Question Isolating coach batteries from house batteries.

[Too Little Too Late]

I called freightliner some time ago and asked them about alternator failures, in particular related to lithium house batteries and they had not heard of any issues.
So I do tend to agree that these are pretty tough units. And to be honest, I have not seen the lithiums pull more than 100 amps across (per the shunt).

While experimenting and trying to get the lithium to play well with others in my coach, I unplugged the combiner and attached the alternator lead to the 460Ah lithium. I started the thing and measured 135 amps from the 170 amp alternator.

 

[Richpatty]

While experimenting and trying to get the lithium to play well with others in my coach, I unplugged the combiner and attached the alternator lead to the 460Ah lithium. I started the thing and measured 135 amps from the 170 amp alternator.

Good data! Something to watch.

I have an all electric rig which came with a 240 amp alternator which gives me a bit more breathing room.

Rich

 

[Trav_lr]

I have a BIM from Precision Circuits installed with FLA chassis batteries and LiFePO4 house batteries. No issues in 3 years

 

[Wemustrv]

My issue is, while traveling we usually run the generator to keep the ACs ahead of the heat, so the inverter is able to charge the house bank. I didn't want the alternator trying to join in and possibly have an issue with two charge/ battery type systems not playing well together. I'm going to install another disconnect to be able to keep them isolated unless needed at some point.

With the generator running the transfer switch directs power from the strongest source to the system so this cannot happen. I would also say that the alternator and the charging system sense when the charge level is correct and stop current flow to batteries. Dual source charging is still parallel 12 volt so it would not hurt the system if they were both properly wired.

 

[Too Little Too Late]

I have a BIM from Precision Circuits installed with FLA chassis batteries and LiFePO4 house batteries. No issues in 3 years
View attachment 29917

Ya know this is the first I've seen the specs on this little green thing. I thought is was a duty cycle timer. Ya can't always believe what YouTube says. Thanks for posting the details on this weird shaped amazing little green box. Takes the place of the isolation relay, and gives back the emergency start switch. Remarkable!

 

[SGilbert]

This is as good of a place as any to ask a dumb question!
I see owners talking about DC-DC chargers often. WHY??
My alternator charges the start and coach batteries while driving; same for when on genny or plugged in through the converter.
What does a DC-DC charger do that I can't do now?

 

[Too Little Too Late]

This is as good of a place as any to ask a dumb question!
I see owners talking about DC-DC chargers often. WHY??
My alternator charges the start and coach batteries while driving; same for when on genny or plugged in through the converter.
What does a DC-DC charger do that I can't do now?

When I connected the alternator directly to the lithium battery it was charging it at 135 amps. The lithium battery wants to see 14.4 ~ 14.6 volts at no more than 90 amps. So no matter what go's into the DC to DC charge controller, what comes out the other side is exactly what the battery wants, and this will make it last a really long time.

 

[Chuggs]

This is as good of a place as any to ask a dumb question!
I see owners talking about DC-DC chargers often. WHY??
My alternator charges the start and coach batteries while driving; same for when on genny or plugged in through the converter.
What does a DC-DC charger do that I can't do now?

So,… the big deal with DC-DC chargers is control. The thing that is driving this is the very low internal resistance that LiFePO4 batteries have, Lead Acid batteries have much higher resistance and this actually limits the amount of amperage the Alternator can effectively pass along. And example of low resistance would be comparing hooking up a light bulb to a power source…vs shorting a metal bar with a power source. The low resistance of the metal bar would allow so much current to pass that it would exceed the rating of the conductors and blow or trip the protection for the circuit. Most alternators just put out as much power as they can produce at a regulated voltage…and this normally works out when connected to lead acid batteries. The Lithium batteries do two things…accept massive amounts of amperage, and then when the BMS decides enough, it suddenly stops, spiking the circuit. A DC-DC converter can be configured to the voltage and amperage limitations the LiFePO4 manufacturer recommends for the batteries BMS…protecting your alternator and batteries.

There are work around…like the Li-BIM225 bi-directional relay…which works on a duty cycle principle. DC-DC chargers are just another way. Depending on the robustness of your alternator and the regulation built-in to the lithium batteries you have, some folks get along without. But it raises the question …is it the best and safest practice? Lithium batteries are only safe because of careful practices relating to temperature and rates of charge or discharge. Going outside of the parameters could expose you to a bad situation.

