Replacing OEM batteries with LiFePO4's

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
My OEM batteries are now toast, and I'm replacing them with SmartBattery SB100's - BCI27 size, 100 AH, 28# each. Installation will be simple since they're slightly smaller than the SRM-27's and have bolt terminals. Charging won't be quite so simple, and I need to replace the 55A charger section in my power center. About time - the power center is a Parallax 7355, which doesn't have a great charger.

I don't know of any drop-in replacement charger that'll charge LiFePO4's. At the moment I'm looking at Progressive Dynamics' 9100L series - they're straight 14.6v chargers designed for lithium batteries - voltage is adjustable, and in any event the batteries want 14.4-14.6. They make several sizes including 60A (input rated 1000W), 70A (1250W) and 80A (1300W). I'd like to install a 70A or 80A charger since the batteries can easily handle the higher charge rate, but the RV has a 30A service.
QUESTION: do I run into problems if I install a larger charger?

The big items are obvious:
Microwave - 1500W convection, 1150 normal (12.5, 9.6A)
AC - 1500W compressor rated load (12.5A)
Charger - 1000, 1250 or 1300W (8.3, 10.5 or 10.9A)


I'd never think of running both microwave and A/C simultaneously (even with the 60W charger I'd be over 30 amps if I ran all 3). So I'm inclined to install either the 70W or the 80W charger. Am I missing anything here? I think some of you have better electric design credentials than I do!

There are 2 drawbacks to the 80W charger - it's physically bigger and it requires a 20A breaker instead of my existing 15A breaker. Both of those issues seem manageable, but the easiest compromise seems to be the 70A charger.

One oddity I saw during my reading: The Dometic site lists specs for the 651815 Penquin II heat pump, and they say the minimum generator size is 3.5 kw. Interesting, given our 2.8 kw units...........
 
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nwboater

New member
Peter, switching to AGM batteries is smart. The battery charger has nothing to do with powering the microwave or anything else electrical in your couch. It only charges the battery. Since the Unity has a single 12v battery with a capacity of 220 Ah, and most charging occurs at lower amperage, a 4 stage regulated charger producing 50-75 amps is more than adequate. Check out the IOTA IQ4 Smart Charger (55A).
 

jackfish

Active member
Instead of looking at a new charger to replace the converter, why not get a Magnum inverter/charger which has a fully adjustable charging profile. Then you could use microwave sans shore power.
 
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Peter Tourin

2020 Unity RL, ex 2012 Unity MB
The catch is that I'm getting lithium ferric batteries, not AGM's. I can't use the Iota or similar chargers because the lith batteries want 14.4-14.6 volts until the reach a certain voltage, and that's it. So I have limited choices for chargers.

I guess I didn't make it clear - but lith batteries can be charged at high rates - a 200 AH battery bank will easily accept an 80A charger - and it would be nice to have the ability to plug into shore power and charge the batteries quickly quickly. So it's appealing to have a high-capacity charger, but I have to stay within the limits of my 30A service. That's what I'm trying to figure out. For example, suppose I install that Progressive Dynamics 80A charger. Let's say I've run my batteries pretty far down - I plug into shore power and the charger is working away and drawing 10.9A. Now the air conditioner comes on. I'm not sure how much it draws when the compressor first comes on, but I don't want to install a charger that causes my main breaker to trip every time my A/C fires up.... It's the A/C that confuses me - I don't know how they behave and how much short-term load they put on the system each time the compressor kicks in.

BTW - my MB has 2 batteries - the OEM batteries were Interstate flooded cell batteries that each were about 95 AH - so I had around 190 AH, and I always tried to keep them more than half charges, so I figured I had around 90 AH available capacity. The lithium batteries don't get damaged as much by deep discharge, so I can use more of their rated capacity without shortening their life much - I'm expecting I'll have about 160 AH capacity from the 200 AH lith battery bank, maybe even 180 AH capacity. How lith batteries behave is another story - I won't get into a rant about that here. But you can see why they're appealing:
1. My 200 AH battery bank gives me about double the useful capacity that a 200 AH standard or AGM battery bank would give me - twice as much boondocking before I have to recharge.
2. The lith batteries weigh 28 lbs each - my Interstates weighed 58 and FullRiver AGM's would weigh over 70 each. I want more cargo capacity, not heavier batteries.
3. The lith batteries life span should be MUCH longer than any standard or AGM battery. Their rated life cycles at 50% discharge would be anywhere from 250 to maybe 900 cycles. Lith batteries are rated 2000 cycles. But the definition of lifespan is different. For flooded cells, end of life is when the battery has deteriorated down to 50% capacity - that is, when a battery rated 100 AH only has 50 AH capacity left. For lith batteries, end of life is when the battery is down to 80% capacity. So if I try to calculate the lith battery lifespan the same way as they do for flooded cell batteries, they're around 4000 cycles - so to 5 to 16 times the lifespan.

