LITHIUM BATTERY UPGRADE, PART 3: CHARGING / INVERTER SYSTEM

Following on Part 1 which described the general configuration of our DIY electrical system, and Part 2 which dealt with the AC system, here's the diagram for the charging and inverter portions of our system.  Relevant forum cross-references:  My husband's original electrical system development Air Forums thread here, and a Sprinter Forum lithium thread here.

As with my first two posts, I'll recap the general operation by describing the components individually.  The superscripted component numbers on this diagram correspond to the numbered paragraphs below it.

Click or tap image to expand size for clarity.  Blogger downsamples embedded images. 

13.  Engine alternator - For as long as we retained the original OEM single-AGM Lifeline house battery in our rig, we had no need to change or upgrade the alternator.  Shortly after converting to lithium iron phosphate (LiFePO4) battery (described below), we installed a Bosch 200 A alternator which, when used with a battery-to-battery charger (also described below), theoretically should be able to integrate seamlessly and charge the lithium battery when the van's engine is running.  However, as is often the case in cutting-edge conversions, there have been complexities, most notably the premature failure of the alternator's clutch pulley, which I describe in this post.  I will have more to say about alternator management in the future.

14.  Chassis battery - Nothing fancy here, just an ordinary EverStart truck battery from Walmart.  Chassis batteries don't last in the deep south, and with their climate-limited lifespans, I've never seen fit to buy anything more expensive.

Welcome to the red zone.  Diagram courtesy of Tiresplus and yes, I've always found it to be accurate.  Not changing a battery at 30 months here is to risk being stranded.  I often do it at 24 months. 

15.  Battery relay - The purpose of this device is to isolate the house system from the chassis system when the engine is not running.  This is a Mercedes Benz (MB) component that was already present in the vehicle as OEM equipment.  It had been used by Airstream to isolate the original lead-acid single house battery.  These types of relays are sometimes called battery isolation modules (BIMs).

16.  Battery-to-battery charger - This Sterling limits the load to 60 A and bumps the voltage up to 14.6, which is what the lithium battery prefers to charge at.  There is sometimes uncertainty about which of Sterling's products we've used (they don't seem to number their products very clearly), so here is a close-up photo:

17.  Lithium charger - This Progressive Dynamics Inteli-Power PD9100L Series unit accepts 120V AC and converts it to 14.6V DC with a maximum input current of 60A (again, 14.6V DC is the voltage at which the lithium battery charges most efficiently).  We installed this to retain the option of charging the lithium battery from a shore power source.  As we have learned again and again, it is essential that off-grid vans incorporate as much systemic redundancy as possible.  Something will fail - it's just a matter of what and when - so you better have a Plan B.  In our case, we also have Plans C and D, because our system allows 4-way charging - solar, alternator, generator, and shore. When we originally installed this charger, we never actually intended to connect to shore power - it was just there for emergencies.  But, heh heh, we've done it, especially following the alternator clutch pulley failure described above.

18.  Solar Panels - Our Grape Solar GS-S-100-TS 100 watt monocrystalline panels are wonderful, and I'm very glad we managed to get them before supplies were exhausted.  They are no longer manufactured in the size (aspect ratio) that was ideal for installation on our roof.

They straddled our OEM stainless steel roof rack like they were made for it.

These monocrystalline panels were also noteworthy in the high efficiency that they offered (18.3%).  Knowing that they would be discontinued, we bought two extras and put them in storage for potential future use.

19.  Switches - The Blue Sea manual battery switches were inserted so that we would have the option to disable charging on PV1 or PV2 circuits on the BMS described below.  We like Blue Sea products, which are ruggedly made for marine applications and are good quality.

20.  Battery Management System (BMS) - This is an Electrodacus SBMS-100, currently out of production but its builder still offers comparable models.  This particular BMS was chosen because we were able to run every load except the inverter through it, a configuration which allows the BMS to disconnect the DC loads when the battery is in a low state of charge (SOC).  In other words, it would cut power to the whole DC circuit array (distribution) once the SOC fell to a predetermined level, or when an individual battery cell voltage becomes critically low to the point where battery damage could occur (lithium batteries are expensive and they cannot be discharged all the way down to 0%).  This load-shedding does not disconnect any of the charging circuits, so the SOC can be recovered while the system is in low-charge shut-down mode.  Some other commercially-available BMSs disconnect both charging and discharging when the SOC falls to unacceptable levels, triggering a greater demand for human interface.  The Electrodacus is programmed to protect the battery from both overcharging and undercharging conditions.  There is no need for manual re-set buttons to remedy these conditions.

