Head spinning planning build for a LiFePo4 battery system for RV, no solar.

[PHRoG]

?

Greetings all! Been lurking for awhile, learning as much as possible and about ready to pull some triggers here. However, I'd definitely appreciate some feedback/suggestions/thoughts on what I'm about to try and do here.

My application is for a ~30ft RV I will be living/traveling in for the next year or two while working remotely. I expect a fair amount of boondocking in shady areas and I will mostly be working (graveyards)/traveling at night so I don't think solar would be worth the investment for me. Most of the builds I'm seeing incorporate solar, so I'm not certain my plan is feasible now.

I'm interested in going with either a 4 or 8 cell 280/302 LiFePO4 build. My power needs while working are likely in the 11 to 15 amp range for 10 to 12 hours, 4 days a week. I currently have a 3200W inverter generator I'd plan on using for charging when shore power isn't available. If I'm doing math right, that'd be 11ish hours for 4 cells or 22ish for 8 cells...which, isn't ideal.

I've read that as an alternative, I could do a DC to DC charger with an upgraded alternator in my truck (60A Renogy or even 120A with 2)...so I'm considering that maybe I should build them as portable battery packs that I could swap in/out of the truck to do the bulk of my charging instead?

Next issue is powering the RV 120V system, I'm looking at a 2000w/3000w inverter. I currently have the WFCO-8955PEC power center. If I'm going to have to do the bulk of my charging using my truck, does it make any sense to upgrade it to the Li compatible one? Is there another way for me to be able to charge the LiFePO4 packs using shore power when I have access to it while also powering the RV?

Or, is there a better way to go about all this that I'm missing?

 

[sunshine_eggo]

?

Greetings all! Been lurking for awhile, learning as much as possible and about ready to pull some triggers here. However, I'd definitely appreciate some feedback/suggestions/thoughts on what I'm about to try and do here.

My application is for a ~30ft RV I will be living/traveling in for the next year or two while working remotely. I expect a fair amount of boondocking in shady areas and I will mostly be working (graveyards)/traveling at night so I don't think solar would be worth the investment for me. Most of the builds I'm seeing incorporate solar, so I'm not certain my plan is feasible now.

Shading can be overcome with a deployable array - lightweight panels you can set out in the sun. They can be angled and orientation changed a couple times a day to optimize harvest. Even 200W of PV could get you > 1kWh/day.

I'm interested in going with either a 4 or 8 cell 280/302 LiFePO4 build. My power needs while working are likely in the 11 to 15 amp range for 10 to 12 hours, 4 days a week.

11-15A at 12V? at 24V?

I currently have a 3200W inverter generator I'd plan on using for charging when shore power isn't available. If I'm doing math right, that'd be 11ish hours for 4 cells or 22ish for 8 cells...which, isn't ideal.

No idea what math you're using. Generator power doesn't matter. Charger current does. Either way, 11-ish and 22-ish hours sounds absurdly high.

Assuming you could charge a 12V battery with the 55A from the WFCO, that's like a 6-7 hour charge. Unfortunately, you couldn't due to the nature of most converters and the poor wiring in most RVs.

I've read that as an alternative, I could do a DC to DC charger with an upgraded alternator in my truck (60A Renogy or even 120A with 2)...so I'm considering that maybe I should build them as portable battery packs that I could swap in/out of the truck to do the bulk of my charging instead?

Vehicle DC-DC charging is pretty common.

Next issue is powering the RV 120V system, I'm looking at a 2000w/3000w inverter.

So how much of the aforementioned 11-15A @ ?V is for 120VAC?

I currently have the WFCO-8955PEC power center. If I'm going to have to do the bulk of my charging using my truck, does it make any sense to upgrade it to the Li compatible one?

Probably not. It charges at 14.4 bulk/13.6 absorption/13.2V float -all of which are fine for lithium. The issue is the wiring and distance between the converter and battery usually mean there is a LOT of wiring loss, so the converter will "see" a notably higher voltage than the battery's open circuit voltage. It will hit 14.4V when the battery is probably closer to 13.8V and then drop to 13.6V and simply trickle current to the battery.

