LiFePO4 for full-time boondocking?

So the scenario that we're talking about here would be my wife and I full-time boondocking in a self-converted Sprinter 170 extended 3500 dually. It was suggested that I start a separate thread (this one) when the subject of LiFePO4 came up.

We were talking about using a 2,500 watt Cummins-Onan propane generator under the floor behind the rear axle, powering a smart charger, to recharge the LiFePO4. No solar. The Sprinter alternator(s) would be invited to the party.

And specifically, I had asked about using a smaller LiFePO4 battery. It seems to me that I would not need several days of battery capacity, since we'd be living in it, and thus able to use the generator every day to recharge, and the alternator would be there to contribute electrons on a daily or near-daily basis. Is there any reason that I'd need more than 24 hours of usable battery capacity?

Our biggest power hog would be a high-efficiency compressor-type chest refrigerator/freezer that reportedly draws 1.3 Ah/h (31.2 Ah in 24 hours). We'd have 12VDC LED interior lighting, periodic use of a 12VDC fresh water pump (for showers and such), and one or two 12VDC roof fans. Our laptops and cell phones would be charged off the 12VDC panel. No toaster, hair dryer, electric water heater, electric stove, toaster oven, electric space heater, or anything else like that. Our hot water, oven, and stove would be on propane. We'd have a 120VAC inverter of some sort for any incidental electronics beyond what I've already mentioned, but probably not a pure sine wave one, probably not very big, and it would be shut off when not in use. Our 10,000 BTU heat pump would be run directly off the generator, through the 120VAC panel.

A 150 Ah LiFePO4 had been suggested, using it all the way down to 5-15%, but I'm wondering if we could comfortably get away with something smaller.
 
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brianszero

Active member
I have roughly the same draws as you and use a 200ah lithium. My average usage is 25-30 ah per day. A friend of mine lives full time and drives to work everyday and only has a 100ah lithium. The 20 minute drive to and from work tops his batteries off everyday if he is lacking sun(who wants to park in the sun when its hot). Your set up seems fine and I wonder what all these people are doing with all their huge battery banks that they are installing.
 

john61ct

Active member
If travelling every few days, then a bigger bank takes more advantage of alternator charging.

A mostly-solar setup is what really requires a 3-5 day buffer in sub-optimal conditions.

Some people prefer to have to fire the genny up as infrequently as possible.
 

HarryN

Well-known member
Agreeing with John61CT's comments above.

It somewhat depends on if you want to run your battery pack at it's limits so it is worn out in 2 years, or have it still useful after 2 years.

Using a slightly conservative approach:
- C/2 charge / discharge rates target
- Useful charge cycle running 20 - 80% SOC for most cycles
- probability that you will eventually want toast with your eggs and coffee without turning on the generator

In that case, a 200 amp-hr LiFe bank size is mathematically more balanced for the use.

If you use a smaller charger - perhaps 50 amp instead of 100 amps, then 100 amp-hr is reasonable but marginal with the toast.

I am assuming that you would purchase a self contained LiFe, example battle born. They are good, just don't ever go over the 100 amp limit and into the surge territory.
 

Totaled108

Captain Slow
Does the RV in question already have a house battery? What’s the current batteries ah capacity?

If you’re really planning on firing up the generator or driving daily, I think you could get away with 100ah of this chemistry.

~150ah would give you a bit more wiggle room, but not NEEDED.

My only experience is with our 320ah LFP, charged via solar (400w) the vast majority of the time, alternator charging sometimes, shore power rarely.
 

calbiker

Well-known member
Your 100-amp smart charger will need power factor correction. Without power factor correction the load may exceed the Onan 2500 VA limit.

How large is your propane tank? The Onan eats 0.6 gal/hr.

The alternator won't fully charge the LiFePO4. Alternator drops to 13.3V, LiFePO4 needs < 14.5V.

How much usage do the laptops get? This power draw is not insignificant.

Do not underestimate power usage.

We were talking about using a 2,500 watt Onan propane generator under the floor behind the rear axle, powering a 100-amp smart charger, to recharge the LiFePO4. No solar. The Sprinter alternator(s) would be invited to the party.
 

john61ct

Active member
A good charger for LFP should be user adjustable, both voltage setpoint (I never go over 13.8V in daily cycling) and to de-rate current, in order to protect upstream sources.

A stock alt setup requires a good DCDC charger, Sterling BB series.

A properly installed, configured and cared for LFP setup should last well over a decade, I'm shooting for 5000+ cycles. Certainly longer than the vehicles it's used in.

So-called drop-in batts, inaccessible BMS sealed up inside, will most likely not get there.

