New years resolutions: loosing weight in RV 190AH lead replaced with 100AH LFP. -100lbs 47kg

[eXodus]

My Class A AGM batteries started to loose capacity especially when cold. So I got my self one of the newer group 24 LFPs.
Elefast 100ah with Bluetooth. My 12V system does only provide power to the lights, furnace etc. The 120V side is covered by a higher voltage solar system

Currently testing if this is really just a plug'n play install.

1. 120V dumb 35A converter - single voltage 13.8V
- check the LFP charges to about 80% and gets very slow.

2. Start 5500W Onan Generator
No problem - pulls 110A for a few seconds generator starts faster then on Lead.

Todo:
3. 100W Panel PWM controller change to LFP, don't care too much about peak performance - just to keep it topped up when stored

4. Alternator Charging - 110A Alternator - which charges the house battery in parallel with the starter battery.
Since this battery is only 100AH - I don't think it should overheat the alternator. It is not a big bank, but lets measure.
Probably do a few tests:

Discharge to 80% - alternator charge current: Temperature:
Discharge to 50% - alternator charge current: Temperature:
Discharge to 30% - alternator charge current: Temperature:
Discharge to 10% - alternator charge current: Temperature:

Any other idea what I should test with that battery in an RV? My goal is to figure out what really needs to be changed over when switching to LFP or if this all just marketing and selling us extra stuff.

 

[12VoltInstalls]

goal is to figure out what really needs to be changed over when switching to LFP

I’d put a ClassT or at least a known brand circuit breaker on the pos(+) terminal.

120V dumb 35A converter - single voltage 13.8V
- check the LFP charges to about 80% and gets very slow.

I’d bypass that for a ‘proper’ charger maybe?

 

[eXodus]

I’d put a ClassT or at least a known brand circuit breaker on the pos(+) terminal.

that is a wise idea.
I was thinking about Terminal fuses:

https://a.co/d/7GU5zUF

https://a.co/d/6r3cqVy

something directly on the battery. How much amp should I go with? 200A ? 300A

I’d bypass that for a ‘proper’ charger maybe?

I usually don't use that thing, just wanted to see if it's a hazard to the battery. It's charging with max of 10A. So pretty slow.

I have a 100W panels which chargers the 12V battery.

 

[12VoltInstalls]

was thinking about Terminal fuses:

https://a.co/d/7GU5zUF

“10,000 AIC at 14V DC satisfies ABYC requirements for DC Main circuit protection on large battery banks. Requires the use of Terminal Fuse Blocks PN 5191 or PN 2151-do not mount the fuse directly to the battery terminal 1,000A Interrupt Capacity.”

Just for educational purposes, @Hedges @Supervstech is an adequate approach versus Class T? I don’t know

something directly on the battery. How much amp should I go with? 200A ? 300A

that is a function of several factors like ‘surge’ but you probably want 125% of the amp rating of your battery cables so they don’t become the fuse and wind up starting a fire.

 

[Supervstech]

Those appear to be similar fuses to automotive, have 1/10th the aic of class T.
I would feel comfortable in a 24V or less using them, but 48V systems scare me.

Those have a 58V rating, but i can see the element and there is little if any arc protection there.

ClassT have a sand extinguish feature.

 

[Hedges]

Good enough if battery voltage and short circuit current is within its limits.
Maybe OK for 12V lithium bank of smaller size.
Class T takes care of higher voltage and moderate Ah. If multiple strings of batteries, might need one class T per string. I haven't seen short-circuit current documented, we have only estimated it from other specs.

Blue Sea 250A MRBF

"

• Interrupt Capacity (The fault current that a device, normally a fuse or circuit breaker is capable of breaking without damage): 10000A @ 14V DC, 5000A @ 32V DC, 2000A @ 58V DC

"

250A Terminal Fuse | West Marine

Check out our 250A Terminal Fuse and more from West Marine!

www.westmarine.com

MRBF Terminal Fuse - 250A - Blue Sea Systems

Space-saving ignition protected fuse for 30 to 300 Amp loads. Must use with Terminal Fuse Block.

www.bluesea.com

"

Interrupt Capacity

20000A @ 125V DC

"

Class T Fuse - 250 Amp - Blue Sea Systems

Use with Class T Fuse Blocks for circuit protection of devices including inverters. high interrupt capacity for large battery banks including Lithium-ion and TPPL batteries.

www.bluesea.com

 

[Supervstech]

Just to clarify the above post from hedges

I know which fuse id want on my system...

 

[eXodus]

Good enough if battery voltage and short circuit current is within its limits.
Maybe OK for 12V lithium bank of smaller size.

Those appear to be similar fuses to automotive, have 1/10th the aic of class T.
I would feel comfortable in a 24V or less using them,

yes it is just for a single 100AH 12V. No expansion planned. Just running lights and furnace.

