How do you determine your charging voltage?

With cells that have a nominal voltage of 3.6  or 3.7, the max voltage for that cell is 4.2 volts.  Most 18650 cells are have this nominal voltage.  So if you have 10 connected in series, the maximum charging voltage would be 10 x 4.2 for 42 volts.

For the larger 26650 cells, which are often a different chemistry, LiFePo4, the nominal voltage is 3.2 volts and the max charged voltage is 3.65 volts.  So, for four in series, the max voltage would be 14.6 volts.

Do not use charge at a voltage higher than your batteries rating.  This is not a loose number.  Over charging will at best dramatically shorten the life of your cells and at worse cause a fire.

When should I undercharge? 

Under-charging extends the life of cells dramatically.  To get the most life out of your battery,  live in the middle of it’s voltage range.  For 3.6 nominal voltage cells that would mean charging to 4 volts and stopping discharge at 3 volts.  This can more than triple the usable life of your battery as most of a batteries stress occurs while at the highest and lowest voltages.  The trade-off is a reduction in capacity.  You can lose 30% of the batteries potential capacity operating in this voltage range.  For transportation with small batteries you may well want to fully charge.  For stationary applications probably not.

LiFePo4 is usually fully charged to help compensate for the lower energy density of the chemistry.

How much power should I charge with? 

A safe rule of thumb is to never charge your battery fast enough to take if from flat to fully charged in one hour.  The faster it is charged the more stress on it.  No harm in charging slowly if you have the time.    I shoot for three to five hours on my batteries.  You start with the number of watt hours your battery has.  So a 36 volt 10 amp hour battery has 360 watt hours- and should not be charged with a charger that has over 360 watts of power.

Balancing Rules of Thumb.  

Balancing is the process of bringing the voltages of each parallel group to match.  You want a well balanced battery for a longer battery life and for maximum capacity.  Charging and balancing are different concepts, though often done at the same time.

     The BMS

A Battery Management System is commonly used on ebike sized batteries.  It is sized according to how much current in amps you want to flow, and how many parallel groups in series it can manage.  So a 10S BMS with 30 amps would manage a 36 volt battery (10S x 3.6 volts) that had the ability to dump about 1000 watts of power.  (36 volts times 30 amps is 1080 watts)  The amp rating is to protect the BMS- the amp rating is the max amount of power it can handle before frying.

Advantages of BMS’s= cheap, long history.

Disadvantages of BMS’s.  Poor balancing capacity- not good for batteries other than made with new identical cells.  Not good for large batteries for the same reason.   Most won’t charge to 4.2 volts and cannot be programmed to start balancing at 4 volts to extend battery life.  Tendency to shut off the battery inconveniently.  Most have no monitoring system so you don’t know if they are working, so you need to geek one up.

    Hobby Chargers

are very common in the Radio Control world.  They have displays of each parallel group so you can verify your battery is charged and balanced.  They also have a much greater balancing capacity and so are good for larger parallel groups, and for homemade (recycled)  battery packs.  The disadvantage is that they don’t charge large batteries with high voltages- the most common stop at 6S, 10S is available.  That means electrically splitting larger packs into two packs to balance.  Or using two balancing chargers at once.  You can program these to balance at 4 volts per cell, extending battery life.

     Active Balancers

These keep getting better and less expensive and are what I recommend and use on larger packs, or packs made of more that 12 cells in each parallel group.   An Active Balancer moves the power from the highest voltage parallel group to the lowest voltage parallel group.  It balances all the time, so the pack stays balanced during it’s complete cycle.  You can charge to any voltage, it still balances.   This extends the capacity of the battery and the cycle life.   They do have good power transfer capability, but the amount of power being transferred depends on the difference in voltages.


For ebike sized batteries made up of matched new cells and ridden by civilians, use a BMS.  For the hobbyists and battery geeks making smallish batteries under 8S- especially those using mixed cells- use a Hobby Charger.   For power walls or storage batteries or any battery made up of large parallel groups-say over 12 cells- use Active Balancing.

Questions and better ideas welcome at [email protected]

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