How to Fix the Slow Charging?

[WearyWanderer]

Long-time lurker, first-time poster. I'm running into an issue with my battery system in my travel trailer, and I was hoping that y'all might have some insight.

I'm running into an issue with very slow charging of the 500Ah of LFP batteries in my travel trailer, and I have hunch of where the problem may lie.

On the DC side of my travel trailer, I have 2 x 250Ah 12v Chins self-heating/ Bluetooth LFP packs wired with a couple feet of 4 AWG pure copper cable (great crimps on the lugs, I can assure you!) to distribution bus bars (a 60-amp DC breaker between between the battery and the bus bar for each battery). Then, from the bus bars is a long run (20ft on the negative side and about 30ft on the positive side as it runs to a battery disconnect switch) of 6ga copper wire that the camper came with from the factory. At the end of that is the WFCO panel that my travel trailer came with along with a "converter" unit that charges the batteries.

The problem that I'm running into is, despite putting in a 55-amp PowerMax PM4 converter (output adjusted to 14.6v), the batteries are only receiving right in the neighborhood of 40 amps combined while charging (as indicated by the BMSs inside the batteries, not by a clamp meter). Thinking that perhaps the PowerMax was less-efficient than I thought it was, I stepped up to a 75-amp PM4, but I'm still getting a charge current right around 40 amps. I should note that the PM4 is set to constant voltage output rather than charging mode (I watch my batteries very carefully during charging), and I shut off the converter when the batteries are full.

How can I get that charging current stepped up to be closer to the output of the converter? I spend most of my year fully off-grid in my travel trailer (why do I even own a house anymore?), and I use my Honda EU2200i to juice things up every few days. With the lower charging current, I have to run my generator much longer to get those batteries charged. I was hoping to charge my LFP cells at a fast enough rate that they complete charging around the same time as my Bluetti power station, but with the slower rate of ~40 amps, the Chins take such a long time to get fully charged.

My thinking is that the problem might be the long run of 6 AWG that the camper came with from the factory. I was tossing around the idea of stepping up to either 4ga or 2ga, but I'd like to get some opinions before redoing the main run.

 

[WearyWanderer]

Perhaps I was a bit optimistic on the charging speed. While it does occasionally hit 40 or even 45 amps when the batteries are quite low, usually charge rate is around 35 amps.

 

[LakeHouse]

I think you've discovered the Achille's Heel of 12V systems: voltage drop!
Next time you're charging, measure the voltage at the battery, and then measure the voltage at the charger. My guess is the charger is putting out the full 14.6 volts, but because of the voltage drop along the long run of 6 AWG, plus connection losses, etc., that's only enough to push the 35 Amps you're seeing. There are lots of voltage drop calculators out there, but the one I'm using gives a voltage drop of 0.9V for the 50' circuit of 6 AWG at 35 Amps. Then add in some more for connections, and your batteries are seeing a lot less than 14.6V.
You probably want to upgrade the 6 AWG to something quite a bit bigger. And if you're looking to charge at 75 Amps, 6 AWG would be insufficient anyway.

 

[DIYrich]

After checking the voltage drop, you can try to increase the voltage to push more current, but be careful not to exceed voltage at battery.

 

[MisterSandals]

My thinking is that the problem might be the long run of 6 AWG that the camper came with from the factory.

You can easily tell if this is an issue by feeling if it is warm or hot (that heat would be your loss).

 

[Browneye]

I junked my Powermax. It would charge pretty close to rated capacity, but eventually wouldn't properly sense battery/bus voltage and would not charge. I got a MeanWell NPB charger instead - flawless. And quiet too. The Powermax sounded like a dental drill. LOL

 

[WearyWanderer]

You can easily tell if this is an issue by feeling if it is warm or hot (that heat would be your loss).

Wires aren't warm, but with such a long run, the small absorption from the wires could cause enough loss without a lot of heat I'd think.

@DIYrich - One of my problems, actually - converter is far (electrically) from the batteries but not from the other loads, so adjusting the voltage upward would up the voltage at the batteries, but I'd be pushing over 15v to my 12v appliances like my fridge and such, so that's a no-go sadly.

@LakeHouse - As I feared, sadly. I was hoping to at least get close to the 55 amps, but alas, here I am. I'm thinking I'll probably step up to 2 AWG copper, but it's not a swap that I can make while deep in the mosquito-infested national forest, so it might be a month or two before I'm home and can change the cabling. HOWEVER, I do have some 20ft jumper cables with me, and they are much thicker (probably CCA, but that'll work for testing). Perhaps I'll clamp those to the converter and to the bus bars to test what charging rate I'd get if I stepped up cable size.

What is very odd is that the charging rate seems to have dropped over time. When I put in the 55A converter a few months ago, I was getting about 22 amps per battery. However, here I am today, batteries about 50% charged, and I'm getting 17 amps per battery. That worries me that there is a connection point that is heating and contracting, thus perhaps getting looser (and causing a potential hazard), but I've checked the known connection points and none are hot, so hmm...

