safe dual battery w/ half power tap

Hello I have a old moped that we have modded, and added custom headlight and turn signals, as well as a new stop and running light.   Mostly LEDs.

The first problem I had is the bike has 6v charging, and lighting.  I have been unable to find a replcement coil/stator that will supply the 12v.   I was told that if I just hooked up a 12v batt the charging coil would over heat and die.So I came up with a rather weird way around it.    I have a few relays that the 6v. output from bike, (after the rectifier, So its 6v DC)  will "trigger".  This will allow 12v dc from a 12v batt. source, to flow through the relay to now power my 12v accesories.  Less than 5A draw is planned, so the relay should take it.  I know seem like lots of extra work, but...So next thing I need to know is can I run the original charging output (6v DC) to a 6v sealed, battery?  Then wire a 2nd in series, to get a 12 volt out?

Can I tap off/on  the middle.  To at least charge one battery, more so the power has a place to go and not blow a regulator or coil.

I think I need to use diodes or something to Isolate the point, that the bike is hooked up to one of the batterys.Can I put the charge to the first battery, then feed through a diode into next battery?  Will this make the bike "think" its attatched to 6v and not damage the bike?   Because the lighting draw will be low will this put "SOME" charge into both batteries?I'm not sure I am explaining it well enough. let me know if I am WAY off on this one.

Also thinking now.The original 6v headlight, ran of a seperate wire not connected to the battery  or rectifire circut.   This would be because the headlight don't care if A/c or D/c right?  Also why we can blow a brand new bulb at high speeds. But she gets real bright first!!! LOL.I could retain the original headlight if need be....     But the singals are led and I need to use a electronic flasher for them. (because low power draw) I can't find a 6v flasher.   The LEDs are 12v, BUT such low draw that a 6v 2A battery will light them fine.  But can't get them to flash.....  Running and brake also 12v but will work on the 6v battery.  The one thing that needs 12v is the speedo and tach unit.  It is digital, also has low power draw, but needs full 12v d/c. to turn on.What if I added a regulator & rectifire to the headlight circut, then used it to "charge" the 2nd 6v battery?  Then I just need to figure out the proper way to get 12v from both, while keeping each isolated?

Frankly, this sounds like it would be a lot simpler to run everything as 12 Volt!

Agreed!  this is kinda what I am trying to do...    But the bike cannot go 12v.!!  They don't make a oprtion to convert to 12v.

So I did the real world test>>>   I have (2) 6v 5Ah batteries.  hooked in series.  and just like the pic up there. I does show 12v across both,  BUT 6v on each battery seperately.  My concern is that there will be feedback of 12v back into the charging system."  So could I use some diodes in between the charging coil and the battery?  What size?  is it based on the power going TO the battery,  or the power That gets drawn from the battery?I'm not to worried about charging both at once. I will charge as a Total loss system if need be.  But I don't know what to expect  yet.   with @ batteries in series.  how will the batteries discharge?  Evenly or on more or faster than the other.I'm not sure about what will happen to the "charge" if only charge one at the time.  In parralel one battery would supply power to the other right?  What ever battery is lower the power would find it?  Also in series I know I double the voltage, but what about Amps? is it now just 12v  @ (5Ah)?  More or less?

You can't wire it so that you can charge both batteries. And if you only charge one battery, both will fail before they should (unequal current charge/draw in series batteries - akin to "don't mix batteries" in any series-battery connected device - eg, remote controls etc.) The options you have are: - a 6V to 12V converter; - depending on the stator winding, it may be possible to reconnect for 12V output. IE - if 2 coils with a GND centre tap, break the GND and GND one of the ends. That's 2x voltage output, but at half the current. (Forget transformers - the varying output frequency makes dc-dc converters more compact & efficient. And I don't think voltage-doublers would be suited, though that depends on the Amperage required.) As to 6V windings charging 12V, they should not fry. The insulation is likely to handle 12V, and 6V systems use twice the current of 12V systems for the same power/Wattage loads. However other components might - eg, it may be a charge dumping system using a ~6V zenor (probably 7.5V ).     Is the alternator controlled - ie, by a voltage-regulator driven rotor? Chances are however that it's a "stator" system - eg, a permanent magnet rotor whose output voltage is determined by the number of stator windings and RPM. They either dump excess voltage (hence current) or only pass thru the output when needed.

