Hi,

I have a project for a motorcycle.
I would like to attach a 5V device and it's charger to the main 12V battery, but I do not want it to drain the main power.
I've found many approaching threads on this forum and elsewhere, but nothing satisfying so far.

The main ideas are to either :

• build a timer, powered by the main 12V battery, or autonomously, that will act as a circuit breaker, and close the circuit on a defined time.
• or either to build a circuit, evaluating the target battery power and closing the circuit as long as the target battery does not reach the target voltage.

The main constraints are to : use as less power from the 12V battery as possible, and to completely cut the power from the 12V battery for it not to drain.

I'm a beginner with some skills in electronics, like modifying synthesizers. But I'm very bad on designing circuits.
Would some one help me with the circuit design ?

I've sketched the 3 ideas (attached).
Thanks in advance for your help !!!

Have a great day.
Alex

 

Use option 3, monitor the battery voltage and charge when needed.

 

Another option:

Draw power from the motorcycle battery only when:
1. the 5V device needs it, based on its own requirements, such as the 5V battery is getting too low...
2. the motor cycle battery still has enough charge left to start the motor cycle, or a more conservative approach, still has >90% of full charge.

Vehicular batteries are typically slightly over-charged while the vehicle is running, and are very close to being fully-charged when the vehicle is first parked. Goal is to minimize parasitic draw from the battery while parked...

It is very bad for lead-acid chemistry batteries to be idle with less than ~90% of full-charge. An idle battery with less than 90% of full-charge is subject to sulphation, which permanently reduces its capacity. The principle goal of vehicular batteries is to maintain the battery >90% of fully charged even when parked....

 

I agree with Mike. Take option #3 and draw a line from the 12 volt battery to the voltage probe. As long as the 12 volt battery stays above 12.5 volts allow the 5 volt charger to operate. The 5 volt charger can go to stand by if the 5 volt battery is at 100% charge.

 

The deep-cycle marine batteries are more tolerant of deep discharge and recharge.

 

Thank you all for your inputs... I was thinking about this option, but as I am a newbie ... complex actions require a complex circuit.

So, as far as I understand, I need :

• a probe on the 12v battery side (monitoring the voltage above 12V),
• and a probe on the side of the 5V battery (monitoring the voltage between 3,5V and 6V).

The thing is I do not want the probes to draw power from the tested batteries, would it be possible to power the probes with a long life cell ? The main sensitive battery is the 12V one, I was thinking about a true circuit breaker on its side, like a relay. But as I search, it tend to be that the relays are power wasters.
Which direction should I take ?
Would someone help me with the circuit schematics ?

Thanks for your help.

 

Can you do the 5V charging only when the bike engine is running? If so, that would eliminate battery drain.

 

Can you do the 5V charging only when the bike engine is running? If so, that would eliminate battery drain.

Thanks for your reply.
Well this is not exactly the idea. The device has to stay always powered. I need to be sure that the 5V battery always keeps enough power for the device to work. So when the engine is not running, the system has to monitor the 5V battery and take some of the 12V battery power to maintain an acceptable level.

 

There is two ways you could do it.
- Use ultra low power op-amps to build your monitoring circuit.
- Use some microcontroller, like a PIC, that has a low power sleep mode. It wakes up once a minute, 10 minutes and tests the condition of the batteries.