Lithium-based rechargable batteries store enormous amounts of energy. They need electronic protection for safe operation:
- Fire Hazard: the stored energy can erupt in flames or explode when the battery is physically damaged or short circuited - or when it is charged below or above safe voltage thresholds where the battery cannot store the excess energy anymore.
- Permanent Battery Damage: permanent damage (loss of battery capacity or complete destruction) may occur when the battery is fully discharged. Discharge must stop at a certain minimum voltage. Else, the battery chemistry can suffer irreversible structural damage.
A BMS is important both for charging and discharging a battery.
Typical Risks Ruining Your Day
A BMS (Battery Management System) is an electronic circuit that protect from many risks. Without a BMS, you may easily run into serious problems while using lithium batteries:
- You charge the battery and leave the charger connected. Without a BMS (or a clever charger), your lithium battery will eventually catch fire.
- You connect a device to your battery and leave it on, then walk away. Once the battery is completely drained, it also is permanently damaged.
- You accidentally connect both poles of the battery and produce a short circuit. Enormous currents will flow and can hurt you, or the battery explodes.
That’s why most quality lithium batteries come with a basic BMS already integrated into them.
Important Protection Features
A sufficient BMS must provide these key protection features:
- Over-Current Protection
- Over-Charge Protection
- Over-Discharge Protection
- Load Balancing
What is over-current protection?
Over-current protection cuts off the battery once the current exceeds a certain threshold. Typically, over-current protection kicks in when the current roughly exceeds double of the continuous (normal) maximum current.
When this happens, i.e. due to a short circuit, the BMS enters lockdown mode. To reset it, connect it to a charger.
Over-current protection protects fire hazards: every battery can provide a maximum discharge current. If more current is drawn, the battery releases so much energy in such a short time that resulting heat can set the battery on fire.
In addition, this also protects your load. Should it encounter a failure or cause a short circuit, power will be quickly removed.
Most BMS have a latency of 100-200ms before over-current protection cuts off power.
What is over-charge protection?
This kind of protection becomes important when you charge your battery through the BMS.
During charging, the voltage raises steadily until the battery is fully charged. The BMS monitors the voltage. If the voltage exceeds a threshold that indicates that the battery is fully charged and cannot accept any more energy, the BMS cuts off the battery.
Over-charge protection prevents fire hazards from over-charging. If you keep charging an already fully charged battery, the additional energy coming from your charger can no longer be stored by the battery and is now converted to heat. Without over-charge protection, your battery would gradually heat up until it eventually causes a fire or explodes.
What is over-discharge protection?
When you draw energy from your battery, its voltage slowly falls. When the battery is almost empty, and you continue to draw energy, its internal cell chemistry will suffer irreversible damage, and your battery will permamently lose capacity or stop working at all.
The BMS monitors the battery voltage. When it falls below a threshold, indicating that the battery is almost empty, the BMS cuts off the load from the battery.
If the battery voltage has already fallen below this threshold, and you start charging the battery, over-discharge protection unfolds a second protection mechanism: it limits the current that the charger can deliver to the over-discharged battery until it reaches safe voltage levels.
This prevents yet another fire hazard: a lithium battery that is completely empty cannot store energy in the same way a healthy battery can. If you would charge it at normal current, most of the charging energy could not be stored and would start to heat up the battery. Eventually, the battery could catch fire or explode.
By limiting the current until the battery reaches a safe voltage level (and thus normal storage behavior), even completely drained batteries can be safely charged.
What is balancing?
When you connect more than one battery in series, the overall voltage is higher than the voltage of the individual batteries.
If you charged all batteries together, you would no longer be able to monitor individual battery voltages. Since batteries are not 100% identical, chances are one battery would be fully charged while another battery would still need energy.
Balancing protects from unevenly charging batteries by monitoring the voltages of all connected batteries individually, and adding extra charge to individual batteries if they lag behind other cells.
When a BMS includes balancing, each battery is connected individually to the BMS. Typically, at the begin and at the end of your battery string, there is a large solder pad. This is where the bulk of charging current is fed.
