Hello, I would appreciate advice and enlightenment with the following application:
The question is: what size capacitor do I need between a 12V vehicle battery and a 12VDC to 28VDC 20A converter/charger?
The application is as follows: I am using a vehicle 12V lead acid battery to maintain charge of a 24V 45Ah Lithium Battery using a 12VDC to 28VDC Converter/Charger. The system powered by the 24V battery pulls an average of 10A which varies between 8A and 12A with brief spikes at 20A.
The Converter/Charger is set to cut off when the 12V battery voltage drops below 12.4V (to ensure the 12V battery does get too low). The converter/charger is meant to turn on when the vehicle engine is running and provides charging power to the 12V battery, which is a good working set up for my application. However the chargers tends to turn on and off as the 12V battery tends to drop below 12.4V when the Converter/Charger turns on and pulls 40A (to provide 20A to the 24V battery) when the vehicle is in idle. When the engine is moving, the engine RPMs increase and provide stable power, keeping the converter/charger on. I am hoping that a Capacitor between the 12V battery and the Converter/Charger could resolve this.

 

Forget the capacitor and add hysteresis to the switching. Without hysteresis, it's guaranteed to oscillate on and off because of the voltage change when the load is connected and disconnected.

Try, for instance, disconnecting the load at 12.2V and not reconnecting it until the voltage is 12.8V. These values are just a wild guess - a little experimentation should get you zeroed in. It'll help to measure the voltage delta directly by manually connecting and disconnecting the load. Your hysteresis needs to be larger than that ∆V.

 

Thank you for the quick response. Much appreciated.
Unfortunately, the voltage at the 12V battery drops below 12V and then goes back up to 13V fairly quickly.
I might have an alternative option which is to modify the power supply from the vehicle's engine so it would provide more stable power but there are several down sides to this option.
I'm still interested to know what specs capacitor would be adequate based on the numbers I described above.

 

Make sure that the wiring to your Inverter/Charger is adequate for at least ~50-Amps ( 6-Gauge wire ).

Do NOT use the Car-Body-Sheet-Metal for a Ground connection under any circumstances.

You need to install a more powerful Alternator on your Engine.
The Output of the Alternator should never drop below ~13.8-Volts under any and all conditions,
if it does, then it is too small for the Loads being applied to it.

It would also help to install a larger, and/or, a second Battery, in your Car.

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I re-read your first post and it occurs to me that the 24v battery must be inadequate. It should perform the role you’re looking at for the capacitor, to supply the big load when needed. It then recharges at a lower, steady rate most of the time.

If instead the load is bringing down the entire system voltage, that 24v battery isn’t cutting it. As noted, you can add capacity there or at the 12V battery. No capacitor is going to get you through a red light at idle.

 

I read the replies from "W" and it seems that a contributing factor might be in the connection between the two batteries, since that will certainly have a big effect on the 24 volts output at the higher load current. Certainly assuring that there is no voltage drop in that connection string will be simpler than any other option that may not even solve the problem. That link was the point of failure in a few dynamotor powered 2-way radio installations many years ago. The DM35 inrush caused those problems.

 

Thank you for the input.