Is a BMS after the 12th printed circuit board possible?

Victron Lithium "LiFePO4 Smart" battery FAQ

Welcome. You are interested in Victron Lithium (LifePO4) Smart Batteries with external BMS in the motorhome, or have you even bought a lithium system? With this article I want to help you. The first part of the article is about planning in advance so that you can decide for yourself which parts to buy.In the second part of this article I will describe the installation procedure. Expertise about the motorhome electrics and safe working is a prerequisite - these are not instructions for the layman, but are only intended to provide assistance for the connection, so that you know how all parts are to be connected to one another.

All of this is too complicated for you and you just want to replace the lead battery in your vehicle? There are also lithium batteries with an integrated BMS.

LiFePO4 battery in the motorhome

In the meantime, word has got around that lithium batteries are almost the perfect battery for motorhomes. LFP (LIFE) batteries are light, extremely durable and extremely resilient. To do this, they do not have to be constantly fully charged like with batteries, but can also be used for weeks with a medium charge level. All optimal conditions if you would like to be self-sufficient with the motorhome.

Conversion of LiFePO4 in the mobile home

You are interested in lithium batteries and of course a few questions need to be answered before buying. The most frequently asked questions now follow in a short list.

  • What capacity should the new lithium batteries have
  • What is needed to operate a LiFePO4 battery in the motorhome
  • Does the charger have to be replaced?
  • Can the solar regulator stay in?
  • Do you need a charging booster for lithium batteries?
  • Can lithium batteries be installed horizontally?
  • Can I retrofit additional batteries later?
  • Can lithium batteries with different capacities be combined?

How many Ah in lithium can a lead battery replace?

Which lithium battery should I buy? That is probably the most asked question. Here is the answer: For a long life, lead batteries should not be discharged more than 50%. This then results in the usable capacity of half an ampere-hour amount, which is printed on the battery. A 90Ah GEL battery would give 45Ah of usable capacity. 45 Ah at 12V are 540 Wh. Lithium batteries can be discharged down to 0% and even then there are still 1000 cycles according to the manufacturer's data sheet. If you only use 80% of the capacity, the number of cycles increases to 3-5000 - a gigantic difference to lead-acid batteries. As a rule of thumb you can say that 50% of the capacity of a lead battery would be in lithium. If you want a little more security, I recommend 60%. Here is an example: You want to replace a 100Ah lead battery with a lithium battery: 100Ah x 0.6 = 60Ah You should therefore consider a 60Ah lithium battery. If you have enough battery capacity so far, you can count on these numbers. However, if the old lead-acid batteries were too short, you will need more capacity. Below is a small formula for determining the capacity for your power consumption. Roughly said: Daily consumption x days of self-sufficiency without battery charging x 1.2 = required battery capacity. There's no point in asking me what capacity you need. YOU need to determine your power consumption, then you will also know which capacity to buy. At you will find a guide article on planning a self-sufficient motorhome - there is a lot of information that can help you with the planning.

What is needed to operate a lithium battery in the motorhome?

A battery management system (BMS) is required for the smart lithium battery. In this system, Victron has not integrated the BMS into the batteries, which leads to significant cost savings if you want to install more than one battery in the vehicle and increase operational reliability. An external BMS has the advantage that there is no electronics in the battery that could prevent the battery from delivering current in the event of a technical defect. Victron Energy lithium batteries are therefore fail-safe as long as none of the 4 cells say goodbye. But do not worry, such defects are a rarity. Victron Energy has two BMS systems to choose from: The BMS 12-200 is the most common BMS for motorhomes with a 12V electrical system. It is only looped into the negative line directly after the battery and then ensures safe operation of the batteries. The VE-BUS BMS is much more complex to install, as some additional switching components such as battery protect, cut-off relays for chargers and control lines have to be connected. It is only worthwhile on a 12V system if the inverter has to deliver more than 2000 watts. The preferred use for the VE.BUS BMS is battery systems higher than 12V or complex systems with special requirements. The lithium battery is ready for use with the battery and a BMS. More is not necessary for pure operation! The BMS12-200 takes over all protective functions fully automatically.

I summarize: lithium battery in motorhomes with BMS

  1. Victron Energy LiFePO4 battery (60-300Ah)
  2. Victron Engery battery management BMS 12-200

Recommended accessories, if not already available:

I recommend a battery computer, namely the BMV712 from Victron Energy and not the Votronic battery monitor. The Victron BMV can be set very precisely to the lithium battery and works here with perfect precision, even for weeks without a full charge. The BMV712 was specially developed for lithium batteries and impresses with its extremely low power consumption.

