Since writing this article I have developed a BMS that exactly meets the protection and longevity needs of a lithium battery for power from an isolated site (boat, camping car, House, industrial site). At the request of friends navigators I industrialized it. It is now for sale on my website: https://www.taobms.com
In this article we will review :
- What you need before you start
- The Preparation of the installation
- The purchase of the lithium cells
- The commissioning of the battery
- Life on board with lithium batteries
See bottom of page links to previous articles.
What you need before you start
- Chargers, alternators and regulators compatible with the tensions and the lithium LiFePo4 batteries charge cycles (see article N ° 4 - the battery charge)
- A BMS (Battery Management System) or equivalent to trigger alarms and control relays at various levels in each cell - function voltage balancing is not useful (see article N ° 3 - the protection of the equipment and the crew)
- Correctly dimensioned fuse Class T (Tao : 400With 20kA breaking capacity)
- A solenoid that is sized correctly to isolate the battery (Tao : 250A)
- Relays to control the break of shippers and consumers if something goes wrong
- A battery controller (current, voltage, SOC, Ah consumed)
All support systems are connected to the lithium battery and must be set for this optimal battery charge. A separate system must be provided to the engine battery :
- either a DC/DC charger that charges the battery engine from the lithium battery when the engine is running
- either the put two batteries in parallel (in this case the engine battery is kept charged in Float mode with a maximum voltage of 13, 8V and will consume current permanently)
- of strong cable to replace those who are too "light" in the existing installation
- lug crimping tool for these cables and other connections
- copper bar or aluminum (6 x 30 mm) If the inter-connecteurs provided with the cells are too short depending on how the cells will be assembled
- device to maintain the cells together and compress them (Tao : two plates aluminum 4mm taking cells in sandwich and six threaded rods coming to tighten them on the batteries through aluminum top and bottom angles to spread the efforts)
- connections protection against accidental short by falling metal object (Tao : a plate of 4mm plywood that covers the battery)
- device to calibrate the battery in its compartment and prevent any movement on the three axes
- Digital multimeter of good quality (able to measure with a precision of 1mV around 3 to 4 Repeatedly volts)
- Clamp for current continuous
- Infra-red thermometer
- Stabilized laboratory power supply (15V 30A) with voltage and current control
The Preparation of the installation
The configuration of the battery
By buying cells lithium 3, 2V nominal, It will take to assemble several to get the voltage and capacity desired for Battery Park.
For installation 12 Volts, the easiest way is to put four cells in series (configuration 4 S) with the BMS which oversees the voltage and temperature of each cell. You can find high capacity cells, but more capacity, more plates inside the cells are large. However our batteries that can be strongly heckled by heavy sea, These large plates could move or deform… and possibly get in short circuit. It is advisable to limit the capacity of each cell to 200 300Ah for a marine installation or.
If more capacity is needed, We need several cells at the same time before series. In this configuration the BMS control the voltage of each group of cells in parallel.
Another option would be to assemble the cells first in series to get the modules at the nominal voltage of the Park, then to put the modules in parallel to obtain the desired ability. The disadvantage of this approach is that it takes as much of BMS there are modules in order to control the voltage of each cell.
The location of the battery
To choose the location of the battery on the boat Park. The easiest to minimize cabling is to lead instead of the old batteries… If this location meets the following criteria :
- dry and ventilated
- temperature not exceeding 30 ° C - which excludes the engine hold
- no risk of freezing
- easy access
The design of the facility
Is not put in place the new battery and connect the wire to the questions of installation design. It is advisable to carefully plan the installation so you have to follow the plan at the time of installation.
Depending on the degree of modification of the existing installation the following diagrams might be useful :
- block diagram of the facility with the most significant energy consumers and producers, safety devices…
- battery wiring diagram (service and engine) with Chargers, security systems…
- scheme of connection of the BMS and the relay they command retail
- details of any new equipment installation scheme (for example alternators)
Given the currents that allows the lithium battery, It is good to check if the section of the existing cables is suitable. Provide for the replacement of low section. Define section, length, and the lugs on each end in order to cut and crimp in a professional manner (crimper to hexagonal JAWS).
Tracking of electric cables can be a good idea… with a table to explain the function of each connection.