Then there are smaller solder pads for connecting each junction of two batteries. These connections feed the (much lower) balancing current to individually boost the charge of a particular cell that lags behind.
Balancing is a protection for charging and not used during discharging.
When you review specs for BMS and their voltage threshold for over-discharge protection, you may be surprised to see the threshold voltage often about 0.5V below the typically recommended thresholds. Keep in mind that over-discharge protection monitors the battery under load. When you draw energy from a lithium battery, its voltage drops by as much as 0.5V. In order to not accidentally cut off power prematurely, over-discharge protection voltage thresholds take this into account.
Battery Voltage Thresholds
The threshold voltages a BMS needs to monitor vary based on cell chemistry and manufacturer. That’s why you need a BMS made specifically for your battery type.
| Chemistry | Min V (Empty) | Max V (Full) | Nominal (During Operation) |
|---|---|---|---|
| LiIon | 2.7V | 4.2V | 3.7V |
| LiPo | 3V | 4.2V | 3.7V |
| LiFePo4 | 2.5V | 3.65V | 3.2 |
(typical values, consult your battery data sheet)
As you see, LiIon and LiPo batteries display roughly the same behavior and voltages. You can use the same BMS for both battery types.
LiFePo4 batteries display significantly different voltages and thus need different BMS that honor their voltage thresholds.
Using a Balanced BMS
When you connect multiple batteries in series (to get a higher output voltage) and plan to charge your batteries through the BMS, you should always select a BMS with balancer capabilities.
A balancer is used exclusively for charging and is not used for discharging.
With balanced BMS, each battery string is connected separately to the BMS.
The balancer makes sure each cell voltage is monitored individually. Without balancing, one battery could already be fully charged when another is not, resulting either in only partially loaded batteries, or risk of over-charging a battery.
BMS Is Not a Charger
One of the most common misperceptions is that BMS can also be used to charge your battery.
This is only partially true and in fact can damage your batteries. A BMS can assist a dedicated charger through its balancing capabilities but you should avoid charging batteries directly through your BMS.
While you can connect a power supply to the output terminals of a BMS and feed energy back into the battery, the BMS only limits the input current.
Typically, this current limit is much higher than it should be for charging your batteries. Chances are you will be over-charging your batteries with currents that are much too high.
BMS also typically do not employ sophisticated *CC-CV-charging cycles that should be used with lithium batteries to ensure a full load.
To charge a battery with a BMS alone, you would need to supply the proper charging voltage for your batteries:
| Strings (batteries connected in series ) | LiIon/LiPo | LiFePo4 |
|---|---|---|
| 1S (one battery) | 4.2V | 3.65V |
| 2S (two batteries) | 8.4V | 7.3V |
| 3S (three batteries) | 12.6V | 10.95V |
| 4S (four batteries) | 16.8V | 14.6V |
When building battery packs, always add a proper BMS and a separate charger board that takes the desired charging input voltage (i.e. USB 5V) and supplies it to the BMS output terminals for even distribution to the batteries.
BMS Needed Or Already Built-In?
Quality batteries come with a simple BMS built-in for basic protection. You can’t typically see the BMS. It is located inside the battery.
Since you can’t see the BMS, make sure your battery has one. Cheap batteries from untrusted sources may come without BMS and can easily explode when there is a short circuit or when you charge them for too long.
Adding a BMS
Adding your own BMS for DIY projects is always a good idea since you probably do not know the specs of the built-in BMS and whether it covers all important risks.
Once you start building your own battery constructs, i.e. by combining multiple batteries, you should always add your own BMS.
Choosing a Ready-To-Use BMS
You could be intrigued to build your own BMS circuit. Don’t do it.
There are plenty of ready-to-go BMS breakout boards available that are much cheaper than even the total component cost if built your own.
There are 4 questions to be answered to select the appropriate BMS:
- Battery Chemistry: you either need a BMS designed for LiIon/LiPo, or for LiFePo4.
- Strings: How many batteries do you want to connect in series? In other words: what is the intended total output voltage of your battery pack?
- Current: What is the maximum current you want to draw? In other words: at which current should the BMS disconnect the batteries to protect them (and also protect your load should it cause a short circuit or suffer another failure)?
