BFL13
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Joined Feb 15, 2006
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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-not888Views0likes0CommentsRe: 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.587Views0likes0CommentsRe: Renogy 20A dc to DC report
theoldwizard1 wrote: BFL13 wrote: #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.580Views0likes0CommentsRe: 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.551Views0likes0CommentsRe: Renogy 20A dc to DC report
Lwiddis wrote: 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.553Views0likes0CommentsRe: 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" )!314Views0likes0CommentsRe: 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.553Views0likes0CommentsRe: More on LFP Charging
Durb wrote: 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! :)328Views0likes0CommentsRe: 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.552Views0likes0CommentsRe: 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.338Views0likes0Comments
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