Location and wiring of components
Four objectives for installation and wiring of the various components :
- to simplify and eliminate all the superfluous (battery selector, battery, automatic relay…)
- put the most distance between the positive and negative connections
- reduce the length of the cables and the number of connections
- have a good accessibility of the connections, Relay, fuses, circuit breakers…
For example on Tao :
- all the connections (positive and negative) were on a same wall - I moved all the negative connections on another bulkhead "negative".
- the two buses were on a Clipboard with only 10 cm between the positive and negative connections - I moved the positive bus to the "positive" part and put in a junction box
- three Cup-batteries and a battery selector - I kept the Cup-batteries to the engine battery and another to possibly start the engine on battery service
Provide the location of the BMS (close to the battery) and relays it controls (usually near the circuits ordered)
The purchase of the lithium cells
There are several reputable manufacturers… all based in China : CALB, Sinopoly, Winston, GB Systems (GBS)…
By buying the cells at a retailer it is important to specify that these cells must come from the same batch and if possible with consecutive serial numbers. In order to have the minimum number of characteristics and behavior gap between these cells. Indeed, if a cell has, Like what, resistance internal very slightly different from the other cells, It will load and will be discharged differently from others, and an imbalance between cells occur with premature aging of some.
Thinking about ordering a sufficient number of connectors inter-cellules for all the cells in parallel.
The new cells must be loaded at the factory between 50% and 60% SOC, be out of the factory less than six months ago, and have never been connected to what.
Especially do not buy so-called "pre-balanced" cells because if the distributor not has not unloaded them at 50% SOC immediately balancing achieved, the cells have suffered.
The cell reception it is important to measure the voltage of each cell with an electronic multimeter of good quality. The voltage of each cell must be in the order of 3, 3V with a voltage difference between cell less of 2mV. Any difference greater than 2mV is an indication that the cell has different characteristics from those of the other cells or cell could be used until you…
Tao I bought eight cells Winston 300Ah directly at the Winston in China distributor. No particular problem to order, What to pay by bank transfer before delivery… But no surprise. In 2016 it cost me US $ 3000 plus US $ 370 for shipping in the USA (I am responsible for the consolidation and forwarding to St Martin).
The cells were packed and chocked in a wooden box with foam on all sides. I received eight cells with consecutive and immediately unpacked serial numbers I checked their blood to 3, 307V without gap between cells.
The commissioning of the battery
Balance cells and put in place the battery please make sure that all necessary components are in place and that the wiring is carried out in accordance with the plan.
- Disconnect the existing batteries
- Implement the new components (solenoid, fuses, Relay, BMS, Chargers, converters, alternators…)
- Connect all in agreement with the plan (replace the too thin cables, move the connectors to the sandpaper and toss dielectric grease, identify the cables in line with the plan…)
- The BMS and its peripherals are wired, and ready to be connected
- Program the regulators for the existing batteries and Chargers
- Reconnect the existing batteries
- Fuel solenoid control so that it is closed
- Check that everything is in order and working normally
My Victron MPPT solar controller 150/70 has no separate voltage probe. It regulates the load based on the voltage at its own terminals… which means that the control voltage is equal to the voltage at the terminals of the battery less voltage drops in the cables and connections (a 25mm 2 wire has a resistance of 0,7 milliohms per meter and a connection has a resistance of the order contact 0,1 0, 3milliohm according to its quality). With a charge current of 50A these voltage drops are not negligible and may reach 0,3 0, 4V if we're not careful (This corresponds to a total resistance of the circuit between 6 and 8 milliohms) … and you can never charge your battery.
Indeed for this example, Suppose that the Bulk/Absorption voltage is set at 13, 8V. When the voltage reaches this level (at the level of the solar controller), the battery voltage is only 13, 4V at 13, 5V… It is not enough to charge the battery !!
On Tao, I measured a drop in voltage between 0, 2V 0, 3V with a load of 40 current has. To reduce this fall of tension to 0, 1-0, 2V, I connected the regulator directly between the output of the shunt and the output of the solenoid and I spent all the connections to the sandpaper + dielectric grease. By adjusting the Bulk/Absorption voltage at 13, 95V regulator goes into Float mode when the battery is charged between 90% and 95%.
