Charging LiFePO4 from 12V

Rando

Explorer
Not likely. You still fuse the wire below the rating for the wire.

With 15 feet each way of 8 gauge (guessing here), it will drop about 0.8V at 20A, which would drop the ~14V alternator down to ~13.2V at the battery, at which point the battery is not charging anymore. So the effective max charge rate you would get with this system is somewhere between 0 - 20A, not a worry for either wire or alternator.
 

john61ct

Adventurer
You'd be replacing the fuse a lot then.

I would put a Sterling in there at my desired amps and run a wire spec'd for that load.

My LFP bank deserves precision in its charging.
 

Rando

Explorer
No you won't be replacing the fuse. Look at the actual numbers - you will not draw more than 20A in this situation.

I can actually speak to this, having done the math, and running my LFP battery in essentially the same configuration. The peak current I see with this configuration is ~15A, as expected. I have never tripped the 30A breaker on this wire.

If you want to spend more money for more control - great! But it is not necessary, you are not going to melt wires or burn out your alternator.
 
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john61ct

Adventurer
Sorry did not realize talking such low rates.

My much larger installs can pull 15kW easy if left to their own devices, have burned up a few firetruck alts before discovering Sterling.
 

DiploStrat

Expedition Leader
Sorry did not realize talking such low rates.

My much larger installs can pull 15kW easy if left to their own devices, have burned up a few firetruck alts before discovering Sterling.

This may be the most useful comment here for those of us with larger battery banks who are considering lithium.

I had suspected, and postulated, that a B2B might get a whole new lease on life with lithium, not as a voltage booster, as is usually the case with lead acid, but as a voltage reducer (and here Sterling, with the old gel profiles, has an edge) and, perhaps more importantly, as a current limiter.

Would love to see real details of the installs you refer to. You may have the real world experience to prove my point. (And have saved me $$$!)
 
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john61ct

Adventurer
The Sterling B2B units voltage setpoints are user customizable, not just limited to canned choices.

Using one to front-end your LFP bank, so that's the only charger directly touches your bank

lets you avoid having to upgrade any existing charge sources, can coexist with a Reserve / load dump lead bank,

lets you turn the LFP into a portable powerpak to use on another boat, camper, off-grid cabin whatever.

e.g. Wired one up to drive a killer loud stereo setup in the back of my mate's 60's VW beetle camping in the bush, recharged in a couple hours driving.
 

john61ct

Adventurer
more importantly, as a current limiter
Exactly, even old fixed-voltage 100A DC power supplies off Craig's or eBay turn into a perfect charge source, don't need to worry about their built-in overcurrent protection, just dial the B2B down to 80A and bob's your uncle.

An iCharger 4010-duo will do the same if you also need automated 20-hour load testing and balancing of individual cells, but it's limited to only 40A.
 

DiploStrat

Expedition Leader
...
Using one to front-end your LFP bank, so that's the only charger directly touches your bank
...

Hmmm. Are you proposing to plug the output of your solar controller into the input of the B2B? That's novel. But it might be interesting if your solar controller's profile was not very good.
 

LeishaShannon

Adventurer
If you can't set a suitable output voltage on your alternator , a simple on/off relay works fine too... If you want to get tricky you can automate it based on SoC, Voltage, or when going up hills if you're massively underpowered like in my case :)
Its pretty rare our alternator is turned on , solar provides plenty of power during the day and the night time load with headlights is minimal (<250W)
 

john61ct

Adventurer
Hmmm. Are you proposing to plug the output of your solar controller into the input of the B2B? That's novel. But it might be interesting if your solar controller's profile was not very good
That is the whole point!

If your SC is modern, already has full adjustability, then the B2B is pointless.

But say you're visiting a friend at his off-grid cabin, whose large array is pumping 15V into his bank.

Plug in your LFP bank with the B2B in between, no worries, they get their 13.8V.

Same from that old RV's "converter", or the alternator of the semi-truck moving your boat, or or

Well you get the idea.
 

loudboy

Observer
Having read through this and many other threads, and looked back and forth at solar charge controller ratings, I'm left with what is either a dumb question or a simple solution:

Why can't OP (and I) just have a switched supply to the solar charge controller? When the vehicle is off and the sun is out, the solar panels supply power to the charger and thus the batteries. When the vehicle is on, a relay switches the input from solar to alternator. Solar panels output about 13-15v, alternators output about 13-15v. How would the solar charge controller even know the difference?
 

loudboy

Observer
To split ’Expo hairs, ”12volt” solar modules put out around 17Voc.

When driving on a sunny day, I would rather have solar doing all it can to assist reducing fuel consumption & wear on alternator. Otherwise forsee no reason it wont work.

Thanks for the info. The panels I've been seeing say ~17Voc, but usually have a voltage regulator to bring that down to 13-15. That may just be the el-cheapo models though?
Either way, good to know that it should work. I'm going to try it. I agree that utilizing solar when driving does make sense in general, but in my case (and I suspect the original poster's), I'll only be charging one relatively small battery instead of a whole RV house bank. Also, I'll likely have the roof of my truck covered by kayaks anyways.
 

john61ct

Adventurer
Many 12V panels are rated 21+Voc

Higher is better if you go MPPT, often get better boost from 24V (actually 40+V)
 

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