installing lithium batteries
the latest in battery technology
 

lithium batteries in place in battery compartment

We recently upgraded from our four and a half year old Vista RV Crossover to a new Crossover XL. As part of the process I spent around three months investigating battery options. The original Gel batteries had done a good job, but I thought I should check out some of the newer options. I looked at Lead Crystal batteries and Lithium cells. My final decision was to opt for Lithium cells. 

theory

Lithium cells are around half the weight of lead batteries, yet can be safely discharged to 20% State of Charge (SOC), as opposed to only 50% SOC for lead batteries. This means Lithium batteries can give 160 useable Amp hours out of 200 Amp hours total instead of 100 Amp hours from the same 200 Amp hours of lead batteries, a 60% gain.

They also maintain their voltage down to 20% SOC  so things like fridges keep working efficiently. The voltage of a lead battery drops off as the battery drains. Lithium cells can also safely accept up to 1C (one times the rated Amp hours of the cell) so, in this case, 1x100 = 100 Amps as a charging rate and can handle high discharge rates as well, easily 100 Amps.

They are also very efficient at charging, almost 100%. The down side is the initial cost of roughly twice the same nominal capacity Gel of AGM batteries. This however is offset by their long life, usually 10 to 20 years.

There is much spirited – if not downright vitriolic! – debate online as to how to go about setting up Lithium cells. In the end, after talking to several suppliers, I opted to get the cells and monitoring equipment from EV Works in Perth and follow their advice. I put together a reasonably simple setup which monitors individual cell voltages and automatically attempts to rectify any problems.

putting it all together

Lithium cells come as individual 3.7 Volt cells with different Amp hour ratings. You can make up a “battery” out of however many cells of the same Amp hour rating as you need to get the voltage you need. To get a 12 Volt battery, you need four cells. I decided to go with eight Winston 100 Amp hour cells, arranged as two 100 Amp hour, 12 Volt batteries. I joined two sets of four cells together in series in mirror image. The end positive and negative terminals are then side by side and can be easily joined together in parallel, giving 200 Amp hours at a nominal 12 Volts.

To monitor the individual cell voltages, I used a pair of ZEVA BMM8 Battery Monitor Modules from EV Works. You need one module per battery. These modules use a wire attached to each positive terminal of a battery to monitor the voltages. You also connect a negative wire to the module. The module can then monitor each cell in a battery individually and detect if one cell is going too high or too low in voltage. If this happens, a small solid state relay on the circuit board is triggered, causing it to open. There is one for high voltage, another for low.

These relay outputs are, in turn, daisy chained, high with high and low with low, over the two batteries. The cables are then run to a pair of solid state relays, one connected to the charger outputs and the other to the loads – fridge, lights, etc. These relays will disconnect either the chargers or the load on the batteries, depending upon the problem. If a cell is too high, the chargers are disconnected, leaving the load still connected to draw the voltage back down. If a cell is too low, the load is disconnected, leaving the chargers connected to try to bring it back up. Once the problem is fixed, the relay is reconnected automatically.

I fitted a pair of 100 Amp solid state relays from Jaycar. These were cost effective and have a very low current demand. I made up a pair of wiring looms with four wires each, each wire terminated with an 8mm eye terminal. The wires were from a Cat5 network cable with braided strands. The eye terminals were attached to the positive posts of each cell, one loom per battery. The other ends of the wires were soldered into the male plugs to connect to the BMM8s.

I made a second set of positive terminal wires running from both batteries into the electrical cabinet of the Vista Crossover XL. These connect to a pair of plugs, one per battery, which I can attach to a pair of Cell Log8s which give a readout of each individual cell voltage when connected. This I did for peace of mind – and because I am a technoweenie and I could! The Cell Log8s came from Hobby King who have an Australian outlet.

A section of 12mm ply was fitted to the top of the batteries to protect the terminals and to the front of the batteries to allow attachment of the BMM8s. The batteries were fitted into the battery compartment on the Vista Crossover XL and secured in place with a ratchet strap running around the cells to hold them together and another strap over the top to hold them down. The BMM8s were screwed to the ply and then a clear cover was screwed in place to protect them. I used a couple of clear plastic lids from NARVA component boxes, but a piece of clear perspex would do.

Two positive cables were attached to the battery’s common positive terminal, one running to the charger solid state relay, one to the load one. One negative cable was attached and it runs to the shunt for my Victron BM700 intelligent battery monitor.

at first

The cables were then all attached and the system commissioned. I discovered that the charge relay disconnected upon commissioning. After checking I found out that Lithium cells, like lead batteries, can hold a high residual charge – a “surface charge” – when they are charged initially. I simply turned on a few LED lights in the Vista Crossover XL for a while and the relay turned on as the surface charge was taken off the cells. It worked as advertised.

One of my biggest worries was finding appropriate chargers. I spent a lot of time examining the specifications of some very expensive chargers, several of which claimed to be ideal for Lithium batteries. Upon much research I worked out that most of them were not. After talking to people who install Lithium batteries for a living, I worked out that charging is not such a problem after all.

charging

Charging in the Vista Crossover XL is done by three separate chargers. I have a Projecta 50 Amp AC-DC charger to handle charging from 240 Volts when it is available. This charger is rated to 0.5C of the 100 Ah cells, so is very safe for a fast charge – under four hours if the batteries were down to 20% SOC. I have it as “Power Supply” so it delivers 13.8 Volts to the batteries. This is considered a very safe charging voltage.

There is a Ctek D250S Dual to work as a DC-DC charger when driving and as an MPPT solar controller for my two 135 Watt semi flexible portable panels. This charger is not user configurable at all. It supplies 14.4 Volts as a bulk charge and 13.8 Volts as a float.

The third charger is a Projecta 20 Amp solar controller to look after the 105 Watt fixed solar panel on the rear of the XL. This panel can easily be removed to allow me to put it out in direct sun. The charger is set to “Gel” so it supplies 14.2 Volts on bulk charge and 13.8 Volts as a float.

overview

The system is working well with all the cells staying within 0.01 of a Volt of each other – a very good thing. Losing 35 kilos and gaining 60 Amp hours is a wonderful thing!

The total cost, including monitoring hardware, was around $1700 for 200 Amp hours, 160 Amp hours useable, with a weight of around 30 kilograms. To get 160 useable Amp hours out of lead batteries you would need roughly 300 Amp hours of capacity, costing around $900 for good quality batteries and weighing around 100 kilograms.

links

EV Works http://www.evworks.com.au
BMM8 module http://www.evworks.com.au/zeva-8-cell-bat-monitor
Jaycar Solid State Relay http://www.jaycar.com.au/Electromechanical-Components/Relays-%26-Accessories/Solid-State/Solid-State-Relay-4-32VDC-Input%2C-30VDC-100A-Switching/p/SY4086
Cell Log8 http://www.hobbyking.com/hobbyking/store/__44896__Cell_Log_8M_Cell_Voltage_Monitor_2S_8S_Lipo_AU_Warehouse_.html

two batteries made up on the work bench	individual cells in a crate
battery compartment
how cells are connected to make two batteries	the three chargers in the electrics cupboard
circuit diagram

Thanks to David Jones for this article

july 2015