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gcrank1
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# Posted: 28 Jun 2021 08:58pm
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Can anybody tell me why LFP dc to dc chargers are so much more expensive than grid wall plug chargers? Seems the dc to dc should be simpler/cheaper to make.
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ICC
Member
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# Posted: 28 Jun 2021 10:20pm
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Product example links to see just what devices you are comparing?
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FishHog
Member
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# Posted: 29 Jun 2021 08:39am
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I put a sterling dc to dc charger in my camper van. Yup they are expensive and I assume it’s just because it’s a speciality with low demand
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gcrank1
Member
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# Posted: 29 Jun 2021 10:12am
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Ive been looking at the ac-dc and dc-dc chargers in the lit for a couple of different LFP battery companies, Rhinovoltz and Miller.
The ac-dc, 20a chargers are about $80, the dc-dc (iirc) are about double that. That doesnt make sense to me, the only more hd part should be the 12v input vs the 120ac and there is no step-down transformer needed, just volt & amps control; ie, its just a voltage regulator.
Actually, I cant even see the ac-dc should be more than $40......
My 'smart charger' at home doesnt have an LFP setting, though it does have FLA, AGM and Gel. from the readout Ive watched with FLA and AGM it looks like AGM should meet the charge criteria for LFP.
BUT....It could be an expensive educated guess!
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Steve_S
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# Posted: 29 Jun 2021 11:07am
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A tad clarity that is missing here.
FLA/AGM are not "digital" they are analogs. And charging / discharging them can be brute force managed and they won't care, unless it's something really stupid.
Lithium, all varieties are Not Brute forceable ! They are voltage sensitive down to the MILLIVOLT ! remember in Lithium Land we always talk about 3.000-3.400Vpc (which BTW is the majority of the Voltage Working Curve representing 87-90% of the overall capacity of a cell and the pack it belongs to.
Lithium also is (2-Stage) CC (Constant Current) which fills them to 85%+/- and the CV (Constant Voltage) with decreasing amps to top off to "designated 100%".
No Compensation for temp, No Equalization EVER. Float can be used for topping off at 0.5V lower than Bulk C.C. (terms vary by maker) charge voltage.
End-Amps, TailAmps fro LFP is also 0.05C ! NOT so with FLA/AGM.
Example:
100AH cell @ 0.05C = 5A for cutoff.
280AH cell @ 0.05C = 14A for cutoff.
* In a bank with multiple packs, always use the cutoff C-Rate for the highest capacity pack in the bank.
Using LFP (LiFePO4) as an example. Full Operational Voltage range is from 2.500Vpc to 3.650Vpc, while Nominal is 3.200 and the "working range" is 3.000-3.400. (Vpc = Volts per cell) Very long flat voltage curve, similar with all Lithiums relative to their chemistry.
PEOPLE even have trouble with Proper Bench Power supplies that have poor wires or those gator clips not providing full voltage & amperage, many end up, upgrading the wires to high grade silicone and installing Ring Terminals to ensure proper contact to minimize resistance & losses. When there is a .5V difference between the cell terminal and the posts on the bench supply, overcharging a cell is way too easy.
The moment an LFP cell goes over 3.650V it is being damaged, cross 4.000 and it's really getting hurt, hit 4.500 - chuck it ! or be prepared to, unless you can get it down and FAST !
I use a TekPower Bench Supply, pretty darn good and not very cheap and even that had to get better wires & connectors and still must be cross verified with a DVOM.
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gcrank1
Member
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# Posted: 29 Jun 2021 01:34pm
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So, the smart money is to buy the ac-dc LFP charger from the LFP company at the $80ish, it should be right, no guesswork/risk; or their dc-dc at basically twice the price.
Do you have a less expensive, suitable, charger to recommend?
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ICC
Member
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# Posted: 29 Jun 2021 02:03pm - Edited by: ICC
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I am curious why are you looking at both AC-DC and DC-DC charging?
I do have both AC to DC and DC to DC chargers in my bus-rv. I use the AC-DC when the pv panels can't do the whole job. AC from grid connection or generator. Or at home I may have reason to boost the bus lithium from the solar powered house... so I use AC to DC beause that is easy even though technically inefficient. Done so selfom it does not matter. It might be needed once in a long while as the bus is inside when parked at home.
The DC-DC is necessary as the engine driven charge system does not have a high enough voltage to charge the buses house batteries which are lithium. That needs the go between DC-DC.