BUT - you pay for those advantages. The batteries cost $1300 instead of $250-300 for a good quality AGM battery.

End of rant....
 

OrioN

2008 2500 170" EXT
The catch is that I'm getting lithium ferric batteries, not AGM's. I can't use the Iota or similar chargers because the lith batteries want 14.4-14.6 volts until the reach a certain voltage, and that's it. So I have limited choices for chargers.
Incorrect.

That is a maximum charge voltage. They will charge more than adequately with voltages below that and even down to 13.8V.





.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
That's confusing, as their spec sheet says "Charge voltage 14.4V - 14.6V" - no mention of maximum. If I charge with a lower charge voltage, will they charge to 100% but just slower? Or will they not fully charge? Also, I understood that they didn't want a float charge - they wanted the charger to simply stop charging when they reached a certain voltage. So I understood that I couldn't use a multi-mode charger like I would for a flooded cell battery bank. I'm trying to reach them now to ask exactly what they expect for good charging.
 

Custommm

Member
Peter ,Im VERY interested as well... It's also seem that LiFePO4 or those with Mg requires more careful monitoring of each cell during charge and a good temperature management to archive their expected lifespan (just like a smartphone)
I would love to swap my interstate batteries in my MB as well but it's seem that we are more or less in R&D.
Keep posting your finding!
 

Don Horner

2012 Unity IB
Peter, same advice I give to everyone when the subject of charger/converters and inverters is raised. Talk to Randy at bestconverter.com. Click here to see his page about Lithium Ion charger/converters...
 

smiller

2008 View J (2007 NCV3 3500)
Peter, same advice I give to everyone when the subject of charger/converters and inverters is raised. Talk to Randy at bestconverter.com. Click here to see his page about Lithium Ion charger/converters...
Interesting, from the brochure/manual it seems that their line of lithium battery chargers are basically just a fixed voltage supply and the local battery BMS takes care of the actual charging regimen. If I understand that correctly than any high quality fixed voltage charger should work, such as the Iota DLS series, etc.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
Don, I called Best Converter but didn't get Randy. I'm thinking of that 9100L line of chargers - the PD9160AL, PD9170AL or PD9180AL. They were a bit confusing - the man I talked to said that although they list the converters, they don't actually sell them there - he referred me somewhere else.

Smiller, that's also what I understand - that the charger is a fixed voltage supply that puts out 14.4 - 14.6 volts. All the Progressive Dynamics lith chargers default to 14.6 I don't yet fully understand how the BMS works. It sounds like when you're driving or connected to shore power, the BMS relay opens at full charge - then the batteries are out of the circuit and house 12 vdc is provided by the converter/charger. When you're not driving and not connected to shore power, the BMS relay is closed and you start drawing battery power. If you look at the battery curve, the voltage drop is so low that when the relay opens on 8V low voltage, you're down at very near what we'd call full discharge - around 95% depth of discharge. I'm not saying this is a good idea in daily use! But the battery manufacturers are saying that if you drag the batteries down that far, the BMS will open the relay to protect the battery from damage. And they're saying that going down to 80 or 90% DOD won't damage the battery chemistry as is the case with flooded cells. That extra usable capacity is one of the things that makes the lith batteries interesting.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
BTW - if I understand this correctly, it means that whether the batteries are being charged at 14.6v or the BMS disconnects them, the house is being run on 14.6v since the converter/charger is holding the house there. I assume that 12v equipment can operate on 14.6 with no ill effects, though I never thought about it before now. I hope that's the case...

