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Old 06-04-2016, 07:16 PM   #1
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My Interstate Lithium Battery Adventures

This is long, if you need a TL;DR, this thread isn't for you. In short, I don't like most solar/lithium combos on the market today and have chosen what I hope is a superior solution even if there is little to no market support. Hi, I'm LB_3 and I'm a recovering engineer.

I'm still a couple months away from doing any real work rewiring our Interstate and converting to lithium batteries but as we've discussed ad nauseum (here, here, here, & here, etc...), the Interstate wasn't born as a great boondocking platform. The AI was built with the assumption that people will want to stay in campgrounds/RV parks with electrical hookups or be able to run the generator when hookups aren't available. This may work for most occasions but generators are noisy and the propane they use is a finite resource which merely extends the duration between when we must plug in.

To solve this problem, we need more solar and more battery capacity. Boxter1971 did a great job addressing the need for more batteries by slinging several additional AGMs under his chassis. Unfortunately, the T1N AIs are already near their weight limit so adding 300 lbs of lead and steel under our vehicle is problematic, particularly when InterBlog already has me looking to adding another 200#s to our vehicle to support a custom hitch mounted swing arm and basket.

So this leads to the discussion about Lithium batteries. They're lighter and smaller than lead acid batteries and the Lithium polymer chemistries are much safer than the Lithium ion chemistries that made exploding laptops famous. Yes, they're expensive but when reviewing the lifecycle costs, they are more than cost competitive, they're actually cheaper.


We're not on the vanguard of this technology the way Technomadia was 5 years ago, however it is still a developing market and while it is no longer bleading edge technology, it's still in its infancy. Lithium batteries differ from lead acid batteries in some fundamental ways. The most critical of these is that not only must each cell not be drained too low (like lead acid batteries), they must also not be overcharged or cell damage will occur. To ensure this doesn't happen, a sophisticated battery management system is used to monitor the cells when charging and discharging. At first this may sound daunting but it really it isn't any more complicated or expensive than the sophisticated 3 and 4 stage charging systems used with lead acid systems.

Before I jump into BMSs I want to point out that lithium batteries don't need a bulk, absorption, float, and equalization charge modes. Lithiums will take a constant current up to .5 C (half the AH capacity of your battery so for a 300 AH battery they can charge at 150A) until full. This beats the super slow trickle charging required to top off lead acid batteries which wastes the power available from our solar panels, generators, and alternators. This means that independent solar charge controllers are not needed when used with lithium batteries.

MPPT solar chargers had their day when solar panels were crazy expensive and it was cheaper to add a magic circuit that could increase current than it was to buy more solar panels. Today however, (and this is still controversial) MPPT charge controllers are effectively obsolete in most off grid applications. For grid tied systems that added current is money in your pocket so the cost is justified.

In lead acid applications the MPPT circuit is only active during the bulk charging stage. The rest of the time, a pulse width modulated circuit throttles the MPPT solar charge controller's output to maintain the voltage prescribed in the other phases of the charging algorithm. MPPT charging circuits also require electrolytic capacitors which limit their effective life to approximately 10 years depending on use and thermal environment which depending on your intended application, could be a recurring expense with little to no gain. Lithium battery charging rates without MPPT are much quicker than lead acid charging rates with MPPT.

A BMS is a device that turns off the charging source when any battery cell is full and turns off the battery loads when any cell charge is too low. To get maximum capacity out of a battery, it is desirable to make sure each cell is at the same state of charge to ensure that the battery isn't turned off prematurly. In a lead acid battery, this is achieved by slightly overcharging the battery and 'boiling' the cell with the highest voltage until the other cells catch up. Since lithium batteries don't play nice when over or under charged, cell balancing is performed by a circuit in the BMS that measures the voltage of each cell and slowly drains current from the cell with the highest voltage. Or sometimes this is accomplished with external "sense boards" mounted on each battery cell.

The use of inverter/chargers like the Magnum chargers that come equipped on new AIs complicates things since they were designed for use with lead acid batteries. The inverter/charger uses the same battery cable to feed the inverter loads as is does to push current back into the battery when charging. This single power cable for both charging and discharging makes installing a relay to prevent over discharging also turned off the charging option. Early battery management schemes used a push button to manually close the relay when you plugged in to shore power or turned on your generator.

This drives me to selecting a stand alone inverter and possibly a stand alone charger. I say possibly, because with added battery capacity, solar power, a generator, and alternator power I'm not really sure it is really needed given the more efficient 0.5 C charging rates of lithium batteries. Even when stored, lithium self discharge rates are an order of magnitude lower than lead acid batteries, and since I will drive a stake through the heart of all the stupid AI vampire loads when I do my install, I'm not convinced I will ever need to plug in to charge again. Besides, it's easier to tell the IRS that 100% of my charging comes from the solar when I don't have an AC charger connected. (2016 is the final year of the 30% solar tax rebate and it applies to the entire system including the inverter. For batteries, the tax rebate is prorated based on the percent of power derived from solar.)

So, finally we get to the fun part. I have chosen the following BMS, the SBMS-100:

This BMS was designed by Dacian Todea . Not only is he a brilliant EE, he also lives off-grid and probably understands solar off grid applications, electronics, and economics better than anyone. He launched this BMS on KickStarter and built the units in his solar powered home using only a 200AH battery bank. I recommend reading through his Google+ updates to see how progress was delayed when they had a spell of cloudy weather in Canada last winter.


Here is a manual for his BMS. Go ahead an read it even if you aren't interestedin his BMS or if he is sold out of his initial (only?) production batch. He has some good info on sourcing flexible high temp wire and other things in his manual that could be useful.

Dacian built this device for his own needs so there are some things I would prefer be different. In the mechanical room of his home it doesn't matter if he has exposed wires and conduit, but in an RV I would prefer all the connections be on the back of the device instead of on the front and sides. Additionally, the display isn't very large so the percent charge is hard to read from a distance.

Oh, did I mention the unit has Wi-Fi? I guess that solves that second issue. Now you can read your battery from up to 300 feet away.


Dacian was really only trying to build a better BMS that met his own needs so he has shared all his intellectual property with us by making this project open source.


Here is a link to his source code: lectrodacus.com/SBMS100/SW/SBMS100-v01a.zip

All his cad drawings are open source as well (but I can't find the link right now). I'm not an EE so only the mounting hole placements make any sense to me :-) Hopefully, Dacian builds a bit of a community around this product and add on components get built to support this.

I'll post more about my prefered inverter and battery choices and the specifics of how I plan to install the BMS in the next couple days.
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Old 06-04-2016, 10:27 PM   #2
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Thanks LB_3. I like your approach and can certainly understand going with lithium batteries. Looking forward to your future posts.


- - Mike
2013 Lounge EXT on 2012 Sprinter
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Old 06-05-2016, 09:19 AM   #3
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I'm doing the same research. To tide me over, I've essentially copied Boxter's approach and this will get a test run on a 21 day trip with substantial Boondocking later this month. It is clear to me if the Lithium below freezing issue can get tackled the whole game changes. I'll be following closely. One twist I'm looking at is to ditch the generator all together and use that space/weight for more AH capacity.
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Old 06-05-2016, 10:58 AM   #4
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It is clear to me if the Lithium below freezing issue can get tackled the whole game changes. I'll be following closely. One twist I'm looking at is to ditch the generator all together and use that space/weight for more AH capacity.
Go back and read Interblog's blog on our waste water tank: http://interstateblog.blogspot.com/2014/10/interstate-leaking-gray-water-part-2.html?m=1

She has some good pictures of the heat shield around our grey water tank. It is only minimally insulated as you can see in the pictures but that's really only to isolate the silicone heating pad from the metal shield. That is the approach I would take with external lithium batteries. The BMS I'm using has pin outs for a temperature probe which will prevent charging if the batteries are below a certain temperature.

They sell 12"x30" silicone heat pads that can consume up to 1400 watts. If that doesn't warm up your batteries, your diesel will be too thick to flow and you probably already died in you sleep. You'll need to use a PWM motor controller or similar device to throttle that much juice to the heater since you can't run a full 100A while charging unless you add a second alternator.
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Old 06-05-2016, 11:09 AM   #5
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You could also run a coolant line to the rear and install a transmission cooler or other heat exchanger in the battery enclosure. I'm not sure if you could find an automotive thermostat much less than 160 degrees but I've seen other wax based thermostats used on spacecraft with much lower set points so I know they exist and probably aren't that expensive.
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Old 06-06-2016, 07:43 AM   #6
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Speaking as someone who has not materially participated in the lithium research, my first reaction to the warming predicament was, "Is there no way to adapt a combustion heating source?" When I get to the point of having a better conceptual grasp of the energy balance and flow, it's entirely possible that I will strike my own question. But right now, any resistive heating proposal causes me to visualize a snake eating its own tail. It would take only a tiny amount of propane to keep those batteries warmed, but I don't know if any device has been invented that could be adapted safely to that kind of an application.
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Old 06-06-2016, 08:14 AM   #7
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"Is there no way to adapt a combustion heating source?"

An in-line, propane fired instant water heater. Then wrap the tubing around the battery cases like heat tracing.

I was under the rug yesterday and found the silicon adhesive heater mats on the Gray tank. They seem straight forward and easy to install, but again it would be like 2 steps forward and 1.5 back on charging and running heaters.
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Old 06-06-2016, 08:51 AM   #8
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An in-line, propane fired instant water heater. Then wrap the tubing around the battery cases like heat tracing.
....
Yup. I have a qualitative "feel" for how much propane we burn across a variety of usage and travel scenarios (very important because the gauge has never read accurately, either with the old tank or the new one). Even with our T1N's smaller 6-gallon propane tank, a device such as you describe should not have a big impact on propane consumption. In other words, my guess is that it would not insert a new rate-limiting factor into any boondocking equation.
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Old 06-06-2016, 08:52 AM   #9
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If you want to use the water heater, 2 small DC hydronic hot water pumps and a heat exchanger should work.

