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Why Passive-Balancing LiFePO4 Batteries Have a .2C Recommended Charge Rate
One of the most common misconceptions about LiFePO4 batteries is the belief that a battery’s advertised max charge rate and its balancing capability are the same thing. They’re not.
A passive-balanced battery may be perfectly capable of accepting a 0.5C to 1C, charge rate from a safety standpoint. The cells don’t suddenly burst into flames because you charged them at 1C. In fact, many LiFePO4 cells are quite happy to accept high charge currents through much of their state of charge range. The problem begins when the battery approaches full charge, hence the .2C recommendation. While many boat owners may never even get close to a .2C charge rate, the advent of 300Ah batts (300Ah .2C = 60A) and high performance externally regulated alternators makes this entirely possible.
What does a Passive Balance Circuit look Like?
This is what a typical 4S drop-in balance circuit looks like. This BMS balances at 30mA and each of the four balance circuits uses doubled up surface mount resistors. When the balance chip says a cell is getting too high a voltage, the mosfet switches turn on so the resistors can slow charging, to the high cell/cells.
Alternatively here’s a simple schematic of passive balancing:
Terminology
Passive Balancing – A BMS that utilizes small shunting resistors to slow charging of high cells, allowing lagging cells to catch up.
Recommended Charge Current – The charge current that will yield the best cell balance/life
Max charge current – The max charge current the cells & BMS FET’s can handle.
VPC – Volts per cell.
V-Diff – The cell voltage spread needed to initiate balancing eg: 3.4V and a 15mV v-diff = start balancing.
Cell-Drift – No two battery cells are identical, as a result imbalances occur.
C-FET – The FET’s that allow or disallow charging.
D-FET – The FET’s that allow or disallow discharging.
C-Rate – The percent of charge or discharge current: eg: Ah capacity/.2 = 20% of Ah Capacity
Charge Acceptance vs. Cell Balancing
Let’s use a typical 12V drop-in LiFePO4 battery using a passive-balancing BMS. We’re going to assume the BMS initiates balancing at approximately 3.40V per cell with a 15 mV V-diff. In a 4-cell battery, that’s about 13.6V.
A 100Ah battery starting at 0% SoC could easily accept a 50A (.5C – 100A (1C) depending on the C-FET ratings. It could accept this charge current from empty to roughly 13.6V. During this phase, balancing isn’t occurring because the cells haven’t yet reached the balancing threshold.
Everything appears normal, the battery charges quickly & the owner is happy. The marketing department is thrilled. Then physics enters the chat-room.
What most drop-in battery owners see:
The information they make hard to find:
Sadly, some battery makers have been creeping up the “recommended” charge current from .2C to .25C & even .5C. Nothing inside the battery has changed it’s just “A competitor did it so we will too” mentality.
Brands that recommend .2C
-Li-Time
-Redodo
-Powerqueen
-Wattcycle
-Dyness
-Dumfume
Humsienk
Etc.
Passive Balancing is Not Constant
One of the hidden secrets most LFP owners are completely unaware of is that once passive balancing starts it is not constant, it can’t be. We need to remember the BMS is using tiny little surface mount resistors to slow charging of the high voltage cells in order to allow slower cells to catch up. Passive balancing to does this to, burn off current as heat. This is why it can’t be constant or the resistors can burn out. Most owners just assume once balancing begins it is continuous, it’s not, it can’t be. Instead the BMS logic pulses the shunting resistors on & off to avoid over-heating damage. This pulsing only adds to the time it takes to correct cell-drift. Slowing your charge rate helps a passive balance battery remain in balance.
Good Balance
If you don’t have a BMS that allows you to see this information you’re essentially a Mr. Magoo & driving blind. Any cell balance below 15 mV (.015V) is perfectly adequate.
Poor Balance
The Balancing Bottleneck
Once the first cell reaches the balancing voltage, the BMS begins attempting to slow energy to that cell through small PCB mounted shunt resistors.