And now I'm back to the original post - thinking about which charger. If the A/C uses 12.5A (and probably more when it starts up) and one of the Progressive Dynamics chargers uses 10.5 or 10.9A depending on the size, it appears that I'm still fine to run the A/C while the charger is at work. They'd take some wiring changes, and the 80A charger would require a BD2015 breaker to replace the existing BD1515 as that charger requires a 20A circuit, but I wouldn't be overloading the 30A service - unless I then use the microwave, a hair dryer or coffee maker <g>... It would just be a matter of paying proper attention, and you already have to do that in an RV.
 
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jackfish

Active member
Popping a fuse or two to find out your limits is not a bad thing.
 

smiller

2008 View J (2007 NCV3 3500)
When I mentioned to the BMS (battery management system) above I was referring to the module built into the SmartBattery that controls the charging regiment and protects against overcharge and undervoltage. The specs on their website specify 14.4 to 14.6 volts charging supply (at up to 100 amps charge current) so I would assume that any reasonable-quality regulated supply will suffice, in fact your Parallax may work -- it's single fixed voltage is a poor performer when charging lead-acid batteries but with the BMS resident in the battery doing the 'smart stuff' then a fixed voltage supply is OK, so you can adjust your Parallax to the proper voltage and I expect it would work fine. Something like a Progressive Dynamics or Iota would probably be better for severe duty but if all you need is a fixed voltage supply the the Parallax would probably work, although I'd agree that if you are going to invest in Lithium batteries then you might as well get something capable of a higher charge current.

14.6 volts shouldn't hurt anything on the DC buss, in fact any decent lead-acid charger is going to sit around that voltage while charging. Your vehicular electrical system runs at around 14.1 volts and I guess you'd have to ask Smartbattery whether that is adequate for charging while underway.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
Yep, that's how I understand it. SmartBattery happens to put the BMS inside. AM Solar's batteries have it external, as do several of the cell suppliers I've looked into. But I assume the basic BMS design intentions are similar.

I don't know if the Parallax charger voltage is adjustable - I can't find any technical info on it at all. It would be handy, at least for a start, if I could simply jack the voltage up and start driving <g>...

I've been assuming that the SmartBattery internal BMS has to sense an over-voltage in order to open, and I've assumed that this situation would be when the voltage rises to somewhere in the vicinity of 14.4 volts - if it's higher than that, the battery would let the charger remain connected and charging, which I don't think they want. I also assume that if I connected the Parallax charger as it stands (putting out 13.6 volts or so), that it would also stay on forever without the relay opening. But I don't know this - I don't understand the dynamic charging characteristics of standard and AGM batteries, much less these. I wish there were more technical articles around.
 

smiller

2008 View J (2007 NCV3 3500)
I don't know if the Parallax charger voltage is adjustable - I can't find any technical info on it at all. It would be handy, at least for a start, if I could simply jack the voltage up and start driving <g>...
It's not officially adjustable but it's pretty easy to do. If you remove the charging section (located below the circuit breaker/fuse section) there will be a pot at the rear of the circuit board, adjust that to the desired voltage and you're good to go.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
You mean slide it out so you can get to the pot, but don't disconnect it - run it and turn the pot while measuring the voltage? I wonder how high it can be set? That'd be worth doing, as I'm still not sure just what to replace it with. The PD 9100L series are 9" deep - they stick out the back and I'd have to relocate some wiring and an LP furnace duct. Also, it's not a hard-wired unit (it has a plug), so I'd have to wire in an electrical box for a receptacle. All manageable, but I keep wondering if I just haven't found a charger yet that'll work a bit easier...
 

Don Horner

2012 Unity IB
You mean slide it out so you can get to the pot, but don't disconnect it - run it and turn the pot while measuring the voltage? I wonder how high it can be set? That'd be worth doing, as I'm still not sure just what to replace it with. The PD 9100L series are 9" deep - they stick out the back and I'd have to relocate some wiring and an LP furnace duct. Also, it's not a hard-wired unit (it has a plug), so I'd have to wire in an electrical box for a receptacle. All manageable, but I keep wondering if I just haven't found a charger yet that'll work a bit easier...
My replacement converter charger was also set up for a plug; I just cut it off and hard wired it. No big deal.
 

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