DIY Lithium Battery System

Disclaimer:

The comments/suggestions in my write up are specific to my RV and my needs. If your RV and needs are similar to mine then perhaps you will find this write up useful. While I do have some training and work experience as both an electronic technician and RV technician I'm not an 'expert' in the field of lithium batteries. The information I share is based on what I already know about electronics, online research, discussions with vendors and personal observations.

My RV:

I own a Class B van camper. The OEM 12 volt system consists of two 12 volt lead acid batteries ( GEL ) in parallel. OEM charging is accomplished via either alternator or converter ( single stage 110 VAC input / 14.1 VDC 10 amp output ). This OEM system is pretty standard for most RV's - although battery chemistry ( IE: flooded, AGM, GEL ) and charger output ( IE: charge current & single stage vs multi stage ) do vary. 

I've owned this RV for 12 years - during that time I've made 'a few changes' to the OEM 12 volt system - it has been and continues to be an evolutionary process... Most importantly - for me - I've added two 100 watt solar panels on the roof and one 100 watt portable panel. I also replaced the two 12 volt parallel connected GEL batteries with two 6 volt series connected AGM batteries. And I added a 1kw inverter - which I connected directly to the house batteries via a 100 amp DC circuit breaker. 

My needs:

I primarily use my RV to 'escape' the summer heat of southern Arizona - IE: snow bird type usage... Most of my camping is in NFS campgrounds - IE: no hookups - dry camping. June/July along the Oregon coast and August/September in the high country of Utah/Colorado. Typical length of stay one to two weeks. 90% of the time I charge via solar. My typical daily ( 24 hour ) power consumption is about 450 watt hours. My biggest DC loads are the refrigerator ( Danfoss compressor ), Espar airtronic diesel furnace & ventilation fans. Cooking and water heating is via propane. Light usage of microwave - typically less than 1 minute run time per day. 

Why lithium:

There are many write up's discussing the advantages of lithium vs lead acid batteries for 'off grid', RV & marine applications. IE: Smaller, lighter, more deep cycles, 100% of the rated capacity is usable, faster charging, etc... 

The absorption phase for charging lead acid batteries takes a really, really long time! Solar panels are my primary energy source. Variable overcast, shadows from trees, campsite orientation often result in limited opportunity for solar gain. On some rare occasions I run the generator or engine alternator to charge the batteries. In any case - the less time spent charging the better. Charging a lithium battery takes a fraction of the time it takes to charge lead acid!

Perhaps less understood are the dis-advantages. Lithium batteries typically only die prematurely because they've been abused. They don't like extremes of temperature - room temperature is best. For example: they don't like being charged when the temperature is below freezing. Some articles suggest the possibility of lifespans shortened by 50% when stored and operated at higher temperatures ( 90 degrees F ). They don't like being charged up to 100% and left that way for more than a day or two. They don't like being left hooked up to a charge source continuously ( IE: high float voltage ) for an extended period of time without be cycled. They can fail - sometimes with rather dramatic consequences - if dis-charged below below a certain point or charged above a certain point.

So there's more to it than just purchasing a battery. It's better to think of it as a lithium battery system. To protect the battery, RV and occupants it is prudent to install a BMS ( battery management/monitoring system).  

Why a DIY Lithium Battery System:

When it comes choosing lithium battery systems for RV & Marine applications there are: 

High end systems like Victron ($3,900). If money were no object then Victron would be a good choice. 
Note: some assembly required:-)

And drop in systems like Lifeblue ($1,000). If you want the most bang for the buck and want to keep it simple then perhaps the Lifeblue would be a good choice.

I can't afford Victron and I don't know what's inside those monolithic blocks of plastic commonly referred to as drop-in replacements. The vendors seem reluctant to share that information - which makes me suspicious... If any single component in that monolithic block of plastic fails then you're probably going to end up with an expensive boat anchor! Most of them don't offer any means for 'tweaking' the settings IE: over/under voltage/current disconnect thresholds. Most of them don't offer any provision for monitoring cell voltage or SOC (state of charge).

There are a few BYOB ( bring your own battery ) BMS kit's available. For example: Orion ($425), 123SmartBMS ($325) & Elite ($345). They're all rather expensive and somewhat overkill for my needs... The 123SmartBMS seems like a pretty good product and would be my first choice if paired with CALB cells.

I decided to go with a DIY BMS approach. In choosing components I felt it was important to consider not only cost, but reliability, scalability, ease of integration, ease of use and low self consumption ( IE: how much power does the battery protection system use during normal operation ). There are trade off's and compromises one must make when deciding what components to use. After two years and several design revisions I think I've found a set of components that strikes a fair balance.

