diy solar

diy solar

Big Battery Purchase...A Mistake???

A clarification as there is a touch of confusion.
When looking at AH ratings and trying to do the MATH. With LFP (LiFePO4) we ALWAYS use 3.200V per Cell (Nominal Voltage) X #S of STD Cells for the Range. 3.2 volts X 4 cells = 12.8 Volts. 12.8 Volts X 170AH = 2176Wh or 2.17kWh.
STD Cells meaning 12V=4, 24V=8, 48V=16.

The amount of AH from (2.500-3.000) & (3.450-3.650) typically represents about 8% of total Gross Capacity of the cells (WHEN NEW) and is generally not used as most will use (limit) their battery packs to use between 80-90% of cap[acity for maximized life and reduced "stress".

Please Download this resource (PDF) and refer to pages 12 & 13 on voltage charts & charging matrix. It WILL simplify many things and answer some questions coming out here. Luyuan Tech Basic Lifepo4 Assembly Guide

Some say Float is not needed, well I use float on a larger complex bank of LFP Battery Packs and it most certainly helps get the batteries & their cells topped up. Float will also be keeping the Constant Voltage - VARIABLE AMPS going so the batteries can actually saturate (deep charge to low amps taken which is best) and should the System Demand increase, if there is enough incoming solar, then that increase to cover the demand... IE during the day once batteries are in float, I run my coffee maker and NONE of that power comes from BATT it comes direct from the panels. Float increases relative to demand and can provide all the solar needed, if not capable then the balance is pulled from the batteries.
This is how Float works and always has and to NOT utilize it is Handicapping your systems capabilities, which isn't smart at all... tossing $ in the form of power away.

If you Bulk Absorb @ 13.80V (3.450Vpc) and have an EndAmps / Tailcurrent set to 8.5A (170AH X 0.05) Then float @ 13.60 (3.400Vpc) that will allow to top off at the top of the "Working Voltage Range" which is 3.000-3.400V per cell, Nominal being 3.200 for LFP. Rebulk @13.4V.

A CRITICAL WORD OF CAUTION !
As you see in the voltage charts in my PDF resource above, LFP has a fairly flat voltage curve. This means that your system MUST BE CALIBRATED! Even a 1/2V offset can mess things up ! BMS' are Millivolt & Milliamp, sensitive folks ! The Voltage @ Battery Terminals will not be the same at the SCC or Inverter/Charger, due to wire voltage drop, resistance from connections, fuses & breakers and whatever else is "on the wires".
! The SCC or other "Charger" devices MUST be set to correct for any difference DURING CHARGE so that when the battery pack reaches 13.80V at the terminals and you want to stop Absorb the SCC needs to see that too, but if there is a 0.3V difference (loss) the SCC will overshoot to 14.1V at the battery terminals... So that MUST be corrected / adjusted / compensated. Different gear handles it differently, some have actual offset params, others you have to add the difference to the settings themselves.

! The Inverter MUST ALSO BE CORRECTED for DISCHARGING ! Again if there is a 0.5V Difference (loss) and you want to cut off the battery at 11.0V (2.75Vpc) the inverter will actually cutoff at 10.5V (2.625) remember absolute 0%SOC is 2.50), you will likely hit an LVD (Low Volt Disconnect) by the BMS which could "drop" the Inverter hard (not a good thing). Cutoff points are REALLY IMPORTANT and even a 1/2 Volt over/under can cause the BMS' to Trip and Fault or worse get damaged if a dumb BMS fails and you don't know.

In my signature there is a resource on Calibration and more, much of this would be handy to review and check over your gear.
Hope it helps, Good Luck.
Steve

PS: It is my understanding from reviews / teardowns etc that the BMS' used in those BigBattery are just fairly basic Dumb BMS' without any type of balancer or interface to interact with & set params. Apparently, some of the params as devined by other techies are pretty "loose" and not very optimal. Sorry but to me, this is a RED FLAG especially as they use Used Cells & Packs, and being able to tweak & adjust such is "critical" for healthy long-term operation. It also seriously handicaps troubleshooting capabilities because you cannot "see" what is going on internally with the cells.
 
Btw
How did you get a 2/0 cable on the BB. Their internal Anderson connector is a SB175 (I thought) and max size cable for that is 2AWG. 2/0 would require a SB350.
I ordered their BB350 cable and used the one they sent with the battery for my charger. All my cabling is from Country Coach and is 2/0 so the Anderson plug is used from the battery to the shunt and distribution bus bar (about 14") and the 2/0 cables take over after that.
 
There are likely 4 cells in series in the battery. If cell 1 = 3.2v and the other 3 are at 3.4v, you'll still have a nominal 12v battery, but as it discharges the one that is 3.2v will hit bottom way too soon and the BMS will cut-off as soon as the 1 cell out of 4 hits bottom. This can dramatically shorten the useable power.

