DC to DC Charging Questions

[UTTransplant]

Kevin, this is from the Kevin side of UTTransplan. Yup, we don’t worry about alternator charging either. With the solar on the roof, we aren’t dependent on the alternator for charging on the road. Oh, and he just checked the manufacturer’s recommendation for our LifeBlue batteries; they still say 13.8-14.8 charging voltage.

(And WOW, have the LifeBlue batteries gone up in price!!! It would be a big ouch if we chose them today).

 

[Lou Schneider]

A lot the this previous post is wrong, or incomplete, at least when it comes to LiFePo4 Lithium, but may apply to other currently less common forms of Lithium batteries.

First most LiFePo4 manufacturers suggest charging at no higher than 14.2 -14.4VDC, with more and more suggesting 14.2VDC max charging voltage to extend service life. This actually puts them VERY close to the bulk charging voltage rate of Lead Acid.

Also unlike Lead Acid, LiFePo4 will eventually stabilize at very near the charging voltage, though it may take some time, in other words with Lead acid you can float them at a 13.8VDC charge rate, but the batteries themselves will never get over about 12.8VDC once the surface charge dissipates a few minutes after the charger is disconnected. By contrast if you float LiFePo4 at 13.8VDC you will end up with a charge level of 13.8VDC or close enough not to matter (ie 13.75...). In addition to the LiFePo4 has a sharp knee on the charging curve where it shoots up from about 13.4VDC at 98-99% state of charge to 14.2-14.4VDC at 100% state of charge

In other words, the manufacturers aren't wrong when they claim LiFePO4 batteries are drop-in replacements for an equivalent quantity of lead acid batteries. Yes, there are tweaks you can do to hot-rod their performance but for the most part they will act acceptably and provide several advantages over lead acid if you just plug and play.

 

[garyb1st]

What determines how long the max energy flowing into the batteries will last. Based the battery monitor my two 105 aH batteries, each capable of accepting 100 aHs, have never shown more than 40 to 50 amps going in. Then it's only a short time before dropping down to around 20aHs. This is pretty much what I see, even when the batteries are more than 50% depleted.

Does anyone ever watch their battery monitor for more than a few minutes at a time?

 

[quaddriver]

A lot the this previous post is wrong, or incomplete, at least when it comes to LiFePo4 Lithium, but may apply to other currently less common forms of Lithium batteries.

First most LiFePo4 manufacturers suggest charging at no higher than 14.2 -14.4VDC, with more and more suggesting 14.2VDC max charging voltage to extend service life. This actually puts them VERY close to the bulk charging voltage rate of Lead Acid.

Also unlike Lead Acid, LiFePo4 will eventually stabilize at very near the charging voltage, though it may take some time, in other words with Lead acid you can float them at a 13.8VDC charge rate, but the batteries themselves will never get over about 12.8VDC once the surface charge dissipates a few minutes after the charger is disconnected. By contrast if you float LiFePo4 at 13.8VDC you will end up with a charge level of 13.8VDC or close enough not to matter (ie 13.75...). In addition to the LiFePo4 has a sharp knee on the charging curve where it shoots up from about 13.4VDC at 98-99% state of charge to 14.2-14.4VDC at 100% state of charge

no no no pay attention to what I said.
(different than me starting with no! no! no! in which extreme danger is present)

First, consult the spec of what you are charging! our chargers, the ones found in our rvs, unless they SPECIFICALLY have a lithium charge mode (some do today) will NOT adequately and or fully charge a lithium battery. Since they are designed to be plugged in essentially forever, or a camping season, the new smarter ones float down to 13.4 (or even quiesce) and bulk at 13.8. I think you are confusing the fully charged steady state with the charging state - your charging voltage must always be higher (duh, or no current flows) but consider on the rapid bulk charge state of a DC which may rise UP to 14.8v in some 'not smart' cases, the time spent here is measured in minutes as a short time rating because as we all know, above 13.8 volts a secondary reaction in the battery chemistry becomes primary and the water is split into base elements (i.e. hydrogen offgassing)

Today,, they are building the charging part of the converters for lithium, if most of us have rvs newer than 10 years, most of us are using 8955 (or compatible converters) or paralex 7155s (or compatible). There are drop in replacements for the charge board in non-compliant equipment - ymmv depending on the maker - lots of imported clones. I do a lot of these conversions for people and while not rocket science, the work needs checked.

All of my newer standalone chargers now have lithium modes - tho mostly manually triggered. if you use a simple set of probes, you will see the time, current/ voltage program graphs are different. Im not saying you reach a dangerous runaway condition at all no, but it will never reach 90%-100% SOC and a) you wont get what you paid for and b) life will be reduced. What cost cost right?

and, fwiw, a lead acid battery, fully 100% charged, OC, high impedance DMM or FET meter, will read 13.2v - it HAS to. God said so. The internal condition of the battery (read: resistance, and nothing else) is what causes the shown battery voltage to drop. A new battery with no cycles ever below 70% SOC will read 12.9ish. If you drop to 12.7v your reserve capacity is prolly around 50% of new. It seems like a little number, but in a battery its the world. For park queens, this is ok. for boondockers...wellllll....

If someone likes to play rv tech in this area, I HIGHLY recommend getting a velleman (or quivalent) O-scope for pc /laptop - USB run and putting it on graph mode to record your charge program. the somewhat smart charge controllers we have have simple circuitry that periodically vary the voltage and use a simple rise/time equation to make an educated guess on battery condition, tempered by the selected charge program - and nothing more. its also how they determine dead cells.

the short is: because a 'smart' charger has multiple steps to charge a flooded battery - the DC batteries in our coach are these - it spents as LITTLE time as possible in bulk over-voltage mode. It a fixed time period in just about all of the non-lithium nameplate converter/chargers.