I also recommend a manual one Main switch directly after the battery in the negative lead. To be able to store the system safely and unattended. If a solar system is installed - another switch should be installed in the solar line so that it can also be switched off when the vehicle is not in operation.
IF the vehicle is in a dark hall and the battery is not disconnected from the BMS and battery computer, the minimum power consumption of the BMV and BMS ensure that the battery is discharged at some point. The BMS will switch off at some point, but the self-consumption of the BMS AND BMV will continue to discharge the battery until death - therefore a main switch to prevent this. (The same thing happens with lead batteries if they are not charged, but their loss is usually easier to get over.

In order to be able to connect all parts together, you still need Battery cable. I can deliver these ready-made.

The following is a suggestion for a shopping list with a lithium battery, BMS, battery, battery disconnect switch and battery cable:

Matching battery cables:

  1. Battery to circuit breaker. For Victron batteries up to 200Ah: 22mm² 50cm M8 / M10 (the 300Ah battery has M10 connections !!: 22mm² 50cm M10 / M10)
  2. From isolating switch to BMV shunt: 22mm² 30cm M10 / M10

To clarify, the following is a sketch of how the lithium battery is connected in the motorhome. Please note - that the battery may really only be connected if you want to continue using the original electrics in the motorhome.

As you can see, after installing the battery with the BMS you only have 2 poles where you can connect the cables of the motorhome - we used to do the same with the lead battery.

Does the 230V charger have to be replaced?

No, as long as you only use shore power for charging! Most of my customers upgrade to lithium batteries so that they don't have to be plugged in every few days.
However, if you want to connect the charger for longer than the actual charging time, then the charger must be able to output a trickle charge voltage of 13.5V. Most pure lead chargers can only handle 13.8V and are therefore unsuitable. All 230V chargers here in the shop are suitable for lithium batteries - I would recommend the devices from FRARON.

Can the solar regulator stay in?

If the solar charge controller cannot be set to a float charge voltage of max 13.5V, it is not suitable for charging lithium batteries. All MPPT solar charge controllers here in the shop have a characteristic curve for lithium batteries. I would prefer to use the charge controllers from Victron Energy. More on this in my blog article on the correct charging of lithium batteries.

Do you need a charging booster for lithium batteries?

The higher voltage of lithium batteries increases the problem that the battery is not properly charged while driving. In new vehicles with Euro 6, the lithium battery can even discharge while driving. Then a charge booster is definitely recommended.

LiFePO4 batteries cannot be used below 5 ° C?

Very often panic is spread unnecessarily about lithium batteries that cannot be charged below 5 degrees. The old LiFeYPO4 cells with yttrium are very heavy and have a low energy density. Victron Energy is gradually converting the lithium system to light LiFePO4 cells - this has the disadvantage that the cells should not be charged below 5 ° C. Since the batteries are installed in the interior of the motorhome, it can be assumed that whenever you charge the battery (because you are on the move) the temperature will also be above 5 ° C. Is the vehicle parked over the winter? If the battery is switched off anyway and is not being charged - the temperature does not play a role either.
If you have a Put a cold vehicle into operation, the BMS system of the battery will slow down the charge until the critical temperatures are exceeded. Then it will be charged normally. The batteries are therefore protected by the BMS.

Can lithium batteries be installed horizontally?

No with Victron Energy, yes with the FraRon and Liontron battery

Can I retrofit additional batteries later?

Yes, an additional battery can be added even 2 years later.

If I connect more than one battery, how does that work with the data cables?

In this case, the data lines are connected in series; this is the only way for the BMS to react to all batteries. The batteries themselves are connected in parallel as usual with plus to plus and minus to minus in a 12V network.

Can lithium batteries with different capacities be combined?


Why do you need a Victron BMS?

The LiFePO4 battery from Victron Energy consists of 4 lithium cells which are connected in series. The whole thing is surrounded by a robust, blue housing with a plus and a minus pole at the top and two “data cables” going out on the side. There are no electrical components that could fail between the lithium cells and the external battery poles. Parallel to the large number of lithium cells, balancers are installed in the battery, which can even be called up via Bluetooth. Should these electronic components fail, the battery will still be able to deliver electricity.

