Convert to LiFePo4 batteries

Reading time 7 minutes

Updated - January 3, 2025

Converting to LiFePo4 batteries is worthwhile for two reasons: Weight saving and service life. Both are welcome reasons for replacing the old AGM batteries in motorhomes.

An AGM battery should be recharged after 60% discharge. A LiFePo4 battery actually lasts for 100% of its nominal capacity. In addition, an AGM battery requires recovery phases if high currents are demanded continuously, e.g. in inverter operation.
In the test, a 120 Ah AGM battery lasted 31 hours with a defined permanent current draw, while the LifePo4 battery with 110 Ah lasted 54 hours.
The two types also differ in their charging behavior: the AGM battery needs 12 hours, the LiFePo4 battery 6 hours to replenish the energy used.
Regarding the weight: the AGM battery weighs around 26 kg, the LiFePo4 battery 10 kg.

Not everyone is on familiar terms with this technique, which is why people are often reluctant to take this step. However, as always, knowing how is the key to the supposed secret behind it.

Step-by-step instructions should help you to lose your inhibitions and master the exchange successfully.

Victron devices are used for all components, with the exception of the batteries themselves.

Shopping List

Depending on the power requirement, the required ampere-hours (Ah) are selected, e.g. 280 Ah from EVE. As we assemble the battery ourselves and use a single Battery cell 3.35 V, but we usually need 12 V on-board power supply voltage, four of these are added to the shopping cart. For 24 V, e.g. in truck chassis-based motorhomes, 8 cells must be purchased.

If larger capacities are required, cells can also be connected in parallel. For example, 2 x 280 Ah cells connected in parallel to 4 x 3.35 V result in a capacity of 2 x 280 Ah = 560 Ah.

LiFePo4 batteries require a so-called battery management system (BMS), which protects them from voltages that are too high or too low and - in the event of a fault - disconnects the negative line electronically in good time.

Furthermore, a Balancers which ensures that all cells always have approximately the same voltage.

Charger / Charging booster are used to charge the LiFePo4 battery with shore power or via the alternator while driving.
Such devices, if already installed for the AGM battery, should be suitable for LiFePo4 batteries. They are either manually adjustable or automatically recognize the battery type and charge with the corresponding charging curve. If neither the one nor the other feature is provided by the charger, a suitable new device is required.

If you intend to use an inverter, you may not need a separate 230 V charger, as the larger inverters, such as Victron Multiplus II, already have a built-in, very powerful charger.

The power output of the alternator must be taken into account for the charging booster and a more powerful one installed if necessary. A charging booster with a charging current of 30 A is considered standard and can generally be installed without a more powerful alternator.
The use of thick and short cables in accordance with the manufacturer's specifications must be observed in order to minimize voltage drops from the alternator to the charge booster and from the charge booster to the battery to be charged. Good charge boosters compensate for voltage losses from the alternator to the charge booster by automatically increasing the output voltage.

A calculation tool for cable cross-sections can be found here.

Always ensure that all screw, clamp or crimp connections are tight!
Loose connections generate high resistances, which can lead to excessive heating and even cable fire if the corresponding currents are present!

Options

If you don't just want to check the charge status of the batteries via the Bluetooth interface using the app, but also want to be constantly informed about incoming solar energy, charge status and consumption values, the Victron Cerbo GX with Touch 70 Monitor, for example, can display all relevant values, configure devices, etc..

Cell balancing

Due to manufacturing and material tolerances, the properties of a cell are never exactly identical. As a BMS assumes a discharged cell when a certain cell voltage is reached and switches off the entire battery, even if the remaining cells are not yet discharged, it is important to keep all cells at an ideally identical voltage level.

Cell equalization takes time. It does not hurt to set up the cells in parallel in a row immediately after delivery, connect all negative poles with the busbars supplied and do the same with all positive poles. In this way, a cell with a lower charge can take over the charge from another cell overnight and thus achieve a balanced voltage potential across all cells.

The battery

Voltage measurement

In the example above, 16 cells have been equalized, which are now checked for their respective voltages. A digital multimeter with a resolution of 0.001 V ±0.03 % or better should be used for this. A max/min function makes it easier to record the voltage deviation of a series of measurements, as shown here for 16 cells.

Once all cells with largely identical voltages have been confirmed as balanced, the cells can be arranged as required. Here, four times four 3.35 V cells are to be connected in parallel (resulting in 4 x 280 Ah = 1,120 Ah) and then the resulting four-packs of 3.35 V each are to be connected in series to form a 13.4 V battery.

Construction

Here the blocks of four, which ultimately each represent ONE 3.35 V cell, were connected in series to provide a total of 4 x 3.35 V = 13.4 V.

The pink foam inserts are used to dampen vibrations. Total weight incl. housing 96 kg. Weight saving compared to 4x AGM DeepCycle battery 222 kg! Dimensions (W) 619 x (H) 407 x (D) 283 mm.

