some of these is just a lot if else statements thus don't always play nice with lithium imo
if you find the right settings that fools its if else statements to give you what you want all is golden
 
FrozenToast said:
Interesting that I paid an electrician quite a lot of money to setup everything and they put the wrong size cables in.

I will speak to the electrician to upgrade the cables.

Thank you for your time.
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yea gotta realise a lot of the graphs ie like wingnut used is from overseas
electricians tend to work on ac , and may be rusty on dc in sa

i mean the sans regulations surrounding solar is still in draft phase

cable size we don't know what you have , normaly they have some writing somewhere
though with a short cable you could have piece inbetween the writing

a 10mm cable the copper diameter is about 6mm with casing mine is 10mm
and 35mm cable about 7mm with casing mine is 11.5mm


cable run lentgh is a thing with such a short run according to calculator 25mm is fine for 1% drop (but yea that is 0.24v)
and over 1.25m is where they recomend 35mm and over 1.69m it recomends 50mm

DC Cable Sizing Tool - Use The Correct Sized Cables - Free Calculator

Use our free cable-sizing tool to establish the correct size of cable for your DC power system cables. This is especially important for low voltage, high current systems.
www.solar-wind.co.uk www.solar-wind.co.uk

voltage drop will always be a thing it is more about managing the voltage drop for fussy devices

ie limiting how much you get
 
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checking my bike battery this morning the voltage on the cells is 13.49
and on the bms 13.19 , so i would not use this daly bms values as a guide

as that volttage drop tells me if i want a true resting voltage i would need to disconnect the bms
 
wingnut771 said:
Why is there 0.4V voltage drop over 0.5m with no load?
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remember a 10 mm circumference cable means squat the conductor could be 3.6mm squared, ive seen people recommend you use 10mm surfix ,there is no such thing ,surfix only goes up to 4mm and 6mm a 10 mm multistrand cable is rated at around 40amp max without ant factors involved as its only a 4 mmsquared conductor .
 
soulie said:
Thats not in the regulations, not that ive seen
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What regulations? The guy used an electrician and still has ****.

What do you recommend without being so cryptic or won't you say it's a trade secret or something?
 
wingnut771 said:
Why don't you help @FrozenToast then.

Everything is so classified with you.

Like drawing blood from a stone.
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I gave you the SANAS regulation for dc cables in a solar sysytem as well as the correction factor ,you clearly dont understand cables,and then you question them,i have now deleted them .you would rather google junk from goodness knows where and try apply that .I dont give advice or tell people how to do it i will give a suggestion according to SANAS and thats all , I dont know any factors about any install other than my own so cannot give advice on any other install ,thats why certified electricians with IE certification are the only ones approved to work on solar systems . he must approach his installer for the info .
 
soulie said:
I gave you the SANAS regulation for dc cables in a solar sysytem as well as the correction factor ,you clearly dont understand cables,and then you question them,i have now deleted them .you would rather google junk from goodness knows where and try apply that .I dont give advice or tell people how to do it i will give a suggestion according to SANAS and thats all , I dont know any factors about any install other than my own so cannot give advice on any other install ,thats why certified electricians with IE certification are the only ones approved to work on solar systems . he must approach his installer for the info .
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Thanks for the assistance.
 
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