Replacing OEM batteries with LiFePO4's

wade5979

New member
Peter thanks for your reply. I live in southwest Florida and will be able to do some long term testing once I get everything to the level you are at I'll try to take the next step and add the relays to fully disconnect the charger. We will be coming through Tampa Bay, FL on our way back from picking up our unity in Las Vegas and I plan to stop at Smart Battery and try my best to get eye to eye with an engineer to see if I can get a warm and fuzzy over leaving the charger on all time when plugged in to shore power.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
It might be very useful if you can stop by and talk with them. Communication is quite poor by phone. I talked with Victor in sales - some time ago, and then again after some months had gone by, when I actually ordered. I could usually get him by calling his cell, but I believe he's really sales and not in any way an engineer. When I've called and asked either for en engineer or a support engineer, I think I've gotten support guys who answer phones in addition to prepping batteries and fixing bad ones - they don't sound like engineers in the sense I mean it.

About the batteries - the biggest question to me is what's inside them. As I wrote - their website now says that they have "automatic internal cell balancing", but twice they've told me on the phone that there's nothing but over- and under-voltage protection and that's all. The other time, I asked them if they meant that they had top balancing circuitry, and they said yes, that there was top balancing circuitry at the bank level. I asked if that meant that they provided current to the weakest bank to bring it up into balance to as not to overcharge the other banks, and they said yes. It would very very nice to know what's actually in there.

About charging - I've been busy, but I've been thinking about how to build a charge control system. Interestingly enough, the StarkPower website implies that there's communication between the Progressive Dynamics chargers and their batteries and that they can shut off and reconnect - but when I called them, I got an engineer who was very familiar with the issues, and he said that there is no communication as such - the charger works as it's designed to work, and it's a simple CCCV charger. SmartBattery hasn't said anything like that, they've simply said that they recommend the PD chargers because they've tested them with their batteries. They have been consistent about the chargers - they've always said that leaving them constantly connected won't overcharge or hurt the batteries in any way.

Again, I do feel that the company is primarily a battery sales company - that's why I'm being so cautious about their claims.

My charging circuitry relies on the battery monitor that has the capability of watching return amps (charging amps) and that has an internal relay that I can program - then it's just a matter of how to do it. My Victron
monitor can do this - I don't know about other chargers. I'm not thinking of anything complex - I'd like the internal relay to activate an intermediary relay that can handle a bit more power (the internal relay is 60v max, 1 amp max on the contacts). I'd like a relay that'll engage when 120vac is present, so that when there's no 120, no relay draws power except the internal monitor relay. Finally, the intermediary relay will control a power relay that opens the AC feed to the charger. This is pretty simple stuff, and should be reliable - but I haven't done any tests yet with the internal monitor relay. It'll be a long winter, shouldn't be had to do the testing.

It would be nice if the system could also auto-start the generator when the battery gets low and there's no shore power - I'll think about that, but it's not a big requirement for me - I only want to cut charging at 100% SOC until there's more consensus about whether constant connection does cause battery problems or not. I also haven't thought much about what happens if you drive for 4 days, 10 hours a day - with the alternator output at 14.2 - 14.4v, I'm assuming that the battery will never get overcharged by the alternator, but I may be naive about how alternator charging works.

Anyway - happy to trade ideas and charge control wiring diagrams if you'd like - and I hope that you get a lot of useful info from the SmartBattery folks.
 

wade5979

New member
Read over the web site again and twice in print and once in the design video they claim internal cell balancing so I will ask them to "show me" before I leave there so maybe we can put that item to bed. Even if there is no danger to the batteries if the charger is left on continually I'd still feel better with a monitored circuit that I could turn off at 100% SOC.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
I've been talking more over on the Class B Forum about controlling charging. I picked up something very useful, and thought I'd post it here in case anybody is following the thread. The issue is that some people worry that you can damage lith phosphate batteries by continuing to leave the charger connected after they're fully charged. I've been trying to figure out how to turn the charger off automatically - up till now I've been simply throwing the circuit breaker that goes to the charger - and when I drive, I always have the alternator line connected to the batteries, even if LTV's DC switch is off.