I asnwered the OP because his 2011 is nearly identical in chassis/coach setup as our 93 was and we did the exact same thing. Except for the lithium part, the old 7600 series was NEVER gonna charge lithium, nor would the year version of the dual bank NOCOs

 

[Isaac-1]

I will admit it depends on the exact model of converter, setup, etc. but my nearly 4 years experience charging my pair of 210AH LiFePo4 using a standard Progressive Dynamics 9260 converter (installed in 2015) says differently.

I don't have time at the moment to go into a detailed reply, but here is the short version:

The 9260 will bulk mode output 14.4VDC for 4 hours then drop to 13.6VDC for the next 28 hours, and finally if no activity is noted it will drop to 13.2VDC storage mode. Though using the optional charge wizard pendant I can manually bump it back into 14.4VDC bulk mode.

13.2VDC is actually a great long term storage voltage for LiFePo4, as it is around 60-70% SOC, this will extend their service life vs storing at near 100% SOC

60 amps for 4 hours is 240 amp-hours of charge going into the batteries before it drops down to 13.6V float mode, which on a typical day fully charges the batteries, if I am there and in a hurry I can bump the button on the pendant and get another 4 hours at 14.4VDC which will fully charge my 420AH battery bank, and if I am not there at 13.6VDC they will then get to better than 95+% SOC within perhaps 8-12 hours.

I am not concerned about top balancing as I also have solar panels, and even just 100-200 watts of solar is more than enough to finish up that last couple of percent of charge and provide top balancing.

p.s when it comes to instrumentation, I prefer to stick with my Fluke handheld meters and my Tektronix bench O-scopes. Note also while you describe the chemical reaction for charging Lead Acid, it should be noted that LiFePo4 stores electricity as Ions not as Chemical reactions so the mechanisms are different.

 

[CharlesinGA]

Hi Kev. Hmmm... I ordered the Victron Orion TR Smart charger and it says it has algorithms for charging lead-acid AND Lithium batteries, with six pre-programmed Lithium settings or user defined settings, so maybe this version is newer. After all this, I certainly hope it'll charge lithium batteries. If it doesn't - honestly - at this point I'll just let the solar finish it off.

Question - when you installed yours, where did you connect the Victron's output? Was it directly to the House battery terminals, or somewhere in between the Chassis and House batteries? Say hi to Pam

Kev

Just interrupt the lead from the BIM to the house batteries, and put the bat to bat charger in line. It will receive power from the vehicles charging system, and send it to the house batteries.

 

[Kevin Means]

Thanks everyone for your help - I really appreciate it. I have not yet installed the DC to DC charger, but I'll get to it soon. (Thanks CharlesinGA) The lithiums are a nice upgrade. I tested their charging characteristics on 50 amp service, generator power and solar power. What a difference over lead-acid! The lithium's charge a lot faster, even though their AH capacity is larger.

I ran a set of wires from the BIM's compartment to the driver's side panel, so I could manually connect, or disconnect the BIM's ground wire. That will prevent alternator-charging while driving, which should reduce a lot of stress on the alternator.

An interesting thing happened after I wrapped the project up, and I'm not sure I understand it. Maybe someone here can explain it to me. After testing the battery's charging characteristics, I parked the RV in the garage and plugged it in to 50 amp service. A short time later, the battery monitor indicated that the batteries were at 100% SOC, and the charger (a Magnum MS 2812) went into Float mode. Over the next week, however, I noticed that there was a steady discharge of .8 amps until the SOC reached 90%. I was just about to type for help when the charger started charging with a steady 10 amps. Huh?

Before installing the lithium batteries, I had fully charged them and balanced their cells per the manufacturer's guidelines, but I'm starting to think that they really didn't get balanced. In addition to replacing the battery bank, I also had to replace the older Magnum ME-MR-L remote with a newer ME-RC remote, cuz the older remote didn't have lithium settings, nor did it have customizable settings. The ME-RC has those things. I've double-checked the settings and everything looks fine.

I leave the inverter on when the RV is in the garage, and the residential fridge is also left on. As the fridge's compressor cycles on and off, combined with normal parasitic loads, I've always seen an amp draw of about 7 to 12 amps, which the charger easily overcomes. However the charger never let the lead-acid batteries discharge when the RV was plugged into shore power. It always kept them at 100% SOC.

For some reason, the charger allowed the lithium batteries to sloooowly discharge to 90% SOC, then started charging them with 10 amps. Now they've been at 100% SOC for a few days, and they seem to be staying there. Could that have been caused by the system balancing the battery's cells? Any other ideas as to what might have caused that slow discharge, then recharge? Thanks

BTW Gary... when I was testing the battery's charging characteristics, our battery monitor indicated a steady charge rate of 120 amps per hour while on shore power, or generator power. That's the maximum output of the Magnum MS 2812. On solar power, our 1300 watt array was putting 60 to 65 amps per hour into the batteries on a sunny day at about 1:00 PM. I don't know what converter/charger you have, but the charger might be limited to a maximum of 40 amps.

Kev

 

[garyb1st]

Kev, if your coach has a battery disconnect for the chassis battery turn it off and see if that stops or slows the discharge. Our Pace will do the same thing that you're experiencing if I don't disconnect both the house and chassis batteries.

With both the house and chassis batteries turned off, my battery monitor shows a discharge rate of about .04 aHs. With just the house batteries turned on the discharge is about .8 aHs. If I forget and leave both on, it will discharge my lithiums at a rate of 2+ aHs or more and my lithiums will be down by 50% in a few days.