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RE: More on LFP Charging
Some graphs here of interest. Scroll down. One has the Absorption SOCs for some lower C rates.
These guys seem to have a clue, lots of interesting (IMO) posts on some of the things being discussed here.
https://forum.solar-electric.com/discussion/351996/charging-lfp-lifepo4-batteries-to-90-soc-or-not
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BFL13
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06/30/22 11:06am |
Tech Issues
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RE: Renogy 20A dc to DC report
PT the alternator should be supplying enough to run the V-10 etc, plus do some charging to the house. So if the front is drawing 8 amps from the house to keep up, what is going on?
And if you isolate the house from the front, how will it find that 8 amps it needs?
I don't think I have the story straight since you know all that.
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BFL13
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06/29/22 09:27am |
Tech Issues
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RE: Renogy 20A dc to DC report
#8 might not be enough. I had to change my input neg path to use the frame (as a really fat wire) to get my voltage drop small enough to keep input voltage in the 13s.
The whole point of a DC-DC charger is that it will accept low input voltage and still provide the correct output voltage.
Yes, it has "boost" (and "buck" if needed) but you cannot control the input, only the output setting voltage. The 20 amps is built in. Once you set the output voltage, you now have the output watts it needs to "supply"
So now the unit will "demand" the watts it needs for input. You have no control over how many amps that will be. All you can do is tinker with input voltage to try and keep the amps demand from being too high for the alternator. Also you don't control the vehicle's voltage, just the size of the input wiring.
The alternator should not have more than about 1/3 of its rating demanded ("pulled") from it, so you have to know your alternator's rating before you pick your DC-DC "size" in output amps. We have had several threads about that on here.
Some on here have reported how they use really fat wire on their inputs and the truck frame as needed, and have their DC-DC pulling only a few amps more than their rated output amps "size". The Renogy unit says it can pull as much as 30 amps to output 20 amps, but you can use fatter wire to get higher input voltage and thus have fewer amps being pulled to get the same input watts.
Going back to the example output setting of 14.7v and 20 amps for 294w and spec 90% efficiency, it will "demand" 327w input.
If the vehicle is running at 14v after warming up, and you have 13 amps showing at the DC-DC input (1 volt drop) it will pull 25 amps. ( if you did get 14 at the unit, amps would be 23.4.
My truck alternator is rated at 105 amps and 1/3 of that is 35 amps, so I am good with that pull of 25 amps. That leaves a few amps to run the truck too (as was mentioned by one of the guys on here recently)
People with vehicles that have alternators rated at over 200 amps can use the Renogy 40 amper which might pull 60 amps and they will be ok as long as they have fat/short enough wiring for input.
On the output wiring, you can set the unit for your battery spec of say 14.7v for charging. But how many volts reach the battery of that 14.7? You want it all, but if the battery can still get charged with 14.5v (the specs usually give a range of ok voltages eg 14.5-14.8) So you have to use fat/short enough output wire to get 20 amps at 14.5v at the battery posts, allowing a 0.2v drop.
PT--I thought you did a bench test and it was not installed yet. Confusing. Anyway, same thing--try it out and if it does not hold 20 amps at the set output voltage, you must improve the input so it will.
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BFL13
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06/29/22 07:45am |
Tech Issues
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RE: Renogy 20A dc to DC report
"Rated Max power is 250 watts."
Yes but what is that rating based on? 20 amps output means 250/20 = 12.5 volts.
You can choose a charging profile that is 14.7v at 20 amps = 294w.
That is what it is supposed to do, never mind its "rating" of 250w. I have no idea why they have that in their specs. It is like those converters that are "rated" for when they are at 13.6v and not when they are at 14.4v. Some kind of "Sales" stuff maybe, who knows?
Meanwhile you can install it and see if it holds 20 amps steady when set at 14.7v. If it does, you are good. If not, you will have to use fatter input wiring or use the frame until it holds its 20a.
Also when they specify the output wiring they seem to forget that the batteries also have some resistance, not just the wiring. So you might have too much voltage drop between Renogy output and battery post voltage when you measure each in real life. IMO use your #8 for the output and #4 for the input and hope for the best.
You won't really know until you install it and try it out. Measure everything and be ready to adjust the installation as required.
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BFL13
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06/28/22 10:17pm |
Tech Issues
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RE: Renogy 20A dc to DC report
That’s terrible efficiency! Thank you for posting.
85% is typical for converters and chargers so not "terrible" at all.
Plus the "efficiency" of a solar controller varies with how much buck or boost it is doing, so I am guessing it is the same with the DC-DC units of whatever brand. Although solar controllers are in the 90s % not the 80s like converters.
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BFL13
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06/28/22 07:48pm |
Tech Issues
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RE: More on LFP Charging
"Thank you for the information. I have a Battle Born 100AH battery and a Progressive Dynamics 60amp convertor, so I guess .6C. The combo has been working well for two years now. I do unplug the PD once the battery is charged."
Seems right although BB allows 14.6 for a while until balancing has happened at least.
How do you tell your BB "is charged" ? Also what do you mean by "is charged"?
I see 3-tons also disconnects after his batt bank is full and if on shore power, he runs on LFP batt for 12v and shore power for 120v, even though he has a PD converter that can sit at 13.6 doing the 12 v while also floating the LFP at 13.6.
One complaint with the Dakota Blurb is he says a normal charger will only charge an LFP to 80% and then shut down. What weird "Sales" talk is that? He explains it, but it turns out he is not talking about any "normal" charger/converter at all. Oh well--"Sales" has its own rules ("all's fair in love and war" )!
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BFL13
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06/28/22 07:34pm |
Tech Issues
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RE: Renogy 20A dc to DC report
"The whole idea of the dc to DC is to boost the voltage going into the battery bank. Therefore voltage drop to the charger doesn't matter--so long as the ampacity of the wire size is not exceeded."
Yes it does matter.
The unit is "buck/boost" and the output voltage will be higher than the input's in many situations so it has to boost. The voltage drop to the charger is vital to keeping the input watts high enough to keep the watts up. It also affects the amps draw on the alternator, which you want to keep as low as possible.
I had a problem where my output amps fell off from 20 and it turned out the input voltage was too low. The unit has a boost spec from about 8 volts but not quite so in real life. Anyway, you improve your input voltage "somehow" until the unit can sustain its full output amps at the chosen output voltage. You cannot control the input amps directly.
I am using the thin and long positive wire in the 7-pin as input so to get the whole circuit better, I had to swap over to using the frame instead of the neg 7-pin long thin wire. If that had not worked, I would have had to run a fatter positive wire too.
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BFL13
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06/28/22 05:24pm |
Tech Issues
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RE: More on LFP Charging
What does "C" mean when referring to current?
Relates to the Capacity of the battery bank in AH and how much of that figure to use as your initial charging current (which holds at that until it tapers)
Edit--this all does not apply very much to solar, where the amps rise in the morning and taper in the afternoon.
So for a 100AH bank 0.3C is 30 amps.
Many LFPs will say they can take 1C as "max" but prefer you keep it at 0.5C or less. Some specify max of 0.5C if say their BMS wiring is too thin to take more amps. (That would also affect their max discharge amps I guess---not sure--anybody know? )
FLAs and AGMs can also handle 0.3C, but 0.25C is preferred, so "fast charging" comparisons at that level turn on when amps start to taper in each case, not on the amount of initial charging amps. LFPs supposedly taper later giving them more time at the higher amps.
Still no info on when amps start to taper with LFPs if you use a reduced charging voltage from the 14.6 max. Looks like charging/generator run time comparisons will need to specify what voltage was used as well as what initial amps were, but still no good info on that.