Cell balancing and protection:

Perhaps a brief explanation of what cell balancing is might be in order. Typically the way this works is once the battery gets into the upper part of the knee on the charge curve there is a tendency for the individual cell voltages to go their separate ways. Invariably there will be winners and losers in the race to the top. Unfortunately - in the case of a lithium battery pack the prize for being the winner can be a DEATH sentence! So to prevent any cell[s] from 'redlining' a resistor is placed across the cell[s] that are getting too far ahead. In theory this allows the 'laggard[s]' to catch up. The goal is to push the maximum amount of energy back into the battery. In reality this generates heat, wastes energy and prolongs the charge cycle. 

Most lithium battery packs for use in 'off grid', RV &  marine applications consist of four LFP cells (Lithium Iron Phosphate LiFePO4). 

Perhaps the most important decision is what level of protection is needed - IE: cell vs pack. All of the aforementioned choices ( Victron, Lifeblue, Orion, 123BMS, etc...) offer both cell and pack level protection. There are different school's of thought as to whether or not cell level protection is necessary. 

In my opinion:

And that's all this is - just my opinion - based on what I already know about electronics, online research, discussions with vendors and personal observations.

IF

1) The battery pack consists of 4 cells with similar characteristics ( IE: consecutive serial numbers ) 

2) The cells have been properly top balanced prior to use ( responsibility of end user )

3) The battery pack voltage is restricted to the flat part of the charge/dis-charge curve ( 12 ~ 14 volts ).


THEN - Cell level protection and balancing aren't absolutely necessary. 

It's only when you're trying to push that last 10% of energy into the cells or pull the last 10% out of the cells that you run into trouble. 

Battery protection and management are insurance against damage to the battery, the RV and it's occupants. How much to spend on insurance is a personal choice. But spending more on insurance than the battery itself seems rather wasteful to me. I feel that $150 ( battery monitor & protect ) is a reasonable amount of insurance to spend on a $400 battery. 

Hooking up a Cell Meter and monitoring it during the charge/dis-charge cycle is an inexpensive yet effective method of knowing whether the cells are balanced or not. Across the flat part of the curve I typically see less than 5mv difference between individual cells. As the voltage approaches the upper or lower knee the difference increases. A large difference in cell voltages is a good indication you're getting into the danger zone. 

When my battery was new I performed some tests. I used a bench top power supply to charge the battery ( 14 volt's @ 10 amp's ). I used a Cell meter to monitor the individual cell voltages. I noticed that the difference in cell voltages started to become noticeable ( > 10 mv ) at around 13.6 volts. I stopped charging and began dis-charging ( @10 amp's). I noticed that the difference in cell voltages started to become noticeable ( > 10 mv ) at around 12.6 volts. I defined that as the flat part of the curve for my battery. I repeated this test three times - staying between 12.6 and 13.6. The measured battery capacity during these tests was consistently 900wh. The rated capacity is only 780wh! 

So I'm not sacrificing anything by avoiding the upper and lower knees of the curve. Your mileage may vary... These tests help give me confidence that cell level balancing and protection aren't absolutely necessary and probably not worth the extra expense. Moral of the story - stay on the flat part of the curve and no worries:-)

One final comment regarding balancing. When purchasing a new set of LFP cells it is highly recommended to top balance them before use. The method I used:

1) Connect cells in series and charge to 14 volts - termination current 6 amps

2) Connect cells in parallel and charge to 3.65 volts - termination current <= 2 amp ( the lower the better ).

3) Connect cells in series and dis-charge to 50% SOC for safe storage.

My lithium system components:

Battery: 

GBS 60ah LFP Battery pack ( $412 including shipping and tax )

I purchased mine from Starlight solar - for support I've worked with Larry who has been very helpful.

I do like that the GBS is offered as a pack - IE: all four cells have consecutive serial numbers and are both physically and electrically tied together.

FWIW:  CALB CA100FI might be a better choice than GBS.

Charger: 

Kisae DMT1230 ( $177 including shipping & tax )

I purchased mine from Inverter supply - the cost has since gone up to about $270

For support I've worked with Ricardo at Kisae who has been very helpful.

Regardless of which battery system you purchase ( Victron, Lifeblue, BYOB 123smart, Orion ) you'll need to carefully consider charging. While all those systems offer balancing and protection they all lack charge control ( except perhaps for Victron). Whether you're charging from solar or alternator the charge voltage and current must be carefully controlled. That's the role of a charge controller.