Is there another way for me to be able to charge the LiFePO4 packs using shore power when I have access to it while also powering the RV?

Yes. Depends on 12V or 24V and the rest of the system's config.

Or, is there a better way to go about all this that I'm missing?

Yes. There always is a better way, but many ways are plenty good. I don't really understand what your needs are from the information presented.

 

[PHRoG]

Hey there, thanks for the response!

Shading can be overcome with a deployable array - lightweight panels you can set out in the sun. They can be angled and orientation changed a couple times a day to optimize harvest. Even 200W of PV could get you > 1kWh/day.

I considered that....but, I'll be sleeping during the bulk of the daytime...so, not ideal either.

11-15A at 12V? at 24V?

12V

No idea what math you're using. Generator power doesn't matter. Charger current does. Either way, 11-ish and 22-ish hours sounds absurdly high.

Assuming you could charge a 12V battery with the 55A from the WFCO, that's like a 6-7 hour charge. Unfortunately, you couldn't due to the nature of most converters and the poor wiring in most RVs.

Sorry, was calculating at full discharge and made an error giving me 11. It's a 30 amp generator...assuming I'm burning the max estimated 15Ah for 12 hours, That's 180Ah I'd need to be replacing each day...so, about 6-ish hours (which, is better...but, would like to half that if possible?).

So how much of the aforementioned 11-15A @ ?V is for 120VAC?

All of it and is currently an overestimation.

Probably not. It charges at 14.4 bulk/13.6 absorption/13.2V float -all of which are fine for lithium. The issue is the wiring and distance between the converter and battery usually mean there is a LOT of wiring loss, so the converter will "see" a notably higher voltage than the battery's open circuit voltage. It will hit 14.4V when the battery is probably closer to 13.8V and then drop to 13.6V and simply trickle current to the battery.

Hmm...I'm able to install 4 cells right next to the converter...unsure if I could make 8 work in that space though. Would that make a difference?

Yes. Depends on 12V or 24V and the rest of the system's config.

12V...haven't really don't much reading up on benefits of going 24V to be honest. It's doable if it would be a better solution.

The RV is currently wired for 30 amp supply.

 

[sunshine_eggo]

I considered that....but, I'll be sleeping during the bulk of the daytime...so, not ideal either.

Even if you just had a deployable 200W array sitting there without moving it would be enough for a microwave running for 30 minutes.

12V

Assume worst case, 15A:

15A * 12V = 180W

180W * 12h = 2160Wh

The aforementioned 200W deployable array could offset over 1/3rd of the above as it should be able to generate 800-1000Wh/day.

Sorry, was calculating at full discharge and made an error giving me 11. It's a 30 amp generator...assuming I'm burning the max estimated 15Ah for 12 hours, That's 180Ah I'd need to be replacing each day...so, about 6-ish hours (which, is better...but, would like to half that if possible?).

30A at the generator is 30A of 120VAC. That's roughly the equivalent of 300A @ 12V and 150A @ 24V.

280Ah/304Ah cells can typically be charged at 0.5C or 0.5 * capacity, i.e., 140A/152A.

If you charge at near those limits, you could charge in as little as 2 hours.

A 12V BMS that can handle 150A of charge current may be a little hard to find. 100A is a much more common number.

All of it and is currently an overestimation.

Okay. Over is way better than under.

Hmm...I'm able to install 4 cells right next to the converter...unsure if I could make 8 work in that space though. Would that make a difference?

Very much. If you connect to the converter with heavy/short cables, you can minimize the voltage drop and likely get to full charge using the 55A output of the WFCO. 55A would replenish the 180Ah you used in just over 3 hours. Also, it's worth noting that you don't necessarily need to get to 100%. LFP do just fine with partial charges provided they are regularly charged to full to allow the BMS to balance the cells a few times a month.

12V...haven't really don't much reading up on benefits of going 24V to be honest. It's doable if it would be a better solution.

Batteries are also BMS limited. 280Ah cells may be able to discharge at 280A, but the BMS must also allow this.

