Charging LiFePO4 from 12V

[PonyOnMyBoat]

I'm setting up my truck (2018 Tacoma Off Road) for a Snap Treehouse shell that's on order. Rather than go dual batteries I have a 40Ah LFP in a battery box (https://powerwerx.com/pwrbox-portable-power-box-large-bioenno-batteries). The battery takes 14.6V to charge. My plan was to keep the battery's AC charger plugged in to the 400/100 W outlet in the bed of the truck so it will charge whenever the truck is running, and keep the fridge's DC cord plugged into the battery box. Only other loads I anticipate are the camper's LED lights and USB charge ports which will be hooked to the powerports on the battery box.

Complication is that the factory inverter isn't powered unless I hit a reset button on the dash every time I start the engine. I can see forgettting to do this, in which case I will arrive in camp with the house battery depleted. The options I see are:

1) Don't forget to hit the reset button (simplest, cheapest, least reliable)
2) Hack the momentary reset switch to come on automatically. I may lose the capability for 400W when the truck is in P if I do this.
3) Add another inverter on a switched circuit to power the charger. Inverter could be in the cab (less power drop running to the back) or in the bed.
4) Run 12V to the back, add a step up converter, and a solar controller (https://www.bioennopower.com/collec...-charge-controller-mppt-for-lifepo4-batteries).

Each of these options involves two steps of conversion (up from alternator output and back down to battery input), but Option 4 eliminates the DC to AC inversion and AC to DC rectification. It's probably the most expensive but I'm likely to add solar at some point so it would be a step toward that.

I'd appreciate any thoughts/advice. Also, suggestions for the boost converter if I go Option 4; I see some on Amazon for around $20-35 but I wonder about quality.

 

[DiploStrat]

Making this too hard. Your "magic" box is still nothing but a battery in a plastic case.

Ditch the AC adapter and simply wire it up as you would a normal, isolated dual battery. Bring the feed from your alternator into the same Anderson plug point that the AC adapter uses. If you don't have solar, then a simple key controlled relay is probably all you need.

Possible Bonus: I believe that some Tacomas run at about 13.9v, which is ideal for lithium batteries. Their 14.6v number seems very high, but their charger is very low output. Your Tacoma should be able to provide 13.9v at 20A or so all day long. More than enough to feed a 40Ah battery.

But if you insist on 14.6v, then you could spend money on a small B2B (DC to DC) charger from REDARC or Sterling. Both have settings that will get you 14.6v and the REDARC has the advantage of including a solar controller.

Don't waste time/money/energy futzing around with DC>AC>DC perversions.

 

[Rando]

I have a 150Ah LiFePO4 battery that I happily charge away at 13.6V. Your Tacoma will charge it just fine - but do wire it up with at least moderately large wire to your truck battery, not for current carrying capacity but to limit voltage drop as you don't have a huge amount of margin between the alternator voltage and charge voltage.

 

[john61ct]

Yes lower voltage is better for longevity.

I stop at 13.8V myself.

 

[pdxfrogdog]

Is amp draw an issue? I've heard that Lithium batteries can pull lots of current so you have to be careful about wire sizing. How does one account for that in terms of wire gauge and fusing?

 

[luthj]

Plan on the bank pulling its capacity (C) in amps at 14.1V+ (at the battery terminals). So a 40AH battery can pull 40A. So wire and fuse for that amount. In reality it will probably be lower unless you have a very high alternator voltage, or a large gauge/short cable run. The 40A pack mentioned will probably accept 30A with 13.8V at the terminals. This means that to limit the charge current some, you need to choose wiring that will provide enough voltage drop to drop the alternator voltage to 13.6-13.8V at 30A (for example.

So with a 14.1V alternator you need a 0.5V drop at 30A. V=IxR (ohms law). R=17 mOhm (milli ohms, or 0.017 ohm). So the resistance of your complete wiring loop (including fuse resistance) should be no less than 17mOhm in order to limit peak current. 10 gauge is ~1 mOhm per foot, so a round trip of 17ft in 10 gauge would work. A 30A fuse and 90C wire should be safe with a run this long. Use a quality fuse and connectors. A longer run is okay too, but it will reduce charge rates, not a big deal really.


