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Old 10-21-2018, 06:59 AM   #1
The Grand Tour
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Default 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.

Last edited by The Grand Tour; 10-21-2018 at 06:27 PM.
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Old 10-21-2018, 12:50 PM   #2
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 12:58 PM   #3
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 02:23 PM   #4
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 02:51 PM   #5
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 03:11 PM   #6
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Default Re: LiFePO4 for full-time boondocking?

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.

Quote:
Originally Posted by The Grand Tour View Post
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.
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Old 10-21-2018, 03:57 PM   #7
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 03:58 PM   #8
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 04:07 PM   #9
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Default Re: LiFePO4 for full-time boondocking?

Quote:
Originally Posted by calbiker View Post
The alternator won't fully charge the LiFePO4. Alternator drops to 13.3V, LiFePO4 needs < 14.5V.
Incorrect. All three sentences, period.
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Old 10-21-2018, 04:17 PM   #10
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Default Re: LiFePO4 for full-time boondocking?

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.

Last edited by OrioN; 10-21-2018 at 06:09 PM.
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