Simple Lead Acid to Lithium Iron Phosphate

[Willingham]

Setting up my 2001 F350 for extended camping trips. The F350 diesel has dual lead acid starting batteries with constant voltage 200A alternator. I want to add a 200ah Lithium Iron Phosphate battery to power my frig and other accessories. I would also like to be able to charge my entire system with solar, AC, or alternator. Do I need to get one of the expensive DCtoDC systems or can I simply get an isolator and/or other less expensive power management devices?

 

[john61ct]

You need a DC-DC, nixing chemistries means not "simple"

Ideally with a current rate around 60-70% of what the alt can stand cintinuously putting out in hot weather without stress.

And with the ability to tweak the output to 13.8V.

Depending on the BMS you might need to go higher to kick off balancing, but that does not need to be fed by the alt, use a low amp AC-DC overnight set at 14.4V for that, or via solar.

All your charge sources should primarily go to House, alt output can be wired to both, with an IGN switch / solenoid / relay on the Starter side.

If you need to draw down Starter while parked (bad idea) use an automatic combiner / VSR / ACR in between

to feed charge current to keep the Starter batt topped up from your solar or genset off grid, or mains (powered campsites)

 

[4000lbsOfGoat]

The only way to keep that setup "simple" is to keep the house battery completely isolated from the starting batteries and charge the house from solar only. If you don't draw anything from the starting batteries you don't need to worry about keeping them topped up while camping.

 

[pugslyyy]

The issue is the strain that the LiFePO4 batteries place on the alternator, forcing it to run at 100%+ of rated output.

I actually ran a test direct connecting 400Ah of LiFePO4 batteries to my 2006 Dodge Ram charging system through a Blue Sea ACR. Within the first month or so my alternator failed. Fortunately with the house batteries fully charged I had about 20 hours of run time on the truck without an alternator. I replaced the alternator and am still running the direct connection to see how long the new alternator will last. (The one that failed was the truck's original one.)

I may do stupid electrical things on my personal vehicles, but for customers we do the Sterling DC-to-DC chargers.

 

[john61ct]

Yep Sterling's best.

But riding bareback like that's not doing your LFP bank any good either once the charge rate goes over 0.4C or so, depending on temperature in the heat maybe 0.6C

 

[pugslyyy]

But riding bareback like that's not doing your LFP bank any good either once the charge rate goes over 0.4C or so, depending on temperature in the heat maybe 0.6C

Yep, I'm also using my little test as a way to see how well the BMS in the Battle Born batteries does it's job.

 

[john61ct]

BMS usually just burn if inflicted with overcurrent.

Best case hiccup protection, maybe full latching, need a reset.

Never heard of a cheap one actually limiting current.

 

[Deleted member 9101]

Why not just run a 3rd lead acid battery? They are a fraction of the cost and an F350 won't notice the weight difference.

 

[Bart_from_Antigravity]

Run a REDARC BCDC 1250 with that setup and you should be steller.

 

[pugslyyy]

Why not just run a 3rd lead acid battery? They are a fraction of the cost and an F350 won't notice the weight difference.

Once you have tried LiFePO4 it is really hard to go back to lead acid. The weight savings is nice, but even if they were just as heavy as lead acid I would be a fan.

 

[Deleted member 9101]

Once you have tried LiFePO4 it is really hard to go back to lead acid. The weight savings is nice, but even if they were just as heavy as lead acid I would be a fan.

I use them at work.... They cost waaaay more and do the same job. The only reason we use them is the light weight.

 

[broncobowsher]

Providing the battery has an internal BMS things get easier.
Running solar with a charge controller set correctly for Lithium is a simple given.
Tying into the vehicle charging system can get interesting. For the simplest run an isolation relay. Next up is to downsize the wire gauge and lengthen the wire size between the truck/isolation relay and the house battery. 20 feet of 10 or 12 gauge wire will work nice to limit charge current. Yes, putting resistance in the charge wire is what you want, just a little. Under load, there will be a voltage drop (just the difference in batteries so a volt or two) and this will limit current. As the house battery charges the voltage differential drops, and eventually they will come to a balance. The limited current will keep from overloading the BMS as well.
Everyone things that the huge 0-gauge wire is the only way to do. But if you actually understand wiring, and use voltage drop in your favor, you can find smaller wire can be a great answer. The smaller wire will make some heat, leave it out to breath and make sure to get some good insulation. Don't do the cheap insulation and bury the wire where it can't reject heat.
Hint: this is the exact same setup that OEMs use to charge house batteries in trailers using the charge wire in the 7-pin trailer plug.

 

[john61ct]

No, that advice is a recipe for disaster if implemented by no professionals.

The wire gauge MUST be sufficient to safely carry the full ampacity, as per the thermal rating tables.

You are correct that the voltage drop is NBD, but that is because with LFP you need a current limiting DC-DC charger located at the bank end

which also happens to ensure delivering the correct voltage no matter what the input V is.

 

[ducktapeguy]

I recently switched over to LiFeP04 batteries and so far I am very happy with them. Twice the usable capacity, half the weight, charges much faster, and 10x the lifespan of SLA. The initial cost was hard for me to overcome at first, but when I did the cost per Ah calculations over it's lifetime, lithium is the cheapest solution in the long run.

I went through a few iterations for my charging. For now I'm using a Renogy DCC30S. It's a combination DC/DC charger and MPPT solar controller. If your alternator and wiring can handle it, you could go with the 50A version and always set the current limit in the app. Having a combination charger/controller is nice because it keeps the wiring simple and takes up less space in the engine compartment. As an added bonus the solar panel automatically switches to charge the starting batteries after the house batteries are done charging. I tried a separate DC/DC charger with a Victron solar controller, that was nice because IMO the Victron is a better controller with a more user friendly app, but for my simple setup it wasn't worth the added space and complexity.

 

[pugslyyy]

I use them at work.... They cost waaaay more and do the same job. The only reason we use them is the light weight.

Maybe in your application they do the same job, but for me it is the ability to quickly charge them to 100% and deeply discharge them without damaging them that are game changers.