Simple BMS devices usually prevent cells from under-charging, over-charging and cell balancing, when cell’s voltage reaches certain level.

Such BMSs have pretty low balancing current because of spacing and thermal issues, usually around 50-100 mA, which is pretty low when coming to bigger cell sizes and/or not equalized cells. Offcourse we must know, that additional power creates heat, which depends on a batery case design, so usually low balancing current must be a compromise between requested balancing current and heat removing captabilities.

When making own battery pack we can dimension casing with more cooling. A BMS must be positioned in a way, that generated heat won’t affect battery cells, which can destroy them in a long term. Many low-cost battery packs has BMS attached directly to cells and in 90 % first damaged cell is the one, to which BMS is attached. This is due to cell overheating.

Let’s take a look at such “standard” BMS for 8 cells:

1

PCB’s top part has power MOSFETs (MOSFET can be represented as a switch), which disconnect the battery in case of under- or over-voltage. In some BMS design solutions charging and discharging MOSFETs are seperated, however most of them has MOSFETs in s.c. bidirectional connection, which means that they can switch current in both directions (charging and discharging). MOSFETs also have thermal switch (white), which disconnects the battery in case of overheating because of overcurrent. On the left side, between P+ and P- pins (this are pins, where load/charger is connected) there is a protective diode, which blocks reverse-polarity spikes, generated by inductive loads (motors, coils …)

At the bottom part of PCB (black text) there are 8 identical modules (one per each cell), which consist of balancing resistors, cell’s control IC and MOSFET, which activates cell discharging through resistors.

Because of lower price most of BMSs is equipped with SMD resistors, which are not designed for greater powers. Additional disadvantage is their location directly on PCB, which means that they can only be cooled on one (air) side. With low powers it’s not such a problem, but with greater power this is a cause of many problems.

Let’s see this BMS. For each cell there are three 47R resistors connected in parallel. This gives total resistance of 15,7 R. This is a LiFePO4 BMS, where balancing voltage is usually set to 3,60 V. With such voltage and resistance balancing current is 229 mA (I=U/R) and power 0,83 W (P=U*I). A little less than 1 W – not much, but on the other hand enough, if the heat has nowhere to go. We must keep in mind, that balancing can simoultanously run on 7 of 8 cells! (n-1)

And if we would like to increase balancing current? It’s not such a complicated solution: current balancing resistors must be replaced with a smaller ones (I=U/R – lower resistance = greater power/current), which must be properly dimensioned (regarding to power). In some cases also MOSFETs, which turn on balancing must be replaced with more powerful ones.

3Selecting proper resistors

Based on target balancing current we calculate resistor valve. Because voltage for LiIon and LiFePO4 is different, we must take a proper one when doing calculations. (3,60-3,65 for LiFePO4 and 4,15-4,20 for LiPo/LiIon).

In our case, target current was 0,5 A, which gives resistance of 7,2 R (R=U/I = 3,6/0,5). Expected heating power is  1,8 W (P=U*I = 3,6*0,5).

2Because there was no stock of such resistors, we took closest valve 8,2 R, which means a little lower balancing current/power (440 mA, 1,58 W). Resistor must be declared at least for this power, but we suggest taking at least 100% more powerful resistor. Here we took 5 W resistors. First old resistors are removed and new ones are soldered over their contacts.
Because of higher discharging current also MOSFETs were replaced. Basically any “logic level” NFET is suitable (SOT23 casing).

4After this procedure BMS has new balancing resistors, which are quite bigger and because of their square shape they can be simply attached to metal casing and therefore additionally cooled.