can I add a second battery??

I am wanting to add a second battery to the 77 ltd 1000, the reason is I am adding LED light bars to it and want them to run off the second battery while parked,

The real question is how to isolate it from the main battery and still be able to charge it when riding?

Also is the charging system going to be able to keep up with a second battery.

I have a small sealed battery, listed at 5ah

Any thoughts

To my thinking it would be more efficient to run the LEDs off your original battery and use a separate circuit with a switch and relay. There is inefficiency when charging so to charge two batteries would add this inefficiency and your stator would be under greater stress to keep both charged. If the stator is capable of charging two batteries and you do it, you will still be on the separate circuit with a switch for these LEDs. I would think you would want to shut that stuff off if dragging around at low speeds when charging isn't as robust as when out on the open road.

Personally, I like the idea of an isolated battery for parking. I liked it so much I designed a possible circuit for it.

It's far better than taking a chance on killing the main battery. You just have to make sure it won't interfere with the charging of the main battery when the main battery is low. It may slow down the re-charge of the main battery, but only after the main battery is above a certain voltage. This will require an active circuit.

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This is a circuit designed to recharge a second battery on a motorcycle. The purpose of the secondary battery is to power an auxilliary light for parking only. The operational criteria of the circuit is as follows:
1. The auxilliary light will not drain the main battery at all.
2. The secondary battery will not drain the main battery at all.
3. The main battery will not drain the secondary battery at all.
4. The secondary battery will not use any alternator power for re-charging until the system voltage is at least 13.5v or higher. That is, the secondary battery only gets charged while the bike is revving at higher RPMs such that the system voltage on the bike is 13.5v or higher. This will ensure the main battery is getting charged before the secondary battery starts to get charged.
5. If the auxilliary light is left on accidentally, it will not draw any power from the charging system until the bikes system voltage is over 13.5v. This ensures the main battery is getting a charge before any power is diverted to the aux. light.

The design is such that a careful selecting of R3 can act as a crude current limit for the secondary battery. 300 ohms is a good starting point. To determine the ideal value of R3:
Wait until the secondary battery is somewhat drained.
Place an ammeter inline with the battery (with the light off) to monitor the charging current.
Increase the resistance of R3 until the charging current tapers off to the desired value. 1 or 1.5 amp should be good.

Hint: If you buy a pack of 150-ohm 1/2watt resistors, you can put 2 of them in series to make the 300 ohm, 1 watt resistor, since a 300 -ohm, 1 watt may be hard to come by. You can also add more in series to make the crude limiter.


EDIT: I attached a revised drawing as mentioned below, to reflect the connection to the main battery.
Also, I added other choices for R1 so you can experiment to get the ideal threshold voltage range of 13.6v to about 14v.

Here is a very easy way to hook up a second battery:

Get a relay and wire the coil contacts to the 12V (switched) line coming off the ignition switch.

use this relay to connect the second battery to the bike's main 12V battery. I would use a direct connect to the battery going only through the relay. Put an inline 20A slow-blow fuse in that line going to the second battery.

Wire all your "parking" electrics to the second battery directly through a switch.

When the ignition switch is turned, that will power the relay and connect the two batteries in paralell. The bike's alternator will charge both batteries up while you ride.

WARNING:

Whenever two batteries are in parallel, they affect each other. If you deeply discharge the smaller battery when you leave it parked, then when you go to ride again you will be connecting a discharged battery in parallel with a full one and the big one will dump current into the small one to charge it up. Make sure the smaller battery can handle a high surge from the big battery.

You could limit the surge by inserting a small amount of resistance (like maybe 2 Ohms) in the series line from the main battery to the smaller one to limit the peak current to about 6A. The resistor should be rated for about 3 Watts or higher.

The advantage of this method is there are no semiconductors and you don't have to worry about power dissipation in a transistor feeding the battery.

loudhvx wrote:

Hint: If you buy a pack of 150-ohm 1/2watt resistors, you can put 2 of them in series to make the 300 ohm, 1 watt resistor, since a 300 -ohm, 1 watt may be hard to come by.

Digi- Key always used to have a good selection of power resistors in the 1 - 3 Watt range. The metal-oxide kind are usually cheapest, wirewound are more expensive.

