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

[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.

 

[eXodus]

If ya dont, you will be needing a new alternator soon.

On my good old Chevy engine - those alternators are cheap, should it die - I'll get a larger one, but then I also have to upgrade wiring...

I don't know why they connect them to the house bank. [shrug]

I always wondered about that on RVs. This is my 2nd Class A and it has the Generator 12V in the house side.

The motorized RV system is designed with protecting the chassis starting capability in mind is my guess.
The generator fuel pickup tube - only extends down to 1/4 - the house system can charge the chassis when then engine is off - but not the other way around.

Maybe the generator is connected to the house is related to that safe starting mindset?
If you would run down your house batteries - you could just start up the engine and then after a few minutes of charging you can start the generator.
If you would run down your chassis batteries - you could just start up the generator and then the engine.

 

[Vigo]

I would guess that the 50a is just a result of three main factors: the alternator's target voltage, the cabling resistance between alt and lifepo4, and the fact that the lifepo4 has such a flat voltage curve.

Given that the alternator is trying to hit a pretty fixed target and the lifepo4 has a pretty flat voltage curve, the available voltage differential is going to be pretty flat and create a pretty steady current, unless you get close to the top or bottom of the lifepo4. That's my assumption.

 

[eXodus]

Given that the alternator is trying to hit a pretty fixed target and the lifepo4 has a pretty flat voltage curve, the available voltage differential is going to be pretty flat and create a pretty steady current, unless you get close to the top or bottom of the lifepo4. That's my assumption.

yep the alternator is starting with 14.4V and then dropping to 13.8V after a few seconds.

I have to plot the voltage against the charge amps, it looks like I am getting the maximum (unhealthy) amps during the high voltage. Then it levels off.

Going to test with and without the LFP in parallel to the Lead Starter battery.

 

[Browneye]

A generator doesn't do any charging by itself. It draws from it's starting battery to start, then supplies 120VAC. I think most RV's have a converter/charger/12V center which does charge, but not generally the starting batteries.

And most of them have a connecting solenoid that allows the engine alternator to charge both starting and house, which is usually a bad idea as they could be at vastly different states of charge. And most have a switch on the dash that manually operates this solenoid so you can jumpber both battery sets for starting.

Going LFP I isolated my two systems completely. Junked the combiner solenoid, and removed the converter/charger. I added a LFP compatible charger - Meanwell is nicely programmable - a Victron Orion for charging the house LFP when the engine is running, solar panels to charge the house LFP, and a separate 100W panel to maintain the starting batteries.

I moved the house battery inside, out of the outside battery tray in the step - that's where they usually put the house and starting batteries together. But not a good place for LFP. So I took that opportunity to double up on starting batts with their own solar charging. Doing it again I would skip the starting battery panels and utilize a trickl-start or similar, although the latest I saw they have closed up shop.

Anyway, charging AGM's is the same as lead-acid batteries as far as your alternator is concerned. It's generally accepted practice not to use a direct to alternator charge circuit for LFP due to their low internal resistance. If you run your house battery down and then get in a traffic jam, it's likely you'll overheat your alternator.

My two batteries...house LFP - 230Ah, and double starting batts, 32' winnebago with 8.1L V8:

 

[eXodus]

A generator doesn't do any charging by itself. It draws from it's starting battery to start, then supplies 120VAC. I think most RV's have a converter/charger/12V center which does charge, but not generally the starting batteries.

true, I understand that. Still - for the backup case my logic still applies - can start the generator from the house batteries in case the chassis is depleted for whatever reason. As soon as the 120V-12V charger kicks in the chassis batteries get charged as well.

Anyway, charging AGM's is the same as lead-acid batteries as far as your alternator is concerned. It's generally accepted practice not to use a direct to alternator charge circuit for LFP due to their low internal resistance. If you run your house battery down and then get in a traffic jam, it's likely you'll overheat your alternator.

My 12V LFP is tiny, it is done charging within a couple of minutes usually. The absolute worst case in my configuration would be - 100AH battery at 50A charge rate - 2hours
My AGMs where double the size and I had ran them down many times, started the engine, drove away the alternator was putting 30-50A into the AGM for 4-6 hours.
It has been doing that for years, not sure why the same charge rate should be an issue just because it's LFP.
Would I double my house battery to 200AH+ LFP - presumable the charge rate would increase to 100A - that would be a problem. I agree.

 

[Supervstech]

true, I understand that. Still - for the backup case my logic still applies - can start the generator from the house batteries in case the chassis is depleted for whatever reason. As soon as the 120V-12V charger kicks in the chassis batteries get charged as well.

My 12V LFP is tiny, it is done charging within a couple of minutes usually. The absolute worst case in my configuration would be - 100AH battery at 50A charge rate - 2hours
My AGMs where double the size and I had ran them down many times, started the engine, drove away the alternator was putting 30-50A into the AGM for 4-6 hours.
It has been doing that for years, not sure why the same charge rate should be an issue just because it's LFP.
Would I double my house battery to 200AH+ LFP - presumable the charge rate would increase to 100A - that would be a problem. I agree.

LFP draws considerably faster than agm does.
100Ah can regularly draw 100A...

 

[eXodus]

LFP draws considerably faster than agm does.
100Ah can regularly draw 100A...

Potentially, true.

But I have not seen this in my configuration yet, mostly below 50A.

Been driving around with the battery for a while now and the alternator is working fine and I don't see high amps.

But I'm slowly getting to the point considering a DC DC to protect the battery.

A few times the battery BMS now disabled charging FET while driving because it hit high voltage disconnect for a cell even when just running at 13.8v - 14.2v

 

[Browneye]

Understand that full charge is right about 3.5Vpc.
My battery is a DIY with LFP cells from china - they were top-balanced when it was built, and remains so until pushed over about 13.5V - then one or more cells will reach full and begin to spike. As soon as charging terminates and it settles back to resting state, they all come into perfect alignment. I don't tend to push charging volts any higher than 14.0V with my system - I still get my 200Ah of usable regardless.