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Old 10-21-2018, 06:04 PM   #11
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 06:07 PM   #12
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Default Re: LiFePO4 for full-time boondocking?

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Originally Posted by Midwestdrifter View Post
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...
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Old 10-21-2018, 06:11 PM   #13
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Default Re: LiFePO4 for full-time boondocking?

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.
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Old 10-21-2018, 06:15 PM   #14
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Default Re: LiFePO4 for full-time boondocking?

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Originally Posted by OrioN View Post
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?
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Old 10-21-2018, 06:22 PM   #15
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Default Re: LiFePO4 for full-time boondocking?

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Originally Posted by OrioN View Post
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.
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Old 10-21-2018, 06:23 PM   #16
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Default Re: LiFePO4 for full-time boondocking?

Quote:
Originally Posted by Gski View Post
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.

Quote:
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?

Quote:
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?
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Old 10-21-2018, 06:25 PM   #17
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Default Re: LiFePO4 for full-time boondocking?

Quote:
Originally Posted by OrioN View Post
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... :)
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Old 10-21-2018, 06:27 PM   #18
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Default Re: LiFePO4 for full-time boondocking?

Quote:
Originally Posted by Gski View Post
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.
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Old 10-21-2018, 06:36 PM   #19
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Default Re: LiFePO4 for full-time boondocking?

Quote:
Originally Posted by Gski View Post
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.
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Old 10-21-2018, 06:37 PM   #20
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Default Re: LiFePO4 for full-time boondocking?

Quote:
Originally Posted by john61ct View Post
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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