What is your peak power? Let's say 2000W.

2000W/12V/.85 = 196A = the battery and BMS must be able to handle a 196A discharge (bump it up to 250A for safety), and you'll need some HEFTY battery cables.

@24V, you will halve those numbers and be able to use thinner cables.

The RV is currently wired for 30 amp supply.

You could supplement your existing converter with a second one. I've had great luck with these:

PM3 12V Series - PowerMax Converters in 12, 24 & 48 Volts

AC to DC Power Converters can be used for RV Converters, Car Audio Power Supplies and Battery Charging for 12, 24 & 48V - Lithium Compatible

powermaxconverters.com

Plug it into an outlet and make a connection to the battery, and you can supplement your charging. You can actually SET the absorption voltage or even set it to a fixed voltage, so it will never taper during generator charging.

 

[MisterSandals]

All of it and is currently an overestimation.

Why not figure out what you are actually using (and/or expect to use)?

Then, if you want to overbuild (great idea!), you can do it right the first time.

And, if you work in terms of watts and watt hours, you will better understand the relationship between DC (solar, battery and charging) vs AC shore, generator and inverter output.

 

[PHRoG]

Even if you just had a deployable 200W array sitting there without moving it would be enough for a microwave running for 30 minutes.



Assume worst case, 15A:

15A * 12V = 180W

180W * 12h = 2160Wh

The aforementioned 200W deployable array could offset over 1/3rd of the above as it should be able to generate 800-1000Wh/day.



30A at the generator is 30A of 120VAC. That's roughly the equivalent of 300A @ 12V and 150A @ 24V.

280Ah/304Ah cells can typically be charged at 0.5C or 0.5 * capacity, i.e., 140A/152A.

If you charge at near those limits, you could charge in as little as 2 hours.

A 12V BMS that can handle 150A of charge current may be a little hard to find. 100A is a much more common number.



Okay. Over is way better than under.



Very much. If you connect to the converter with heavy/short cables, you can minimize the voltage drop and likely get to full charge using the 55A output of the WFCO. 55A would replenish the 180Ah you used in just over 3 hours. Also, it's worth noting that you don't necessarily need to get to 100%. LFP do just fine with partial charges provided they are regularly charged to full to allow the BMS to balance the cells a few times a month.



Batteries are also BMS limited. 280Ah cells may be able to discharge at 280A, but the BMS must also allow this.

What is your peak power? Let's say 2000W.

2000W/12V/.85 = 196A = the battery and BMS must be able to handle a 196A discharge (bump it up to 250A for safety), and you'll need some HEFTY battery cables.

@24V, you will halve those numbers and be able to use thinner cables.



You could supplement your existing converter with a second one. I've had great luck with these:

PM3 12V Series - PowerMax Converters in 12, 24 & 48 Volts

AC to DC Power Converters can be used for RV Converters, Car Audio Power Supplies and Battery Charging for 12, 24 & 48V - Lithium Compatible

powermaxconverters.com

Plug it into an outlet and make a connection to the battery, and you can supplement your charging. You can actually SET the absorption voltage or even set it to a fixed voltage, so it will never taper during generator charging.

Really appreciate the feedback on this...things are making much more sense now!

Peak usage I was thinking 3000 watts in very rare circumstances and for very short periods of time (say, a minute or two). Generally, I'd be using the generator for that stuff...but, could see wanting to zap my coffee in the microwave for 45 seconds to warm it up in the middle of my work shift occasionally. I planned on getting a 3000 watt inverter anyway.

So, I measured...and, I don't think I can squeeze 8 cells into the area next to the existing converter.

However, I'm now thinking maybe 24v might be the way to go? I could do a 8 cell, 24v build in the area where the wet batteries were no problem.

I believe the existing wiring from the wet batteries to the battery disconnect (6ft) and then to the converter inside (20ft) is 6 gauge...would that support a 24v build? If not, think would 4 gauge do?

I like the idea of a 2nd converter I can use just for the batteries if I really need to just charge them up quickly...I'd prefer that over having to to a DC to DC setup with the truck.