BMS connection

The BMS has two thick cable connections, P- and B-. B- is connected to the negative terminal of the battery, P- leads to the negative connection of the consumers. This cable cross-section should also be used for the positive cable to the battery and should not be smaller than this.

The two 35 mm² The thick black cables (top) are connected to the ground rail (-) of the Lynx shunt, the blue cables (bottom) to the negative terminal of the battery.
Normally there is only a black and blue 35 mm cable for BMS smaller than 400A.² Cables. From 400 A, two identically colored 35 mm² cables are connected in parallel in order to make the cable diameter physically sufficiently flexible but electrically twice as resilient.

The BMS is activated by applying an external charging voltage. This switches the BMS on first. Instructions are available here Download available.

The XENES-BMS has an integrated balancer.

The SOC value of 64% (second column from the left, third value from the bottom) represents the current state of charge and should NOT be adjusted manually in order to avoid incorrect values.

The only values to be adjusted are the nominal capacity and, if applicable, the values for maximum charging/discharging current and charging protection at low temperatures (0 °C), as LiFePo4 batteries must not be charged below 0 °C because lithium ions may then deposit on the electrodes and damage the battery in the long term.

Discharge can also take place at temperatures below -20 °C, but at such temperatures the total capacity should be assumed to be reduced to around 70% compared to the usual 100 %.

Balancer connection

There are four cells per battery. Either a cell actually only has ONE 3.35 V cell or several (four in this case), which in turn each have a (common) negative and positive terminal.

The black cable of the white plug (-) at the bottom left is connected to the negative pole of the first (here common) cell, and the four red cables (+) are each connected to the positive pole of the first cell (this is the cell / cell group that is the first to be connected to the BMS). B- in the front view shown above, the cell block at the top right), second, third and fourth cell(s).

If the cables are too short, they can be extended. However, it is important to ensure that ALL five cables are the same length in order to avoid falsifying the measurement results!

The sockets on the lower long side of the BMS, labeled NTC and UART are used to connect the temperature sensor (NTC) and the Bluetooth module (UART), which is used to establish the connection to the BMS app.

Battery and distributor connection

With the Victron Lynx Shunt and Distributor, Victron enables the measurement of incoming and outgoing currents up to the 1 kA range, as well as the pre-fusing of connected load strings.

The data collected is displayed on the touch panel via Cerbo GX.

The battery is connected to the inputs of the Lynx shunt using short, thick and short cables designed for the expected currents.

All M8 screw connections must be tightened to 14 Nm.

Lynx Shunt and Distributor together weigh 3.6 kg.

MPPT controller connection

The output of the MPPT controller is connected to one of the distributor outputs. If several MPPT controllers are in use, they can be combined on one terminal of the distributor.

The MPPT controller weighs 4.5 kg.

Charging booster / charger connection

The outputs of the charge booster and charger are connected directly to one of the distributor's outputs.

The charging booster weighs 1.8 kg, the charger 1.3 kg.

Consumer connection

The sum of all currents of a supply line multiplied by 1.35 gives the size of the back-up fuse to be used. If 105 A are expected, the fuse is 105 x 1.35 = 148 with 150 A.

Consumers are always protected individually via appropriately calculated fuses.

Cerbo GX and GX Touch connection

The Cerbo GX "data collector" is connected to the GX Touch Panel via the supplied connection cable. Devices that communicate with the Cerbo GX are connected to it via a VE.direkt cable or via a USB-to-VE adapter. Both must be ordered separately if required.

External battery connection

An external battery connection can be used, for example, to connect two vehicles, both of which have PV modules and a battery pack, in order to make energy that is not required in one of the vehicles usable for the second vehicle with higher consumption.

A further, external MPPT controller PV connection can be used to connect additional external PV modules to the input of the MPPT controller in order to achieve a higher solar yield and greater power parameters. Provided that the installed MPPT controller has the corresponding power reserves.

Here is the Victron Smartsolar MPPT controller 250/100 Tr VE.Can which generates a charging current of up to 100 A at 12 V DC. According to the data sheet, the maximum PV open-circuit voltage is 250 V.

A fuse of 80 A should be looped into the respective positive line of the external connection socket in both vehicles in order to intercept excessive currents on the one hand and to provide short-circuit protection on the other. A circuit breaker in the positive line is also recommended so that the external battery connection sockets can be switched off when not in use.

Conclusion

A solid unit, which contains all components in one block, ultimately provides 1,120 Ah, weighs around 145 kg and, despite its compactness, allows access to all connections of all components without dismantling individual devices.

Notice

All electrical installation work must be carried out by qualified personnel in compliance with applicable regulations. When working on electrical installations, they must always be kept safely disconnected from the power supply!
To inform uninvolved third parties, appropriate information signs must be attached to the shutdown devices during the work.

ps If you need personal support in the implementation for a fee, you are welcome to Ticketing make!