Lithium Iron Phosphate: Charging Parameters question.

[XERTYX]

So I'm thinking of just putting a small lifepo4 battery in to just run the fridge and lighting. Maybe cell phone charging. 

I've been watching a lot of videos of will prowse dismantling batteries and giving pros and cons. He's even cracked open quite a few teensy weensy ones.

There are a couple that I find intriguing that he mostly hated upon exploratory surgery but generally it was because they only had high temp charging protection. No low temp protection. 

The charge controller I'm planning to use has a temp probe. No worries there.

Here's the question. I know the BMS limits the discharge current and charge current. Usually a 100ah lifepo4 will discharge at 100A. Sometimes less depending on the BMS. The battle born for instance will discharge AND charge at 100A but they reccomend 50A max charging.

The smaller 30ah range lifepo4 batteries usually discharge at their rated capacity. I.e. 30ah can discharge 30A etc. BUT many of them will only charge at 20A or 10A even.

Is that hard wired in the BMS? Will it shut down charging if you go over? Will it merely only accept the 10A rated charge current?

I know battle born reccomends slower charging to prolong the life of the battery. Do they say 10A max charging so they can keep the claim of 2k or more cycles?

It seems kinda counter productive to me to only be able to charge a smaller battery with a single 100 watt panel. 2 100 watt panels would just barely meet one of the batteries I'm considering specs @ 15A max charge.

 

[jonyjoe303]

The reason they recommend a slower rate of charge is for balancing reasons. If you charge at high amps it will reach the max cell voltage too fast and trigger the bms to stop the charge, if it has balance issues you would never get a full charge. But at slower charge rates the bms built-in balancing resistors have time to bleed the high cells.

On ebay I see one of the lifepo4 30ah battery, its a 4s3p with a recommended 10a charge rate. So that one you might be able to charge at higher amps, since its only recommended. But definitely use a cc/cv charger that will throttle down the amps as it gets full. Those kind of batteries (including battleborn) where you can't monitor indidvidual cell voltage I recommend the makeskyblue 50a or 60a mppt controller. The controller claims to prevent voltage surges if the bms activates. Only use a controller that states it wont produce voltage surges with lithium.
Even if you set the controller bulk setting at a lower rate to keep it from ever activating the bms, eventually the bms might activate. That might damage 12 volt devices with voltage surges.

 

[jasper]

I've been using a 100ah AGM house battery for over four years to power a DC fridge, diesel air heater, small TV and some LED lighting.  I also kept two small 9ah agm batteries on standby. Monitoring alternator current is depressing to see how slowly that lead acid (AGM) charges while driving.

On a lark, last fall I bought an Eco Worthy 30ah lifepo4, mainly due to its advertised fast 30 amp charge rate, as I like alternator charging while driving.  Another reason is to limit alternator load to its 30amp charge rate, vs. a larger lithium.

30Ah LiFePO4 Lithium Iron Battery
Battery Capacity: 30AH
Battery Voltage: 12V
Max Charge Voltage: 14.5V
Discharge Cut-Off Voltage: 10V
Continuous Charge/Discharge Current: 30A
Peak Charge/Discharge Current: 30A
Operating Temperature: -4°F to 140°F
Size: 7.1*6.3*3inch
Weight: 7.2lbs

I know nothing of its BMS operation.

Notice the 14.5 max charging voltage. At that voltage, from an adjustable DC power supply, mine exceeded 30 amps charging (34), although I only left it there for about a minute. Reducing the voltage to 14.3 reduced charging to 30 amps.  My meters get checked against a local alternator shops bonafide calibrated meters.

Reducing to 14.0 results in 25 amps.  At a fixed voltage, the current remains constant, then very slowly reduces before dropping from about 15 amps suddenly to single digits and fluctuating slightly until finally zero. ??? Balancing ?????

I have two small, 30amp, switching power supplies, adjustable to 14.8vdc.  I'm thinking of buying another identical battery to allow charging both from my inverter generator in about an hour.

Voltage cut-off works.  My crude testing indicates the 30ah rating is close enough for me. So far so good.  I just want the cycles.  If true, it might outlast me.

 

[ldsreliance]

I know battle born reccomends slower charging to prolong the life of the battery. Do they say 10A max charging so they can keep the claim of 2k or more cycles?

The short answer is yes.  As Jony already explained it is better to give the BMS more time to do its work.  But also the internal wiring of the battery is designed around a max continuous current and in order to prolong the life of the battery they prefer you not use the battery to its limits every day with a 1C charge cycle.  The biggest failure points on LiFePO4 batteries are the BMS and the soldered connections.  So keeping max current off of those components will help decrease failures.

BattleBorn doesn't sell a 30Ah battery, though, to my knowledge.  If you are set on that particular size range and are interested in considering other brands, I carry MillerTech 33Ah LiFePO4 batteries in my web store (store.ldsreliance.com).