There are two charging inputs on the Electrodacus:

• PV1 which accepts current from the alternator, shore power, and the propane generator
• PV2 which accepts current from the 3 x 100 watt solar panel array 

Each of these routes charging current to the lithium battery (described below).  PV1 and PV2 are configured so that all inputs can be handled simultaneously; the BMS can safely handle up to approximately 100 amperes total input (hence the "100" in its model name).  In other words, solar charging can (and does) continue while alternator charging is also occurring, and/or shore power, and/or generator charging (!).  However, in practice, it is typically not necessary to stack charging sources.

21.  Lithium battery - Three GBS 12V (4-Cell) 100Ah LiFeMnPO4 cells were used deconstructed and reconfigured into a 300 AH battery.  The cells were rearranged to fit available space under our closet floor.  Here's a pic of how that was done:

22.  Electrical inverter - Xantrex Freedom Xi 2000 watt Pure Sine Wave Inverter was chosen for two reasons.  First, this model doesn't have a charger, and it was important that we separate our charging and discharging circuits for better battery management.  Secondly, this model had a built-in automatic transfer switch (ATS) which saved space in our tight build area.  Thirdly, the device's form factor meant that it could be positioned conveniently in an available space between the top of the lithium battery and the underside of the closet floor.  This inverter, when coupled with an EasyStart (described in Part 2), also allows us to run our 11,000 BTU Dometic roof air conditioner using the lithium battery alone for up to approximately 2.5 hours at a time (i.e., with neither generator nor shore power) before battery recharge is required.

23.  Positive busbar - The positive busbar allows the connection of multiple high-amperage wires to a single device with multiple terminals.  Without this device, the wires would instead need to be crimped in a way that did not reflect optimal design.

24.  Remote battery switch - This is a Blue Sea ML-RBS switch that isolates the inverter for safety reasons.  A short circuit developing in the inverter or in the high-amperage cables feeding it could cause a fire in the van.   This switch is connected to a pair of manual buttons on the control panel.  When we are done using the inverter, we isolate it by punching the "off" button and creating an open circuit at this point in the system.

Do you see those red and green punch buttons in the center of this control panel?  That's the remote battery switch.

25.  Shunt - The shunt is a 50 milliohm resistor.  The BMS reads the voltage drop across this resistor to calculate how much current is being used by the inverter.  All other loads go through the BMS itself, so it already "knows" how much current is being consumed for the other van circuits.  However, the external load from the inverter does not go through the BMS, so this device is needed.

26.  Negative busbar - This is where the chassis is tied to the negative terminal and return legs of all the major loads.  The negative busbar is tied to the chassis in a location behind the black water tank, under the cabinetry.

27.  Chassis ground - My husband describes the chassis ground as "the pool of electrons for all circuits".  The alternator, chassis battery, house battery, and all loads directly or indirectly connect to a common ground.  A ground is the means by which electricity is able to complete its journey and get back to the current source.

For those of you who are still on the steepest portion of the electrical learning curve (as I am), I highly recommend this webinar which has been reproduced as an unlisted YouTube video below, especially for understanding issues of grounding (earthing) and electrocution generally (HT this Sprinter Forum thread).  The video describes on-water mobile scenarios (boats), but similar principles hold true with on-land mobile vehicles (vans and RVs).  The person who presents the information is a parent who lost a young child to electrocution.  He was not a technician, but rather learned electrical engineering after his personal tragedy.  He uses clear lay language to describe complex constructs as a result.

The direct link below has the video continuing from around the 1 hour 13 minute mark, where the presenter is talking about electricity getting back to its source.  But IMHO, the entire video is worth your time, so I've embedded the full video below the timed link.  There are a lot of valuable lessons in it.

https://www.youtube.com/watch?time_continue=4396&v=O7-s_mdEPb0