Backup power for 12V accessory circuit

[romkal]

I need to build a backup power source for 12V accessory circuit in my motorcycle to power a camera after I turn the key off, so it can close the file it was recording. It takes like extra 10s at around 0.4A.

Normally you connect the camera to the motorcycle battery, that provides 12V and when you turn the key off, it can still draw from the battery, but this is an electric motorcycle and it doesn't have separate 12V battery, but just 14V accessory circuit. It goes off, when I turn the key off so the camera cannot close the file with the recording properly.

The camera can be powered by anything from 7V - 14V properly as it converts to 5V anyway.

I wanted to build a small battery, that would charge from the bike when I ride, and when I turn the key off, the battery will provide enough power to properly shut the camera down.

I bought some Li-Ion 3.7V ICR10440 350mAh and cheap 3S protection board (hx-3s-fl10a-a) but the description on the item says "Charging voltage 12.6V - 13.0V". I have 14V in the accessory circuit, so I'm not sure if the protection board will be able to correctly charge the battery and if it will be safe. Will it shut the charging off when it reaches 4.2V? Will battery like the higher charging voltage? Or maybe the board will limit it? It is on a motorcycle. The last thing I want is to have a bonfire under my butt...

Another option is to use 4S configuration. That should be charged up to 16.8V, but maybe I could use 4S protection board to charge it with 14V (3.5V per cell). Of course it is a waste of capacity, but I need to power the camera for 10s only anyway. I don't care that much about capacity, but much more about safety. Would that work?

Any advice? Maybe there is some better solution than this?

Thanks

 

[amberwolf]

I need to build a backup power source for 12V accessory circuit in my motorcycle to power a camera after I turn the key off, so it can close the file it was recording. It takes like extra 10s at around 0.4A.

You could use a large capacitor, with a diode feeding it from the MC power system so it doesn't discharge back into the MC power system, but is still charged by it. Not sure how big a cap it would take, but you can calculate that based on the time you need the voltage to remain above the camera's minimum input voltage and the rate it is drained at.

I bought some Li-Ion 3.7V ICR10440 350mAh and cheap 3S protection board (hx-3s-fl10a-a) but the description on the item says "Charging voltage 12.6V - 13.0V". I have 14V in the accessory circuit, so I'm not sure if the protection board will be able to correctly charge the battery and if it will be safe. Will it shut the charging off when it reaches 4.2V? Will battery like the higher charging voltage?

If the spec says 13.0v, you're probably much safer not exceeding that. You'd have to test it yourself to find out what happens beyond that. It might fry electronics, but probably not. However, if the protection board does not have an HVC and/or fails to cut the cells off, they'll charge up to nearly 4.7v each (and if they're not balanced then one cell could overcharge even higher than that.

Another option is to use 4S configuration. That should be charged up to 16.8V, but maybe I could use 4S protection board to charge it with 14V (3.5V per cell). Of course it is a waste of capacity, but I need to power the camera for 10s only anyway. I don't care that much about capacity, but much more about safety. Would that work?

It should, as long as the cells stay balanced over time. It's a safer option than potentially overcharging the 3s setup.

 

[MJSfoto1956]

Just use a relay. Simple and reliable. Wire the coil up to your key switch and when you turn off the key, the relay will shut off the power for you.

M