Battery power switch for 15a battery

6 posts / 0 new
Last post
tilt2468
Offline
Last seen: 8 years 2 weeks ago
Joined: Wednesday, May 27, 2009 - 06:57
Points: 76
Battery power switch for 15a battery

I have a 24v 15a lithium battery in a case and I want to put a voltage cutoff switch on it so that it can't spark when I reseat it on the bike after charging.

I found this switch on e-bay that is rated for 20a: http://cgi.ebay.com/ebaymotors/Heavy-Duty-20-amp-Toggle-Switch-with-Waterproof-Boot_W0QQcmdZViewItemQQhashZitem439aaa64edQQitemZ...

I plan to run the positive power lead from the battery to the switch and then on to the fuse at the contact point where power is transferred to the bike. I can't reach the fuse before the switch due to the fuse mounting spot, but I could insert an in-line fuse before the switch.

Should the switch do the job I want?

Should I add an in-line fuse for protection?

marylandbob
Offline
Last seen: 3 years 9 months ago
Joined: Monday, June 22, 2009 - 12:24
Points: 524
Re: Battery power switch for 15a battery

Your problem is, actually fairly complex! If you REALLY want to "eliminate sparking" the addition of an ordinary switch will not do so, it will only relocate the sparking to the inside of the switch! If you want to eliminate the sparking, it will be necesssary to add a solid-state "inrush current eliminator" to gradually ramp up the input voltage at time of circuit energizing. Whichever you do, the device chosen should be rated to handle the current demanded, and this current may be much more than 15 amps! If your vehicle will not operate at full speed, on an uphill course, for an hour or more with your 15 ampere-hour battery, it is likely that it can demand more than 15 amperes. (A typical 15 ampere-hour lithium battery can power a device demanding 75 amperes or more)--Have you measured the actual worst case current and voltage? Using a too small switch may eventually leave you stranded!--Bob

Robert M. Curry

tilt2468
Offline
Last seen: 8 years 2 weeks ago
Joined: Wednesday, May 27, 2009 - 06:57
Points: 76
Re: Battery power switch for 15a battery

Let me better explain the situation and see what you advise.

I have a Merida 550 with the stock controller and battery case (it now contains a lithium battery, but the case and key switch have not been altered). Under ordinary operation, the battery must be remounted to the bike before the key can be turned to activate the controller board-- I assume that there is a limiter on the inrush current built in to the stock system.

My battery has more capacity than I need for my commute (10 miles fairly flat between charges). I want to hook up some lights that draw from the pack, so that I am able to ride in the winter when it is otherwise too dark on the road for safe riding. Is there a good way to tap into the battery power? If I put a switch on the light system (less than 24v 25 watt draw), it won't have incoming rush protection, but it does not appear to need it-- I have been testing the lights with a 24v 12ah sla pack with no adverse effect on the lights-- the only problem is that when I make the connection to the battery for a test, there is a spark when contact is made. For testing purposes, I used a household switch to open and close the circuit and everything worked smoothly. My thinking is that if I wire the switch to the battery then I can follow this sequence when I ride: a) mount the battery with power cut off by the switch, b) throw the switch to open power to the lights, but no power will be going to the bike until I c) turn the key for the bike to power up as it always has.

The load on the battery when the switch is thrown will always be less than the 24v 25 watts maximum of the light system. I am fairly certain the motor on the bike is rated at 400 peak output, but I am not certain whether that figures into the equation.

If it would be better, I could place the switch downstream of the battery connection where it would no longer need to transfer all the load for the motor system, but it would still produce a no load situation when the battery is being connected. This actually would be the easiest solution from a wiring perspective.

Basic question for someone with the knowledge-- what ampere fuse is appropriate for a 24v 25 watt draw?

tilt

marylandbob
Offline
Last seen: 3 years 9 months ago
Joined: Monday, June 22, 2009 - 12:24
Points: 524
Re: Battery power switch for 15a battery

If your TOTAL lighting load, at a nominal 24 volts, is 25 watts, I suggest that you utilize a 2.0 ampere fuse, before the switch, as the normal load for the lights will be about 1 ampere, with a short, but higer amperage surge when first turned on. The purpose of the fuse is to protect the WIRING, so be certain that all wiring is capable of at least 5 amperes for best results. (18 guage or bigger wire will suffice, & remember: 16 guage is BIGGER than 18 guage, 20 guage is SMALLER--Lower guage # is BIGGER wire size!) Wire the switch so that it has NO EFFECT on your MOTOR operation, and all should be fine!(Use the switch only to power the lights) The motor will demand at least 20 amperes, possibly MORE for brief intervals, and it is an inductive load, likely to cause arcing/burning of switch contacts, so it is best NOT to switch it.-Bob

Robert M. Curry

tilt2468
Offline
Last seen: 8 years 2 weeks ago
Joined: Wednesday, May 27, 2009 - 06:57
Points: 76
Re: Battery power switch for 15a battery

The wiring is all 18 gauge primary wire and the connections are all soldered and protected by heat shrink, I will find a 2.0 ampere fuse and place the switch beyond the motor lead. Am I correct that it is much preferable to have the arcing damage take place inside the easily replaceable switch instead of at the battery connection terminals?

Is there any cumulative/ potential damage to my battery from facing a small load when the switch is opened without an inrush current eliminator? The battery is a stock 24v 15ah battery with bms I got from pingbattery.com and it has worked well for the months I have been using it.

Thank you greatly for the input!

How does the 'kudo' system on this site work? Sometimes I get an option to offer 'kudos' for assistance and other times it is not available. I don't know how concerned you are with points, but I would like to be able to consistently recognize those who provide me with advice.

tilt

marylandbob
Offline
Last seen: 3 years 9 months ago
Joined: Monday, June 22, 2009 - 12:24
Points: 524
Re: Battery power switch for 15a battery

The wire should be OK for your lights, the small arcing from switching the lights should not be a problem, as lights are a resistive load, with very little inductance. If an inductive load, such as the motor, were switced, great amounts of arcing could occur, and some switches would quickly fail, or even catch on fire! I Would not worry about an inrush current limiter for your lights, but one could be simply made, by installing a 10 watt wirewound resistor, of about 3 ohms in series with the lights. This resistance will very slightly dim the lights, but would make a noticable reduction in the current spike at switch-on, and increase the life of the bulbs. As far as I know, clicking the "Kudo" selection will award one to the party concerned with that reply. Ride safe, ride far, ride happy!--Bob

Robert M. Curry

Log in or register to post comments


Who's online

There is currently 1 user online.

  • aymgoa

Who's new

  • aymgoa
  • wodnik7
  • Xaavier
  • rsementi
  • locori

Support V is for Voltage