battery in parallel with DC DC converter?

[Circuit Learner]

I have a load that sometimes exceeds the DC DC converter max current output.

If I put a battery in parallel with the DC DC converter, will this work?

I will set the voltage of the DC DC converter to the max voltage of the lithium battery.
I can also use a CC (constant current) DC converter also.

What I am worried about is can the battery harm the DC DC converter if the DC DC converter voltage falls below the battery voltage? Like current reversing into the DC DC converter or something?

Thanks

 

[chuckey]

This can be done in theory but it can be dangerous. If due to temperature the power supply voltage drifts high and the current taken by the load goes low, now the battery could be overcharged and if its the wrong type, burst into flames. Providing the power supply is working it is most unlikely that the battery could damage it. The first component to be overloaded might be an electrolytic capacitor across the output of the power supply. i.e. if its a 12V PSU, the capacitor might be rated at 15V, so it would be safe up to almost a 15V battery voltage. The same sort of limits apply to the power supply diodes.
Frank

 

[Audioguru]

Don't you know that Lithium batteries explode and/or blow up if they are charged or discharged wrong? The latest Boeing airliner: Lithium battery fire. Hoverboards: Lithium battery fire. Most couriers will not ship lithium batteries: because they might catch on fire. The metal called Magnesium also ends with "ium" and it also burns like crazy. Same with Titanium and Aluminum.

 

[Circuit Learner]

I will use 2 lithium ion batteries/cells that have a 4.2 volts each cell. In series these will have a max charge of 8.4 volts.

I will set the DC/DC converter to around 8.2 volts, a little under the maximum voltage of these cells.

I don't know how quick the DC/DC converter will charge the battery as it falls in voltage.

The load will have a voltage cutoff so that the batteries can't fall below 6 volts or 3 volts per cell which is safe.

Overcharging is the only issue. I don't know how much the DC-DC converter voltage will fluctuate from its set voltage. I can set the voltage to from 2-32 volts. I will have to experiment to see if the value starts to go higher.

 

[Audioguru]

It looks like the Lithium battery will catch on fire when its voltage is less than the converter's voltage and the load is turned off because there is nothing to limit the excessive charging current.

 

[Circuit Learner]

Audioguru: Yes that is a very good point and one I actually only solved recently.

Do you know if there is a way to calculate how much current will flow between two different voltages? Like if the DC converter is 8.2 volts and the battery is 7 volts? Is there some kind of formula to figure this out?

However, this is solved because I will use a CC CV DC-DC converter. This will allow me to limit current at say 3 amps and voltage also can be limited.

 

[Audioguru]

I do not know if the lithium battery will survive a charging current of 3A. But it will be damaged and might catch on fire if its charging is not turned off when it becomes fully charged. All lithium battery charger circuits detect a full charge and turn off the charging. It is about 70% fully charged when its voltage reaches 4.2V per cell. Its charging current drops to 1/25th to 1/40th its mAh rating when it is fully charged.

 

[c_mitra]

You can always connect two voltage sources if you use suitable diodes. The diodes will be essentially protection diodes.

If you also want the DC-DC converter to keep charging the battery whenever the voltage (and current from the DC-DC converter) is sufficient, you need to add more protection.

Yes, your worries are well founded and you must not use battery in parallel with a voltage source without protection.

Many consumer electronic devices have a battery backup; when power is available, the battery is kept in a float charge condition. When the power goes, the battery runs the device. The voltage source must be well regulated.

 

[Circuit Learner]

where do you see the need to use a diode?

 

[c_mitra]

If you use a diode in series with the battery, the available voltage will be reduced by the diode drop but if the DC-DC converter voltage increases (for some reason) the diode will get reverse biased and prevent charging. If you also put a diode in series with the DC-DC converter, a low output voltage at the converter will prevent the battery to discharge via the DC-DC converter.

This is the basic diode resistor logic.

If you want to charge the battery to be charged by the DC-DC converter, you need to replace the diode in series with the battery with a transistor (or better, a fet). The FET should be off when the converter voltage is lower (or absent) than the battery voltage.

 

[FvM]

I think we have to assess that the design as far as presented up to now will not work correctly. Some battery types can be charged with simple CC/CV characteristic, but not Li-ion. They also would need a charge balancer with more than one cell in series circuit, by the way. Besides the discussed disconnection circuit, can be diodes or controlled MOSFET switches, a Li-ion charge controller would be needed.

Supercaps are a possible option that is suited for direct parallel circuit with a current limited power supply. The popular types have 2.3 to 2.5 V maximum voltage, you need four series connected cells for the 8V range, also a simple balancer option, parallel connected resistors would be sufficient.

 

[Vbase]

The advice you received above are valid but you may find that in most cases it will be easier to replace your converter with a more powerful one.