It turns out That Blue Sea has a relay that can be closed by a 12v signal and opened when the signal goes away, but once the contacts close, the relay uses a negligible amount of current. It's their 7713, and it's a cool relay alright. I called them up and here's what I found out...

It's a remote battery switch controlled by a microprocessor module. When you ask it to close (by giving it 12v on its control line), it uses ~7A to latch the relay, then it drops down to less than 13Ma to keep the relay closed. So it's an almost-latching relay - it has some current draw when latched, but less than 10Ah/month, which is negligible. When you remove the 12V signal, the relay opens again. They have an LED indicator that's a good idea - in addition to lighting when the contacts are closed and going out when they're not, it will give a double-flash error signal for a contact closure error (where you expect it to be closed but it's not).

So if your battery monitor has an internal control relay like mine does (mine's a Victron; Blue Sea's monitor also has a control relay - I don't know about other brands), you can program the control relay to send 12v to the switch whan you want to charge and no 12v signal when you want it to open. You can put the relay down by the battery bank and feed both the line from the converter/charger and the line from the alternator through it. Then you can be either plugged in to shore power or driving, and if the batteries are charged the switch stays open - but if the batteries are below whatever low setpoint you program, the switch closes and the batteries charge until you get to your high setpoint - then the switch opens again. Very cool, sez me...
 

wade5979

New member
Peter, Which class b site have you been posting to. My Unity pickup was delayed till mid December so have gotten to Smart Battery yet.
 

turbopilot

New member
Been following this thread with the intention of replacing my lead acid batteries with LiFePO4's. LiFePO4's sure look like the right solution at 44% of the weight of lead acid. However as I started to do some deeper analysis of the conversion costs I discovered it is not a slam dunk to make the conversion.

We are cautioned not to deplete the lead acid battery below 50% capacity to preserve life, however this is not a bright line, just a compromise balancing battery cycle life against discharge. One source rates LiFePO4 life at 3,000 cycles with discharging the battery to 20% capacity. Discharging a lead acid battery 20% remaining will result in a life time of around 675 cycles.

My 2016 Unity MB comes with two 6v 232ah lead acid batteries. Discharging those batteries to 20% capacity would provide 186ah of power on each cycle, costing around $.47 per cycle. Drop in LiFePO4 batteries (2) would provide 200ah, yielding a useable power of 160ah discharging them to 20% capacity costing around $.87 per cycle.

So putting aside the significant weight advantage of the LiFePO4 batteries, it would appear to be very cost effective to simple routinely deplete the deep cycle lead acid battery to 20% capacity since the cost per cycle with such a deep cycle is so low in comparison to LiFePO4. The lead acid batteries would have to be replaced much more often but they would provide 26ah more with each cycle than the SB100 LiFeP04's that fit in the same place depleting them down to 20% versus to the traditional 50% for lead acid batteries.

If the goal is to have as many amp hours available in the Unity on each cycle of the battery, then it appears to be a much better deal to just press the lead acid batteries harder and replace them more often. From a cost per cycle perspective you would be saving much more money than buying the LiFePO4 and have 26ah more per cycle. What am I missing?
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
You're right on! I've thought about that quite a bit. There is an argument for buying the cheapest decent wet batteries, taking them way down and replacing them often - you're absolutely correct that it's simply a cost vs. usable capacity issue. Another thing you can add to your argument is that 3000 cycles is a lot of cycles - unless you keep your RV a very long time and use it a lot, you may never get to the end of life for lith phosphate batteries.

There are certainly other issues to consider. Weight is a big one for me, because I have an MB - by the time the two of us climb in with our belongings and a few bicycles, there's very little cargo capacity left - an extra 100 lbs is extremely desirable.

