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Charging/discharging LFP question

Bigreddogs

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May 20, 2021
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I'm charging my LFP 12V 170Ah battery with a Victron Blue Smart IP-22 charger on custom setting of Bulk @ 14.6V - constant current at 30A - absorption @ 13.8V with variable current down to 0A - Float @ 13.6V charger shows no current flowing into the battery.

The report at the end of charging shows the battery received 176.3Ah over a 6 hour 4 minute time frame

I connect the battery to a 25A load - voltage drops down from 13.8V to 12.8V and operates at that voltage for about 3 hours before voltage starts to drop off. (that seems like a large drop in the initial voltage to me but I'm new to LifePo4 chemistry)

I discharge the battery down to 10V - that's resting Voltage (shunt shows 9.5V working and based on prior tries I know the BMS will shut the battery down at 9.3V working voltage)

Shunt reads that the battery has provided 95.6Ah at 25amps over 4 hour period - so based on the math and the 2nd shunt on my inverter I believe that to be accurate
(keep in mind that as the voltage drops off the amperage load varies from 25amps down to around 23amps)

I recharge the battery as above and it again accepts 176.3Ah as before...that is 80.7Ah more than was discharged

My inverter is rated at 94% efficiency and my bus is 20 years old so I know I'm going to have some additional loss and I assumed that figuring 10% to 15% total would cover that - so 170Ah rated battery minus 15% for inefficiency should provide 145Ah - operation shows 95.6Ah provided so that still leaves me around 49Ah missing.

What am I missing here...possible that there is more than 15% inefficiency in the system...or a battery that is under performing. How could I test this to find out which is causing my loss. And is a drop of 1V under a load common? I assumed the battery would operate a 13.6V for a bit then drop and operate at 13.4V for a bit then drop again...and so forth. Not go from 13.8V down to 12.8V immediately.

Any insight or advice would be appreciated
 
I recharge the battery as above and it again accepts 176.3Ah as before...that is 80.7Ah more than was discharged

Something is very wrong.
The round trip efficiency for an LFP battery is typically >99%.

Please post pictures of your setup.
 
Something is very wrong.
The round trip efficiency for an LFP battery is typically >99%.

Please post pictures of your setup.
I'm replacing my gel cell acid batteries with LifePo4 so the installation is inside the battery bay of my 40ft. Motor Coach. The battery tray is located in the right rear bay, directly behind the rear wheels. The inverter is located directly above the battery bay - distance from battery to inverter is approx. 24" - distance from inverter to DC panel is approx. 18" - distance from inverter to AC panel is approx. 12. The only method I have to place a load on the battery is to run appliances through the AC and DC circuits so I'm going to see some losses through out the system. My coach is an upper end model - all wiring was custom engineered, manufactured and installed at the factory and I have not experienced high losses when operating my lead acid batteries on the system. Photos would be difficult to provide as you can imagine. I guess I'm thinking that I should find a method to test the capacity of the battery other than using the above so I can rule out high loss due to inefficiency in my wiring circuits. So I'm looking for input on alternate methods of testing battery capacity because right now I'm guessing that the battery is deficient and not operating to specification, or my system has really high inefficiencies. It just seems to me that I should see at least 145Ah performance from a 170Ah battery, but I can't seem to get close to that. I would like to be able to bench test it and see that the battery will in fact provide 170Ah.
 
Photos would be difficult to provide as you can imagine.
Lots of people manage fine.
Lots of people capacity test their battery with an inverter and a heat gun and a shunt based battery monitor.
If you're system efficiency were really 50% something would be getting very hot.
My instinct says your shunt is the problem.
 
Another option which you may already have is a ceramic heater.
They typically draw ~1440 watts.
 
Some of the installs that are done by RV "professionals" are pretty bad.
 
Any chance your Battery Isolator is closing and also charging the engine batteries at the same time?
Mine is wired to add the engine batteries when their voltage drops below float, so maybe that is the additional charging you see in your reports.

What brand/model of coach and inverter?
 
When you place a load on a LFP battery, how far down does the voltage drop? Is 1 full Volt normal?
Any chance your Battery Isolator is closing and also charging the engine batteries at the same time?
Mine is wired to add the engine batteries when their voltage drops below float, so maybe that is the additional charging you see in your reports.

What brand/model of coach and inverter?
No chance of that because I remove the battery and charge it in my garage so it's independent of the entire system.
Another option which you may already have is a ceramic heater.
They typically draw ~1440 watts.
Yes and I have one, I'll give that a try on the outlet next to the inverter
Any chance your Battery Isolator is closing and also charging the engine batteries at the same time?
Mine is wired to add the engine batteries when their voltage drops below float, so maybe that is the additional charging you see in your reports.

What brand/model of coach and inverter?
I have Country Coach Allure with a Xantrex 2012 2000w inverter/charger and have had no issues with the inverter in the past
Lots of people manage fine.
Lots of people capacity test their battery with an inverter and a heat gun and a shunt based battery monitor.
If you're system efficiency were really 50% something would be getting very hot.
My instinct says your shunt is the problem.
Shunt is new Victron 500amp Smart Shunt purchased new for this application, but I will check the torque to make sure the connections are tight
 
of my 40ft. Motor Coach.
A 170Ah (12V?) battery to run AC and DC appliances in high end coach? Sounds oddly undersized to me.
What is the standby power use of your inverter? Maybe that draw is lower than the threshold measured by your shunt?

Everything in and out of the battery is thru he shunt, correct?
 
One of the classic blunders is a power loop between the converter and the inverter.
I would try a charge discharge cycle with the converter administratively disabled.
 
A 170Ah (12V?) battery to run AC and DC appliances in high end coach? Sounds oddly undersized to me.
What is the standby power use of your inverter? Maybe that draw is lower than the threshold measured by your shunt?

Everything in and out of the battery is thru he shunt, correct?
1 - 170Ah now and one more if this works out. Have a 8000KW Onan for large power needs/use. Yes everything is through the shunt. Don't have the standby on the inverter active.
 
+1 to checking with a clamp meter.
Double check that everything is going thru the shunt. The charging report you get is using shunt amps & not charger amps?
In my limited experience 12.8 is reasonable for an LFP under load.
 
+1 to checking with a clamp meter.
Double check that everything is going thru the shunt. The charging report you get is using shunt amps & not charger amps?
In my limited experience 12.8 is reasonable for an LFP under load.
I will pick a clamp meter up and check, don't have one of those yet.

Charging report is from the charger (Victron LP-22) Shunt report is directly from the Shunt (Victron 500amp.) Both provide reports via Bluetooth. Battery charged outside the RV, report is directly from the charger. Battery is then installed and I take usage & discharge readings from shunt.
 
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