 

[WearyWanderer]

I junked my Powermax. It would charge pretty close to rated capacity, but eventually wouldn't properly sense battery/bus voltage and would not charge. I got a MeanWell NPB charger instead - flawless. And quiet too. The Powermax sounded like a dental drill. LOL

I'm hoping that's not the case in my scenario as I'm not using battery charge mode but rather constant voltage output mode. The fan sure is annoying, though...

 

[Browneye]

Yeah, that's what I had to use - set the POT for voltage. The 2-stage is a fixed output of 14.6V and would over-volt my pack.
For fast charging while monitoring, I would set it for 14.2 and turn it off when it got down to about 10A. Did pretty good that way. Leads were short 4awg, about a foot to the main battery leads, on a 80A fuse.
In the end it would just sit there and blink the green charging indicator and charging would bounce on and off. Very annoying. I was never able to figure out what the flashing green indicator meant - nothing in the docs.

 

[sunshine_eggo]

As the owner of multiple PM3/PM4-XXXLK units, I wouldn't hesitate to operate the unit in power supply mode and adjust the voltage as high as is needed to compensate for the voltage drop and get max amps while generator charging. The BMS should protect the battery in the event of over-volt. No point in charging to 100% either.

Would confirm that the wires aren't too warm at either battery terminals or converter terminals after an hour or so. Anything less than uncomfortable to the touch is fine.

 

[WearyWanderer]

What may be prudent for me to do is to just put a charger closer to the batteries rather than ripping out all of the existing cabling (cheaper as well), especially as I have an AC outlet available right nearby.

I originally sized my system so that I would utilize my 1800VA power budget (rated running output of the EU2200i) to charge my Bluetti system (AC200MAX + B300) at roughly the same speed as the 500Ah of Chins cells. However, I've found myself more than once spending extended time at high altitudes (like weeks on end at 5000ft or even 10000ft), so I can't always count on that full 1800VA from the generator (even with swapping in a high altitiude jet). As such, I'd like to get as close to 80 amps of charging being shoved into the Chins, and once I've charged those to where I'd like them, I can juice up the Bluetti (which I can charge at anywhere between 200w and 1500w as I so choose). I never am charging when I'm not around (the Honda is too expensive to leave running unattended), so I'm always able to keep a close eye on everything while things are charging (I like to take an active role in my off-grid life!).

Any recommendations on a power supply that'll put out 14.4v nicely at 80 amps? Or would even two 40-amp power supplies do the trick?

 

[sunshine_eggo]

What may be prudent for me to do is to just put a charger closer to the batteries rather than ripping out all of the existing cabling (cheaper as well), especially as I have an AC outlet available right nearby.

winner winner chicken dinner.

Any recommendations on a power supply that'll put out 14.4v nicely at 80 amps? Or would even two 40-amp power supplies do the trick?

Does the PM3-75 you have have the power supply mode? If yes, you have your 75A charger.

 

[TacomaJoe]

Check voltage drops along the way. Start with one probe on the positive post and measure to each side of your breaker and to the bus under a heavy charging load. All the drops add up.

Do you have a resettable breaker on the frame? I did a re-wire to get that out.

Your thought of getting the charger out close to the batteries is a good one. I moved mine into the pass through and also moved the converter out of the panel to be next to the batteries.

 

[WearyWanderer]

Does the PM3-75 you have have the power supply mode? If yes, you have your 75A charger.

I might just go all the way up to 100A - giving it some thought... Do you know what the specific differences are between the PM3 and the PM4 (besides the PM4's "4-stage charging" that I don't use anyway)? PowerMax's literature isn't the best. I can save a fair amount by getting the PM3 via RecPro instead of PowerMax directly.

Additionally, what other brands of chargers / power supplies do people recommend? I'm not wedded to PowerMax by any means (although the voltage POT is nice).

 

[sunshine_eggo]

PM4 had some extra boost feature where it would force another bulk periodically. I honestly don't remember. I use mine in pure PS mode.

If the model # ends in "LK" that should be dual mode. Two openings in side of case. One to a switch to toggle PS/3-stage mode and one to a pot to adjust boost/bulk voltage.

 

[WearyWanderer]

Interesting results doing some testing tonight. Since I have previously had a 60A standalone PowerMax PM4 and then a 55A PM4 MBA model, I thought I'd do some testing with my new 75A PM4 MBA before I plunked down a couple hundred dollars on the 100A PM3 or PM4.

Using what I had on hand (since I'm camped in the middle of a national forest), I bypassed the camper cabling entirely and grabbed the 25ft 4ga jumper cables I had on hand (CCA, but hey, it's what I had on hand). I clamped them onto the bus bars and then the other ends onto the short (grossly-undersized) 10ga output wires of the 75A PM4 MBA, arranging the contacts so that they had good connectivity. Turned the battery isolator switch so the only thing going on here was the connection between the batteries and the converter.

