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

[eXodus]

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.

I wonder if that alternator burning up is more a urban myth pushed by the DC-DC sellers... there is only that one Victron Video - and they have not put LEAD in parallel (which most application will have as a starter battery)

I searched this forum and found 1 thread where someone killed 4 alternators, in a boat, with only LFP.

While a found 10+ where people had success and mainly complaining about low charge rate. Alternators are running at 13.8V - are just not great at charging LFP.

 

[12VoltInstalls]

Alternators are running at 13.8V - are just not great at charging LFP.

An automotive alternator should make 14.4-14.8ish

 

[eXodus]

An automotive alternator should make 14.4-14.8ish

it does - for the first 10-15 minutes and then the ECM throttles back to 13.8V.
All of my GM-products had a similar behavior.

 

[Supervstech]

Yes, the elefast 100ah has low temp protection.

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

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.

Yes, however… under several conditions, the BMS can fail shorted closed, Especially under a dramatic failure condition.

 

[12VoltInstalls]

you have sources to share

‘Hedges’ recently posted the basic calculation scheme. LiFePo4 IS bms limited but in the circumstances like you relate- the bms burning up- if there is a sustained arc or otherwise a path created then the whole discharge potential can be available to whatever maintains a circuit.

 

[Vigo]

I think the marine situation is different because they have less or no 'ambient' airflow in the engine bay to cool the alternator with, and they are almost exclusively idling which means the internal fan is at its slowest. Cars typically only do alternator charging while moving, or even if they are idling there is a large fan either on continuously (clutch fan) or at least intermittently (electric fan) so the alternator isn't stuck in 'stagnant' air. That's just a guess as i know a lot about alternators but very little that is specific to boats.

As you mentioned i think *in a car* almost all alternator charging 'durability' problems are fixed by just paralleling a lead acid to the lithium because then you never have the possibility of the BMS tripping and the alternator output having nowhere to go and suffering a massive spike. I dont think the situation of someone sitting still for 4 hours trying to alternator charge a 5kwh bank happens very often in land vehicles.

 

[yabert]

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.

I second that mrbf fuse is ok for this small 100Ah battery.
Good info here: https://diysolarforum.com/threads/calculating-the-lifepo4-battery-short-circuit-current.60969/

 

[HarryN]

Are you using these as house batteries or starter batteries?

 

[eXodus]

‘Hedges’ recently posted the basic calculation scheme. LiFePo4 IS bms limited but in the circumstances like you relate- the bms burning up- if there is a sustained arc or otherwise a path created then the whole discharge potential can be available to whatever maintains a circuit.

Yes, however… under several conditions, the BMS can fail shorted closed, Especially under a dramatic failure condition.

Going to look up those and adding a stud fuse with 200A.

Are you using these as house batteries or starter batteries?

House, really basic loads, lights, fridge control. Furnace, and to start the generator. Tried that already a couple of times and it has no issues to get my Onan 5500 started.


My main engine battery is a 12v FLA lead and is going to stay that way.

 

[Hedges]

Yeah, probably good enough for 12V.

Just for fun, here is a 3 Ah lithium ion (doesn't say LiFePO4) battery with 167C discharge rate. 2.42 lbs including packaging.

1000 Peak Amp Lithium-Ion Jump Starter and Power Bank

Amazing deals on this 1000A Li-Ion Jump Starter at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

If you paralleled 33 of those for 100 Ah (a $3000 purchase), you would get 16.5kA cranking amps. 130kW, 188 HP or so. That could crank a huge diesel! Or make a drag cart.

They recommend 30 seconds connected before cranking to put some charge in vehicle's battery, but don't say how long it can sustain 500A. Total of 22 seconds, if I calculated correctly, so you only get a few attempts cranking. Could be good enough for the 1/4 mile, or take-off from a runway.

 

[yabert]

but don't say how long it can sustain 500A.

Less than 10 seconds for sure... if no, the tiny 8 AWG cables will fused

 

[Vigo]

Project Farm on youtube has tested quite a few lithium jump packs using a large adjustable load tester which gives a pretty informed idea of what they're actually capable of.

He calculates the wattage as well AND does a useful comparison to the pathetic voltage drop of 'adequate' and woefully undersized jumper cables!

Linked to the exact moment:

 

[eXodus]

I installed another fuse. So both sides of the wire are protected now.


Did some testing this week and drove around with the Elefast directly connected to the Alternator. (in parallel with Lead)

It maxes out at around 50A charging. Occasional 52-55A for a split second but then back to 50A.


According to the manual - the maximum charging for this battery is 50A.


Is the BMS limiting charge current to 50A?

 

[eXodus]

So it seems like that the Elefast is somehow current limiting the charging to 50A, not immediately, but pretty fast.

I discharged the 100AH Elefast to 20% - started the engine - measured Amps at the Alternator,

Right after crank it spiked up to 110A - The rated current of the alternator. The BMS showed 70A coming in. There it stayed for all of 5 seconds - after that it started dropping - rapidly.

