Balancing a Tesla module

Using a Tesla module from a Model S or X is very dense storage of electric energy. Three main things is to consider: Charging within specs, Keeping temperature stable and Balancing the cell groups. This is going to be about the Balancing of the Cell groups.

The Tesla module consists of 444 individual 18650 Lithium cells arranged in 6 groups of 74 cells. The individual cells are very similar but over time can the groups get out of balance. When imbalance happens is the voltage of each group different.

A Tesla module should never be charged to full voltage 25.2 Volt (6 x 4.2 Volt), any imbalance is going to force the group higher than the 4.2 Volt maximum per cell. Another reason to not charge to 100% State of Charge is degradation of the battery, In my opinion, is staying below 80-90% State of Charge a safe range (we use ~80% as full charge, 24.1 Volt for the module or 4.02 Volt per cell). Staying below 100% State of Charge both reduce degradation and leave room for imbalance.

Already at 1% imbalance is the last cell group entering dangerous territory. This is, of course, an optimal bad case for the calculation, when only one group charges too high. The Tesla modules are very balanced more than 99.9% is my estimate. Simply removing a small amount of energy each day from the last groups can balance the complete module. This happens in the car by the integrated Battery Management System (BMS). Using a single module require a similar function to be applied. Solutions exist that use the Tesla board on the module for balancing too, but there are other options available too. Looking into the options used by Radio Controlled Plane pilots to balance the cell packs is a cheap option.

The first option we deployed was Tenergy 5 in 1 Cell Meter (~$10). This does a great job balancing the groups of cells and is precise to 1 millivolt (0.001 Volt). The downside is that this requires manually to trigger a balance to happen. Very similar to the maintenance task required for flooded Lead-Acid batteries, except no holes in your cloth.

We now use a HobbyKing Battery Medic (~$15). The device has an accuracy of 10 millivolts (0.01 Volt). The device can be set for a threshold voltage and balancing occur automatically above that voltage. We have set ours to 24.0 Volt. So when the batteries are close to fully charged is a small amount of energy removed from the group with 10 millivolts higher than the lowest. This process cycle on and off since the thin wires add a large voltage drop when the resistor is applied, so a few seconds balance, a few seconds stabilizing and measurements repeat all the time.

The device can get hot during operation so it is a good idea to not cover the rear ventilation and keep the device sitting vertically to allow the warm air to rise. We have been using this solution for over a year now with none of our five devices failing. When measuring on the batteries is the balance of the cell groups perfect.

Other option also exists like the Batrium system (~$350) that provide a lot of useful information but has a higher cost and require a lot of wiring since only a single group module exist and a Tesla Module requires 6 LongMon modules and a central control unit.

So balancing a single module is very easy and cheap using the Radio Controlled Plane balancers. What if you want to put 2 modules in series for 48 Volt? This is a case very similar to balancing the 6 groups in a single module, but with a very different level of energy. Using a Battery Medic on each module could seem like a good option, but is not working. Let's say that one module never has imbalances and require no removal of energy, The other module has a slight imbalance and requires removing 0.01% when charged to keep it balanced. Over time is this 0.01% removal of energy from only one module driving the perfect module to a higher voltage and the worse to a lower in each charge cycle. Within months causing the perfect module to exceed 25.2 Volt and entering dangerous territory. A 12S solution must be applied when using modules in series.

A way to reduce the number of balancing modules is also to have all modules on the same balancer. If two modules charged to the same Voltage and perfectly balanced is it possible to parallel the balancing leads and a single balancer used. We decided against this option because a disconnected cable from one module could not be detected with a simple inspection of the balancing devices, but with a more expensive Batrium solution is this both reducing wiring and cost.

10 thoughts on “Balancing a Tesla module”

  1. Hello Per and Susanne. Thank you so much for sharing this amazing knowledge with the word!! I am learning so much just by using your videos and blogs. I am very interested in putting together a similar system for my sailboat. The sailboat and sail community has very similar systems as your mobile home and the needs and the issues are very similar. I am thinking of starting with 2 Tesla modules and hoping that would be plenty for our needs. I wonder what your thoughts on the BMS, chargers, and investors, have you looked at Victron before making your decisions? I also was wondering if you share your electrical diagram? Thanks again!

    1. The Tesla module route is an engineering solution. It is very dense and light for the kWh provided but everything around using that as a House battery solution is about balancing, temperature control and staying within the limits. A Lithium drop-in replacement is a better and safer option for most unless you have the engineering skills to make it work and wnt to run the risk.
      The reason we settled for Magnum inverter was that the current setup wit ha Magnum MS4024 was a drop-in replacement with similar wiring on the 120 Volt side. Starting from scratch would I definitely consider Victron’s equipment. It is highly customizable and delivers great integration.