But the home or the cabin batteries virtually never are charged with anything but the solar. The inverters are inverter-chargers so I can charge with AC from the diesel generator/welder. There is no need for. DC-DC charger.
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Steve_S
Member
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# Posted: 29 Jun 2021 02:52pm
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Like ICC says, DC -> DC for mobile to run between Alternator & Lithium cells is one thing, otherwise is it for say using 24V or 48V to charge a 12V ?
$80 AC->DC charger ??
Won't be much amperage delivered.
12V/100AH pack would take 11 hours from 0%SOC with a 10A charger.
Some PreBuilt LFP batteries can take an AGM profile without issue, these are BattleBorn and such which are built as "Drop-In" replacements because they use BMS' that can handle such. Most after-market BMS' are less tolerant, this is something that causes some issues of course.
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gcrank1
Member
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# Posted: 29 Jun 2021 03:12pm - Edited by: gcrank1
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The 'big' chargers from both Rhino and Miller are 20a @ about $80 for the ac-dc.
My 10ish yr old Blue Sky Solar-Boost 50 (mppt) has some tiny rotating adjusters (what are they called?) to change the charge profile, though it predated LFP and has no LFP update info online. Even now I have equalization switched off for the AGM.
With an ac-dc I can always bring a 100ah LFP home between visits to fully charge. Not a big deal for as little as it weighs.
With the dc-dc I could charge it off the vehicle if needed while making a 'lumber run', etc, or even off the FLA on the solar as it is now. I could even run the ac-dc charger off the inverter from the FLA. Inverter losses, so what, it is free solar.
For my transition to LFP leaving the working and satisfactory solar system c/w FLA intact seems the way to 'ease' into the new tech.
My typical use pattern is to run higher draw during solar charging hours so I am not depleting the battery; the evening and night depletion is not huge so the charge each day to add back some ah's may be a net loss but still potentially extend that stay.
I figure with an LFP I need a charger anyway, one or the other, ac-dc or dc-dc, I just dont want to pay through the nose needlessly.
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ICC
Member
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# Posted: 29 Jun 2021 05:28pm
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My guess is that the Blue Sky unit you have is not suitable for lithium charging if they don't state how to set it up for that. Or you simply go with a drop-in like Battleborn.
A lithium battery will accept more amp-hours charge in the same time as lead-acid will. So just ditching the lead and installing lithium with the proper controller or settings will charge the lithium battery much quicker. It's like using a bigger pipe to fill a water tank.
It sounds to me like you need a lithium safe AC charger more than you need a DC-DC charger. Want is a different matter.
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gcrank1
Member
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# Posted: 29 Jun 2021 06:58pm - Edited by: gcrank1
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This is good input guys.
My inclination is for a min. 20amp ac-dc charger at $85, then there's a 40amp at $180; am I correct that would be ok on a 100ah?
I didnt really want to have to buy a new scc to charge LFP.....ie, the pit just keeps getting deeper.....
Nor do I want another battery charger, think I have 7 or 8 now, but an LFP model to charge and protect the battery is apparently necessary. Guess I can take that easier than a new scc while Im still so FLA heavy.
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ICC
Member
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# Posted: 29 Jun 2021 08:19pm
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The rough rule of thumb for LFP charging is to not exceed 0.5C where C= the amp-hour rating of the battery.
Example: LFP battery with 100 aH capacity.
C = 100
0.5C = 0.5 x 100 = 50 amps maximum charge amps.
Less is okay
If running off a generator a 40 amp charger would need about one half the generator time of a 20 amp charger.
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paulz
Member
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# Posted: 29 Jun 2021 08:44pm
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You may recall I scored a few of these MeanWell power supplies awhile back. I used two of them (cabin and home) to charge my LFPs before I got the solar operational. I want to keep those two but I'm happy to give a couple away if it helps any of you who've helped me along the way. They are the 110ac/15v/40a units. They have a voltage adjustment screw that I set to 14.6, did a wonderful job for me.
https://www.mouser.com/datasheet/2/260/scn-600-spec-1180098.pdf
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gcrank1
Member
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# Posted: 29 Jun 2021 11:19pm - Edited by: gcrank1
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Sounds like what I could use, Paul, as I head into this Brave New World.
I dont see a pm function on this forum, how about you send me a mssg, my e-dress is in my profile, and I'll catch up with it tomorrow morning?
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gcrank1
Member
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# Posted: 30 Jun 2021 08:10pm
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Thanx Paul!
Will let you know when it shows up 
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