30 Seconds runtime - 96A from the Alternator - 65A at the BMS
1 min runtime - 85A from Alternator - 55A at the BMS
3 min Runtime 79A from Alternator 49A at BMS

10 min Runtime - 78A from Alternator - 47A at BMS

That is all at idle - when I revv it up - it goes to 49.xx A

After this testing - I don't see a reason to worry about my Motorhome alternator - even when almost empty LFP - it runs well below spec.

 

[Supervstech]

So it seems like that the Elefast is somehow current limiting the charging to 50A, not immediately, but pretty fast.

I discharged the 100AH Elefast to 20% - started the engine - measured Amps at the Alternator,

Right after crank it spiked up to 110A - The rated current of the alternator. The BMS showed 70A coming in. There it stayed for all of 5 seconds - after that it started dropping - rapidly.

30 Seconds runtime - 96A from the Alternator - 65A at the BMS
1 min runtime - 85A from Alternator - 55A at the BMS
3 min Runtime 79A from Alternator 49A at BMS

10 min Runtime - 78A from Alternator - 47A at BMS

That is all at idle - when I revv it up - it goes to 49.xx A

After this testing - I don't see a reason to worry about my Motorhome alternator - even when almost empty LFP - it runs well below spec.

 

[Supervstech]

This shows that the alternator is overheating from extended amp draw.

79ish amps at idle is due to voltage drop. At highway speeds the alternator will be near max output.

A 110A alternator can barely supply 80A for long.

Get a dc to dc controller...
And a bigger alternator.
If ya dont, you will be needing a new alternator soon.

 

[Browneye]

For the hundred bucks or whatever, a victron orion will likely save you from having to replace your alternator.
100Ah is really very small for a house battery - my furnace blower pulls 10Ah running, so a cold night can pull 50-80Ah. I didn't notice if you had a 120VAC inverter.

Connect your generator to your starting batteries - you'll thank me later. In my electric bay it was just a matter of swapping over to another terminal/post, once I figured out which wire it was. I don't know why they connect them to the house bank. [shrug]

 

[Vigo]

I dont think any 'limiting' is going on whatsoever.

In the first place, most BMS's have no way to limit charge current. Some can communicate a 'desired' charge current to a compatible charge source, but the only thing the BMS can do is totally disconnect the battery from its terminals, but nothing in between.

I think the time vs amps behavior is exactly as expected. Any car will have high charging amps right after startup as the battery 'recovers' from the cranking event, but it tapers drastically within the first <30 seconds, and then tapers more as the alternator lowers its 'target' voltage sometime in the first several minutes. Typically alternators will try to hit ~14v immediately after startup but most modern charging systems then taper off to something in the low 13s. There's a ton of variability in the wide world of cars but this is a fair generalization in my opinion.

Notice that the current going to the lead acid is tapering much more quickly than the current going to the lifepo4. The chunk of SOC consumed by cranking represents a larger change in terminal voltage on the lead battery, AND the lead battery recovers more slowly to its 'resting' voltage, so it 'accepts' more charge current from the source than the LFP which had much less voltage sag from the cranking event and then recovered to its 'resting' voltage almost instantly.

My 120ah LFP in my conversion van will accept about 60-70a continuously until it reaches the alternators voltage setpoint which is low 14s. Your 100ah is accepting 60+ very briefly before tapering to a very steady ~50a, which is in exact proportion to the results on my van. I personally think all is well here. You MIGHT have a heat issue with the alternator but i dont think the changes in current you're seeing represent the alternator 'derating' from heat. If it hasn't done anything more drastic in 30 minutes of idling i dont think it ever will as that is plenty of time to reach a steady state temp. However, it is true that the brushes in the alternator will wear out quicker if they pass higher current, so in general using the alternator harder does result in the brushes wearing out quicker. But quicker might be in 50,000 miles instead of 100,000 miles. I'd be surprised if you didn't get a pretty decent lifespan doing 78A out of a "110a" alternator, even at idle.

 

[eXodus]

A 110A alternator can barely supply 80A for long.

Get a dc to dc controller...
And a bigger alternator.

Thank you for your concern, I halved my battery capacity - the AGMs were charging at similar rates -for many hours.
The alternator did not mind.

This was an extreme test. Usually I keep the 12V system topped of via my large systems and a 35A charger.

100Ah is really very small for a house battery - my furnace blower pulls 10Ah running, so a cold night can pull 50-80Ah. I didn't notice if you had a 120VAC inverter.

It is just a backup 12V battery - it is charge via my main primary 24V system with 1300W of Solar and 3000W Inverter. Most of the time the 12V battery is full.

I could probably make due with a 24V-12V DCDC step down instead of having the battery. But I like redundancy.

 

[eXodus]

. But quicker might be in 50,000 miles instead of 100,000 miles. I'd be surprised if you didn't get a pretty decent lifespan doing 78A out of a "110a" alternator, even at idle.

yep, my RV gets 3000-5000 miles in a year. It probably dies of something else

Most of the time the alternator is only at 30-40A - since the LFP is full. I usually never idle it, pack,, get everything ready and go.

I still do not understand why the battery is always just under 50A charging? Pure coincident ? It apparently does not matter what SOC is. It could be 60A or 40A - but no it is 50A - like in the brochure.