luthj
Engineer In Residence
As mentioned, charging to a voltage at just the beginning of the knee is the best approach. If you must get that last 4% in, use a lower rate. Staying mostly out of the knee at the end of the SOC curve means you need much less balancing. For example, if your cells are within 5% of each other with regards to internal resistance, and 1-3% capacity, you could go hundreds of cycles between 90 and 10% SOC, without balancing. This assumes a charge discharge rate well under .5C, ideally .2C charge and .3C discharge.
Some rough math indicates that a well matched bank treated such as the above, needs less than 250mA per 100AH of capacity in balance current, and even then its probably more than enough. The full-time balance boards could probably be safe with as little as 100ma per 100AH, assuming you keep the charge/discharge rates as described above. The high current balancers are usually for EVs, Ebikes, etc, which see very high charge/discharge currents.
For a faster charge, using say 14.0.1-14.1V and terminating at around .1C return current works as well. For higher voltage chargers you may want to terminate at a slightly lower voltage.
Any charger that is setup for LFP, really needs a return amps termination for optimal longevity. Especially if charging at voltages over 14V. If a lower voltage is selected (say 13.8-14V), this is not as important. It is possible to use absorb timers instead. For example 3 minutes at 14.1V for a charger rated at 0.5C or lower. This should be set experimentally while monitoring the return amps. There really isn't a whole lot of benefit from going above 14.1V, that last ~4% comes at a much higher lifecycle cost than the rest.
The renogy unit described seems like a good deal though. Does it have a voltage sense wire? That would be very useful for folks with long charging runs.
Some rough math indicates that a well matched bank treated such as the above, needs less than 250mA per 100AH of capacity in balance current, and even then its probably more than enough. The full-time balance boards could probably be safe with as little as 100ma per 100AH, assuming you keep the charge/discharge rates as described above. The high current balancers are usually for EVs, Ebikes, etc, which see very high charge/discharge currents.
For a faster charge, using say 14.0.1-14.1V and terminating at around .1C return current works as well. For higher voltage chargers you may want to terminate at a slightly lower voltage.
Any charger that is setup for LFP, really needs a return amps termination for optimal longevity. Especially if charging at voltages over 14V. If a lower voltage is selected (say 13.8-14V), this is not as important. It is possible to use absorb timers instead. For example 3 minutes at 14.1V for a charger rated at 0.5C or lower. This should be set experimentally while monitoring the return amps. There really isn't a whole lot of benefit from going above 14.1V, that last ~4% comes at a much higher lifecycle cost than the rest.
The renogy unit described seems like a good deal though. Does it have a voltage sense wire? That would be very useful for folks with long charging runs.
Last edited:
