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
, 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 weathe
r 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.