LIfePo4 charging?

[wandererr]

Solar battery... Was tired... Meant panel

 

[john61ct]

So it is possible you might be OK if your wiring / infrastructure in between is preventing the depleted LFP batt from pulling too many amps and frying your alternator system.

But that may be endangering your wiring / infrastructure somewhere else in between.

Key is getting an ammeter in there if not a coulomb counting wattmeter as jonyjoe suggested, so you can measure that.

Also checking the heat production at peak amps, not only at the alt and its electronics but the various points of your wiring / infrastructure in between, connections, isolator etc.

 

[SailRedemption]

To answer one of the previous questions the battery is 100AH.
The 8 gauge cable is fused at 20Amps. I don't plant to discharge completely, and if I do, it was an emergency . So while my alternator would support this, the fuses would blow (fused on both ends). In a long run, yes - charger is on the 'need to buy list', but funding isn't there at the moment

Also, I do have a solar battery that will help me keep it topped off when the circumstances allow.

Oh ok, 100ah nice. What brand did you go with now that I'm interested?

8 gauge is adequate, it will limit your amps but that's fine. We bought a Amazon special 4ga jumper cable for our wire to the bed of the truck. It's not Marine grade, but for this use it's plenty good enough. I wouldn't size your fuse to prevent the alternator from working hard. You will just blow fuses. You should size your fuse for wire protection. 8 gauge wire would be better suited for a 30 or 40a. But if you aren't blowing fuses you could be fine. Just keep some spares so you aren't without a charge in the boonies due to a blown fuse.

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[mechengrsteve]

"solar battery" is not a thing outside of marketing blurbs, please link.

And explain what you mean exactly, charging one battery from another is very rarely a good idea.

A 100Ah LFP battery can easily pull 400-600A from a higher voltage charge source if there isn't anything limiting the current in between

Many drop-ins and powerpacks do limit charge current, sometimes even lower than 1C

But there IS a number of things limiting the current in between. ......A discharged LFP battery will pull a lot, but nowhere close to the 400-600A suggested....Lets calculate what might be possible.....You can't just use battery resistance alone as the system has a lot of equal or larger resistances. For example; Assuming the battery has an IR of ~15mOhms, and is connected to the alternator by a small but adequate cable (assume 8ga @105C rated 80A, with 20ft pos & neg wires to the battery) which have additional resistance of 25.6mOhms (note the wire resistance is ~2x the battery). If in an automotive system operating at 13.8V, putting a very heavy load on the system (at idle) would pull the system voltage way down (unless the vehicle has a high capacity alternator AND is run above idle), system voltage will probably in the 13.2-13.4V vicinity. SO THEORETICALLY, The system could draw (13.2/.0406) 325A BUT that is only if something could provide it. Since I assume, the vehicle has a 'standard' alternator, rated at 120A or so....that is 120A at 6000 (alternator) RPM.....I'd assume the engine WOULD be started and idled, providing ~80A (at idle). Nothing comes from the starting battery as its voltage is below that of the alternator.....that's all the LFP battery is going to possibly be delivered.... The high draw is a short period of time but the IR raises pretty quickly from 0% SOC to 20% SOC. If it was at 0% SOC, it does pull this excessive current for 10-20 minutes. The challenge is living through those 10-20'. Further assuming the alternator were to be left at idle for these 10-20 minutes AND the engine is cold when started (not contributing engine heat to add to the alternators burden, assuming this is the first start in the morning, it would be), I can honestly forsee, that if the operator was aware of the danger of revving up the engine, that this is a very survivable situation. Yes, there is nothing explicitly limiting the current other than the intelligence of the operator but it would not take much to understand the situation. Things that could make the situation worse, more batteries (longer time to 20% SOC), a bigger alternator (higher system voltage/amperage even at idle), 'high idle' (moving the alternator to high production levels) or driving at any speed (same effect as high idle)...Avoid those and I think you would be OK.

 

[mechengrsteve]

Oh ok, 100ah nice. What brand did you go with now that I'm interested?

8 gauge is adequate, it will limit your amps but that's fine. We bought a Amazon special 4ga jumper cable for our wire to the bed of the truck. It's not Marine grade, but for this use it's plenty good enough. I wouldn't size your fuse to prevent the alternator from working hard. You will just blow fuses. You should size your fuse for wire protection. 8 gauge wire would be better suited for a 30 or 40a. But if you aren't blowing fuses you could be fine. Just keep some spares so you aren't without a charge in the boonies due to a blown fuse.

Sent from my Pixel 5 using Tapatalk

Putting a fuse on an alternator charging circuit should be sized so that it can never overload and will only function from a short. If shorted, your cable would get LOTS of amps (200A easily) from batteries. 8 GA wire with 105C insulation (welding cable) is good for 80A CD (68A in engine spaces). Since this is not CD, depending on your alternator size, you could assume 100A+ intermittently. Since OCPD typically function at 130% rating,(except ANL which is much higher), I would choose a 120A rated breaker (120x1.3 = 156A operating). If assuming a 120A alternator, this is probably good. If your OCPD is small enough to every function on overload, you have a chance of destroying your alternator.

 

[john61ct]

But there IS a number of things limiting the current in between. ......A discharged LFP battery will pull a lot, but nowhere close to the 400-600A suggested

My point is that LFP cells can and do pull 4-6C

Maybe not in your specific scenario, and certainly not a drop-in, which indeed has low-ampacity circuitry in place making that impossible.

But setting that aside, the bigger point is that, to protect the kind of infrastructure discussed here

unless the alternator is fitted with a specialized current-limiting VR, which few modern vehicles allow

then a DC-DC charger is required.

Relying on kludges like thinner AWG wiring is not a safe approach unless a real electrickery wizard like you is there to oversee the system and any future changes.

Especially going to a larger bank, in that latter scenario would literally be playing with fire