You don't want water flowing in the external loop since it could freeze which is why the heat exchanger and extra pump are necessary.

Here are some links:
https://www.taco-hvac.com/products/h...ers/index.html

http://www.ussolarpumps.com/index.ph...index&cPath=71
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Old 06-06-2016, 09:31 AM   #10
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Fantastic post!!!

I would love to install lithiums to help bring down my hitch weight, but the work involved seems a bit daunting.

Looking forward to reading more about your lithium conversion, and maybe it'll help me to take the leap. Thank you for taking the time to write this up.
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Old 06-07-2016, 03:18 PM   #11
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When I did all my lithium research over a year ago I came across this video
from The Robotics Institute School of Computer Science, Carnegie Mellon University

http://www.ri.cmu.edu/video_view.htm...60&menu_id=387

Jay Whitacre, a Carnegie Mellon professor with a joint appointment in Materials Science and Engineering and Public Policy, gives a quick overview of Lithium-ion batteries. Starting at the chemical level, he explains the properties and mechanics of the battery which give rise to macroscopic behavior, especially focusing on issues relevant to electric vehicles and other high-power systems. He then fields questions about the batteries themselves, ongoing research at Carnegie Mellon, and tricks for improving long-term performance, such as pulse-charging and pairing batteries with similar defects.

This video is over an hour but very informative. I just rediscovered the link while cleaning up the desktop on my PC.

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Old 06-07-2016, 09:37 PM   #12
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Speaking as someone who has not materially participated in the lithium research, my first reaction to the warming predicament was, "Is there no way to adapt a combustion heating source?" When I get to the point of having a better conceptual grasp of the energy balance and flow, it's entirely possible that I will strike my own question. But right now, any resistive heating proposal causes me to visualize a snake eating its own tail. It would take only a tiny amount of propane to keep those batteries warmed, but I don't know if any device has been invented that could be adapted safely to that kind of an application.
The following suggestion is offered in jest....When you mentioned combustion, the first thing that came to mind was to use the heat of the engine exhaust to heat the batteries. Some creative exhaust pipe bending and a thermostatically controlled valve could shunt the engine exhaust into the battery bank below the rig. That would heat 'em up!
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Old 06-07-2016, 10:03 PM   #13
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They do make exhaust cutout valves for hot rods but I still think that running the output from the heater core to the batteries with a 3 way mix valve would be the best solution for anyone trying to charge exterior batteries in sub-freezing temps.

This valve can be set as low as 80F. No thermostat required:

http://www.caleffi.com/usa/en-us/cat...-valve-na16369
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Old 06-07-2016, 11:34 PM   #14
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Back on topic for a post.

So here is the inverter I've chosen:

This is made by Xantrex and is a pure sine wave model named the Freedom Xi 2000 and cost me $636.

This model met my biggest requirements:

1.) I doesn't have a built in charger. As I mentioned above, charging and discharging with the same cable is problematic for Lithium battery management systems because when the battery is low, you must cut off all your loads to prevent damage to the cells and this would also cut off your charging path. Likewise, if your battery is full, you end up cutting off both your charging source and your loads both. There are ways to work around this but it adds unnecessary complexity. Besides, separating the charger from the inverter eliminates an expensive single point failure.

2.) This inverter has a built in transfer switch. The space under our closet is tight enough as it is and wiring in an extra box to switch between the generator/shore power and the inverter was going to add extra wiring complexity and more importantly, take up valuable space.

3.) It has an accessory on/off switch. This is designed to be wired into the accessory power from the ignition switch of your vehicle so the unit turns itself off when your vehicle is turned off. I will wire this into the SBMS I linked to in the first post, but more on that later.

4.) 2000w capacity and 4000w surge capacity. This should be sufficient to run our 13.5k BTU A/C (but not for more than an hour or two). I'm assuming it will be enough to start the compressor but there we can always install a soft start on the motor if the inverter struggles.

The accessory switch turns on when 12 volts is applied. Unfortunately, the I/O on my BMS is only 3.3 volts so I must use a relay to control the inverter. Here are my two leading contenders:

The first is a $5 solid state relay which accepts an input voltage from 3-32 volts:


The next is an optocoupler (optical isolator) for about $6:


The optical isolator has the small benefit of being coupled by a small LED and photodiode so there is no electrical path between the higher current circuit and the IO pin on the BMS. It is also just a small circuit board and would need to be put in a project box so there is some added complexity there.

Do any of the EEs have any preference for a 3.3v relay?
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