Of course to take advantage of the LFP high acceptance rate in amps for "fast charging" claims by "Sales" you need a charger that can do that many amps, plus a 120v source that has enough watts (VA) to run the charger without dropping too far in 120 voltage, or a generator that has enough watts/VA to run that big (in amps) of a charger.
It is all good fun if you don't weaken! :)
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BFL13
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06/28/22 05:08pm |
Tech Issues
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RE: Renogy 20A dc to DC report
#8 might not be enough. I had to change my input neg path to use the frame (as a really fat wire) to get my voltage drop small enough to keep input voltage in the 13s. You can tell by if it can't hold that 20 amps output, that you need more input (ie, less voltage drop)
I can't remember the unit's efficiency, but for instance:
14.7 x 20 = 294w and 85% efficiency (eg) needs 346w input.
If input voltage is 13v after v drop from engine batt, 346/13 = 26.6 amps. If you could keep input voltage at 14, then 346/14 = 25 amps
You are supposed to get input from the engine battery, not from the alternator. You need a common ground like it shows in the manual. With a Class C that would be the frame
The unit turns on and off using the little wire that connects to pos input. I have a switch on that little wire in my set-up, and just ran that wire over to the pos input terminal.
You must not also have the OEM alternator charging to house connected like in a MH, or you will add to the amps demand on the alternator, which the DC-DC is supposed to limit.
Pick the nearest charging profile on the Renogy to what your batt spec is (14.7 on mine) and then set your solar controller to match that voltage so they add their amps best while driving down the road.
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BFL13
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06/28/22 03:05pm |
Tech Issues
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RE: More on LFP Charging
A guy here was selling his Dakota 200AH for $500 (Can) so I was looking into it. Could not find the BMS specs for cut off voltages. Some specs info only.
(I got nervous about buying used because how can you tell if it has been "damaged" by whatever usage? Didn't need it anyway, but tempting price. Somebody got it, so good luck to him.)
It does seem LFP advice can be "brand specific" even if they all have similar type cells.
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BFL13
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06/28/22 12:18pm |
Tech Issues
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More on LFP Charging
Some "sales" statements here, but also some possibly good info of interest.
One thing is they found longest life (most cycles) was to charge at 14.4v but at lower amps at 0.3C.
Some LFP owners on here ISTR have taken to charging at lower voltages hoping for longer battery life. Perhaps it needs to be established which is correct--lower amps or lower voltage?
They say 14.4 vs 14.6 to reduce stress, and recommend their own charger (of course!) which is single stage 14.4--BUT they say shut charging down-disconnect- once the charger says the batt is full. (light comes on---what makes that light come on? oh well)
They are strongly against floating! see why.
Anyway, it covers some of what has come up on here lately.
https://dakotalithium.com/2021/09/22/how-to-charge-dakota-lithium-and-lifepo4-batteries/
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BFL13
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06/28/22 11:12am |
Tech Issues
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RE: Renogy 20A dc to DC report
I got mine to do better with a higher input voltage. What was the input voltage on yours when input amps was 30?
Of course you have to have input watts higher than output watts, so what was the output voltage when it was doing 20a?
I run my output set to 14.7v and it does the 20 amps out steady. I had to improve the wiring from engine battery to Renogy input to reduce voltage drop and so keep input up enough.
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BFL13
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06/28/22 09:55am |
Tech Issues
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RE: Converter voltage drop
My personal choice is a programmable inverter/ charger but my use is much different than most.
If I was downsizing and was going to stay with a converter than it would be one that Time2roll suggested in the 3rd post of this thread. I do have one of those converters hanging in the front compartment just disconnected from the system.
Thanks. Totally agree/concur (not sure which one of us is senior) on that.
Hope the OP is still here and hoisting all this good stuff in! :)
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BFL13
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06/24/22 07:20pm |
Tech Issues
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RE: Converter voltage drop
Itinerant1, what would you advise the OP wrt to his possible converter replacement for his ? brand LFPs?
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BFL13
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06/24/22 06:57pm |
Tech Issues
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RE: Converter voltage drop
Any QUALITY converter/charger will have a specific setting for LiFePO4. Use it !