I do not consider toast important.
 

monoloco

Member
My system uses 1) 100 amp BB LiFePO4, charged by a 360 watt solar panel. It is completely independent from the vehicle electrical. I run an Engel fridge/freezer, lights, Maxair fan, and a 600 watt sine wave inverter for mostly charging lap tops. So far, it seems totally adequate. I generally see the battery at a 100% state of charge by 10am if it's sunny. I'm not sure of the wisdom of relying on a propane generator for charging. In my experience, they go through a lot of propane and would require fairly large tank capacity. The generator combined with the required tanks would take up much more space than I would be willing to sacrifice. If I was going to rely on a generator, I would just get a Honda 2000 which will run for a very long time on a small volume of fuel and take up very little space. As a side note, I have a 3.5 Kw Onan propane generator that I bought almost new as back up power for our off-grid home. It now sits unused because it is an inefficient, total and complete POS, that broke down on a regular basis. I just got tired of fixing it and buying parts for it. I switched to a Kubota diesel that has 3x the Kw's and costs less to operate.
 

OrioN

2008 2500 170" EXT
Here is a consideration...


... the Onan will crank/draw ~65A when starting. Sometimes (more often than not really) it needs to turn over for more than 2 seconds before firing. If your Li's are low say SOC 15%, the voltage will get below LVC stage and trigger the BMS to shut the battery down.

If you have a bi-directional relay between the house and chassis battery, you can mediate this scenario by combining banks to start genset.
 
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Midwestdrifter

Engineer In Residence
13.8V is completely adequate for fast and safe charging of LFP banks. 13.5V won't get the bank very full though, so you will want to see what your alternator produces. The v6 sprinters seem to be in the 13.6-13.9V range from my reading.
 

OrioN

2008 2500 170" EXT
13.8V is completely adequate for fast and safe charging of LFP banks. 13.5V won't get the bank very full though, so you will want to see what your alternator produces. The v6 sprinters seem to be in the 13.6-13.9V range from my reading.
Measured the other day when I was diagnosing a low headlamp:

10/16/18:
VOLTAGE (at idle/low draw):
Alternator: 14.32-14.33
Chassis Batt.: 14.26-14.28

I test and record periodically to see the heath of the wires/terminals/etc...
 

Gski

Member
The standard ‘safe’ charging limit for LiFePO4 batteries is 0.5C, which has two implications.
- The maximum charge current is 50A for a 100AH battery.
- The minimum charge time from 5% to 95% is two hours, irrespective of battery size.

You can charge faster, but I will quote an old sailing proverb here: Storage batteries do not die, they are murdered.

My guess is that your daily usage will be at least 40AH, and increase in warm weather.

This works out to two hours of genset running time every two days, or one hour per day.

Switching to a 200AH battery doubles the charging rate, which means two hours of genset running time every four days, or half an hour per day.

The 200AH bank will also be able to start the genset from a lower state of charge.

If you (and your neighbours) can live with the genset noise, the approach looks like a good fit for your needs. The propane genset should be able to produce 170A at 14V, and unlike a diesel genset, won’t die from running at lower outputs.
 
Here is a consideration...


... the Onan will crank/draw ~65A when starting. Sometimes (more often than not really) it needs to turn over for more than 2 seconds before firing. If your Li's are low say SOC 15%, the voltage will get below LVC stage and trigger the BMS to shut the battery down.

If you have a bi-directional relay between the house and chassis battery, you can mediate this scenario by combining banks to start genset.
The Onan doesn't have it's own starting battery? Doh!

Can (and should) the Onan's starting circuit be wired to the chassis battery instead, independent of the house battery?
 

Gski

Member
Here is a consideration...


... the Onan will crank/draw ~65A when starting. Sometimes (more often than not really) it needs to turn over for more than 2 seconds before firing. If your Li's are low say SOC 15%, the voltage will get below LVC stage and trigger the BMS to shut the battery down.

If you have a bi-directional relay between the house and chassis battery, you can mediate this scenario by combining banks to start genset.


If your chassis battery is AGM or lead acid and the house battery is LiFePO4, combining the banks will result in a large current from one to the other, very likely damaging both batteries.

It is safe to combine battery banks only if they are of roughly equal capacity AND have the same chemistry. Even then, you can get a large current if the state of charge is not similar. Start the engine before combining, just like with jumper cables or the MB cutoff relay.
 
The standard ‘safe’ charging limit for LiFePO4 batteries is 0.5C, which has two implications.
- The maximum charge current is 50A for a 100AH battery.
- The minimum charge time from 5% to 95% is two hours, irrespective of battery size.

You can charge faster, but I will quote an old sailing proverb here: Storage batteries do not die, they are murdered.