For 1 cell to be so 'out of whack' with the other 3 is not good - could mean that it's self discharging (losing voltage over time) more than the others (which is bad) but if it's kind of slower then charging may activate balance and bring it up with the others over time. That's where the suggestion of charge/discharge a few times comes in to play,

But it's not good in my opinion.
I am also thinking that there may be a bad cell. When the battery was delivered it was sitting at 13.3V. I didn't install it for 3 weeks, it just sat on my workbench in the garage while I finished some other projects. When I decided to tackle removing the AGM and installing the LFP battery the voltage reading was down to 13.1- I thought that drop was interesting as I was under the impression that LFP batteries would hold the charge better while in storage. I just wrote if off thinking that maybe storage voltage is more in the 12.8V to 13.0V range. I'm new to this.
 
A clarification as there is a touch of confusion.
When looking at AH ratings and trying to do the MATH. With LFP (LiFePO4) we ALWAYS use 3.200V per Cell (Nominal Voltage) X #S of STD Cells for the Range. 3.2 volts X 4 cells = 12.8 Volts. 12.8 Volts X 170AH = 2176Wh or 2.17kWh.
STD Cells meaning 12V=4, 24V=8, 48V=16.

The amount of AH from (2.500-3.000) & (3.450-3.650) typically represents about 8% of total Gross Capacity of the cells (WHEN NEW) and is generally not used as most will use (limit) their battery packs to use between 80-90% of cap[acity for maximized life and reduced "stress".

Please Download this resource (PDF) and refer to pages 12 & 13 on voltage charts & charging matrix. It WILL simplify many things and answer some questions coming out here. Luyuan Tech Basic Lifepo4 Assembly Guide

Some say Float is not needed, well I use float on a larger complex bank of LFP Battery Packs and it most certainly helps get the batteries & their cells topped up. Float will also be keeping the Constant Voltage - VARIABLE AMPS going so the batteries can actually saturate (deep charge to low amps taken which is best) and should the System Demand increase, if there is enough incoming solar, then that increase to cover the demand... IE during the day once batteries are in float, I run my coffee maker and NONE of that power comes from BATT it comes direct from the panels. Float increases relative to demand and can provide all the solar needed, if not capable then the balance is pulled from the batteries.
This is how Float works and always has and to NOT utilize it is Handicapping your systems capabilities, which isn't smart at all... tossing $ in the form of power away.

If you Bulk Absorb @ 13.80V (3.450Vpc) and have an EndAmps / Tailcurrent set to 8.5A (170AH X 0.05) Then float @ 13.60 (3.400Vpc) that will allow to top off at the top of the "Working Voltage Range" which is 3.000-3.400V per cell, Nominal being 3.200 for LFP. Rebulk @13.4V.

A CRITICAL WORD OF CAUTION !
As you see in the voltage charts in my PDF resource above, LFP has a fairly flat voltage curve. This means that your system MUST BE CALIBRATED! Even a 1/2V offset can mess things up ! BMS' are Millivolt & Milliamp, sensitive folks ! The Voltage @ Battery Terminals will not be the same at the SCC or Inverter/Charger, due to wire voltage drop, resistance from connections, fuses & breakers and whatever else is "on the wires".
! The SCC or other "Charger" devices MUST be set to correct for any difference DURING CHARGE so that when the battery pack reaches 13.80V at the terminals and you want to stop Absorb the SCC needs to see that too, but if there is a 0.3V difference (loss) the SCC will overshoot to 14.1V at the battery terminals... So that MUST be corrected / adjusted / compensated. Different gear handles it differently, some have actual offset params, others you have to add the difference to the settings themselves.

! The Inverter MUST ALSO BE CORRECTED for DISCHARGING ! Again if there is a 0.5V Difference (loss) and you want to cut off the battery at 11.0V (2.75Vpc) the inverter will actually cutoff at 10.5V (2.625) remember absolute 0%SOC is 2.50), you will likely hit an LVD (Low Volt Disconnect) by the BMS which could "drop" the Inverter hard (not a good thing). Cutoff points are REALLY IMPORTANT and even a 1/2 Volt over/under can cause the BMS' to Trip and Fault or worse get damaged if a dumb BMS fails and you don't know.

In my signature there is a resource on Calibration and more, much of this would be handy to review and check over your gear.
Hope it helps, Good Luck.
Steve

PS: It is my understanding from reviews / teardowns etc that the BMS' used in those BigBattery are just fairly basic Dumb BMS' without any type of balancer or interface to interact with & set params. Apparently, some of the params as devined by other techies are pretty "loose" and not very optimal. Sorry but to me, this is a RED FLAG especially as they use Used Cells & Packs, and being able to tweak & adjust such is "critical" for healthy long-term operation. It also seriously handicaps troubleshooting capabilities because you cannot "see" what is going on internally with the cells.
A bunch of great information here! Thanks so much. I will be reading this for a bit trying to digest all of this information. I will reset my charger after todays load test to your suggestion above and see if that does a better job of topping off overnight. I never expected the battery to provide 170Ah to me knowing that there is loss from my inverter of 6% and figured 10% loss for resistance in wiring/connections and distance. Still thought I would be able to pull 150Ah even with the losses.

