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gcrank1
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# Posted: 7 Jan 2022 12:43pm
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And I dont have a charger (off grid- out back...)?
Hypothetical:
My lead acid starting battery is too low to start the rig.
I have an LFP at the cabin.
What if I jumper the LFP to the LA, let it sit a bit....my understanding is the higher voltage LFP should dump amps into the low LA easily boosting it to LA full charge of 12.8.
Disconnect the jumpers and start the rig.
Right?
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paulz
Member
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# Posted: 7 Jan 2022 01:04pm
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Well I've asked this question several times, no reply. The pocket size car jumper boxes they have now, and I have a couple, are Li, and work great, but they obviously have some built it circuitry.
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gcrank1
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# Posted: 7 Jan 2022 01:40pm
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I expect the magic boxes have some built in safeties and the stuff for ubs charging, cig-lighter socket, etc.
My thinking is that 'charging is charging', ie, gotta have enough more volts on the source to move amps to the target. With nominal 12v batteries, the LFP at say 13.3 and a low LA, the current will get pushed by the much higher LFP volts.
Ive done this b2b thing with a fully charged LA to LA, a car bat to a small mc bat; they equalized so both were sub-optimum, but it worked just enough.
The LFP should work much better.
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curious
Member
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# Posted: 7 Jan 2022 02:07pm
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What size of LFP? Why not jump start the UTV with reg jumper cables from the LFP?
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FishHog
Member
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# Posted: 7 Jan 2022 02:22pm
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Quoting: curious Why not jump start the UTV with reg jumper cables from the LFP?
Exactly. Just like boosting another vehicle, hook up the cables, start it, then unhook the cables.
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paulz
Member
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# Posted: 7 Jan 2022 02:37pm
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Quoting: gcrank1 With nominal 12v batteries, the LFP at say 13.3 and a low LA, the current will get pushed by the much higher LFP volts.
That's what I was thinking. I'm not looking to jump start, just jumper a car battery to my cabin solar LFP bank to charge it. Seems like it would be just another draw on the bank. Maybe the amps would be too high. Not about to try it until I know for sure. I guess you could put a 5a fuse in there..
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curious
Member
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# Posted: 7 Jan 2022 02:53pm
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Quoting: paulz I'm not looking to jump start, just jumper a car battery to my cabin solar LFP bank to charge it.
With different capacities and different chemistries needing different charging parameters or limits I prefer to not do that. At times if I was faithfully watching there might be no problems. But I know myself snd know I may lose track of time.....
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paulz
Member
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# Posted: 7 Jan 2022 02:57pm
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I hear you. They do make DC to DC chargers, maybe one specifically for this purpose.
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gcrank1
Member
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# Posted: 7 Jan 2022 03:01pm - Edited by: gcrank1
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Not interested in live starting off an expensive LFP. Once a rig starts what kind of spikes may happen with the charging system? I dont want to be testing/stressing the LFP bms any more than I have to.
FLA batteries charge at higher volts off a charger than the fully charged LFP has, so no hurt to either just hooked up to each other only.
LFP can dump amps under load way more than LA, and there is no running load, just moving some amps into a low LA with the higher LFP voltage. A nice, easy, top up methinks.
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curious
Member
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# Posted: 7 Jan 2022 03:01pm - Edited by: curious
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Yes to paul
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ICC
Member
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# Posted: 7 Jan 2022 03:14pm - Edited by: ICC
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gcrank, I think you worry too much, but ask the LFP manufacturer. I have had an LFP in my ATV for years. Most of the dirt bikes my family members have use LFP batteries. My solar has LFP and has a number of things that use large surges og power b welder, big air compressor. And I have had an LFP battery starting my cub for a few years. I once jump started my Jeep from the cub.
I use dc-dc chargers and also sometimes just plug an AC powered charger in to charge some battery
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gcrank1
Member
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# Posted: 7 Jan 2022 03:23pm - Edited by: gcrank1
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My Rhino Voltz is not recommended to be used as a starting battery.
The applications I often see b2b/dc2dc chargers used for are from a vehicle LA battery to a 'house or rv' LFP where the typical LA charge parameters need to be regulated to not overcharge (or kick the HVD on the bms) the LFP when the vehicle alt is charging the LA.
To use a b2b unit for a simple 'static exchange' scenario seems like inverting from a solar bat-bank to 120 to go a distance and transforming it back to 12vdc to run lights.
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FishHog
Member
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# Posted: 7 Jan 2022 03:23pm
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Quoting: ICC gcrank, I think you worry too much
Nothing wrong with being cautious, but I agree, don't really see a risk here. Lots of people using Lithium starting batteries as ICC mentions.
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paulz
Member
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# Posted: 7 Jan 2022 03:24pm
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This was several years ago now, I was on a Brit bike club ride. A guy had just installed one of the 'new' Li batteries in his old Triumph. Don't know if it was LFP or something else. We had stopped at one of the destinations, looked over and his bike was on fire! Ran over, yanked the melting battery out. Fortunately no big damage, and we had a spare FLA along.
Lucas electrics, either charging too much or not at all. I do know guys now that use Li batteries in bikes without issues. Not sure about old British though.
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gcrank1
Member
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# Posted: 7 Jan 2022 03:42pm
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Lol, Joe Lucas, Prince of Darkness.....cant say he was a friend of mine.
The positive ground may have been an issue? But the 'regulation system' on my 6v alt-equipped '63 Triumph TR6 was pretty basic.
Guys, I assure you I 'do not worry too much', but I am naturally curious and analytical. It has served me well and avoided a lot of expensive mistakes.
Some of this stuff I post up is to engage in the dialogue, brain-storming if you will. I figure if I think of a question I cant be the only one curious, Im not that original 
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gcrank1
Member
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# Posted: 8 Jan 2022 11:15am - Edited by: gcrank1
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Test Results:
I have a little well used AGM 12x8ah bat I keep around for powering up odd projects; it doesnt hold a charge well and was sitting at 12.60v this morning.
So I hooked it up to the RV 50ah LFP at 13.53 (vom).
Just checked it after 20 min, unhooked the AGM and its at 13.00v.
By 10:30 it is at 12.82.
1 pm, still at 12.78
Simple transfer of power, no muss-no fuss; looks like a proof of concept.
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travellerw
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# Posted: 8 Jan 2022 12:24pm
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On our boat, we had BIG solenoids that would allow us to connect our LFP house bank to the FLA start batteries for boosting. Saved my ass at least one time in a big storm when one motor wouldn't start on the FLA.
We didn't even let them sit, just fired the motors on the LFP if we needed (while holding the switch to engage the solenoid).
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