Thank you oldspark, I will look into the stator one day and see what makes it tick.But until then here is my slick trick I will try.  Let me know what you guys think.I know the "generator" is listed as 44w 6v.  As I said before headlight WAS 6v 15/15w.    Running on its own wire A/C direct.The "projector" look alikes I planned to use, have 12v 55w (H3) halegon bulbs.I found a rechargeable 3 millon candle power spotlamp,  Using a 6v 25w bulb!!!!!!  With a (H3) base.So now I can run the headlamps @ 6v just like original. Then I can use the original "charging"  to power the 6v relay & trigger the dual 6v batteries in series to power my 12v LED signals/flashers and the run/stop LED.So I'm still not sure can I "dump" the "charging" output into one of the 6v batteries?  Even though they will be connected in series.   (Nothing will draw 6v, off the batteries)  Is it ok to just install a diode or something to be sure nothing "feeds back", into the 6v charging system? As far as I understand, a battery MUST be connected to avoid damage to the system.  I assume this is because it would "reproduce" little to no resistance, and coil or regulator overheats. I Don't care about charging 1 or both @ this point. if only the LED's are running off the 5A. total loss...   it will run plenty long enough.The next problem I will have is getting replacement 6v (H3 bulbs) under 55w.  I found some 6v 35w, but they will need to come from the UK.  Hope to find them in Canada or USA. and able to accept paypal.Once I make sure these lights work proper.  I will post something in the motorcycle section about it.  Might be a option for us guys with old motorcycles on a 6v system.   Lots of Auto lights out there take the H3 bulb style.  If We can convert to 6v bulbs.  We can find styles that suit the bikes.(I won't argue if someone says the light output may be less.  But if a D.O.T. headlamp on a car or truck needs to illuminate up to 110 meters. While a,  D.O.T.  motorcycle or moped light only has to illuminate up to 30 meters).Some style reflectors will be better than others too.

Have you considered LEDs for headlights? They only use a few Watts, and are getting reasonably cheap. Pushbikes use them. I have a few focusable torches that are ~200 Lumen and run off 3x1.5V AAA batteries, ie 4.5V. Though I got my originals for ~AUD$25, my last was $10 from a computer "swap meet" (no used stuff there!) but I've seen them on eBay for ~$4. I suspect they will be cheaper than your "special" halogen bulbs. Wire a few in parallel if needed, but on narrow beam, they illuminate 100m with ease. And I reckon use a 6V flasher can. They are not difficult to make - a timer and a relay. And if you convert your flashers to LEDs, you can omit the relay and drive the LEDs direct else through a transistor. You could add current sensing to imitate traditional cans (ie, fast flashing if a bulb has failed), but these days the cans for LEDs omit that feature. (LED flasher cans - the new rip-off.) As to the batteries, if 12V is still needed, I suggest a relay to switch them from parallel to series. That's been done in charging two 12v batteries in series for a 24V starter motor, but change to 6V relays and batteries and it's the same. I'll post that link's diagram below. Note that the alternator is not shown - that is connected across the main battery. Note too that there is no break involved wrt the "main" 6V battery. (I had a brain fart and thought there was, and then thought of a workaround, but then regained sanity.) The indicator switch would actuate the series to parallel relay. The main battery would charge as per normal. The "imbalance" caused by the 2nd battery not being charged - which long term will wreck the battery(s) - is not an issue because, upon being returned to their "normal" parallel connection, the 2nd battery will charge at a higher rate and re-balance. (Yes, in case you have read from the many (sites) that say otherwise, you can parallel dissimilar batteries when they are being charged.) There is no need for diodes to prevent feedback... there is no feedback. In short, you'll have the "main" 6V battery across the alternator, and that's that. (The other battery can't "push thru" etc. You'd need a connection from the +12V end to the regulator or something else intimately connected...) Besides, diodes would create other problems. I wonder what your charging system is. Although the lights can be AC, DC must be provided for/to the battery. That can be done by my previously mentioned zenor-diode dumping system, but then I'd expect 2 stator-winding outputs - one for the lights (AC) and another winding for the DC. A single-circuit system (which can still have multiple stator coils) would have a bride-rectifier (diodes) for the DC after the AC take-off point. Confused? (Sorry, it's 11:30AM and I've been up all night. Not that that'll make much difference to my writing!) But in retrospect, I like the LED headlights idea. And why not tails and indicators too? To put LEDs in context, a normal LED uses 40mW (0.04W) - that's 1/75th of a 3W bulb (but a 6V LED is 3x40mW = .12W, & for 12V would be .24W - still ~1/10th of a 12V/3W bulb).    As if this isn't already long enough - now I'm inserting that diagram...

I have considered LED and even have a H3 LED with 9 bulbs that draws .06w @12v  but disappointed with the output even in my reflectors.

Even considered a Full LED light but most don't have a D.O.T. mark and are a "spot light" style.  I WILL get pulled over on this bike and need to be legal while being custom.    I can have 4 bulbs shipped to Canada for about $28.00 can.  If I order more they get cheaper.  (10's, 50's etc.) Our moped guys and classic motorcycles here may want some.  I have installed the 6v 25w in the headlamps I have and they are HELLA bright. @ 6v!!! the 35w that I ould order would be even brighter. While staying under the 44w output.  The wiring diagram is the best match to my original system. 