Reminder on the need to balance the cells
- The usable in lithium battery capacity is set between the time or one of the cells is full and when one cell is empty. If the cells in series are not all exactly the same level of charge when the battery is assembled, the busiest cell will define the upper limit of load and less unit will define the lower limit of discharge. So there is loss of ability.
- the cell which is full first may be deteriorate and over-load
- the cell which is empty first may fall below the minimum charge level and deteriorate
- The cell balancing aims to synchronize the load of the cells so that they are all full or empty at the same time
If the cells are not balanced :
- with a BMS, high and low alarms may be triggered prematurely and disconnect the battery
- No BMS, There is risk of damage or destruction of cells
It is therefore imperative to balance the cells before you assemble them in series.
Principle of balancing
- As two cells do not have exactly the same capacity he must choose if we want them to be full at the same time (balancing at 100% SOC), or if they are empty at the same time (balancing at 0% SOC). In our applications can probable and undesirable discharge the cells at 0% SOC. Balancing is therefore 100% SOC.
- Note : We can minimize the differences between cells by cells of the same batch with consecutive serial numbers
- The cells are connected in parallel and loaded with a voltage to the maximum of what they accept without damage. When the load current is zero, all the cells are full.
How to balance the cells
- This operation must be done just before putting the battery in service because he shouldn't let the cells at 100% SOC for a long time at the risk of damaging them.
- Put all of the cells in parallel using the connectors supplied by the manufacturer. Ensure that the contacts are clean (eventually pass them to fine sandpaper) and tighten bolts according to the instructions of the manufacturer.
- Before you connect to the cells, fix stabilised with a voltage supply of 3,45V and a current limited to 80% of the maximum capacity of power supply (If power 30A, limit the current to 24A)
- Connect the output of the power supply to the cells (never stop feeding as long as it is connected to the cells - to disconnect the cells before stopping)
- The tension will stay around 3, 3V for many hours / days (based on the ability of the cells, their number and their State of charge). This process must be done with a little oversight – I stop when I'm away and night charge.
- Regularly check the temperature of each cell with infra-red thermometer (aim the Terminal cell always to the same place). If a cell is hot this is an indication that either his connection is of poor quality (unscrew, clean and tighten), the cell is sick…
- Wait for the tension to stabilize at 3,45V and that the current is zero (measured with digital Voltmeter and current clamp)
- Disconnect the power supply of the cells, set the voltage to 3,65V and check the setting with digital multimeter
- Connect the cells to the power supply and monitor loan voltage with multimeter
- In a few minutes the voltage should climb to 3,65V and stabilize
- The current (measured with the current clamp) drop - when he sucks your cells are balanced
- Disconnect the power supply
- Leave the cells in parallel for 30 minutes, then measure their blood - if it is below 3, 50V reconnect a few minutes at the power supply set to 3, 65V
Connect the battery
Lithium cells will be damaged if they are kept loaded at 100%. The lithium battery must be assembled and put into service quickly.
- Position the cells in the device you'll have planned to keep sets and compress them
- Connect the cells with the connectors provided by the manufacturer or the copper bars you will have planned for this purpose
- Set the regulators of charges for your lithium battery ( see article N ° 4 : The battery charge – Bulk/Absorption = 13, 8V, Float = 13, 2V, Absorption to a minimum time)
- Put the battery the lithium in place and immobiliser
- Connect the voltage probe and temperature of the BMS
- Connect the BMS and the components it controls (solenoid and relay)
- Connect the battery terminals to the bus negative and positive
The battery controller
This instrument practice, and in my view essential, measure the voltage of the battery and the current that enters or exits. It calculates the cumulative consumption and displays a status of the battery charge (SOC).
In theory, This is perfect… but often the information he gives are entirely false if the controller has not been carefully set up and calibrated.
A bad setting will not appear immediately, but usually after a few weeks. The controller will indicate a wrong state of charge, Like what :
- It may indicate 80% SOC while the battery is at 100% SOC (not too serious because regulators of charges go into Float mode and the controller will reset to 100% SOC.)
- more serious… It may indicate 20% SOC while the battery is at 0% SOC… fridges and freezers are stopped for a long time and, at best, the BMS disconnects the battery ! If there is no BMS… cells are damaged.
Anyway, given the number of parameters, It is impossible that its SOC indication is accurate. In view of the above two examples, I prefer his load indication be understated rather than sur-évaluée.