Another issue is voltage drop as the % SOC drops. I'm looking at charts for one of the Full River deep cycle 6v batteries - that must be similar to what you have. The voltage vs. SOC chart shows ~12.8 at 100% and ~11.75 at 20%. That's usable but getting pretty far down there. I'd expect my batteries to be ~13.4 at 100% and ~12.8 at 20%. So the lith batteries have less voltage drop and higher voltage, both of which are advantageous. Less current to run a device that requires a given wattage, if the voltage is higher. These numbers are rest voltage, not voltage under load.
Full River: http://www.fullriverbattery.com/product/batteries/DC224-6

There's a further issue, though I'm not taking advantage of it. Lith batteries can be both discharged and recharged at high rates without hurting the chemistry. They'd be good batteries for an RV with a 2nd alternator dedicated to battery charging. There's been quite a bit of discussion of that on this and other forums as an alternative to solar. If I can install a 2nd alternator dedicated to battery recharging and capable of 160A charging rate, I can fully recharge in an hour, and the price is similar to what I was seeing for a 400W solar installation. And I don't have to park in the sun with no A/C on <g>...

Added to that, lith recharging is simpler than flooded cell recharge - no multi-mode chargers that bulk charge fast and then take a long time to get the battery fully charged. You use a CCCV constant-current constant-voltage charger that simply puts out its rated current - and the voltage climbs as the batteries get near full charge, until it's up to its rated voltage, 14.6 in the case of my charger. Then the charger remains at 14.6 and as the battery approaches that voltage the charger current drops to near zero. At that point I'm fully charged - if I remove the batteries from both charge and load and let them sit for a day, they'll drop to 13.4, their nominal full charge voltage. There's a possible issue concerning whether lith batteries can be damaged if you leave the charger connected once they're at 100% SOC, but to complex to go into here, and there's no consensus at present on it.

The last issue that comes to mind is shelf discharge. Much of the discussion on solar has been about small solar installations that'll keep the batteries topped up when the RV sits for a month or so. There's less need for this with lith batteries - they don't self discharge very fast, and they don't care (in terms of their chemistry) whether they're sitting at 80% or 30% SOC. So I've just parked my RV in a storage facility for the winter (in VT - long winter!) and I'll probably get it out early in April. I'm not trickle charging, and I'm expecting to find the SOC still pretty far up there next Spring.

The biggest concern for me at present is temperature. Lith batteries can be discharged below freezing, but they don't like to be charged below freezing (battery temp, not outside ambient temp). That Full River page says their battery 100% of rated capacity at 80 degrees F and 76% at 20 degrees. My lith batteries are 100% at 80 degrees and ~82% at 20 degrees - a bit of an improvement. But I can damage the batteries if I charge when they're below freezing, so I need to either remember not to let them charge or provide dircuitry to do this - working on that issue right now! It's easy for the charger - I can simply turn it off by throwing its circuit breaker. But the alternator is wired directly to the batteries - turning off the DC switch doesn't prevent the batteries from being charged by the alternator. So there's some rethinking and rewiring needed there.

Those are most of the issues that come immediately to mind.

Later - another thought. When I read your post, I thought that your life cycle figures were optimistic for both flooded cell and lithium - I'd have to go back to my spreadsheets, but my financial calculations were based on 600-700 cycles at 50% discharge for wet cell and 2000 cycles for lith phosphate. I think the discussions about lith batteries have made the manufacturers and sellers more aware of this issue, and I noticed today that the life cycle figures for both types are more optimistic than they were a few years ago when I started pushing a pencil and comparing.
 
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Peter Tourin

2020 Unity RL, ex 2012 Unity MB
Wade - if you end up going this route, check with me about charging control - I've been thinking about it quite a bit and am going to install charger control circuitry as soon as the RV is out of storage at the end of the winter. It's been an interesting challenge. I started out with a fairly complex and expensive circuit, but it now appears pretty simple and not too expensive. Also not too complex in terms of the actual physical wiring changes. Biggest issue: the alternator lead runs directly to the battery, not like the charger lead - if you open the DC disconnect switch, the battery is isolated from loads and from the charger, but not from the alternator.