I already bought these slick headlamp bodies.  So cheaper @ this point to mod the existing ones.  Even if I rob my spotlights bulbs, LOL they go on sale for 8.99 all the time.   Thats cheaper than buying a 12v 55w H3 fog light bulb at a parts store.

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Trust my they look awesome on this build.  both run same watts, bottom has a shield to block light from blinding oncoming traffic.. "low"

The rear running and stop light is a Oval Clear lens with F%#&ing bright Red superflux LEDs. stop draws less than 1A easy. (D.O.T.)  And the signals are kinda like old school "beehive" amber marker lights.  But again are superflux LED's. Amber lens with chrome visors. (D.O.T.) The "flasher" I found works with these lights is a "electronic flasher" 2 terminal.  I use it in my van to for when a trailer is hooked up with old lights. Cost $2 more than a Standard flasher. and rate is perfect, even if I add a incadesent indicator, or buzzer in line.I also already have the 6v relay  (6v dc coil, 48vdc 5A pass through) and tested the 12v "flip flop".  (nice thing about this is when I have the bike off, the 12v goes to N.O. @ rest. This gives me a 12v power for Show lighting and maybe a alarm). Will go probably go with 12v horn as well. I will look at that diagram you posted, again tomorrow as it is late here to.Can I use a 6v voltage regulator on the head lamp wire, to prevent overrunning the headlight? @ extreame RPM.  Something like this....

https://www.treatland.tv/6-volt-voltage-regulator-p/6v-voltage-regulator.htm

I wonder if that's an SMPS regulator? If it's linear, it'll get hot (35W) and waste a lot of power. SMPS regulators (dc-dc converters) can be quite cheap - see current-logic.com, not that I see a 6V out (9V & 5V yes), but they do have adjustable output versions - albeit bigger and not enclosed - for ~$20. And they can build to order.... As for LEDs, forget the multi "normal" LED arrays. They need to be Cree LEDs or similar. The one you mentioned consists of strings of 20mA LEDs. Crees take higher currents (and need current regulation... speaking of LM317 current limiters {another thread I just replied to}). My 200 Lumen torch is a single Cree LED. It will spot thru my car's headlights! And there are approved LED headlamps, though their configurations may still be limited. (I saw a 55W LED spotting light that blows HIDs into the weeds! But each LED has its own lens - it's not LEDs mounted behind a typical headlight lens, and that's typical for any multi-LED torch etc.) Bear in mind that the 44W output is probably above a certain RPM, and power is probably used elsewhere too (tail & dash lights, plus the averaged power taken by flashers and brakes, and I'd assume engine ignition power). I recon that with a 35W bulb you'll find yourself with a flattery before long.   (As I did with my 120W permanent magnet Ducati alternator - it didn't charge appreciably below 3,000 RPM which happened to be over 100kmh in top gear!) And if you put your wipers on...! (ha ha) I see the solution as either converting the alternator to 12V and hence everything else, else sticking to 6V. Another option is a dc-dc step-up converter from 6V to 12V, probably with a 6V battery across the alternator with the dc-dc converter feeding a 12V battery. A twin-6V battery with only 6V charging capability is IMO not practical for other than short duration use. The "upper" 6V battery (12V output) will be discharged until it can be connected back in parallel with the alternator and other battery. That wears the upper battery quicker, and connecting a flattery to another charged battery isn't good for either in the long term.

Ok so instead of the regulator on the headlight wire.  I think some guys hide a 2nd bulb so the extra current @ high RPM has somewhere to drain off to instead of blowing main bulb.  I'm guessing in seires? After the main bulb? I think this is why I can sometimes run the "high beam" Even where the low would pop.  Both were 15w but the high also has a 5-7w indicator.     Just a guess mind you.The 25-35w headlight bulb wouldn't be running off the battery at all.  So no concern of anything but the LED's drawing of ANY batteries 6v or 12v.  I ran the taillight and the 2 signals off the 2 6v 5A batteries wired in series for over 24 hours and they still had LOTS  juice left.   (keep in mind the signals won't be used 24-7 and also both won't be on at once) I have a charger in the shop I park in to trickle charge the 12v overnight,  Via a lighter style power port. This bike also most likely won't be used every day.I had thought of making a "auto or timed" relay  To flip the charge from one 6v to the other, parralel.  every 20 min. or so LOL.  Could work I guess.  Like I said the LED draw is so little that in 20-30 min not a full volt is lost yet. maybe even less!