12v LFPo to Alternator question

[mtts]

This is a vehicle-mounted system. This is strictly a question about the battery-alternator interface.

TLDR: Question: Has the issue, of a high-drain (dead short) by the LFPo 12v 100ah batteries, been resolved? I.E. so the BMS restricts the charging rate to something reasonable like 20-40 amps. So a DC-DC converter isn't needed, to interface between the alternator and the 12v LFPo.

Intro: My main system is backed by 2x EG4 5kw LFPo's. Growatt 3k. 1800w panels. 3kw gasoline generator.

I left the existing 12v Flooded Lead Acid, 1kw modified sine wave inverter, and basic 12v chassis stuff, alone. I added a simple NOCO-brand 5amp charger, along with a basic mechanical 120v AC plug-in wall-timer, that periodically cut's it's power, so the charging cycle will reset.

The problem is, FLA is junk. The 12v Group 27 Marine batteries from East Penn State (O'Reillys 27-DCM) consistently die after 3-4 yrs, and I want to swap them out with maintenance-free LFPo.

Right now, there's a single house 12v battery, and an engine start battery, and there is a flip-flop no-drain solenoid that can be toggled to disconnect them from each other. Can I just get 1x 12v 100ah Ampere Time, or other brand (please state!), to replace the house & engine start battery? The draw is 300amps max, from the engine's starter, according to what documentation I could find. Of course, that's only for a brief period, but on cold starts after a long sit, there could be many seconds of turning it over.


The thing that held me back from converting everything to LFPo, was the issue with the legacy alternator needing some DC-DC converter to the LFPo. That seems retarded, and I wonder if it's been solved?

 

[BentleyJ]

TLDR: Question: Has the issue, of a high-drain (dead short) by the LFPo 12v 100ah batteries, been resolved? I.E. so the BMS restricts the charging rate to something reasonable like 20-40 amps. So a DC-DC converter isn't needed, to interface between the alternator and the 12v LFPo.

The BMS is not a control device, it has no way to modulate current, voltage or temperature. Its only a safety switch, its either On, normal conditions or Off, when it detects a condition that could damage one of the cells. So unless the vehicle has some kind of built in current limiter a DC DC charger is needed. Note: there is a difference between a DC DC converter and a DC DC charger.

 

[k490]

You can't use a Lifepo4 to start the engine not designed to dump that much current and charge quickly. Lead acid is fine for a start battery I would not bother with those marine dual purpose pay more for them they work just like a starter battery. I would just leave the starter battery alone drop in a DC-DC with the current you want and go lithium on everything but the starter battery.

Renogy makes a 60 amp DC to DC charger biggest I could find Victron has a 30 amps max size if you only want 30 amps Renogy makes 20/40/60. You want to verify your alternator is big enough to handle the charge rate as the lithiums are capable of taking in larger currents and charge faster. You will need 4 gauge wire with one 70 amp fuse at the alternator side and another on the other side of the DC-DC charger. You might want to use circuit breakers as long as they can take the heat being in the engine compartment. Good thing about breakers are you can flip the breaker when you want to work on it no risk of shorting out something by removing a wire from a lug. You should always unplug the ground side of a battery before working on it. I noticed on my Ram 2500 has dual 220 amp alternators the wire is 4 gauge no way it could ever charge at 440 amps almost useless maybe could be used for a winch have to run 4/0 gauge wire from it. It was only a $400 upgrade back in 2014 I got it anyway for redundancy thought it was cool to have dual alternators never had a truck with dual.

RNG-DCC1212-60-US https://www.renogy.com/12v-60a-dc-to-dc-battery-charger/

 

[Supervstech]

The BMS doesn't limit charging...



It is only rated for 40ish amps charging, but that means it will fail if more amps are fed to it...



Don't feed it over 40A ... the alternator WILL feed it all it wants... killing the bms, and the alternator as well.

 

[k490]

The BMS doesn't limit charging...



It is only rated for 40ish amps charging, but that means it will fail if more amps are fed to it...



Don't feed it over 40A ... the alternator WILL feed it all it wants... killing the bms, and the alternator as well.

Is 40 amp max charge rate for all LiFePO4 BMS's? The batteries specs I have seen show 100 Amp max charge rate for 100 AH battery. Has it been your experience that BMS's will burn out if you charge at 1C rates?

 

[Supervstech]

Is 40 amp max charge rate for all LiFePO4 BMS's? The batteries specs I have seen show 100 Amp max charge rate for 100 AH battery. Has it been your experience that BMS's will burn out if you charge at 1C rates?