In many drop-in batteries, the balancing current ranges from 30mA to 100mA. Let’s be generous and assume 100mA. If the charger is still delivering 100A while the balancing circuit is trying to bleed off that 0.1A, to slow charging on that cell, the balancing circuit is effectively being out-muscled by a factor of 1,000! The balancing resistor is basically trying to trim a bonsai tree with a Bulldozer. It simply cannot keep up.
The result is that the highest cell continues to rise faster than the balancing circuit can correct for. Eventually that cell reaches the BMS high-voltage cut-off threshold & charging is cut-off. With most drop-ins, a C-FET cut also stops balancing, a real catch-22. The battery may appear “full,” but in reality only one cell has reached full-charge while the remaining cells may still be lagging behind.
Repeat this process enough times and the imbalance gradually worsens until the passive balancing can no longer correct cell balance and charge cuts occur at lower and lower pack voltages.
Why Lower Current Matters Near Full Charge
The ideal charging strategy for a passive-balanced battery is not necessarily a low charge rate from empty.
The ideal strategy is:
- Charge aggressively through the lower and middle state-of-charge range.
- Reduce charge current significantly once the battery reaches balancing voltage start parameters.
- Allow sufficient time for the balancing circuits to do their work.
If charging current remains excessively high all the way to the top, the balancing circuit becomes a spectator rather than a useful participant.
The problem with this strategy is that it is complex to implement, and typically requires the boat owner to manage this type of charging manually.
Why .2C Became a Practical Drop-In Recommendation
The often-cited .2C recommendation isn’t because LiFePO4 cells can’t accept more current. They absolutely can. The recommendation exists because most passive-balancing systems are extraordinarily slow and need time to do their job. We are just now beginning to see active balance batteries show up on the market a great option!
At approximately 0.2C or lower, the battery spends more time in the upper-voltage region where top-balancing occurs. This gives the BMS a fighting chance to keep the cells aligned. Charging at .5C, 1C, or higher, charging will be completed long before the balancing system has corrected any meaningful cell drift. The cells are fully capable of accepting the charge. The passive balancing system is just not capable of keeping pace with it. Simple stuff.
That’s a critical distinction.
Max Charge Rate = Wrong Choice
When a manufacturer advertises a 100Ah battery as capable of charging at 50A or 100A, they’re describing what the cells and BMS can safely tolerate in charge-current. Most drop-in batts are using 1C rated cells, but BMS design does not always match the 1C cell rating. They are not necessarily describing what is best for maintaining long-term cell-balance, they’re just saying what the cells/BMS can handle in charge-current.
Keep in mind the C-FET’S are often not designed for the same current as the D-FET’s. Many manufacturers have a 1C max max allowable discharge but, only .5C max for charge. Some do have 1C C-FET’s but they are rarer. This is the difference between the C-FET’s & D-FET’s.
How Can I charge Fast?
A passive-balancing battery can often accept .5C to 1C when charging from a low state of charge to approximately 13.6V (depends on BMS spec) without complaint. The major problem with this is, charge systems don’t have a way to reduce current based on balance the balance start voltage. The problem is not what happens prior to the balance start point it’s what happens after the balance voltage is reached.
Once balancing begins, the tiny balancing resistors need time to work. If charging current remains high, the balancing system simply cannot keep up, and cell imbalance accumulates over time.
In other words, fast charging isn’t usually the problem. Fast charging all the way to full charge is.
That’s a distinction many battery marketers conveniently forget to mention.
Bottom Line
– Avoid Max Charge ratings (Unless you want to be a human-regulator)
– Use Recommended or Lower Charge-Rates
– Know your batts balance start point eg: 3.4VPC- 3.55VPC
– Know your batts balance current typically 30mA – 200mA
– Know your BMS max charge spec, if you want to fast-charge below the balance voltage
– Always use a BMS that allows you to visually monitor individual cell balance
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