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Designing a Customized Battery Charger Circuit: Part 2

In our previous tutorial we learned how to design and optimize a constant voltage feature for charging a particular type of battery, in this post we try to understand how to design and include a constant current feature for the same, which is another critical factor to be considered while making a customized battery charger circuit.

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.

For
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.

However
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.

For
implementing a constant current feature, again a LM338 becomes useful
and can be configured for achieving the parameter with a high degree of
accuracy.

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.

As
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:

In
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
voltage level.

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

In
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:

25 amp charger circuit 

In
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.


17 Comments

  1. Hi,

    During a constant current battery charging process, the battery voltage goes, for instance, from 10.8 V to 14.7 V. With this system, it seems that you just set a constant value of voltage in the right part of the diagram, and you put a current limiter at the input. Does this system work if the voltage battery increases during the process? How do you manage to do it?

    Thank you!

    • Hi,
      the next part of the article explains how to add a auto cut off with the above explained constant V and I circuit, please refer the next posts for the detailed info.

    • Hi,

      Thanks for your fast response. I'm not sure if I've understood the whole thing properly. I'll try to explain myself:

      The charging process starts with the battery discharged. From the 10,5 V until you reach the 14,3 V upper limit, is the charging current constant (otherwise, which is the shape of the charging current curve)? The "constant voltage" part of the circuit (the right side of the first figure) confuses me, because if you are controlling the current, you shouldn't worry about the voltage: it will be the needed one to satisfy the constant current imposed during the process. Why do you need, then, this voltage part? Maybe it just acts as a voltage stabilizer? I'm quite confused with this part, because for me there should be two different circuits: one for carry out the constant current process (we impose the constant current value), and another one to carry out the constant voltage process (we impose the constant voltage and after some hours the battery ends fully charged), but here they're put together.

      I mean, after you reach the 14,3 V limit… Didn't you should start the "constant voltage" part of the process where the current decreases exponentially (like in the typical charging curve for lead-acid batteries)?

      Thanks so much for your time

    • OK yes you are right, the charging intake is not constant rather it's reduces exponentially, it's high initially and gradually goes on decreasing as the battery reaches its full charge mark. The term constant current" in battery chargers refers to the limiting current which is never allowed to exceed beyond the specified limit ensuring a safe charging for the battery, otherwise the battery would try to suck-in dangerous levels of current at the initial phase of its charging.

      A constant voltage is important because as the battery reaches its full charge mark it begins consuming lesser amount of current, which implies that if the voltage was not maintained at the given safe level (14.3V) it would force the battery to keep on charging until its internal cells began boiling.

      The above two parameters compliment each other and ensures a perfectly safe and optimal charging fro the battery.

  2. Thank you for the answer. So, actually, we are always doing a voltage control, and we limit the current at the desired value for security reasons, right? However, these IC already have a current limit protection, so I assume that including the extra current limiter allows to put this level below the maximum limitation of the IC (please correct me if I'm wrong).

    My last question is: do you think that this charger would work properly for a 2V lead-acid cell, 600Ah (parallelizing many ICs)? Finally, can you suggest some good post about homemade battery discharger?

    Thank you very much!

    • yes your assumptions are correct!

      The ICs are internally current controlled as per their individual specs, and that's the reason I have suggested using many of these ICs in parallel in order to increase the limit proportionately for higher rated batteries.

      Yes it can be used for a 2V batt also but the input will need to be at least 5V

      🙂

    • Thanks! Regarding the discharger… How would you manage to discharge these 2,2 V cells at a constant current rate? I don't know if it would be possible to use the battery now as a DC supply, since the voltage is too low, but also I don't know how to automatically achieve a constant current discharge rate, since the battery voltage will decrease during the discharge proces…

      Thank you so much

    • as you rightly guessed the charge and the discharge rates cannot be made constant.
      Just like the charging process where the consumption is maximum in the beginning and it tapers down as the battery gets charged, similarly the discharging rate can be higher initially and it would go on decreasing as the battery voltage comes down……unless the load is modified somehow

  3. If I'm intereseted in doing a charge at a perfect constant current rate, would it be possible to use some power MOSFET (or many in parallel) with the collector connected to a DC voltage source, the emiter connected to the battery positive and regulate the gate-emiter voltage (working in the MOSFET linear zone)? The problem here is that there is no security, though. But this could be solved with auxiliary circuitry…

    Could you suggest some suitable power MOSFET for doing this at high currents? I'm not sure about which DC voltage source would the MOSFET need for doing this, but I guess it's about 60-100 V? Maybe you know some manufacturers with these kind of stuff.

    Thank you!

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