No, I was onlcommenting on the op statement that his bms limits charging to 40 amps

 

[fratermus]

Can I just get 1x 12v 100ah Ampere Time, or other brand (please state!), to replace the house & engine start battery?

For the house, yes. The kinds of drop-in batteries discussied on this forum are not meant for starting purposes.

The thing that held me back from converting everything to LFPo, was the issue with the legacy alternator needing some DC-DC converter to the LFPo.

DC-DC is not required in all cases; several people on this forum are direct-charging LFP from alternator. I did it this morning (VSR, 100Ah LFP) and the alternator charged the batt well within voltage and current recommendations. And well within the stock alternator's abilities.

{edited to remove stray word}

 

[fratermus]

the alternator WILL feed it all it wants... killing the bms, and the alternator as well.

The alternator will feed the bank what it wants, subject to voltage delta and resistance on the charging circuit [ I = V/R ]. In practice this often results in far lower charging rates than people fear and warn others about.

I have collected reports (from this forum and elsewhere) of people direct-charging LFP from the alternator. For the most common vehicle setups (OEM alternator, 100-300Ah, vsr/solenoid) the average of the highest observed charging currents was 0.33C, the same as AGM at 50% DoD. Example: the highest I've seen from my own setup was 0.32C, and it was ~0.2C when I was driving this morning.

Having said that: as with all things DIY research and measurement before pulling the trigger can prevent undesirable outcomes.

BN: the spreadsheet linked above was intended for my own understanding rather than public consumption; it is not elegant or even presentable. It does contain links to the report primary sources.

 

[fratermus]

Is 40 amp max charge rate for all LiFePO4 BMS's?

The 40A number was an offhand comment by OP. The Ampere Time 100Ah battery they mentioned is not limited to 40A (0.4C in this case).

The batteries specs I have seen show 100 Amp max charge rate for 100 AH battery. Has it been your experience that BMS's will burn out if you charge at 1C rates?

If I had to make broad generalizations about 100Ah LFP

* 100A max rate allowed by BMS
* 50A max charge rate recommended by manufacturers
* 20A rate used by manufactures to derive cycle life estimates

The oldest well-documented bank I know of has been charged at 0.4C to 13.8v since 2011 and is in good shape. IIRC this was Rod's (mainesail) bank. I don't have the link in front of me but he posted a recent capacity test on the cruisers forum.

 

[GooseF16]

good info.. and with parallel packs, they last longer in general because of the lower charge/discharge amps that are typically delivered since its kinda hard to take a .4+ C rate to 6 batteries.. thats alot of current.

 

[RandyP]

WHen you discuss LiFePO4 (12v) battery charging with an automotive (12v) alternator, you need to include wire connection specs. If you use 4/0 copper wire, alternator to battery, the charging current will be much higher than if you use the 12 gauge wire usually found in the auto wiring alternator to aux battery scheme thru the trailer plug. In some cases the wire gauge limits the current sufficiently to charge the LiFePO4 battery without regulation (without a DC-DC battery charger) in the charging wire loop.

 

[k490]

good info.. and with parallel packs, they last longer in general because of the lower charge/discharge amps that are typically delivered since its kinda hard to take a .4+ C rate to 6 batteries.. thats alot of current.

Working with 12V is the problem the wire size goes up quickly and is expensive I agree with the OP that 20-40 amps is reasonable expectation from alternator charging. Good quality UL listed 1/0 wire cost me $5 a foot.

 

[mtts]

I don't know why I wasn't getting any notifications on your replies. Thank you everyone who did answer with their input.

The existing cabin system had a 4/0 cable running all the way back to the generator. The purpose, was, to turn over the generator, 20+ feet away, with the cabin batteries, when they used to be located up-front, and hung outside the vehicle (in the environmental conditions underneath, either extremely cold or too hot). I pulled that 4/0 into the cabin, up, about mid-way, into the bench where the electrical box is at (near the floor), and put the cabin batter(ies) in the bench.

That made two lines: one towards the rear (generator), and one going forward towards the engine, (110amp alternator, stock rating for this yr make and model), on a 300-amp fuse. The one back to the generator is on a 350-amp fuse.

Now, up at the engine, the cable from the alternator to the positive terminal on the battery is not 4/0, it's something smaller (stock), I would guess 2 or 4 gauge, and runs from the POS terminal on the ALT to the POS terminal on the starter BAT [which is a decent FLA in good order].