  2. Makes sense on Magnum. I completely understand that Tesla battery module requires engineering and very careful management of charging, discharging and close attention to the temps lows and highs. I am currently concerned that Tesla battery module is not truly 24V battery 6P instead of 7P that dominates the marine market. I am concern that the “useful” voltage range is 19 to 24V is too low for anything that has a DC motor on the boat – water pressure pump, refrigeration compressor, emergency water pumps, watermaker pumps and etc. The electronics do not worry me as all navigation equipment is 12V/24V capable, the Victron Quatro cut off voltage is 19V. Do you have any experience with this limitation?

    1. You are not going to run anything directly from the Tesla Battery except the Inverter and a DC 2 DC converter that makes a stable “12” volt, we have a Samlex that do 60 Ampere and 13.8 Volt all the time from 0 to 50 Ampere at least.
      The Tesla Module is in the low range of the inverter but is such a strong performer that it does not have the other Low Voltage issues that Lead-Acids do.

  3. That was a typo “Tesla battery module is not truly 24V battery 6P instead of 7P” should be Tesla battery module is not truly 24V battery 6S instead of 7S..

    1. Depends on that you call a 24 Volt battery. The Tela Module operates from 25.2 to 19.8. The cell has a linear discharge curve and a not like a drop-in LIFEPO4 battery that is stable for 80% of the range.

  4. my boat’s systems are designed for “24V”. I have 8 Trojan T-105 batteries 4 in series and 2 groups in parallel. When the batteries are fully charged the voltage is 25,6V and when they are 50% charged the voltage drops to 24V and drops to 21V when 0% charged. I think all the systems like refrigeration and other DC systems rely on the narrow voltage between 28.4V(charging voltage) to just 24V when the lead-acid batteries 50% charged and need to be recharged again. This would present an issue for marine systems of chronic low voltage. I speculate that most of your systems are rely on the inverter and you don’t have many 24V DC motor driven systems … id that true?

    1. I only have experience from my Magnum inverter and DC to DC converter with 24 Volt. Both are designed to work down to 20 Volt and in the case of the Magnum Inverter instant disconnect at 19 Volt. Only high draw systems should be an issue when you have smaller systems like a GPS is the draw very low and internally is it probably 5 Volt electronics.
      I can see an Anchor Winch maybe having an issue with the lower voltage. But a lot of these things benefit from the more stable voltage from a Lithium battery.
      Even the price looks great on the Tesla modules is the 24 Volt Battleborn maybe a better option. A Tesla module used between 24.1 Volt (80% SoC) and 21 Volt (22%) is 58% x 5300 Watthour = 3.1 kWh for a price of $1200 with supporting electronics. A single Battleborn is $950 and 1.3 kWh used from 100% to 10% (24 Volt) or 2.6 Battleborn Battery for a single Tesla Module $2246 for secure drop-in replacement within normal operating range or $1200 for an engineering solution.
      Unless you are aiming for 10-15 kWh (4 Tesla Modules) or more should you go with the Drop-In replacements. 2 Tesla modules is not a stable enough power source for high loads like a 4000 Watt Inverter. You can see more information here http://offgrid100.com/tesla-module-load/
      You can get a DC 2 DC converter that pumps up/stabilize the Volt, but those are expensive especially as you need to support high loads. Below is a breakdown of the price and weight for the mentioned system.
      5 kWh Quantity Watthour Price Weight
      Tesla 2 6148 $2,400 110
      BB 5 5850 $4,750 145
      Trojan 8 5280 $1,600 528
      10 kWh
      Tesla 4 12296 $4,800 220
      BB 9 10530 $8,550 261
      Trojan 16 10560 $3,200 1056
      15 kWh
      Tesla 5 15370 $6,000 275
      BB 13 15210 $12,350 377
      Trojan 24 15840 $4,800 1584
      20 Kwh
      Tesla 7 21518 $8,400 385
      BB 18 21060 $17,100 522
      Trojan 32 21120 $6,400 2112

  5. Thank you for the info. Yes, I the boat has some BIG loads from a windlass, powered winches and a bow thruster that can consume each up to 250A depending on the load. What do you think if I use 2 Tesla modules in series and have 48 to 24 DC-to-DC converter?
    I wonder if they have one that can supply this amount of amps and how much would it cost?

    1. You need multiple ones in parallel to supply 250 Amp. Only the “Isolated” type of DC2DC converters allow parallel operation and are very expensive like $300 for 60 Ampere. So you need 5 or $1500.

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