How do you balance battery packs in series?

You may be looking for information on one of two things, and we’ve included both on this page. Either:

• Balancing 24-volt systems (connected in series); or
• Balancing series-connected EV modules/cell groups

Both these processes are incredibly important for your fleet operations, although for slightly different reasons. You can learn more about how these processes work below.

Balancing 24-volt series systems

The 24-volt systems used in trucks, buses, tractors, and so on are made up of two 12-volt batteries in series.

Why do the 12-volt batteries in 24-volt systems become unbalanced?

Even two brand new 12-volt batteries of the same make and model contain microscopic differences in internal resistance, charge acceptance and molecular chemistry. As the batteries charge and discharge, these small variations mean each battery responds slightly differently to the same current. Over time, these differences accumulate, causing the batteries’ state of charge to drift apart.

During charging, for example, the alternator regulates the overall system voltage rather than the voltage of each individual battery. Because the batteries are connected in series, the same current flows through both. If one battery reaches full charge before the other due to those internal differences, it may begin to overcharge while the other remains undercharged.

This imbalance means one battery may never reach full state of charge (SoC). Over time, the undercharged battery becomes weaker, which places additional strain on the system during engine cranking and electrical operation.

Important PPE considerations

Take the following PPE considerations into account before beginning the balancing process.

• Remove metal jewellery such as rings, watches, or bracelets before working around batteries. These can cause severe burns if they bridge battery terminals.
• Wear appropriate gloves, overalls and safety goggles to protect against acid splashes and sparks.
• Use insulated tools wherever possible to reduce the risk of accidental short circuits.
• Ensure the work area is well ventilated. Lead-acid batteries can release hydrogen gas during charging, which is highly flammable.
• Avoid open flames, sparks, or smoking near the battery system.
• Be wary of ratchets or spanners short-circuiting across the battery terminals.
• Disconnect the negative terminal first when isolating batteries and reconnect it last.

How to test the balance of your 24-volt system

The best way to assess the balance of your 24-volt system’s two batteries is to use a dedicated tester. One of the best options is the Midtronics CPX-900, which uses built-in algorithms to check each battery’s SoC and state of health (SoH) individually. If it discovers an imbalance, it recommends charging or replacing. Here’s how to use the CPX-900 to test the batteries in your 24-volt system:

1. Select the battery age (New Battery or In Service) and press Next.
2. Choose the battery’s location (Out of Vehicle – removed from the vehicle or In Vehicle – still connected up and in place). Select Next.
3. Enter the fleet ID/VIN. Press Next.
4. Pick the vehicle type (Bus/Truck or Car/LCV). In this case, you’ll choose Bus/Truck. Select Next.
5. If enabled, enter the battery ID. Press Next.
6. Select the battery chemistry (Flooded/AGM Flat/AGM Spiral/EFB/GEL). Push Next.
7. Choose the rating units. Select Next.
8. Use the Up and Down arrows to change the rating. Press Next.
9. When prompted, aim the tester about 5cm from the sides or top of the battery case to read the temperature.

The CPX-900 then conducts an automatic test of the two 12-volt batteries in series. In some cases, the tester may require some additional steps. It walks you through these on the screen, if needed. Results can be automatically uploaded via wifi to the cloud-based battery data platform ROBIS, as well as viewed on-screen and printed via a Midtronics printer.

How to charge the batteries in your 24-volt system

If the tester establishes that the batteries are imbalanced, the next step is usually individual charging and re-testing. The key here is to ensure individual charging at 12V each, not 24-volt charging across both batteries.

Use a workshop charger such as the ChargeXpress Pro 100-2. This dual output charger with battery balancing technology is well-suited to fleet applications because it ensures each battery is charged to the same level as the other.  This process can restore the battery to near 100% state of charge and helps equalise the cells internally. Once the first battery is fully charged, repeat the process with the second battery (or have it charging with a second charger at the same time).

Now, either reconnect the two 12-volt batteries in series in the vehicle and perform another 24-volt test; or use the CPX-900’s Generate Pair function. This checks the two batteries to see how well they match as a pair.

Either way, these re-tests should confirm that the batteries are now much closer in balance. If they aren’t, at least one of them is likely damaged beyond repair, requiring replacement.

Balancing EV battery modules in series

All EVs use different battery pack designs. However, most follow a similar structure. Individual cells are typically grouped in parallel to increase capacity, forming a cell group. These groups are then connected in series within modules or across modules to form the overall battery pack.

Why do EV battery packs become unbalanced?

EV battery packs can become unbalanced in much the same way as the 24-volt systems mentioned above. A modern electric car may contain hundreds of individual cells arranged into parallel groups and modules to form the overall pack.

