can I add a second battery??

BSKZ650 wrote:

Lou,,, WOW,, way cool, I am going to build the system as you have designed it, I really appreciate you taking the time to work that out.

I will do a follow up as soon as it is up and working..

ThanksB)

No problem. Bountyhunter raises some good points, though. In any design, a complete catastrophic failure of the second battery will probably produce some smoke, resistor or transistor. But that type of failure would have to be pretty significant. The battery will more likely fail gradually and slowly lose charge holding capability.

I should mention, you want to set the threshold voltage. I roughly estimated R1 but you can verify it with two voltmeters if you have them. Put one voltmeter on the main bat and another on the second bat. As you rev the bike, watch both meters and note the main bat voltage when the second bat voltage jumps up. That main bat voltage is the threshold voltage (roughly speaking). You want that to be somehwere from 13.5 to 13.9. To raise it, increase R1. To lower it, decrease R1.

run guys,theyre taking techno stuff!:laugh: i might have to haul my bike to you and pay you to wire it when i get that close!:laugh:

loudhvx wrote:

a complete catastrophic failure of the second battery will probably produce some smoke, resistor or transistor. But that type of failure would have to be pretty significant. The battery will more likely fail gradually and slowly lose charge holding capability.

Actually, lead-acid batteries (both wet cell and gel cell) almost always fail when one of the cells shorts out. In a wet cell, the coating on the plates burns off with use and collects in the bottom until it bridges the separator and connects between the pos and neg plates. Gel cells also short their cells when they build up enough sulphation inside.

Just be careful.

As much as I am loathe to recommend a product from the ahole company that laid me off, there is a perfect part for this application that simplifies the design and makes the part protected against overcurrent and excessive heating: The LM2941
www.national.com/mpf/LM/LM2941.html

It's a adjustable regulator rated at 1A with the big PNP and all the other stuff built in. Just select external resistors to programe the output voltage to anything higher than 14V, and the internal PNP will be saturated ON whenever the bikes regular battery is up and connected to the input of the LM2941.

The advantages:

1) Internal current limiting of about 1.5A

2) internal thermal shutdown protects the thing in case the battery has a problem

3) has built-in load dump protection and reverse battery protection.

You can buy these at Digi-Key and probably even Radio Shack. They are very common.

EDIT TO ADD: You can do this even simpler using an LM2940-15 which requires no external resistors. It's internally set to 15V out which means when powered by the bike's 14V sytem, it will simply saturate the power PNP transistor and charge the second battery. It also has reverse input protection, overcurrent protection, and overvoltage protection.

www.national.com/mpf/LM/LM2940.html

I forgot to mention, and draw on the diagram:
The circuit should be attached to a switched 12v source on the bike, not tied directly to the main battery as I show on the first diagram. It would probably be better to run a switch directly to the battery to be sure you get a good source of voltage rather than going through the ignition switch and other connectors. You could also just put in a relay in place of that switch to make it automatic.

If you want a diagram of how the relay would go, let me know.