- Balancer: Do you need balancing capabilities? In other words: do you use more than one battery and want to charge it through the BMS?
Never use a BMS designed for LiPo/LiIon and connect LiFePo4 batteries to it (or vice versa). The safe voltage range for LiIon/LiPo is 2.8-4.2V whereas the safe voltage range for LiFePo4 is 2.5-3.65V. Your BMS must enforce the safe voltage levels appropriate for the battery type you use.
Understanding Strings: 1S, 2S, 3S, 4S…
To select the appropriate BMS, you need to know how many battery strings you have.
BMS types use S as short for strings: a 2S BMS can be used with two batteries connected in series. So for the string count, only the number of batteries connected in series count.
If you connect batteries in parallel, they count as one string.
The reason why a BMS cares only about batteries connected in series is because this increases the voltage, so it directly affects the voltage thresholds the BMS is supposed to monitor. Batteries connected in parallel do not change the resulting voltage.
Here are some “string counting” examples:
- If you connect two batteries in parallel, this would be a 1S system. If you connect the same two batteries in series, you would have a 2S system.
- Should you use a total of six batteries, two of each connected in parallel, and then the three pairs connected in series, you would have a 3S system.
Maximum Current (Power)
A BMS limits the maximum current. That is one of its fundamental protection features (over-current protection).
Selecting the appropriate BMS requires that you know the maximum current you want to draw from your batteries.
One of the risks a BMS protects is over-current. By adding the appropriate BMS to your battery, you essentially also get a fuse: should your load require more power than anticipated, the BMS cuts off power.
Since BMS are not designed to routinely handle over-current and short circuits, still do use a fuse. If the BMS encounters an over-current, it may permanently shut-down the connection until you re-connect it with a charger.
Absolute Maximum Current
The maximum current you want to draw cannot be higher than the absolute maximum technically safe current.
The absolute maximum technically safe current is determined by the batteries you use. As a rule of thumb, lithium batteries sustain a maximum discharge rate of 3C where C stands for the battery capacity.
If your battery has a capacity of 3.000mAh, then this is 1C. The maximum safe discharge current would then be 9A (3x3.000mA).
With 10.000mAh batteries, the maximum safe discharge current would be 30A.
That’s just a rule of thumb. Be careful, and always look up the datasheet of the actual batteries you use. There are huge variations.
-
Cheap LiPo batteries may allow a maximum discharge current of as low as 0.3C.
-
Big LiFePo4 batteries can deliver 10-30C and more.
How batteries connected in parallel increase current
What has been discussed so far applies to one battery. When you connect batteries in parallel, the voltage stays the same but the maximum possible current raises as all batteries now share the load.
- If you connect two batteries in parallel, this doubles the maximum safe current.
- Connecting ten batteries in parallel raises the maximum allowable current tenfold.
If your battery does not sustain the maximum current you need, connecting more batteries in parallel is your option to raise the current to the level you need.
True Maximum Current
The technically possible maximum current often is much higher than the real needs of the load you want to power.
Just make up your mind: what is the maximum current your load will possibly draw under the most demanding conditions?
Add a reasonable safety margin, and you now know the maximum current your BMS should allow.
Balancing Capabilities
Balancing (explained earlier) is not required with just one battery, or when you do not plan to ever charge the batteries through your BMS.
In all other cases, the BMS should have balancing capabilities. The extra cost is marginal.
To find the appropriate BMS breakout board for your project, first select the number of strings, then visit the appropriate section below to select the BMS supporting the maximum current you need.
1S BMS
Most quality batteries come with a BMS built-in, but if you want to make sure you can add a 1S BMS.
This is a must if you decide to connect multiple batteries in parallel because now the total current raises considerably, introducing new risks that your 1S BMS should mitigate.
Here is a list of popular ready-to-use *1S BMS.
2S BMS
Here is a list of popular ready-to-use *2S BMS.
3S BMS
Here is a list of popular ready-to-use *3S BMS.
4S BMS
Here is a list of popular ready-to-use *4S BMS.
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(content created Feb 27, 2024)