There is a latching relay switch on the dash, it either connects the starter battery with the cabin or disconnects it (and because it's latching, it doesn't use any power in either position). I am familiar with how it's wired, having rewired the relay connections, so I could have the starter battery connected to the cabin battery, to help with cold starts, for instance. Or, to boost the cabin battery (the failing FLA) in order to tie-in the starter battery, to start the auxiliary generator: the main generator is the one with a 350-amp fuse, but I never use it, I use the aux gen most of the time, as it's a newer inverter-style. The aux gen has a 40-amp fuse.

@k490 Thank you for the link. https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/ would be sufficient for my needs. Reasoning is, the 12v cabin system provides incidental power, for lights, pump. Normally, when stationary, it charges off the 120VAC growatt solar / generator system. If this vehicle is headed down the road, it's going to be running for hours at a time. And it won't be using a lot of 12v cabin power. What's interesting to me, though, which is what @fratermus got into discussing, gave me the thought that the LFPo cabin battery could drop-in, as-is, and I can put a clamp meter around the alternator POS output, and toggle that latching relay, and see real-time, how much current is being dumped into the LFPo. That would tell me right there, what the deal is, whether I need the DC-DC unit right away.

MOST OF THE TIME, the LFPo is going to be at or near 100% charge.

A related question I have, is assuming the Ampere Time 12v 100ah, mentioned in the OP, is relay-tied into the starter battery, to help with turning over the motor, is that going to kick-off it's internal self-protection when I attempt to start the engine? The existing starter battery (FLA) is closer to the engine, so it's going to bear the brunt of the current drain from the starter, with the cabin battery only 'assisting'. I am wondering if you have an idea of whether it would work, or just trip the LFPo BMS circuit breaker, as soon as the starter is engaged and there is the slightest load trying to turn over the motor? Engine is a 454.

 

[kbeefy]

Why bother tying in the house battery for starting? Is there a reason the chassis battery cannot handle starting duties by itself?

 

[S Davis]

If you go directly from the alternator you need to figure a switch to turn the charging off, LifeP04 don’t like continuous charging once fully charged.

What voltage does your alternator charge at? And what is the voltage drop from the alternator to the batteries. That is where I would start, once you figure that out you can decide if you need a dc/dc charger.

I run a 50amp dc/dc charger and it pulls about 52amps from my 220amp alternator, my charge voltage gets as high as 15.2 so I couldn’t go direct unless I want to baby sit them while charging.

 

[mtts]

Why bother tying in the house battery for starting? Is there a reason the chassis battery cannot handle starting duties by itself?

It's a 454.

Not only that but you start it in the winter with a cold battery and you may be only getting 60% of it's capacity.

 

[kbeefy]

I've had plenty of big blocks that start fine in the cold. Gas engines are easy.
AGM batteries and lower viscosity oil in the winter often helped.

 

[mtts]

I'm not buying an AGM battery. I'm not investing in old, broken technology.

This is not an R22E. 'Gas engines' is a broad phrase.

Moving forward...

 

[scubadoo]

An interesting thread.
We have been travelling full-time in our Mitsubishi Canter 3.9l turbo diesel based class C motorhome for the last 9 years here in New Zealand.

On day one I installed a 4 cell LiFePO4 300Ah Sinopoly battery with no internal BMS circuitry as our only battery. It both starts the engine often multiple times a day and powers all our RV goodies. It still performs perfectly.
The maximum peak starting current ever recorded was c740A but only for a fraction of a second. Starting energy is replaced by the alternator in less than 30 seconds.

Charging is direct from the stock 100A rated alternator although the end voltage was a little high at 14.4V so I added in series a Victron ArgoDiode Battery Isolator to lower the terminal voltage to nearer 14.1V at which point the battery is always 100% SOC. It had a secondary benefit of lowering the charge current just a little.

The alternator has survived all those 9 years charging at 70-80A without releasing too much of that smoke we are warned about. Certainly many hundreds if not thousands of hours.

At fast idle the alternator spins at >3000RPM.

No charging source exceeds 14.1V.
Victron BatteryProtect is set to disconnect at 12.1V and the SmartShut to alarm at 20% SOC. Never triggered.

 

[fratermus]

Charging is direct from the stock 100A rated alternator although the end voltage was a little high at 14.4V so I added in series a Victron ArgoDiode Battery Isolator to lower the terminal voltage to nearer 14.1V

How much current reduction did you see when dropping voltage to 14.1v? I've been thinking about diode-based isolators and LiFePO4 -- the milder voltage and (presumably) current seem like a good fit.