Wish I could make adjustments to the inverter/charger but don't have the means or the knowledge for that even if Xantrex allows it and as far as I know there are no provisions on this inverter to allow the end user to make adjustments to settings. I am thinking that the gel cell setting for the charger will work for LFP batteries on a short term basis as that setting is 14.2V bulk 13.6V absorb and float. Have not used the inverter/charger for charging the battery yet, just using the Victron as it allows me a custom charge setting so I can set my own parameters.
 
You have to be careful with "Factory Preset" Values. For example Lead has Equalize / DeSulphate - NEVER EVER do that to LFP, the BMS' will slam down hard.

These are my settings from "About My System" pages link @ top of my Signature.

All equipment MUST BE Voltage Corrected & Calibrated (VERY IMPORTANT) see link in my signature on how to do it.
Divide Values X2 for 12V. Multiply X2 for 48V.
Absorb: 28.2 for 15 minutes (3.525vpc) (some call this boost)
Equalize: OFF
Float 27.9V (3.4875vpc)
MIn Volts: 22.0 (2.750vpc)
Max Volts: 28.7 (3.5875vpc)
Rebulk Voltage: 27.7 (3.4625vpc)
End Amps: 14A (*1)

(*1): End Amps is calculated from the Highest AH Battery Pack in a Bank. IE: 200AH X 0.05 = 10A 280AH X 0.05 = 14A.
NB: Victron Forum discussion says EndAmps = TailCurrent
This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.

** Coulumbic Efficiency for LFP is 99%
 
I ordered their BB350 cable and used the one they sent with the battery for my charger. All my cabling is from Country Coach and is 2/0 so the Anderson plug is used from the battery to the shunt and distribution bus bar (about 14") and the 2/0 cables take over after that.
Thanks and good idea. I actually bought the Owl Max 2 cuz I love the specs, but then canceled the order because I think the internal connector is only rated to 175A. So it seemed to be a weak link in a battery that can output 350A burst (if remember right) with a fuse that is 300A as well as max cont discharge at 175A. So putting a BB350 connector on a SB175 connector seemed to still leave the SB175 as the weak link. I might be wrong about their internal connector.

Also to note: about their deal to send free Anderson cables rated to 175A ... I asked what gauge cable was used they told me 6AWG. I looked up ampacity of 6AWG and it is not 175A. So I am not sure I trust those guys anymore. That was another reason to cancel the order.
 
Unpopular observation: Wire size seems to be like penis size, it makes some people happier if it is bigger.

Come on, people, the calculations for proper wire size aren't that hard. If you don't understand how to do the calculation, ask someone who does. You will save money and make your construction easier.
 
I ordered their BB350 cable and used the one they sent with the battery for my charger. All my cabling is from Country Coach and is 2/0 so the Anderson plug is used from the battery to the shunt and distribution bus bar (about 14") and the 2/0 cables take over after that.
Does it matter that the wire inside the battery running to the sb175 connector is 4awg? How is that supposed to handle 175a continuous?
 
Does it matter that the wire inside the battery running to the sb175 connector is 4awg? How is that supposed to handle 175a continuous?
Who is ever going to pull or push 175 amps continuous for more than a few seconds if ever?
 
Who is ever going to pull or push 175 amps continuous for more than a few seconds if ever?
I've got a few appliances that will pull 1600w from my inverter + my dc fridge + other 12v loads. If the appliance needs to run for 3-4 minutes that's 175a continuous.
 
Okay, read thru this thread again...it's from last year. What ever happened with OP's Big-Battery quandary?
For @Maw230 - you also bought a 'BigBattery' Owl or similar? And going to run an inverter and appliances in a mobile application? And you're concerned about the interior foot of 4awg wire and output?

If the battery is rated for 175A continuous output then it will probly do it for a time. Yes, things might get warm. If the battery is a 170Ah capacity then figure it might run for a half hour like that before voltage sags and it shuts down. That's a BIG draw in Ah compared the battery's total capacity.

And yes, 1600watts at 120VAC might will pull 160A off your 12V side. I find they run about ten times - I have a Xantrex 600W and it really won't do more than about 500W, and pulls 45-48A on the battery. My battery is a DIY from Eve cells, 230Ah capacity, so it will hum along doing 45Amps for a few hours. We don't typically run it like that, we don't really have high-draw appliances to run on it - charge a laptop, ebike batteries, phones, a box fan, etc. and then all the 12V systems in the RV. For the coffee maker and wifey's hair dryer I got a Ecoflow Delta Mini that will power up to 1800watts for a bit, and can recharge it with the LFP setup and solar. It's a really handy little unit.
 
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