These cells are manufactured to extremely tight tolerances and may appear identical when they leave the production line. However, repeated charging and discharging gradually amplify small differences between them. Over time, some cells degrade faster than others and develop a poorer SoH, meaning they can store less energy than the rest of the pack.

When an EV charges, the same current flows through all cell groups connected in series. To prevent individual cells from exceeding their safe voltage limits, the battery management system (BMS) uses a process known as cell balancing.

In the vast majority of EVs, this takes the form of passive balancing. Each cell group is connected to a small resistor controlled by the BMS. When a cell group approaches its maximum voltage limit, the BMS activates a resistor that bleeds off a small amount of energy as heat. This slightly reduces the net charging current for that group, allowing the other cell groups to continue charging until they reach the same voltage. Passive balancing occurs near the top of the charging cycle, when the charging current is low enough for the balancing resistors to gradually equalise cell voltages.

However, because the cells are connected in series, the charging current must still pass through every cell group. The resistor cannot stop current flowing into a weak cell. It can only dissipate a small amount of energy from stronger ones. If a cell group has significantly poorer SoH, it will reach its voltage limit earlier than the rest of the pack. At that point, the BMS must reduce or stop charging altogether to prevent overcharging and potential thermal runaway.

In other words, your battery pack is only as strong as its weakest cell.

Important safety considerations

The high-voltage propulsion packs in EVs can be very dangerous. Any work on them isn’t something to be taken lightly. Ensure you adhere to the following general safety considerations:

• Wear electrically rated insulating gloves appropriate for the voltage involved. Where required, use leather protectors over the gloves.
• Wear safety goggles or a face shield, along with suitable protective clothing and safety footwear, to reduce the risk of injury from arc, sparks or electrolyte exposure.
• Remove metal jewellery such as rings, watches, bracelets and necklaces before starting work.
• Use insulated tools rated for electrical work, and do not allow tools to bridge terminals or contact exposed high-voltage components.
• Isolate and de-energise the high-voltage system in line with the vehicle manufacturer’s procedure before touching connectors, modules or busbars. Always assume the system could still be energised until it has been proved safe.
• Never work on a damaged, leaking, overheated or swollen battery pack without following the manufacturer’s high-voltage safety process. Battery electrolyte and gases can be hazardous.
• Ensure the work area is controlled, clearly marked and kept dry. Do not work near ignition sources, and do not touch exposed orange high-voltage cables or components unless the system has been made safe.

The best tool for balancing EV battery modules in series

One of the best tools for simplifying balancing EV modules or cell groups in series is the Midtronics xMB-9640.

It uses built-in algorithms to automatically detect individual modules and verify their voltage levels. It can then charge or discharge each module to a manually entered target voltage, at up to 40A below 48V and 20A up to 96V. This makes the xMB-9640 much faster at module-level balancing than lower-current service tools.

Because modules themselves are connected in series within the battery pack, equalising module voltage also helps restore balance across the series chain of cell groups inside the pack.

If you’re looking for a broader EV service tool, including de-powering and rescue charging, consider the GRX-5100. This can be used for balancing, but it has a much lower max current (about 5A), so it takes a bit longer. However, its other benefits may make it worthwhile in your EV workflow.

How to balance EV modules in series

Using a tool like the xMB-9640 is the most effective way to balance your EV fleet’s series-connected battery modules.

1. Before beginning any module balancing procedure, follow the vehicle manufacturer’s high-voltage safety process to prepare and isolate the battery pack. EV battery packs can carry several hundred volts, so modules should only be serviced once the system has been made safe.
2. Confirm that all the appropriate safety equipment and PPE is in place. Choose Confirm.
3. Select Module Balance.
4. Choose the module type.
5. Enter the target module voltage and press Next.
6. Verify the individual cell voltage, temperature, cell delta, and module voltage, and press Next.
7. After everything has been verified, press Next to begin the balancing procedure automatically. You don’t need to do anything at this stage.
   1. You can end the procedure at any time by pressing Stop.
8. When finished, you’ll see the results on the screen. Send your results to ROBIS by pressing Send Results in the bottom left.
   1. You can also send results to a WiFi printer, by email or by USB transfer.

Balance your battery packs with Rotronics’ trusted tools

Rotronics is the leading expert in battery testing, charging and maintenance solutions. We stock a wide range of trusted products from Midtronics, CTEK, and more reliable brands. Work with our team of experts to develop a custom in-house battery maintenance schedule and workflow, and we’ll help you find the perfect tool for your job.

If you’re interested in learning more about our solutions for balancing battery packs in series, contact us today for an obligation-free chat.