# Design a circuit will charge a battery using the charging current as a means of control.

Joined Sep 14, 2018
5
I would like to create a battery charging control circuit which would use 16.5v across the battery cell and would be limited to a current to 3A max.
When the charging cycle has been completed the charging current will decrease to 100mA.
At this point I would like to switch off the charging circuit by the use of a charge enable control in order to not overcharge the battery.
The circuit would then be closed periodically in order to replenish the loss of charge in the battery through use or loss.

I already have a 16.5v supply and I would like to control an open/closed state between two contacts which will then be used in a separate charge-enable circuit.

Therefore the conditions are:
1. Charging current to be limited to 3A
2. Charging current between 3A and 100mA -> "contacts closed"
3. Charging current less than 100mA -> "contacts open"
4. Periodically "close contacts" in order to "top up" any loss or use of charge in the battery.
How can this be designed and what components would be needed?

I would be very grateful for any advice or ideas that are offered.

#### Yaakov

Joined Jan 27, 2019
3,517
What is the chemistry of the battery being charged?

Joined Sep 14, 2018
5
The battery in use is a sealed Lithium Ion battery.

#### Yaakov

Joined Jan 27, 2019
3,517
OK... I don't recognize this charging strategy, why the particular voltage and current?

You mention a "supply", is it actually a charger with some sort of BMS?

Joined Sep 14, 2018
5
The "supply" in question is a DC-DC converter which will be set to a 16.5 volt output.
The current and voltages are set specified by the manufacturer.
The battery is a 12 volt sealed lithium Ion battery which feeds into another DC-DC converter.

The rechargeable battery is being used as a reservoir of charge, which the supply will periodically top up.
The topping up is be controlled by the charging current.
if its less than 100mA the it ceases, after a period of time the charging is to be permitted, if it is less than 100mA then it stops ad infinitum.

#### Yaakov

Joined Jan 27, 2019
3,517
Can you tell us the manufacture and model of the battery?

Does the battery have it's own BMS?

Joined Sep 14, 2018
5
Yes, the battery has a charge enable circuit which will allow charge if a low voltage (in series with a resistor) is applied between two specific pins on the case.

I may use a BB2590 such as:
https://www.bren-tronics.com/bt-70791cg.html

#### Yaakov

Joined Jan 27, 2019
3,517
Here's my concern. Lithium Ion charging requires great care. An appropriate scheme with protection against overcharge and discharge are essential. If the battery has a BMS, you may or may not have to do what you are proposing, and instead simply apply the correct voltage to charge. If it doesn't you may need to do a constant current followed by a constant voltage.

In any case, it does matter which particular you use, and your intention to use current sensing means you expect something to be controlling the current. Are you counting on the constant voltage from the supply to do the current limiting? Do you expect the curve to be the same throughout the life of the battery?

Do you have some reason to adopt this particular charging strategy?
Also, how will you know if the battery is being discharged in order to stop the charging?

Joined Sep 14, 2018
5
Hi Yaakov,

• In any case, it does matter which particular you use, and your intention to use current sensing means you expect something to be controlling the current?
Yes, the charging instructions on the battery reads:
Charge each 12v section (there are two sections in a battery) independently at 16.5 volts constant voltage (3A max). Charge is complete when current drops to 100mA.
Therefore in order to not overcharge the battery I need to determine when the current drops below 100mA and then remove/disable the supply voltage or the charge enable circuit which is separate entity.

• Are you counting on the constant voltage from the supply to do the current limiting?
No, the PSU to be used will apply a constant voltage to each of the sections of the battery (there are two sections in a battery). It can limit current (to Imin= 8A) but this is too big for the maximum charging current of the batteries. Therefore the circuit must limit the flow of current to each section to 3A max.
This charging current is expected to drop as the battery becomes recharged, once it drops below 100mA I need the circuit to control a pair of contacts to go open circuit.

• Do you expect the curve to be the same throughout the life of the battery?
• No, I expect the curve will change, as the battery gets older but the batteries are replaceable.

• Do you have some reason to adopt this particular charging strategy?
• Yes, I don’t want to overcharge the battery.

• Also, how will you know if the battery is being discharged in order to stop the charging?
• I will not be able to tell if the battery is being discharged, so I would like the control circuit after a fixed period of time to “close circuit” the contacts (mentioned previously) to allow the charging voltage across the battery sections.
The condition of sensing when this current falls below 100mA will still apply, so that if the current required by the battery to charge is less than 100mA it is disconnected.

#### BobTPH

Joined Jun 5, 2013
3,674
No, the PSU to be used will apply a constant voltage to each of the sections of the battery (there are two sections in a battery). It can limit current (to Imin= 8A) but this is too big for the maximum charging current of the batteries. Therefore the circuit must limit the flow of current to each section to 3A max.
You cannot supply a constant voltage and limit the current. If you supply a constant voltage, the load will draw whatever current it draws at that voltage. Limiting the current means lowering the voltage.

You should not be messing with charging Lithium batteries. It is dangerous and you do not have the knowledge to make it safe.

Bob

#### Yaakov

Joined Jan 27, 2019
3,517
Hi Yaakov,