I also believe that properly regulated charging adds life to your batteries. If my bi-directional relay closes…the alternator output is not curtailed by any parameters that we set for our inverter charger or solar charge controller. Normally, if ai’m driving during the day…my bi-directional relay doesn’t connect the alternator to my house because it sees my solar is already holding the voltage above the voltage it uses to make the connection. But if we make microwave popcorn, or go under a wide enough bridge to block sunlight…the voltage may momentarily dip and the bi-directional relay connects. I know this because instead of seeing my solar controller pushing 11A to 21A at float voltage…it reads Zero Solar amps and somewhere around 14 volts at the battery. I have to either allow the alternator to keep going until the bi-directional relay says enough…or stop somewhere and shut the motor off for a while to get the relay to open again. I have no idea how many extra months I get out of my bank by pampering the batteries…but we’re enjoying the Balloom Fiesta now…and these almost five year old AGMs get us thru the night, make breakfast, run fridge, lights, fans, chargers toaster oven, blender, espresso machine…and drop to around 70%…then charge themselves on solar thruout the day. We wouldn’t need to even crank the generator if the temperatures weren’t so high during the day. We wind up running the generator for a couple of hours for A/C to cool things down. The sun goes down around 8pm and we shut the generator off again. I’ll eventually replace them out of an extreme measure of caution. But they are still going strong.

 

[Richpatty]

Thanks Charlie. Do you think the sudden/periodic cut off of the LIBim poses a spike risk to the alternator?

 

[Chuggs]

Thanks Charlie. Do you think the sudden/periodic cut off of the LIBim poses a spike risk to the alternator?

Not with the chassis batteries connected. They will act as an accumulator to dampen the spike. And the Leece-Neville 240A alternator is actually a tough cookie.

There was a gent that had a long boat webpage in the UK and was equipping it with solar and LiFePO4. He found he had to add a lead acid battery to the fray to help with the sudden cutoff from the BMS. He evidently had a local Victron Energy supplier that was well versed in this…and posted about it. But I haven’t heard too much about it in RV systems.

 

[Richpatty]

Makes sense! Thanks Charlie!

 

[SGilbert]

Well.....
I have AGMs all around--no lithium & don't plan to.
The house batteries are group 27 deep cycles, but when they die, I'll attempt to go group 31s.
I have no clue of the Ford E450 alternator's output.
I also have no solar.
Between alternator, generator, converter, inverter, and good ol' common sense, I'm sufficiently covered, I think.

My only boondocking I have done & plan on doing is overnights behind Cracker Barrels.

All in all, I still see no use for a DC-DC charger unless I missed some important aspect.

Thanks all for an explanation that is still a tad confusing, but "ignorance is bliss", so they say.

 

[newmarokie]

I have SOK batteries. You can limit the uptake of amps through the app. I have mine set at 40 amps each for a total of 80 amps uptake. No issues in two years.

 

[Chuggs]

I have SOK batteries. You can limit the uptake of amps through the app. I have mine set at 40 amps each for a total of 80 amps uptake. No issues in two years.

Sounds like they are performing the function of proper regulation for you already. I think a lot depends on the design of the built-in Battery Management System.

 

[Chuggs]

Well.....
I have AGMs all around--no lithium & don't plan to.
The house batteries are group 27 deep cycles, but when they die, I'll attempt to go group 31s.
I have no clue of the Ford E450 alternator's output.
I also have no solar.
Between alternator, generator, converter, inverter, and good ol' common sense, I'm sufficiently covered, I think.

My only boondocking I have done & plan on doing is overnights behind Cracker Barrels.

All in all, I still see no use for a DC-DC charger unless I missed some important aspect.

Thanks all for an explanation that is still a tad confusing, but "ignorance is bliss", so they say.

You're good. There is a balance. Components cost and life cycle vs Use vs Dependability/Backup. We all need enough usable aH of DC voltages to run RV specific systems... wether that is starting an engine, a generator...or keeping the lights on, vent fans, outlets, entertainment, wifi, water pumps, inverter, refrigerator, DSI for water heater/Oasis, Furnace/Oasis. All while being comfortable...and if needed spanning the quiet time restrictions.

No one size fits all... We had a fifth wheel before. One battery, no inverter, no generator. I'll just say...it left a lot to be desired! Our Motorhome has 8 house batteries, a 3000w inverter, residential fridge, 8k diesel generator... so, it can do much more and keep us comfortable even if not parked by a utility pole. And since utilities can fail, our we could have a breakdown away from utilities...we decided to put solar on the coach. Do we need it? No...but it is quieter than a generator, doesnt burn fuel, and is a very reliable backup to failed generator or blackout utility situation. It is an escape pod for us when Hurricanes threaten our home...and has been a great traveling machine for our travels and adventures.

We're dry camping now...and the solar has met our needs, except we have needed to start the generator to cool off the rig in the afternoon.