Constant Current (The constant charging rate):
Just like the "constant voltage" parameter, the recommended charging current for a particular
battery should not be increased or fluctuated by much.
lead acid batteries, the charging rate should be approximately 1/10th
or 2/10th of the printed AH (Ampere Hour) value of the battery. meaning
if the battery is rated at say 100AH, then its charging current (amp)
rate is recommended to be at 100/10 = 10 Ampere minimum or (100 x 2)/10 =
200/10 = 20 amp maximum, this figure should not be increased preferably
to maintain healthy conditions for the battery.
for Li-ion or Lipo batteries the criterion is entirely different, for
these batteries the charging rate could be as high as their AH rate, meaning
if the AH spec of a Li-ion battery is 2.2 AH then it's possible to
charge it at the same level that is at 2.2 ampere rate Here you don't
have to divide anything or indulge in any kind of calculations.
implementing a constant current feature, again a LM338 becomes useful
and can be configured for achieving the parameter with a high degree of
I have discussed the subject comprehensively in one of my previous post named universal current limiter circuit.
The below given circuits show how the IC may be configured for implementing a current controlled battery charger.
discussed in the previous section, in case your input mains is fairly
constant, then you can ignore the right hand side LM338 section, and
simply use the left side current limiter circuit with either a
transformer or an SMPS, as shown below:
the above design, the transformer voltage may be rated at the battery
voltage level, but after rectification it might yield a little above the
specified battery charging voltage.
This issue can be neglected because
the attached current control feature will force the voltage to
automatically sink the excess voltage to the safe battery charging
R1 can be customized as per the needs, by following the instructions furnished HERE
The diodes must be appropriately rated
depending on the charging current, and preferably should be much higher
than the specified charging current level.
Customizing current for charging a battery
the above circuits the referred IC LM338 is rated to handle at the most
5 amps, which makes it suitable only for batteries upto 50 AH, however
you may have much higher rated batteries in the order of 100 AH, 200 AH
or even 500 AH.
These might require charging at the respective higher
current rates which a single LM338 might not be able to suffice.
To remedy this one can upgrade or enhance the IC with more ICs in parallel as shown in the following example article:
the above example, the configuration looks little complicated due to
the inclusion of an opamp, however a little tinkering shows that
actually the ICs can be directly added in parallel for multiplying the
current output, provided that all the ICs are mounted over a common heatsink,
see the below diagram:
Any number of ICs may be added in the shown format for achieving any desired current limit, however two things must be ensured in order to get an optimal response from the design:
All the ICs must be mounted over a common heatsink, and all the current limiting resistors (R1) must be fixed with a precisely matching value, both the parameters are required to enable an uniform heat sharing among the ICs and hence equal current distribution across the output for the connected battery.
So far we have learned regarding how to customize constant voltage and constant current for a specific battery charger application.
However without an auto cut-off a battery charger circuit may be just incomplete and quite unsafe.
We learn about this in our next article: How to customize auto cut-off in a battery charger circuit.