Personally, I would just eliminate the converter and buy an inverter/charger/automatic transfer switch.
New to this so hopefully replying in proper format...all LFE specific converter/chargers I've found have a 14.6V bulk phase. My battery mfg says to stay at 14.4 or less in bulk. Are there LFE chargers that start at 14.4? Regarding an inverter/charger, I already have a 2200W pure sine wave inverter that Works great. My best value seems to be using existing inverter and upgrading the converter.
PowerMax says they made their LFP compatible converter (LKL model) according to what BattleBorn told them and that's that. Obviously, PD did the same. That leaves out other LFP makers and their specs, which are not always the same as BB's.
At least the LKL still has the adjustable voltage option, so you can't go wrong there, unlike PD's L model unless your batt's specs are the same. As posted earlier, with your specs you will be ok with a standard 14.4v 3-stage PD with the Charge Wizard, if you find that easier than using the manually adjusted PMX LKL or LK.
Perhaps your batts can stand 14.6 and just have recommended 14.4 like some other LFP specs say? Can you link to your particular brand and model of LFP so we can see its specs?
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BFL13
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06/24/22 05:24pm |
Tech Issues
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RE: Converter voltage drop
The charging specs for this LFP are interesting. They say 14.6 max but 14.4 is ok. BMS cut off is 14.8. Big surprise ? Is balancing at 13.6.
https://www.canbat.com/downloads/CLI100-12.pdf
Also it says resistance is 30 mOhms at 50% SOC. An AGM spec says IR is 5 mOhms, but does not say when (probably at 25C/77F but ? SOC) that 30 mOhms seems way high where the usual story is that LFPs have low IR?
On shore power you can disconnect the LFP and run on converter for 12v. I still have not got an answer for what you do if your rig has an inverter/charger. Last heard, an inverter/ charger will not operate at 12v unless it it is connected to a battery to run its own systems. Not sure if it still would do 120v pass through.
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BFL13
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06/24/22 11:16am |
Tech Issues
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RE: Converter voltage drop
By the time the charger gets the battery to 14.4 volts, amps will have tapered down so there will be little voltage drop.
Standard PD converters do 14.4 not 14.6, so you want a standard PD converter (NOT their LFP model!!!) in the 45 amp range. Make sure it has the Charge Wizard or built in version of that so you can pick and choose between 14.4 and 13.6 yourself.
It is a good idea to have a battery monitor so you can see amps as well as voltage and see AH count so you know where you are at, and not hit the 13.6 button too soon. I have a Renogy 500A one that does the job nicely and is not expensive, eg.
It seems there is no one correct 14.6v that applies to all LFPs, so you are right to look carefully. It is like "AGM" voltage settings that are lower than for Flooded batts--except there are AGMs that specify 14.7v for charging.
However, if Battleborn says to use 14.6 on theirs, I don't know why anybody would choose not to if they have BB LFPs, and use 14.X instead. Up to the individual LFP owner I guess.
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BFL13
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06/23/22 06:20pm |
Tech Issues
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RE: Costco 100W 18V solar blanket: which controller to use?
Apparently nobody watched the video I linked back when.
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BFL13
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06/21/22 07:19pm |
Tech Issues
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RE: Costco 100W 18V solar blanket: which controller to use?
How do you connect that 18v power wire to the 31 battery? or even to a solar controller that then connects to the battery?
You might have a power station that you charge up with that like in the video, and then use a battery charger plugged into the power station, but that seems a bit much.
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BFL13
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06/21/22 09:49am |
Tech Issues
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RE: Costco 100W 18V solar blanket: which controller to use?
It works with a power station, as seen in this video. It is not a "normal" solar panel.
https://sterlingwong.com/2022/02/05/review-costco-massimo-100-watt-solar-blanket-folding-solar-panel/
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BFL13
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06/20/22 04:37pm |
Tech Issues
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