DIY LiFePo4 Balancer

BMS for Lithium Batteries

Many lithium battery packs never reach their potential lifespan because of the lack of a Battery Management System, or more often a very bad one. A good BMS needs to control every single cell and protect them from being either over-charged or deep discharged (See table on page 32). A good BMS also has also a short circuit protection, and limits the output current to the amount needed for its purpouse.

The most common cause of death with lithium battery packs, and lead acid batteries come to that, is like this: One single cell differs a bit in capacitiy, and during some cycles the distance between it and the other cells gets increasingly bigger. Soon this single cell will be either deep discharged (very typical with lead acid batteries) or overcharged (more common with lithium batteries). Lithium cells cannot stand one single overcharge, they blow up and loose their capacity, so it is essential to protect them from that.

Therefore so-called 'balancers', are fixed to every single cell to take away some of the incomming charging current in order to keep the tension under the limit. When they reach the critical tension they suck a definite ammount of current away, depending on the resistance of the built-in high power resistor. They have to be well adapted to the charger current because if the charger gives much more than the resistors in the balancer can take away the cells will be overcharged.

The big transistor and the high power resistor will get hot, so it's a good idea to include them in the charger unit, where they can be cooled, e.g. with a fan, much more easily than inside the battery pack. Therefore you need to have access to every single cell connection inside the battery. The cables don´t need to be very thick, and so they can be combined in a multi plug for connecting with the charger.

Another regulator inside this external charger controls the tension of all the cells together and switches the charger off when the battery pack has reached its maximum tension. Only the deep discharging regulator always needs to be inside the battery pack, in order to switch off the output when the minimun tension is reached.

It's easy to make a balancer at home with a zener diode. The circuit needs only 0.1mA in stand-by, and takes 600mA with a 2.2Ohm resistor. You can adjust the working point over the poty very precisely, to 0.01V, between 3.4V and 4.2V. A 220Ohm resistor gives a small hysteresis of 0,03V to prevent too much flickering. The LED shows when the maximum tension is reached. A BD535, or any other suitable NPN, and a big 2.2Ohm resistor might need to be cooled with a heat sink.

Here you see a set of 4 balancers connected together in series. The more cells you have in a battery pack, the more balancers you need.

16 Balancers for a 48V battery pack of an E-Bike

You can charge it all up with a solar panel, it works very well with one panel on packs of 3 or 4 lithium cells. For 36V batteries use 3 solarpanels, and for 48V you need 4 solarpanels. See other post for more details.

48V-discharge protection:

A discharge controller is the last link in the lithium monitoring. It prevents that the battery can´t be discharged too far. Reaching the minimum voltage it simply switches off the consumers. Professional BMS are monitoring each individual cell. But with a still good battery is usually sufficient enough to do a simply voltage-controlled shutdown of the entire battery. Because of the lack of space the circuit here works with multiple Mosfet`s in parallel connection, so they should not get too hot even with a very small heatsink.

The capacitor buffers false cutt offs at peaks, if its still doing it, (often with old batteries) set the tension even a bit lower. There will be still no damage on the cells.