A simple example with a single parameter incorrectly… Let's say the calibration to 0 Amps of the controller is made while the actual consumption is 0,1 Ampere (the BMS Conso, Solenoid, battery controller…). In one day the cumulative error is 2.4 a.… and in five weeks this cumulative error is 84Ah ! If Battery Park is of 400Ah, It is an error on the NCS in addition to 20% ! But what the controller can measure reliably a current of less than 0.1 A ?
My battery controller is the Victron BMV-702. After several trials setting here is the one I use for very slightly sub evaluate the %SOC :
- battery 600Ah
- voltage at 100% SOC 13, 8V
- current at 100% SOC 0, 025C (2,5% battery capacity)
- time to reset to 100% SOC. 4 minutes
- Peukert exponent 1,00
- Effectiveness of load 100%
- Current 0.01 A detection threshold
- Calibration of current draw (0A) taking care of all disconnect the battery… except, Of course, the controller itself even which consumes a little when its backlight works.
I also set the alarms of the controller in a way more conservative than those of the BMS. It gives me a warning early on which I can act :
- %Low SOC 20%
- low voltage 12, 8V
- high voltage 14, 1V
The lithium batteries never need to be loaded to 100%, but I did all four to six weeks so that the battery controller resets to 100% SOC automatically when the voltage reaches 13, 8V and the current is less than 15A two minutes (see above parameters).
For the last reset (a month after the previous) the controller showed 96% SOC before reset itself 100% SOC based voltage and current measured. For my 600Ah battery that matches a 24Ah error in 30 day, either an error of 0.033 current measurement – quite acceptable and harmless error.
Inappropriate reset to 100% SOC.
While the approach the full battery with a voltage of 13, 8V, It could make the combination of several outdoor events down the current under the threshold from which the battery controller resets to 100% SOC.. For example when the solar panels production drops to the passing of a cloud, the engine spends idling, or a consumer to turn it on…
To limit this risk increase the time during which the reset conditions (voltage >= 13, 8V and the current <= 0, 025C) must be present before the reset to 100% SOC.. A few observations and adjustments will be may be necessary. For my part I set this timer to 4 minutes.
Life on board with lithium batteries
If the installation is properly sized, designed and produced, the battery will forget. The biggest concerns that you have is learning a new behavior for energy management :
- do not panic when the battery charge drops below 50% (It is around 50% SOC she is the better)
- do not try to achieve 100% SOC in a systematic way
- charge the battery when it reaches 20-25% SOC
- the voltage of the battery at rest will give you less information than for a lead-acid battery, but there are still some useful tips to be learned when the battery is almost full or almost empty…
"U" battery voltage
State of charge
U > 13,30V
SOC > 80%
13,20V < U < 13,30V
70% < SOC > 80%
13,15V < U < 13,20V
40% < SOC< 70%
13,00V < U < 13,15V
25% < SOC < 40%
12,80V < U < 13,00V
10% < SOC < 25%
U < 12,80V
SOC < 10%
- When the day is sunny and the battery charge is at the top of 70% I want to put the desalinateur on and bring down the SOC underneath of 50%
Regular maintenance operations
The only operation that I made every four to six weeks in order to reset the controller battery is to charge the batteries to 100%. I stop the load when the voltage reaches 13, 8V, the current is below 15 A and the controller is reset to 100%.
During charging and charge end I control the temperature of each cell to detect any anomaly (This allowed me to detect a slightly oxidized and/or less tight connection on one of the cells and remedy)
Charge end I disconnect the battery (with the solenoid) and I measure the voltage of each cell to check their balance. My battery is in service for one year and the difference in voltage between cells when they are loaded to 100% SOC is stable and less than 0, 01V (on the cell whose connection was defective). There is so little drift on this first year and this information will tell me if and when he will have to balance the cells.
Away from the boat for several weeks / month
If you leave bu boat to ensure that load systems (solar and wind power) do not keep the battery constantly at 100% SOC, or that batteries will not offload below 40% SOC.
The simplest is charge it to 50-60% SOC and disconnect them entirely (attention, Open the solenoid is not sufficient as the BMS is always connected and the battery controller). Disconnect the cables entering the battery.
Menu… Practice > Technique > Lithium
Next articles :
- design, completion and commissioning of the new BMS
- feedback over time…