Turned on the converter (14.6v measured at the end of the output wires of the converter with no load, 14.3v measured when hooked to batteries at around 50% charge), and to my annoyance, charge current was just a hair above 40 amps cumulative as indicated by the batteries' Bluetooth BMSs. Thinking I'd take even MORE out of the equation, I threw a breaker to isolate an individual battery and clamped it directly to the battery posts - a hair under 40 amps charging.

Why am I seeing this and not closer to 75 amps? I'd like to say that it is the crappy 10ga wires coming out of the PM4, but when I had the 60A PM4 a few months ago and connected that to the bus bars via 1ft pure copper 4ga cable (not excellent crimps but the best a girl could do in the desert with the tools on hand), I saw similar results.

Could the bottleneck be crappy BMSs inside Chins 250Ah self-heating batteries? I'm worried I'll order the 100A, hook things up, and see the same result.

As an aside, this may finally be a good enough reason to spend the $100 I've been waffling over for over a year and buy a AC/DC clamp meter. Any one in particular y'all recommend? And don't say Fluke (too expensive)...


P.S. - Something even weirder? The Bluetti was reading over 900w output when the converter was turned on (it was the only thing on except for vampire drain from my TV).

 

[sunshine_eggo]

jumper cable clamps are horrible conductors. You sure the cables aren't CCA? Copper Clad Aluminum? Common for jumper cables. Aluminum conductivity is only 60% that of copper.

Measuring 14.6V @ the charger terminals means the charger is in absorption even if the battery isn't there, so the charger will only feed the current needed to maintain 14.6V. As a test, dial the boost voltage all the way up. The battery will protect itself. You should see near peak current.

BMSs can't regulate/restrict current. They either allow it, or they cut it off.

A DC clamp ammeter is a must-have tool for anyone doing anything on this forum. Period. I have a CL800, and Will recommends another on his site.

 

[WearyWanderer]

Yeah, I mentioned that they were CCA. Sure would be nice if they used solid copper, but everyone is cheap these days. I even saw 15ft jumper cables at Kwik Trip earlier this week that were 10 AWG - my jaw dropped.

I will try running up the voltage later today and cross my fingers that the BMS does what it's supposed to do. I don't completely trust the BMSs in these Chins... A few months ago while charging, a cell inside one of my packs got up to 3.65v, and the BMS went into protect mode and shut off incoming current flow till the cell dropped back down to 3.5v. However, after it dropped to 3.5v and current started flowing again, it let the cell shoot all the way up to 3.75v before I manually cut it off. Shouldn't the BMS have prevented that?

I'll pick up a clamp meter this weekend. It may be a Southwire 21550T - the only good option available locally at Menards. Additionally, I did go and order the PM3-100LK - I'll get it working right one way or another.

 

[sunshine_eggo]

Yeah, I mentioned that they were CCA. Sure would be nice if they used solid copper, but everyone is cheap these days. I even saw 15ft jumper cables at Kwik Trip earlier this week that were 10 AWG - my jaw dropped.

Yes you did. I must have been more tired than I thought.

You need as close to 0V drop between the converter and the battery. It's that simple.

I will try running up the voltage later today

Please do

and cross my fingers that the BMS does what it's supposed to do. I don't completely trust the BMSs in these Chins... A few months ago while charging, a cell inside one of my packs got up to 3.65v, and the BMS went into protect mode and shut off incoming current flow till the cell dropped back down to 3.5v. However, after it dropped to 3.5v and current started flowing again, it let the cell shoot all the way up to 3.75v before I manually cut it off. Shouldn't the BMS have prevented that?

Not sure why you don't trust it, you observed it behave exactly as intended. 3.75V is a non-issue, and most "dumb" BMS protect anywhere between 3.65 and 3.85. >3.65V is not some boogeyman. LFP used to be charged to 4.20 before the industry figured out that significantly decreases cycle life and found you only gain a few % cap above 3.65 and stuck with that for increased cycle life.

If you've noticed these EG4, SOK and other server rack 48V batteries, they don't engage protection until 3.90V.

I'll pick up a clamp meter this weekend. It may be a Southwire 21550T - the only good option available locally at Menards.

Looks fine. If you find one slightly more expensive with an INRUSH feature, that's nice to have if you ever want to measure the surge associated with electric motor startup... which can be 5X and more than the run current.

Additionally, I did go and order the PM3-100LK - I'll get it working right one way or another.

I would confirm that you can get near rated output of the 75A before you commit to the 100A model.

 

[WearyWanderer]

It was less of a "I'm worried about 3.75v in particular" situation and more of a "why didn't protection re-engage when it hit 3.65v again?". Shouldn't it re-protect at 3.65v?

Great call on the inrush feature - that's something I'd actually use. I set my house (my real house that I'm never at it would seem) up with a power inlet and have run the whole place multiple times during multi-day power outages with a big inverter gen. Unfortunately, it was kind of a shot in the dark guessing what size gen I'd need - I could read the LRA rating on the sticker for my central AC but had no idea about the well pump (both worked in the end), so I ended up stepping up a size in generator probably unnecessarily because I had no idea of the start current that I needed.

I will test the 75A tonight, this time connecting the output wires of the PM4 to the bus bars.