The DMT1230 has separate inputs for solar and alternator. It automatically switches to whichever source has the most power to offer. For $180 it's not a bad choice - but @ $270 I think it's over priced for what it does. IE: It does many things but doesn't do any one thing particularly well... 

For example: It occasionally goes into what I call yoyo mode. The charge current rises to a peak then falls to zero and repeats endlessly. Net result is charge time is doubled or even tripled! Turning the charger off then back on sometimes stops this erratic behavior. 

Relevant definition of yoyo in this context: 

1 : a condition or situation marked by regular fluctuations from one extreme to another. 2 : stupid or foolish behavior. 


I could offer a rather lengthy list of other things I've found irritating and/or undesirable with the DMT1230 - instead I would suggest you search for a better alternative - for example Votronic.

BMS: 

1) Thornwave battery monitor ($100 including shipping & tax)

For support I've worked with Raz who has been very helpful.

I feel it's important to be able to tweak the BMS settings and monitor SOC ( state of charge ). This compact/rugged little device can do both via a bluetooth connected app ( Android or Iphone ). It's self consumption  is very minimal - about 7ma.

2) Victron battery protect ($52 including shipping)

Typically when a battery monitor detects an out of bounds condition it either opens or closes an internal switch (typically a small transistor with a maximum current rating of .5 amp). This switch is then used to operate a battery protect relay ( or solid state relay ). The Victron battery protect is essentially a solid state relay. The self consumption is only a fraction of an electromechanical relay. 1.5 ma compared to 250ma for a typical $50 solenoid! 

3) Cell meter  ($10)

4) Bussmann CB185-35 Surface-Mount Circuit Breaker, 35 Amps

My lithium system connections:

Other than the battery, cell meter and circuit breaker there are basically just three components.

Kisae battery charger:

Thornwave battery monitor:

Victron battery protect:

When safe to do so - the Thornwave battery monitor relay port provides a path to ground for a relay coil. If the battery monitor detects an error condition ( for example: high or low voltage ) it opens that circuit and the relay dis-engages.  

The Victron battery protect needs 12vdc connected to it's remote port ( yellow wire ) to turn it on ( it's essentially a solid state relay ). 

The DMT1230 charger has an on/off switch.

The wiring diagram for the Thornwave battery monitor shows a SPST relay - you can use a DPST relay to:

1) Provide 12 vdc to the Victron battery protect to turn it on/off.

2) Provide a remote switch to turn the charger on/off.

NTE R56−7D.5−12D 12 DPST−NO Relay 
( DIP [dual inline package] 26ma current draw)

Connections:
1) 12 vdc
2) 12 vdc
6) Thornwave relay port
7) Victron remote port ( yellow wire )
8) charger on/off switch
14) charger on/off switch

Alternatively you can use a digital transistor ( NTE2360 ) which consumes virtually no power. I chose this approach to minimize BMS self consumption. This approach does not offer the capability of turning off the charger in an over voltage situation.

Connections:

Emitter    > 12vdc.
Base        > Thornwave relay port.
Collector > Victron remote port.

Note: This schematic is for reference only.

The NTE2360 I'm using is an integrated circuit which includes the pnp transistor and bias resistors in a single package.

My lithium system configuration settings:

Battery charger:

Battery type - lithium
Lithium voltage - 14.0
Charge current - 30 amp
Terminate current - 6 amp

Battery monitor:

Low voltage disconnect - 12.4
High voltage disconnect - 14.1

Battery protect:

7 segment display - C ( Li-ion mode ) 

What did I do with the old battery & charger[s]?

I really didn't make any major changes to the pre-existing lead acid battery system.  I simply disconnected the cable between the DC fuse panel and the lead acid battery bank then connected it to  a battery switch. IE: I can run the loads connected to the DC fuse panel off either lithium or lead acid. In my case the loads connected to the DC fuse panel typically never exceed 30 amps total at any given time. Everything else that was connected to the lead acid battery bank remained as is - IE: inverter, generator starter cable, alternator charge line, shore power charger, etc...

A lithium battery pack <= 100ah is relatively small and lightweight. So it's not difficult to find a place inside the RV to secure it. In my case I configured it such that I can easily disconnect it and move it into the house when I'm back in Arizona after my summer travels. Leaving it the RV for 8 months ( October ~ June ) would cut the lifespan of the battery in half due to the heat. I perform a top balance on it then discharge it to 50% capacity for safe storage until the following summer. During the winter months - when I'm only occasionally using the RV - the lead acid battery system reverts from backup to primary.