If your battery mfg has detailed charge current plots for various voltages, you can be more precise.

 

[john61ct]

Is amp draw an issue? I've heard that Lithium batteries can pull lots of current so you have to be careful about wire sizing. How does one account for that in terms of wire gauge and fusing?

You size the cable and CP for the distance and the maximum ampacity of the concurrent loads used on that circuit.

A 12V compressor fridge is under 10A, but a winch might be 400.

Cranking the engine maybe 600A, but only a short time.

Blue Sea Circuit Wizard app is great, come back here for details not cleared up by googling.

____
It's **charging** the LFP that can burn up an alt, pushing it to max rated amps makes it overheat.

A DC-DC charger includes a current limiting function, simple solution.

 

[feltyellow]

I'm also setting up a small, ~40ah lifepo4 house battery in my camper and was also considering using my compatible solar charge controller for alternator charging as in (4). I assume you were planning to do a setup similar to what user "soul" describes here,
https://expeditionportal.com/forum/...lifepo4-in-a-dual-battery-setup.110727/page-2

1) I already have 8ft of 8awg wiring between my starter battery and my house battery, fused at 30amps. It sounds like I might be able to charge the lifepo4 battery directly off this without an expensive charger like the sterling, etc, assuming the alternator voltage is approximately correct. Anything to watch out for here besides fusing and isolation?
2) Any advantages to using the solar charge controller? Any concerns? I hesitate to use the solar charge controller for something it wasn't designed for. Does anyone have a recommendation for a reverse protection diode to prevent the solar panel from charging the starter battery in this setup?

 

[john61ct]

In thousands of setups, hundreds with LFP, I've never heard of a SC needing overcurrent protection on the output side.

Yes the panel output should not go over the SC power rating.

 

[PonyOnMyBoat]

Feltyellow, thanks for posting that link; I've been doing a lot of on-line research but I don't remember finding that thread.

The internet being what it is, there are a lot of confidently and authoritatively expressed statements out there that are at odds with each other.

Soul's SC link no longer works, and the step-up converter link now goes to a step-down only converter.

I contacted the supplier of my battery (Bioenno) about using a Victron isolated step up converter:
https://www.victronenergy.com/dc-dc-converters/orion-tr-dc-dc-converters-isolated

They recommended this:
http://www.westmountainradio.com/pdf/epic-pwrgate-manual.pdf
which Powerwerx (who apparently have a marketing arrangement with Bioenno) sells for $194. My reading of the manual tells me that this unit does not boost voltage; the Bioenno person responding to me (from a sales email address) insists that it does boost. I've asked West Mountain's technical support to comment but so far they haven't responded.

Redarc sells a LiFePO4 charger which has both alternator input and MPPT SC, but they want close to $800 for it.

I'm now thinking about the Victron 12/12-9 or -18 DC-DC isolated converter, feeding a LiFePO4 SC - if I can get confirmation that it works properly. Bioenno has a MPPT for $90 but right now I don't have a lot of confidence in their assurances. For the present I may just go full kludge and run the AC battery charger off an inverter.

 

[john61ct]

Sorry but I've lost track of the specific problem / challenge you're trying to solve here.

Please restate it simply, what specifically you would like to accomplish, what you think the obstacles are, what the solution would ideally look like without compromise or workaround.

 

[PonyOnMyBoat]

Fair enough.

Challenge: Charge 40Ah LiFePO4 in battery box while driving. Provide capacity to later add ~100-120W solar.