My main system has T-Class fuse @ 24V and 10kWh LFP and 3000W AIO. So all high amp draws are on that system.

I know which fuse id want on my system...

true - for a huge system the expense and mounting of T-Class is justified, but my system is only 14V - so that's 10.000A for the Stud mount - vs 20.000A on the T-Class. I don't think that a single 100AH cheap LFP battery can deliver anywhere close to that amount.

 

[eXodus]

that is a function of several factors like ‘surge’ but you probably want 125% of the amp rating of your battery cables so they don’t become the fuse and wind up starting a fire.

2 AWG automotive grade

Battery Cable 2 AWG ( Size 2 Gauge ) Red - Copper Flexible Stranded - By the Foot

www.batterycablesusa.com

Table of Allowable Amperage for copper conductors 50V or less with 105C rating:
Gauge Amps Outside Amps Inside
of Engine Space of Engine Space
6 120 102
4 160 136
2 210 178
1 245 208
1/0 285 242
2/0 330 280
4/0 445 378


So half of the wire is inside the engine compartment, so I should do 125% of 178A ? so 220ish? So a 200A fuse would be appropriate?

That would be also the BMS limit - it allows up to 200A for 5 seconds.

 

[Hedges]

FLA starter battery, about 3000A. AGM 100 Ah, 4000A has been reported.
I have not seen measurements reported for LiFePO4. From IR measurements I've calculated 20,000A, but there may be chemistry effects (ion transport) which limit it. Most measurements have the BMS successfully interrupting, but I want fuses to protect if electronics fails.

Here's a reference showing only 1050A from a 160 Ah battery.

https://www.researchgate.net/figure/The-plot-of-current-and-temperature-during-short-circuit-of-LiFePO4-160Ah-battery_fig5_316171277

I wonder if it has too much series resistance in test circuit? I have a tiny LiFePO4 jump starter rated for a few hundred amps. Haven't measured, but it will crank a car engine.

Such current isn't much above cranking amps of lead-acid automotive battery, 700A, and that is at about 2/3 of Voc.

 

[eXodus]

Here's a reference showing only 1050A from a 160 Ah battery.

Yes part of this exercise here is, how much we are over provisiong.
People in this Forum are - recommending very much on the safe side if things "better safe then sorry" and I appreciate this.

But it also creates expensive systems and people having fear about burning up things.

FLA starter battery, about 3000A. AGM 100 Ah, 4000A has been reported.

Correct, people have been fusing LEAD batteries for decades with ANL fuses. Why should a LFP battery need something better at the same voltage? While not being capable of putting out anywhere close of the peak power of Lead-Bank?

Lets say worst case - the LFP is empty - 12.2-12.4V V That is barely 0.4V above the resting voltage of a Lead battery at 12.8V

To get 300A out of a Lead battery you need to drop the voltage to 8-9V - that gives you 4V drop to actually move that many amps. Otherwise - lead is lazy, high resistance and does not want to give you power.

Starter battery for motorbike, planes

12V Lithium Starter battery. Lithium iron phosphate technology. Lightweight starter battery with easy start of internal combustion engine,. For motorbike, quads, light planes, LSA, Van’s RV7, RV8 airplanes. Has a built-in BMS, It ensures for safe and long operation and provides extreme discharge...

www.mylithiumbattery.com

 

[Reed Cole]

I can report that I had one of those MRBF fuses successfully blow the other day, 12v 2 x100ah battleborns. Though it wasn't from a short circuit, if that's relevant.

 

[Vigo]

Im trying to mentally recap what the questions/issues were..

I think the terminal fuse is perfectly fine for the 12v system. I would fuse to protect the wiring sizes since i have personally found the 'X amps for X seconds' part of BMS ratings to be a grey area in reality.

I think the alternator charging will be fine. I have a 136a (peak measured) alternator charging a 120ah lifepo4 in my conversion van and haven't noted any issues yet. The alternator will not push up to the 14.6 high voltage cutoff and i can barely, under some conditions, exceed 1c charge rate but it is typically more like 0.6C so i have not seen any issues.

As far as the 13.8v converter, you could change it, but.. is there a need? 13.8v will eventually get an LFP to 100% but since you are probably neither A. trying to use all 100ah of capacity, nor B. needing to put 100% of it back in a big hurry from shore power, then.. why bother? It WILL charge at a good rate if the LFP is actually at low voltage, say 12.4 or something like that. It only really slows to a crawl when you are probably already at something like 40% soc. Would need testing to confirm the 'SOC vs charge rate @13.8v' curve, but my guess is it really just doesnt matter for your usage.