Also, I'm looking into temp protection, since the lith batteries shouldn't be charged when the battery temp is below freezing. Haven't gotten very far on this yet, but it shouldn't be too hard.

For accomplishing this, I'm relying on my Victron battery monitor. It's been very good - physically small, easy to install, and it has a programmable internal control relay that can be set to respond to voltage, SOC or a combination of both, with time delays on make and break. Very handy.
 

turbopilot

New member
Later - another thought. When I read your post, I thought that your life cycle figures were optimistic for both flooded cell and lithium - I'd have to go back to my spreadsheets, but my financial calculations were based on 600-700 cycles at 50% discharge for wet cell and 2000 cycles for lith phosphate. I think the discussions about lith batteries have made the manufacturers and sellers more aware of this issue, and I noticed today that the life cycle figures for both types are more optimistic than they were a few years ago when I started pushing a pencil and comparing.
Agree. Found a lot of scatter on cycle depth to life on the internet for both battery types but generally the difference between the two was about the same.

Bottom line run the deep cycle lead acid hard and replace often and you should equal or exceed the amps available from LiFePO4's while saving a lot of money. Obviously as the price of LiFePO4's decline all of this flips.

I wonder how the LiFePO4's would perform in a mixed battery environment? Thinking about maybe adding one LiFePO4 to the adjacent storage compartment and running it in parallel with the two 6volt lead acid batteries. Running the numbers it would appear the amps available would then make it feasible to pull the 3 source refrigerator and replace it with a 12 volt compressor refrigerator.
 
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avanti

2014 GWV Legend 3500 I4
Bottom line run the deep cycle lead acid hard and replace often and you should equal or exceed the amps available from LiFePO4's while saving a lot of money. Obviously as the price of LiFePO4's decline all of this flips.
Exactly. It is so important to realize that the "don't discharge below 50%" thing, while good advice, is simply an arbitrary rule of thumb, not a hard-and-fast rule.

When I did my major electrical upgrade last year, I though hard about going to Li. I decided that it made much more sense to commit to AGM for one more battery-life cycle. I am guessing that when it is replacement time, things will be different. But, that is a decision for the future.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
I must say that you guys are being smart about this. I like sorting out new technology, and in the process of doing this LiFePO4 project I've spent a lot of money on batteries and a lot of hours figuring out how to deal with them. I'm not trying to be a lithium phosphate advocate to the exclusion of all other battery types - just putting the info out there <g>...

It would be very interesting to pull out my capacity/lifetime/cost spreadsheet again, update it with what I know now, and see how it looks. At the time, I was looking at around 600 cycles @ 50% DOC vs 2000 cycles @ 80% DOD.By now it'd look different - and even then I was aware that those 2000 cycles were going to be more than I'd use unless I live a really long time...
 

Dizzee2020

2021 Unity 24MB
Peter. If I remember correctly Advanced RV is using a heating pad/blanket around its installations. I think they mount the batts on the underside of the Sprinter Chassis just behind the rear axle in a proprietary mounting rack. I have seen a video of the installation but it has been several months ago. If interested, try their youtube channel.
 

wade5979

New member
Will do Peter, Our delivery was delayed one month so we are picking up unit Dec 13th been following your posts all month I think you just about have it down. I was also think about battery temp and saw some battery blankets on the net. Also if I use the Blue Seas Relay I might also get their battery monitor
 

Don Horner

2012 Unity IB
Interesting. I've always abided by the 50% discharge point. Thinking about it, we would rarely discharge to 20% even if we adopted a policy of "pushing it". Most of our travels are point-to-point, on the road again the next day, and have never camped in one place for moire than 4 days. That means that we rarely reach even 50% discharged. I'm making an educated guess that our opportunities for a discharge to 20% would not exceed 20 cycles a year, at the extreme.