Issues:
- Do not want 13.3V battery draining into FLA starting battery
- LiFePO4 batteries require significantly different charging regime than Pb chemistries. For example, they can be damaged by trickle charging. BMS -should- prevent this, but I do not want to rely on this (especially since Bioenno does not seem to share details on their BMS)

Ideal solution? Something reasonably priced, along the lines of:

https://redarcelectronics.com/products/dual_input_25a_in-vehicle_dc_battery_charger

or

https://shop.pkys.com/Victron-Energ...MIxJ7NrsOJ3QIVFVqGCh3adQ3cEAQYASABEgK9B_D_BwE
plus:
https://www.bioennopower.com/collec...-charge-controller-mppt-for-lifepo4-batteries
at less than half the price of the Redarc - IF I can be convinced that it works and won't burn out or shorten the life of my $360 battery.

Cheaper would be better, of course.

The workaround I'm hoping to avoid is 12V DC to 120V AC to 14V DC using inverter and wall charger.

 

[luthj]

What is your alternator voltage? If its below 14.4V or so, just charge directly using a disconnect relay driven by the D+ signal from the alternator or ignition switch. LiFePo4 will happily charge (to nearly full) from voltages around 14V. Just pick the right sized wire and fuses. I would include a manual disable switch for the relay, that way you can avoid charging if the bank does not need it.

r example, they can be damaged by trickle charging. BMS -should- prevent this, but I do not want to rely on this (especially since Bioenno does not seem to share details on their BMS)

Nope, sorry, unless the BMS can disconnect the charging source, it won't do anything to prevent damage from floating at 100%. This won't destroy a battery anyway (unless the voltage is way high), but it will accelerate degradation.

 

[john61ct]

Run a heavy enough wire from your alt/starter circuit to an Anderson plug at your battery box.

If you care about the LFP longevity, check the voltage output while driving, you want below 14V.

If too high, use a Sterling DCDC B2B charger, user-adjustable, set to 13.8V. Isolate the input, or just turn it off when that point is reached, no Float.

Going direct can overload some alt setups, watch alt diode temps. The DCDC charger limits amps pulled, another advantage.

If you don't care, just go direct.

___
Now, isolating combining - no need if using the DCDC.

A VSR / ACR will only close the circuit while a charge is present, any charge source. If alt is the only source, a $50 ignition wired solenoid will do.

_______
A cheaper solution will be a kludge. The Redarcs are not adjustable. The Sterling will last decades, and can be repurposed / adapted, very flexible.

 

[DiploStrat]

...
Challenge: Charge 40Ah LiFePO4 in battery box while driving. Provide capacity to later add ~100-120W solar.

Issues:
- Do not want 13.3V battery draining into FLA starting battery
- LiFePO4 batteries require significantly different charging regime than Pb chemistries. For example, they can be damaged by trickle charging. BMS -should- prevent this, but I do not want to rely on this (especially since Bioenno does not seem to share details on their BMS)
...

Again, this really isn't hard.

-- Add solar: Two options, use a controller with integrated solar, e.g. REDARC (25A if your 40Ah battery is all you will ever have, 40A if you may go to something larger) or a stand alone solar controller with an acceptable lithium output. Again, 200w isn't much.

-- Don't drain FLA starting battery: Key controlled relay will do this perfectly. Some VSR/intelligent may not work because of the higher resting voltage of the lithium. (Vilctron makes a VSR for just this purpose, but, unless you need trickle charge of your starter battery (a contentious issue all on its own), you need not pay extra for this.)

-- Proper profile for lithium: Actually, it is only lead acid that really worries about a charging profile, lithium simply wants volts (lower) and amps (higher) until the battery reaches a target voltage and then nothing; it doesn't really need floating. So, assuming that your Tacoma runs at 14v or less (a good bet) and assuming that you use a simple, key controlled relay, you are perfect.

Probably your cheapest answer is:

-- Key controlled relay, and,
-- Simple PWM solar controller, e.g. Bogart. (http://www.bogartengineering.com/products/solar-charger.html But there may be even less expensive options)

If you read the replies you have gotten you will see that all are saying basically the same thing. (Minus perhaps, a bit of phanboi devotion to REDARC or Sterling! )

The REAL question is whether your 40Ah battery will be enough in the long, hot run.