The reason I chose the circuit as I did, was as you said, the second battery will feed off the main battery after the second battery is drained with a possibility of a large current. I wanted to protect the main battery from this. I assume the second battery will be considered somewhat expendable since it's going to get deep-cycled and it's life will be short. Although, maybe a small marine-type deep-cycle battery would be good.

I also thought of the resistor idea, but that would reduce the charging current. A 2 ohm resistor will drop the voltage too much when the second battery needs charging the most. Also that resistor will heat up. It would have to be a 5w or 10w resistor to be safe.

The transistor shouldn't get any hotter than the resistor idea, and in fact should be in saturation or cutoff most of the time since the charging system on the bike has a lot of ripple. That means there will be very minimal power dissipation in the transistor. It acts like a duty cycle when the bike's voltage is near the threshold of the zener detector circuit. If the current limiter option is tried, then it will act like that resistor idea.

The main benefit is that the circuit isolates the two batteries from each other, and doesn't put a drain on the main battery.

Seems like just using diodes should be another possible solution for isolating the batteries, but I couldn't think of a way.

how many amp's are your led's going to pull they don't normally pull too much i'd say you would be alright with the stock charging system if your battery is up to snuff. add up all the wattage draw when the bike is running and see if your charging system can handle it, every thing but the starter check out all the light's that some of the garbage barge owners[goldwings] run with stock charging systems.i'd say your probably worried about nothing.another thing to considered is how long you will be leaving these lights on while you are parked.an led dosen't draw a hole lot of juice.

loudhvx wrote:

The transistor shouldn't get any hotter than the resistor idea, and in fact should be in saturation or cutoff most of the time since the charging system on the bike has a lot of ripple.

The voltage across the transistor will be the bike's system voltage (appearing across the main battery) which is 14V minus the voltage of the small battery. The voltage of the smaller battery would be about 11.5V after it has run lights awhile, putting at least 2V across the transistor. The transistor's charging current will be not well controlled and will have the dangerous effect of increasing as the small battery's voltage is lower since the gain of the transistor increases with increasing VCE (the transistor pulls out of saturation). The real catastrophic failure will occur when the small battery ages and gets a shorted cell holding it's voltge down. Unless the transistor has a good heatsink, it will cook.

timebomb33 wrote:

how many amp's are your led's going to pull they don't normally pull too much i'd say you would be alright with the stock charging system if your battery is up to snuff. add up all the wattage draw when the bike is running and see if your charging system can handle it, every thing but the starter check out all the light's that some of the garbage barge owners[goldwings] run with stock charging systems.i'd say your probably worried about nothing.another thing to considered is how long you will be leaving these lights on while you are parked.an led dosen't draw a hole lot of juice.

Another thing, you can usually get higher A-Hr rated batteries for extra $$ with thinner insulators and more plate area to cover higher drain applications.

I had a lot of led's on my 78 and never had a problem even when the bike sat for hours with the lights on.

You can kind of see them shining down on the motor here.

bountyhunter wrote:

The voltage across the transistor will be the bike's system voltage (appearing across the main battery) which is 14V minus the voltage of the small battery. The voltage of the smaller battery would be about 11.5V after it has run lights awhile, putting at least 2V across the transistor. The transistor's charging current will be not well controlled and will have the dangerous effect of increasing as the small battery's voltage is lower since the gain of the transistor increases with increasing VCE (the transistor pulls out of saturation). The real catastrophic failure will occur when the small battery ages and gets a shorted cell holding it's voltge down. Unless the transistor has a good heatsink, it will cook.

If the transistor is in saturation, the current to the battery may be high, but the emitter-collector drop will be near .1 or .2v. I predict 4 or 5 amps on a small battery at most. That's only about a watt. Only if the current limit option is used will the transistor get hot. Lets assume we set the limit around 2 amps. The battery may have near 12v at that point. Then the transistor may have close to 2.5v on it. That's still only 5 watts. The transistor is rated at 65 watts... plenty of safety margin. With a shorted cell, you would add 2 volts. That's still only in the 10 watt range. Let's take a worse case scenario without the current limit. Even at 10 amps, the transistor is only dissipating 20 to 30 watts in saturation.

But of course this ignores the fact that if you're sucking up high amperage from the charging system for the second battery, you're not going to get those voltage drops because the charging system is still powering the bike and main battery. The bike's voltage will drop making the circuit even more safe. I doubt there will be any problem.

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)