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

[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.