Active Balancing on Roamer LiFePO4 batteries

Roamer are currently the only UK and European brand to use Active Balancing on all 12V and 24V batteries. We have used active balancing for two years (introduced with SMART3 and improved with SMART4 / SMART5) and have sold thousands of batteries in this period. It has been a resounding success, virtually eliminating cell over voltage (COV) issues commonly found on the LiFePO4 leisure batteries used in campervans and boats.

LiFePO4 batteries are made up of multiple 3.2V cells, connected in series. In order to deliver the best performance, life expectancy and useable capacity, its important that these cells stay balanced and that they all hit 100% state of charge at the same time. All modern Battery Management System (BMS) will have some sort of balancer fitted however like the battery packs themselves, not all balancers are created equally.

Passive Balancing (often referred to as bleed balancing) burns off excess energy stored in a cell as heat, via a resistor on the BMS board. Passive balancing is simple and cheap to implement but it is inefficient and often struggles to keep up with the demands of balancing the cells in a high capacity LiFePO4 battery, especially after a few years of use.

Active Balancing involves moving energy from the higher charged cells to lower charged cells. Active Balancing is more complicated and costs more to implement but it is more efficient and capable of operating at much higher power. This is especially useful in a campervan or boat leisure battery which is typically stored for long periods and charged irregularly (unlike home solar which would normally be cycled from top to bottom on a daily basis).

If a LiFePO4 battery its not charged and cycled regularly then it is common for cells to drift slightly, even if cells were perfectly matched and top balanced at the start of the batteries life. In practise, you are unlikely to notice this imbalance until you charge the battery to full, raising the cell voltages above 3.4V. At this point you might see one cell reach the Cell Over Voltage (COV) protection threshold before the other cells. This protection mode will interrupt charging and if using a passive balancer that only operates while charging, this will also interrupt the balancing process.

In the short term this imbalance and charge interruption is not a big deal. It is only going to reduce overall capacity by 1-2% at most and charging is only being stopped when the battery is full. Interrupting a high amp charger or alternator can cause voltage spikes however, so it is better to turn down the charge voltage to prevent this from occurring repeatedly. Of course it is better to keep cells in balance and prevent COV protection mode from activating but an imbalance on a high capacity battery can be difficult to correct with a passive balancer.

Consider a 460Ah battery with one cell that is 1% (4.6Ah) lower than the rest of the pack. A passive balancer (commonly working at 50-100mA) would take up to 4 days of continuous balancing to burn off the excess energy stored in that cell. In practise most Passive Balancing BMS only operate during the absorption period of the charge cycle, typically one hour per cycle. This balancing would therefore take multiple charge cycles to complete, if at all.

Active Balancing is able to work at much higher currents because it is transferring current along a cable from one cell to another, it doesn't have to burn energy off as heat. The Roamer BMS Active Balancer balances at 2A so that same 460Ah battery is able to balance the cells in just over 2 hours. Active Balancing is also able to operate throughout the charge cycle, not just when charging.

It's not essential but it does overcome one of the pitfalls of passive balancing meaning its more forgiving of irregular charging with lead acid chargers or high charge voltages.

Cost - The main reason other brands do not use Active Balancing is the upfront cost of the balancer module. Passive balancing is cheaper and while performance is not as good as Active Balancing, this reduction in performance is seen as acceptable. We take the view that if you buy an expensive sports car, you should spend a little extra to keep it maintained. If you buy the decent fuel and change the oil regularly, it'll last longer and perform better.

Added complexity. More components mean more to go wrong right? The Roamer SMART4 / SMART5 BMS has Active Balancing built onto the BMS board itself so actually it's not that complex at all. Our batteries use a modular construction and the BMS is hot swappable but they're so well built, we don't expect to see any back for repairs for a long long time.

Bottom balancing - Most people will charge their battery to full much more often than they discharge to empty so if there is any difference in cell capacity then it makes sense to keep cells balanced at the top of the charge cycle curve (called top balancing) instead of the bottom of the curve (bottom balancing). Active Balancing operates throughout the charge curve which means that fully discharging the battery may end up bottom balancing the pack instead. The Roamer SMART4 / SMART5 BMS has configurable balance settings to prevent this. By default, balancing only takes place when cells are above 3.0V and the difference in cell voltage exceeds 0.01V.

High energy consumption in standby This is a weird one as it's not actually true. We only mention it because some companies state in their literature that Active Balancers consume more than 1Ah per day. If this was right, a 100Ah battery would have a self discharge rate of 31% on top of the usual self discharge rate of 3-5%. This is nonsense of course, Roamer SMART4 / SMART5 overall self-discharge rate is typically less than 5% per month and this drops to 1% if using the BMS sleep mode.