 

[PHRoG]

Why not figure out what you are actually using (and/or expect to use)?

Then, if you want to overbuild (great idea!), you can do it right the first time.

I did that to come up with that figure...2 monitors, the laptop dock and maybe the TV = 10 amps on the 120v side. On the 12v side, occasional usage of lighting, water pump and maybe the refer (don't intend to use it that much though).

Hot water/showers or A/C usage would all be solely shore power or generator powered. I wouldn't expect to use the heater as I'll be back before the cold weather hits.

I will take it out in late October, but, I won't be working during that time and last year I did just fine with two wet batteries, a 1600 watt generator and propane.

 

[sunshine_eggo]

Really appreciate the feedback on this...things are making much more sense now!

Peak usage I was thinking 3000 watts in very rare circumstances and for very short periods of time (say, a minute or two). Generally, I'd be using the generator for that stuff...but, could see wanting to zap my coffee in the microwave for 45 seconds to warm it up in the middle of my work shift occasionally. I planned on getting a 3000 watt inverter anyway.

What loads will reach 3000W

So, I measured...and, I don't think I can squeeze 8 cells into the area next to the existing converter.

However, I'm now thinking maybe 24v might be the way to go? I could do a 8 cell, 24v build in the area where the wet batteries were no problem.

I believe the existing wiring from the wet batteries to the battery disconnect (6ft) and then to the converter inside (20ft) is 6 gauge...would that support a 24v build? If not, think would 4 gauge do?

You can check the voltage drop here:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

Use the Estimated Resistance tab.

You would not run a 24V battery to a 12V converter. The converter can either be removed or you could power it with the 24V battery powered inverter for separate 12V battery.

I like the idea of a 2nd converter I can use just for the batteries if I really need to just charge them up quickly...I'd prefer that over having to to a DC to DC setup with the truck.

With 24V, you would need to purchase two converters to get a second. The WFCO is not usable for 24V.

 

[PHRoG]

What loads will reach 3000W

None...the idea behind going with a 3000W is future planning. I only plan to keep this RV for a couple years at the most and plan to sell it and purchase a much larger 5th wheel with larger power needs such as 120v refer, on-demand water heater, etc. I also plan to build out solar and upgrade to a 16 cell battery at that time.

Since it's only $100ish more, figured it makes more sense to get the bigger one now then having to deal with selling the old one then buying the bigger one.

You can check the voltage drop here:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

Use the Estimated Resistance tab.

Thanks for that link...any suggestion to what kind of voltage drop is acceptable? Not much I'd think...if I went with 4 gauge, looks like I'd lose about 0.6v.

You would not run a 24V battery to a 12V converter. The converter can either be removed or you could power it with the 24V battery powered inverter for separate 12V battery.

With 24V, you would need to purchase two converters to get a second. The WFCO is not usable for 24V.

Yeah, I noticed that too...PD has a 24v/40AMP one. But, I'd then have to convert back down to 12v for my 12v stuff and it looks like inverter options are a bit limited for 24v too.

I ran across the LiFePO4 doc showing different configurations that can be done and it looks like I actually can fit 8 cells right next to the inverter. All things considered, that seems to be the best route to take for now. I just need to decide if the 55A from the WFCO is good enough to do a permanent mount...or, if I should make two, 4 cell packs that I can swap out and charge directly from the generator (or, wire something up to run that 100 amp PM converter you linked above inside the RV).

Assuming the above, here's what I believe I'll need:

4 or 8 cells with busbars (likely 302Ah B grade from 18650batterystore).
BMS (any suggestions?).
DC to AC inverter (Likely 3000w Renogy).
Automatic transfer switch or manual switch.
Shunt
Battery kill switch.
Fuses
Appropriate gauge wire/connectors.

Maybe:
PM 100AMP converter.

 

[MisterSandals]

Thanks for that link...any suggestion to what kind of voltage drop is acceptable?

The explanation on that calculator says not to exceed 5%. The number bandied around here are to stay under 3%.