High operational reliability through external BMS

The lithium batteries from Victron are therefore considered to be absolutely fail-safe as long as all cells in the battery are functioning. Other manufacturers install the battery management in the housing and intervention in the event of a technical defect in the monitoring unit is only possible to a limited extent. Even if I am not aware of any failure of a Victron BMS, there is still the certainty that the battery can still be operated without the BMS “in an EMERGENCY” and that one would not sit dry.

The BMS is not a charger!

The battery management is actually just a battery protection. The BMS receives information about the status of the individual lithium cells in the battery via the 2 data cables of the battery. Should one of the four cells reach a critical voltage, be it undervoltage or overvoltage, this is reported to the BMS and it can act accordingly.

The BMS will not have to intervene during normal operation of the Victron lithium batteries. The chargers alone are responsible for correct charging. Should a charger fail and deliver too high a voltage to the battery, overcharging is prevented before a critical cell voltage is reached by switching off the charging current.

The BMS can act as deep discharge protection: Just like with overcharging, the BMS will also switch off loads to prevent deep discharge.

However, the following should be noted: After an emergency shutdown, the battery has a remaining capacity of 1-2 ampere hours. This buffer was created to allow recharging for the first 10 days without damaging the cells.
Because the BMS and the battery computer continue to discharge the battery with a minimal amount of current even after an emergency shutdown.

That is why I always advise that a lithium battery system is completely switched off when not in use in order to prevent discharge by so-called silent consumers. A battery that is disconnected from the mains can be stored for 6 months without any problems with a charge level of more than 50%! You can query the battery voltage at any time via Bluetooth.

NOTE:An unused battery will continuously discharge itself over time due to the power consumption of the BMS and BMV (Battery Computer). The BMS will switch off when the end-of-discharge voltage is reached, but its own consumption will continue to discharge the battery!It is therefore essential to ensure that lithium batteries are not connected to the BMS if they are not used for a long time without supervision.

Function of the BMS in detail

The battery management does not take over the function of the charger, but it is a pure protective element, which intervenes in a protective manner in the event of a cell undervoltage or overvoltage in the battery. The BMS switches off the consumers by suppressing the currents from the battery when a critical state of charge is reached.

Or vice versa, charging currents are switched off when the voltage of a cell in the battery reaches a critical level. In normal operation, however, the BMS does nothing and waits silently until it receives a command from the battery.

In normal operation, the BMS is most likely to intervene as protection against deep discharge - if the battery is discharged, the BMS switches off consumption so that the lithium battery is not damaged. In order for the BMS 12-200 to work properly, it must be looped into the negative line directly after the battery. Only a main switch and the battery computer are permitted between the battery and the BMS.

BMS12-200 two devices in one

The BMS12-200 for Victron Energy lithium batteries is actually a device for 2 tasks. There is a right side for consumers and chargers, which is marked with LOAD. And there is a left side - marked CHASSIS, which can be used as an isolating relay. BMS12-200 as battery management: The main function of the BMS 12-200 is to protect the battery from all operating conditions.

  • Temperature protection The BMS12-200 will cut off the current flow if the battery temperature exceeds 50 degrees. The BMS12-200 will also limit the charging current to the battery at temperatures below 5 ° C. Operating the battery in winter is therefore harmless.
  • Overvoltage protection If a cell reaches a critical voltage, the BMS will reduce the charging current so that the cells cannot be overcharged. In normal operation, when all chargers are working, no intervention by the BMS is necessary.
  • Undervoltage protection If the battery is discharged, the BMS will prevent further current draw. The remaining capacity in the battery is then still sufficient for the battery to be recharged within 10 days without damage.


All of these functions are available without any additional intervention. The BMS works fully automatically and also automatically releases the current flow as soon as the error message has disappeared from the battery. As soon as you have charged the battery a little, it will be possible to draw power again after it has been switched off due to undervoltage.

LOAD connection of the BMS 12-200

This connection works in both directions, so the current flow to the battery can be interrupted due to overvoltage, overtemperature or undertemperature or the current draw from the battery due to undervoltage or overtemperature. It is a bidirectional actuator that not only knows ON and OFF, but is also used PWM modulated to limit the current.

Chassis connection of the BMS12-200

The chassis connection is purely a charging input. It has nothing to do with the classic ground connection in the vehicle. This connection allows the battery to be charged when the voltage exceeds 13.5V.