My guess is that your daily usage will be at least 40AH, and increase in warm weather.
I'm not sure if the ambient temperature is going to affect our consumption of electrons- if it's hot, we're gonna be using the heat pump, powered directly off the 120VAC panel by the generator.

This works out to two hours of genset running time every two days, or one hour per day.

Switching to a 200AH battery doubles the charging rate, which means two hours of genset running time every four days, or half an hour per day.

The 200AH bank will also be able to start the genset from a lower state of charge.

If you (and your neighbours) can live with the genset noise, the approach looks like a good fit for your needs.
Which one are you saying would be a good fit? A 100Ah LiFePO4 with a 50-amp smart charger, or a 200-Ah LifePO4 with a 100-amp smart charger?

The propane genset should be able to produce 170A at 14V, and unlike a diesel genset, won’t die from running at lower outputs.
Just wanna make sure we're talking about the same thing- we're talking about using a smart charger in between the generator and the LiFePO4, right?
 
Measured the other day when I was diagnosing a low headlamp:

10/16/18:
VOLTAGE (at idle/low draw):
Alternator: 14.32-14.33
Chassis Batt.: 14.26-14.28

I test and record periodically to see the heath of the wires/terminals/etc...
Well, that's encouraging. A lot better than 13.2/13.5/whatever... :)
 

OrioN

2008 2500 170" EXT
The standard ‘safe’ charging limit for LiFePO4 batteries is 0.5C, which has two implications.
- The maximum charge current is 50A for a 100AH battery.
- The minimum charge time from 5% to 95% is two hours, irrespective of battery size.

You can charge faster, but I will quote an old sailing proverb here: Storage batteries do not die, they are murdered.

My guess is that your daily usage will be at least 40AH, and increase in warm weather.

This works out to two hours of genset running time every two days, or one hour per day.

Switching to a 200AH battery doubles the charging rate, which means two hours of genset running time every four days, or half an hour per day.

The 200AH bank will also be able to start the genset from a lower state of charge.

If you (and your neighbours) can live with the genset noise, the approach looks like a good fit for your needs. The propane genset should be able to produce 170A at 14V, and unlike a diesel genset, won’t die from running at lower outputs.
170A DC is not obtainable or feasible from the Onan 2500 LP. It is rated at 20.8 A ac. However, if anyone has tried a 20A load on these 'Microlites' you find they are quite finicky or stressed at this load. 16-18 continuous is best. My inverter/charger derates the current or charge output to 75% of input. That produces a 1A ac to 7.5A dc ratio. At 20A load share, the best I can get or the maximum really for the Xantrex Freedom SW3000 is 150A dc. I run the Xantrex to draw 16A ac continous, and input 120A dc into my 400aH Li bank.
 

OrioN

2008 2500 170" EXT
If your chassis battery is AGM or lead acid and the house battery is LiFePO4, combining the banks will result in a large current from one to the other, very likely damaging both batteries.

It is safe to combine battery banks only if they are of roughly equal capacity AND have the same chemistry. Even then, you can get a large current if the state of charge is not similar. Start the engine before combining, just like with jumper cables or the MB cutoff relay.
:thumbup:
 
A good charger for LFP should be user adjustable, both voltage setpoint (I never go over 13.8V in daily cycling) and to de-rate current, in order to protect upstream sources.

A stock alt setup requires a good DCDC charger, Sterling BB series.

A properly installed, configured and cared for LFP setup should last well over a decade, I'm shooting for 5000+ cycles. Certainly longer than the vehicles it's used in.

So-called drop-in batts, inaccessible BMS sealed up inside, will most likely not get there.

I do not consider toast important.
Yup. And if we needed toast, we could just fire up the generator.

What smart chargers have adjustable set points and can de-rate current? What protection does the generator and alternator need from the LiFePO4?

So the DC/DC charger (Sterling BB series) that you're speaking of, would that be a SECOND charger, in between the alternator and LiFePO4, in addition to the smart charger in between the generator and the LiFePO4?

What LiFePO4 batteries are "drop-ins" with the integrated BMS, and which ones are not? Would I run the LiFePO4 without anything to perform the BMS functions, or would those BMS-type functions be performed by some external component(s)?

And I'm not sure if this is related to my questions above, but what protections does the LiFePO4 itself require? I'd hate to make a mistake in using it, and end up having to bite a $2,500 bullet in the form of a new LiFePO4. That would be an unacceptably expensive learning curve. I can afford to do this correctly ONCE. I cannot afford to be torching stuff, learning through trial and error. That's one of the reasons that I'm trying to pick everyone's brains here, while it's still just a sketch on paper, before I start writing checks.
 

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