 

[Hedges]

Correct, people have been fusing LEAD batteries for decades with ANL fuses. Why should a LFP battery need something better at the same voltage? While not being capable of putting out anywhere close of the peak power of Lead-Bank?

I believe LFP can deliver 5x the current of same amp-hour lead-acid. So higher AIC rated fuse should be used.
That is based on IR; 100 Ah cell rated 0.25 milliohm, tested 0.17 milli-ohm. 3.4V / 0.00017ohm = 20,000A.

Testing with BMS isn't sufficient, will be lower current if BMS successfully shuts off.
I question that one test report I linked. Didn't see what wiring and its resistance would be.
Saw a video linked on the form where a lower AIC rated fuse interrupted short circuit of a large bank, but it had enough wire resistance to reduce current to its capability.

My jumpstarter is many times smaller than 100 Ah, yet can deliver some 400A for cranking, at voltage. I would project > 1000A short circuit, and for 1000Ah 10kA to 20kA.

But it is possible ion transport makes a non-linear voltage drop, and short circuit current isn't that high.

Maybe somebody here could wire a few cells in series, make a very low resistance test circuit, and measure current. Also test a low AIC fuse. My test capability is 2000A. Perhaps several shunts with balanced parallel wiring would do the job. Or a higher amperage hall-effect probe.

How much "insurance" in the form of expensive fuse you want to buy will depend in part on what around the system could suffer collateral damage. In your dwelling is different from elsewhere.

 

[eXodus]

I think the alternator charging will be fine. I have a 136a (peak measured) alternator charging a 120ah lifepo4 in my conversion van and haven't noted any issues yet. The alternator will not push up to the 14.6 high voltage cutoff and i can barely, under some conditions, exceed 1c charge rate but it is typically more like 0.6C so i have not seen any issues.

This it what I've been seeing - Up to 60A when almost empty and about 40A when just idling. Nothing which should kill a 110A alternator.

At a 100AH battery that comes out to 0.6C and 0.4C a simple math.

 

[Vigo]

My conversion van has the lfp battery in the engine bay as the starter battery which means shortish wire lengths.. In my rv when i have tried to charge the 'house' batteries from the alternator, the wire length in between 'throttles' it down to something pathetic like 10-20a unless the LFP is very drained, in which case i'll get the 60-70a you noticed as well. So wire length/size is the controlling factor there.

 

[Brucey]

View attachment 190437

View attachment 190438


My Class A AGM batteries started to loose capacity especially when cold. So I got my self one of the newer group 24 LFPs.
Elefast 100ah with Bluetooth. My 12V system does only provide power to the lights, furnace etc. The 120V side is covered by a higher voltage solar system

Currently testing if this is really just a plug'n play install.

1. 120V dumb 35A converter - single voltage 13.8V
- check the LFP charges to about 80% and gets very slow.

2. Start 5500W Onan Generator
No problem - pulls 110A for a few seconds generator starts faster then on Lead.

Todo:
3. 100W Panel PWM controller change to LFP, don't care too much about peak performance - just to keep it topped up when stored

4. Alternator Charging - 110A Alternator - which charges the house battery in parallel with the starter battery.
Since this battery is only 100AH - I don't think it should overheat the alternator. It is not a big bank, but lets measure.
Probably do a few tests:

Discharge to 80% - alternator charge current: Temperature:
Discharge to 50% - alternator charge current: Temperature:
Discharge to 30% - alternator charge current: Temperature:
Discharge to 10% - alternator charge current: Temperature:

Any other idea what I should test with that battery in an RV? My goal is to figure out what really needs to be changed over when switching to LFP or if this all just marketing and selling us extra stuff.

Do you have cold temperature charge protection in the battery or somewhere else? You mentioned your FLA having trouble in the cold, most LFP cannot be charged below freezing.

 

[time2roll]

I stuck with about same size and weight. Gained 6x usable capacity
Solar does the charging... simple adjustment to the program. Converter is off unless difficult conditions persist.

 

[12VoltInstalls]

can see the element and there is little if any arc protection there

want fuses to protect if electronics fails.

eXodus don't think that a single 100AH cheap LFP battery can deliver anywhere close to that amount.

It’s not its operating current that’s the issue at hand. It’s the catastrophic failure event that is at issue.

Why should a LFP battery need something better at the same voltage?

Because it can “discharge” in failure to potentially 20kA

 

[eXodus]

Do you have cold temperature charge protection in the battery or somewhere else? You mentioned your FLA having trouble in the cold, most LFP cannot be charged below freezing.

Yes, the elefast 100ah has low temp protection.

I searched for a long time to find group 24 size with that feature.

Because it can “discharge” in failure to potentially 20kA

I want to learn more, do you have sources to share? I would assume the BMS. PCB would burn out before that much current could get out.