The chassis connection can also limit the current. A maximum of 80% of the fuse value is allowed through as maximum current. With the factory-fitted 100A fuse, this corresponds to 80A charging current. You can set the charging current yourself with the enclosed circuit board, on which 2 flat fuses can be inserted. The 100A fuse is then exchanged for the fuse holder, which can be fitted with appropriate flat fuses. E.g. 2x 30A for a current limit to 48A.

Danger:This connection is not used in normal motorhomes! Chargers and consumers often run through the EBL and come to the battery in a cable. A separation is not possible here, and therefore ALL cables are connected to the LOAD connection and the chassis pole remains unused!

Installation of the lithium battery with the BMS12-200 in a mobile home

The connection of a lithium battery system from Victron Energy is quite simple.The battery and the BMS form a unit. The cables from the vehicle are connected on the plus side directly to the battery pole and the minus lines are connected to the BMS12-200, which is the minus pole. Between the BMS and the battery, an approx. 50cm long connection cable is required, which should have a 22mm² cross-section to cover all possible currents.

Installation of the lithium battery in 7 steps:

  1. Removal of the old battery and preparation for the lithium battery Remove the mains plug on the motorhome and shut down the solar system by removing a cable from the modules on the controller. Disconnect the battery: To avoid confusion later due to different cable colors, I recommend first removing all negative cables from the battery terminal and bundling them with adhesive tape or a cable tie. The same is then done with the cables of the positive pole. Dismantle the battery and remove the holder that is in the way for the lithium battery
  2. Test sitting the LiFePO4 battery including the BMS Of course, it is useful if you know beforehand that the new battery will also fit the desired location. Nevertheless, after the battery compartment is empty, I would first try to put the lithium battery in the new place and see whether the cables reach the connections and where the BMS can be mounted. According to Victron, the battery must be installed in an upright position. The BMS can be installed on the ceiling, on the side or on the floor. It is important that the data cables from the battery reach the BMS and, ideally, the battery cables from the vehicle also extend to the BMS and the positive battery terminal. Don't forget: With the option with a battery computer or an additional main switch, the two components also have to find a place in the battery box. The battery must also be secured against sliding and flying. This can be solved with brackets screwed to the floor. A tension belt over the battery, which is also either screwed to the vehicle or connected with appropriate lashing eyes, later holds the battery securely in place. Creativity is required here, the installation situations are very diverse and I cannot give instructions here.
  3. Installation of the components Installation usually takes place in the following order. I mount the BMS12-200, the shunt from the battery computer and the battery disconnect switch in the battery compartment. Then the lithium battery comes into place and is secured with a tension belt.
  4. Electrical connection All negative cables that go to the motorhome electrical system. I really mean all of them, including those from the solar charge controller, the inverter, the charge booster, etc., are connected to the LOAD connection (the outer right M8 bolt). Establish power supply for the BMS and BMV: Lay the thin, red cable enclosed with the BMV from the shunt of the BMV via the BMS to the positive pole of the battery. In order to be able to supply the BMS with power, it must be cut at the appropriate point, both ends stripped and connected together with the blue flat plug that can be found in the small bag. You sketch! All positive cables, including the thin cable from the BMV / BMS, are now connected to the positive terminal of the lithium battery.
  5. Connect the negative battery cable: Attach the M8 / M10 connection cable between the middle M8 bolt (Li battery) on the BMS and the battery computer shunt (LOAD). Attach the M10 / M10 connecting cable between the shunt (BATTERY) and the isolating switch. Set the isolating switch to OFF. Attach the M10 / M8 or M10 / M10 connecting cable between the isolating switch and the negative pole of the battery. and check all nuts / bolts for tight fit. Above all, tighten all the nuts on the BMS, even those to which no cables go. Checking whether plus and minus have not been swapped anywhere can not hurt!
  6. Connect the data lines from the battery to the BMS. If there are several batteries, the lines are connected in series so that only 2 cables remain. These are then connected to the BMS.
  7. Turn on the main switch, then reconnect the solar controller to the solar modules.

The system is now in operation and, ideally, everything should work again in the motorhome. The battery computer must now be set and all chargers should be set to lithium characteristic or LEAD WET (Lead Accid). So not AGM and not GEL. In order for the battery computer to display the correct values, the battery must be charged to 100% once. The end-of-charge voltage should be between 14.1V and 14.5V. When delivered, Victron LiFePO4 batteries have a charge level of around 40%, which corresponds to around 13.15V. Depending on the charger, it can take several hours until the battery is charged to 100%.