#160475  by scubadoo
 Wed Apr 01, 2020 2:51 pm
You could safely repeat the test allowing the battery pack to reach 12.0V and report back. It should provide a few more Ah. ;)

Please monitor carefully though.

I assume the quality inbuilt BMS will disconnect well before any damaging voltage/capacity levels are reached. :?

My annual capacity check ceases when the first cell drops to 2.8V. i.e. c11.2V for a well balanced 4 cell pack.
My 300Ah battery pack is set to isolate at 12.1V. It has yet to occur during normal use in 5 years.

I look forward to accurate capacity testing of these cells from Darren aka Bused As.
 #160599  by divxmaster
 Sat Apr 04, 2020 1:42 pm
I've found cheap chinese voltage meters to be very inaccurate, the one I have can be 30% out at times. Also, from all the soc charts I have 12.9v is still 20% charged and the main chunk of power is 13.0v - 13.2v (30% - 70%). I drew 72ah out and was sitting at 13.19v. The only way I have to really test these is my hobby charger which will only discharge these batteries at 0.7a.
so thats 385 hours! Oh well, as said before if I get 200ah out of each battery it will be enough, or even 150ah.
My current thoughts on these batteries are, they are fake (they say ATL not CATL), they are second hand and also very badly assembled. Oh well, you get what you pay for.
 #160605  by Indinial
 Sat Apr 04, 2020 10:24 pm
I intend to test the battery again, I have noticed the voltage drops very quickly from 14v to 13.5v which is what I have read. I will start at 13.8v (which I will take as 90% SOC) and take it down to 13v (25% SOC) using my 60watt bulb (5amps) . I have a Fluke meter so I should get accurate readings.
I calculate I should get175Ah between these two reading If this battery is 270ah, this will take a slow 35hrs (what else have we got to do in a lockdown?) I think I will use the "Load Out" on my Epever controller so this will give me a second reading. I will disconnect the solar input.
If the total capacity turns out to be much below a total of 230Ah I wont be happy and as I only live a few kilometers from Micromall I will pop around there for a discussion once the lockdown ends.
I would appreciate any suggestions of what else I could use as a load to shorten the discharge time, or in case my bulb burns out and I have to start again
 #160610  by divxmaster
 Sun Apr 05, 2020 8:24 am
note that 13.8v is 99.95% charged and 13.6v is 99.9% charged according to the charts I have. Something like that.
Also this battery was never 270ah, 240 if you are really lucky.
 #160614  by Neddy
 Sun Apr 05, 2020 8:54 am
The easiest and best "test load" that I have found is to run 230v bulbs from an inverter. Use of a multitap box enables the selection of a load appropriate to your battery capacity and inverter. Another big advantage is that the inverter will automatically disconnect the battery and stop the test when its "low voltage cutout" threshold is hit. You can set the test running and go to bed without worrying about over-discharging your batteries and always have the same stable, reproducible end point. Simply read the battery Amphour capacity from your Battery Monitor in the morning.

If you have no inverter you can use 12v halogen light bulbs. Five 50w 12v QI light bulbs would give you a test current of 20 amps, for example.

Forget about voltage and focus on what really matters - Amphours.

If you haven't got a Battery Monitor, get one. That is the only way you can really get a handle on your battery performance.

Neville.
 #160617  by scubadoo
 Sun Apr 05, 2020 9:49 am
Jonnny

Do not rely on the inverter low voltage cutoff point for LiFePO4 batteries. My 1000W cheapie cuts off (I hope) at 9.5V. :?
You are in serious territory if a single cell in the pack drops much below c2.8V.

LiFePO4 batteries and voltage readings in an attempt to guess the SOC is a futile exercise. i.e. xxV = xx%SOC = +- c30% dependant on the cell manufacturer, use and age.
A shunt (at the battery) based battery monitor is the only reasonably accurate method that I am aware of.

Ignoring the unknown BMS wizardry inside the box 13.8V at the battery terminals of a 4 cell pack will charge to 100% SOC as quickly as the current from the charge source allows, well for my battery pack.
13.8V to 14.2V measured at the battery = <2 minutes at my typical charging currents. Not many extra Ah. ;)
 #160623  by Neddy
 Sun Apr 05, 2020 11:35 am
Because LiFePO4 batteries are so easily damaged by over-discharging, it is very important that some sort of "Battery Protector" is fitted. These disconnect the battery from any load if/when battery voltage drops below a pre-set level. Johnny could set his Battery Manager to, say, 11.3 volts and run capacity measurement tests safely and automatically. An inverter based "low voltage cutout" would never be triggered.
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