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Charging 12v system with 6v magneto?


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So, building a small dinghy to replace my current one and to use as a prototype for a larger version...

 

Anyways, question being, the dinghy is going to have a small 12v system (to run a small bilge pump, little flood light and 12v socket) but the outboards (tomos 4.8hp) charging output is for a 6v system at 17 watts (so I assume 7.2 volts at a little under 3 amps), could I get away with using a small dc step up converter (for example https://amzn.to/345Ay1E) and setting its output voltage to 14.4v to charge the 12v battery with the outboards 6v output? or is this a horrible idea for some reason?

 

The battery is only going to be a little 7ah unit, and will mostly be charged by a cable connected to an outdoor socket on my boat which is just connected directly to a 100 watt panel on the roof, so the small power output of the outboard isn't a concern, just figure it would add a little bit of run time to the battery as it would at least be getting something back in....

 

Thoughts? Is there a much better way to do this? Sorry if i've explained it stupidly or rambled lol. Cheers for any advice....

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Just now, stegra said:

I don't know the answer to your question but you say that your 7ah battery is going to be connected directly to the 100w panel. I trust that you are putting it a controller in there. I'm sure you will need one. 

Haha no, the panel is on the roof of my narrowboat isolated from my main 600 watt bank, runs into a seperate charge controller on the boat, that then feeds into a 2 way switch, one output will then charge the starter battery, and then the 2nd output goes to a waterproof socket outside which I can then plug my dinghy into outside when not using it to charge it up... if that makes sense lol

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3 hours ago, lifeintheslowlanes said:

Haha no, the panel is on the roof of my narrowboat isolated from my main 600 watt bank, runs into a seperate charge controller on the boat, that then feeds into a 2 way switch, one output will then charge the starter battery, and then the 2nd output goes to a waterproof socket outside which I can then plug my dinghy into outside when not using it to charge it up... if that makes sense lol

Some solar controllers can be damaged if they are disconnected from a battery while they are receiving power from the solar panel. There is a risk you might wreck your charge controller it if you accidentally toggle the two way switch with the dinghy battery disconnected, or if you trip over the wire to the dingy and pull it out while charging, or forget to flick the switch back to the big boats starter before disconnecting. One way of avoiding it might be, instead of having a toggle switch, use a multiway socket with two plugs. Wired as below. Trailer sockets are seven pins, so two more than required, water resistant and cheap. This will disconnect the solar panel from the solar controller while swapping between boat and dinghy batteries and any other time no plug is inserted.

 

                                         Socket     Plug 1 Dinghy charging     Plug 2 Boat starter charging.

+ve from solar ----------------|===    ---|---A connected to B       ---|--- A connected to B

+ve solar to controller--------|===    ---|---B connected to A       ---|--- B connected to A

charge +ve from controller--|===    ---|---C to dinghy batt +ve  ---|--- C connected to D

charge +ve to boat batt------|===    ---|---D not connected        ---|--- D connected to C

charge -ve to boat gnd-------|===    ---|---E to dinghy batt -ve   ---|--- E not connected

 

When plug one is inserted, then the dinghy battery is charged. When plug 2 is inserted, then the charging current from the solar controller is fed to the boat starter battery. If either is pulled out, then the solar panel is disconnected from the solar controller, protecting the controller fro m having no load on its output.

 

Jen

Edited by Jen-in-Wellies
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6 hours ago, lifeintheslowlanes said:

So, building a small dinghy to replace my current one and to use as a prototype for a larger version...

 

Anyways, question being, the dinghy is going to have a small 12v system (to run a small bilge pump, little flood light and 12v socket) but the outboards (tomos 4.8hp) charging output is for a 6v system at 17 watts (so I assume 7.2 volts at a little under 3 amps), could I get away with using a small dc step up converter (for example https://amzn.to/345Ay1E) and setting its output voltage to 14.4v to charge the 12v battery with the outboards 6v output? or is this a horrible idea for some reason?

 

The battery is only going to be a little 7ah unit, and will mostly be charged by a cable connected to an outdoor socket on my boat which is just connected directly to a 100 watt panel on the roof, so the small power output of the outboard isn't a concern, just figure it would add a little bit of run time to the battery as it would at least be getting something back in....

 

Thoughts? Is there a much better way to do this? Sorry if i've explained it stupidly or rambled lol. Cheers for any advice....

My thoughts are that the output from the outboard is likely to be pretty ripply. It will be rectified AC and thus (depending on the number of poles) may momentarily fall to near zero. When connected to a battery via diodes, this isn’t a problem because the battery doesn’t care that at some number of Hz, the battery is alternately being supplied by 7.2v and at the next moment, very little v. The battery will still get charged.