So, even if pushing to 20% would decrease our total cycles to 300, or less than half of what has been mentioned, that would still provide a theoretical battery life of 15 years! Obviously, lead-acid batteries don't last that long under any circumstances, but it leads me to believe that in reality I might have to replace every 4 years instead of 5, as an example, or every 5-1/2 out of 7 (as a guess).

Unless there's an enormous flaw in my reasoning, I'm not going to worry about deep discharge as much as I have previously.

I still plan to replace my standard Interstate 12V marine/deep cycle batteries (standard in 2012 models) with true deep cycle batteries next year, in order to achieve more amp hours,, but I may stick with lead acid instead of AGM.
 

turbopilot

New member
Unless there's an enormous flaw in my reasoning, I'm not going to worry about deep discharge as much as I have previously.
I think you are seeing it right. I have a golf cart with the same Interstate batteries as my 2016 Unity. I deep cycle these batteries all the time and they just keep going. I am guessing you can plan on two years of reliable use without worrying at all about how far you discharge these batteries. Just recycle every two years for around $380 dollars.

 

Boxster1971

2012 Sprinter 3500 Ext
Exactly. It is so important to realize that the "don't discharge below 50%" thing, while good advice, is simply an arbitrary rule of thumb, not a hard-and-fast rule.

When I did my major electrical upgrade last year, I though hard about going to Li. I decided that it made much more sense to commit to AGM for one more battery-life cycle. I am guessing that when it is replacement time, things will be different. But, that is a decision for the future.
I also did a lot of research on going LiFePO4 for my battery upgrade. The temp issue was a killer for me as I use my van year round as a second vehicle. With 400 watts of solar on the roof and parked outside it is always charging in daylight. I have stayed with Lifeline AGM since I could mount them on there side under my van and not give up any internal storage space to get 440 AHs.

The basic charging of LiFePO4's may be simpler, but the need for a reasonably complex Battery Monitoring System to protect batteries offsets any charging simplicity. I do think lithium batteries of some form will someday dominate in the smaller RVs (class B and C). But it will require a whole system approach to be as flexible as lead-acid batteries.
 

jostalli

Member
My father used to sell fork trucks. He'd never heard of DOD % as they just run them completely out before charging.

By the way, I have my 320ah LiFePO4 battery performing great with just my solar charge controller. No BMS, no temp control. You don't need expensive battery management for LiFePO4 batteries if you keep the charge and discharge within battery specs.

I am, however going to wire up my HousePower mini BMS for security purposes.
 

Peter Tourin

2020 Unity RL, ex 2012 Unity MB
>The basic charging of LiFePO4's may be simpler, but the need for a reasonably complex Battery Monitoring System to protect batteries offsets any charging simplicity. I do think lithium batteries of some form will someday dominate in the smaller RVs (class B and C). But it will require a whole system approach to be as flexible as lead-acid batteries.<
Couldn't agree more!! I also agree that the temp issue is a hard one - so what if I can discharge them under freezing, if I can't recharge them! Looking into that now. But OTOH - my batteries are inside, under the steps - I'd expect that a bit more insulation outside and a heat pad underneath for emergencies would fill the bill - we don't camp when it's way below freezing.

TurboPilot - that $380 every 2 years is pretty appealing, compared to $2600 every whenever <g>... I'm also assuming that the cost will go down with time - and in fact, it'd already be a lot lower if I built up cells like what AM Solar is selling, instead of buying drop-ins.

So I'm talking against my own system here, but it IS appealing to use batteries you can get cheaply anywhere, use them as hard as you choose, and just replace them when they die. I'm a techie type and I don't personally like that route, but as long as you understand what you're doing, it can be a very cheap route to go. And I think it's what many RV'ers do, though by ignorance or inattention <g>...
 

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