However when connected to some electronics, viz a step up converter, this ripple may be a big issue and stop the step up device from working properly. Or not, hard to tell without testing it.

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11 hours ago, nicknorman said:

My thoughts are that the output from the outboard is likely to be pretty ripply. It will be rectified AC and thus (depending on the number of poles) may momentarily fall to near zero. When connected to a battery via diodes, this isn’t a problem because the battery doesn’t care that at some number of Hz, the battery is alternately being supplied by 7.2v and at the next moment, very little v. The battery will still get charged.

However when connected to some electronics, viz a step up converter, this ripple may be a big issue and stop the step up device from working properly. Or not, hard to tell without testing it.

Would it be better for the dinghy to have a 6V 20Ahr motorbike battery, rather than a 12V 7Ahr one, then charge it from the engine alternator as normal? The 12V stuff would be operated from a step up converter, connected to the battery. Say something like this. American Amazon, but something similar shirley available in the UK. The battery will then smooth the input in to the step up module, rather than having to operate from the spiky output from the alternator.

For solar trickle charging, use a small 20W panel, connected to a 6V charge controller. Have a separate panel and 12V solar controller for the big boat starter battery. 100W is way to big a panel for the starter battery trickle charging. 20W is enough, even in winter to keep it topped up. Have the dinghy 20W panel either on the dinghy, or the main boat. The 6V solar controller on the dinghy.

 

Jen

Edited by Jen-in-Wellies
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18 hours ago, nicknorman said:

My thoughts are that the output from the outboard is likely to be pretty ripply. It will be rectified AC and thus (depending on the number of poles) may momentarily fall to near zero. When connected to a battery via diodes, this isn’t a problem because the battery doesn’t care that at some number of Hz, the battery is alternately being supplied by 7.2v and at the next moment, very little v. The battery will still get charged.

However when connected to some electronics, viz a step up converter, this ripple may be a big issue and stop the step up device from working properly. Or not, hard to tell without testing it.

Right okay, some points noted, basically just give it ago and see if it works, i've ordered a step up converter so I can experiment lol.

8 hours ago, Jen-in-Wellies said:

Would it be better for the dinghy to have a 6V 20Ahr motorbike battery, rather than a 12V 7Ahr one, then charge it from the engine alternator as normal? The 12V stuff would be operated from a step up converter, connected to the battery. Say something like this. American Amazon, but something similar shirley available in the UK. The battery will then smooth the input in to the step up module, rather than having to operate from the spiky output from the alternator.

For solar trickle charging, use a small 20W panel, connected to a 6V charge controller. Have a separate panel and 12V solar controller for the big boat starter battery. 100W is way to big a panel for the starter battery trickle charging. 20W is enough, even in winter to keep it topped up. Have the dinghy 20W panel either on the dinghy, or the main boat. The 6V solar controller on the dinghy.

 

Jen

Previous points about the charge controller noted, didn't realize that was a thing to be honest, surprised I haven't killed a controller yet knowing that! Your suggestion about using a 6 volt battery and then stepping up to 12v for any accessories isn't a bad one actually, my old man (used to teach electronics at cambride uni in the 80s and worked as an aircraft electrician for the raf prior to that) said exactly the same thing about running a 6 volt battery to stabilize the voltage better... As for the 100w panel, I know it's overkill, I had it kicking about from my old boat and just stuck it on the starter battery to give it a use. 

 

If I do go down the 6v route I guess what I could do, is buy a 2nd starter battery (i've wanted to fit one for a while now), have the 100w panel charge both those batteries, have them connected to the engine by a 2 way isolator switch so i'm only using the main battery for starting and the backup one for redundancy, then use a 12v to 6v battery charger off of the second battery for powering the dinghy etc...

 

Need to get a few bits and have an experiment as to what works well I think...

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10 minutes ago, lifeintheslowlanes said:

Previous points about the charge controller noted, didn't realize that was a thing to be honest, surprised I haven't killed a controller yet knowing that!

Some are more susceptible than others. I didn't know about this and regularly disconnected my Morningstar controller from the battery with the panels still connected, fortunately with no ill effects. Other makes/models don't like it and can die horribly.

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1 minute ago, Jen-in-Wellies said:

Some are more susceptible than others. I didn't know about this and regularly disconnected my Morningstar controller from the battery with the panels still connected, fortunately with no ill effects. Other makes/models don't like it and can die horribly.

Ah right interesting, done it a handful of times with my mppt controller and not had an issue, the other one I used for the 100w panel is just some £6 ebay special and that's had it done almost every day for 2 years lol

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