Outboard and split charging

D

Deefor

Well-Known Member
Joined
16 Aug 2011
Messages
422
Location
Marlow
Visit site
I've recently purchased a split charging diode (input + 2 out) for use with my 15hp Tohatsu outboard as I wish to install a leisure battery in addition to the starter battery.

Currently (with just one battery), the alternator charge lead output goes to the +ve on the starter battery via a simple key operated on/off switch to enable both starting of the engine and charging of the battery. I understand how to wire the new/additional leisure battery but not how to wire the starter battery via the split charger.

All the wiring diagrams I've found on the net pertain to inboard engines with an alternator and starter motor and I'm somewhat confused as to how it all works. The nearest thread I've found is http://www.ybw.com/forums/showthread.php?t=84456 but I'm still unable to understand everything when relating it to an outboard.

Any help would be appreciated :o.

Many thanks :).
 
You will have to identify the DC output from the engine's rectifier. Separate that from the rest of the engine wiring and take it directly to the input terminal of the diode splitter.

Connect the two batteries to the output terminals of the splitter.

Leave the existing connection between the starter battery and the engine ... that will now be purely the supply for starting
 
Deefor said:
Thanks Vic, will have a look under the hood at the weekend ;).
Click to expand...

However on reflection if i were you i would consider using a VSR rather than a diode splitter.

It wont suffer from the volts drop that a diode system will and it will be easier to connect , requiring no interference with the outboard's internal wiring. It wiil simply connect between the two batteries

See here for a circuit that also includes battery isolator switches and an emergency crossover switch.
 
Have you considered how long it's going to take to charge 2 batteries? The output from your outboard is not going to be all that much. I found that a small solar panel produced a useful amount of charge. But it depends on how you use your boat (and how often)
 
I assume that a 15 hp and a location of Marlow the Op has a small River based motor boat so outboard running time will be a few hours at a time .

the exact engine has not been disclosed but the current 15 hp 4 strokes have a 12A alternator ... that should give a reasonable amount of charging.

Is a two way VSR was fitted then a solar panel could be connected as well, with the leisure battery given priority.
 
Spot on Vic. 18 footer with a 15hp Tohatsu, new in October 2011 and, yes, on the Thames ;).

Hmmmm, solar panel :). Wish we had more sun up here sometimes :D.
 
Last edited:
VicS said:
However on reflection if i were you i would consider using a VSR rather than a diode splitter.

It wont suffer from the volts drop that a diode system will and it will be easier to connect , requiring no interference with the outboard's internal wiring. It wiil simply connect between the two batteries

See here for a circuit that also includes battery isolator switches and an emergency crossover switch.
Click to expand...
I would be tempted to move the connection for the emergency start isolator to the other side of the start battery isolator. Reason: Defective start battery, close isolator, both batteries flat.
 
Split battery charging

Yes definitely unless you can find the battery sense wire to the regulator for the alternator you won't get much charge into either battery if you use a normal diode splitter. (some electronic type splitters have 0 volt drop which would be oK but a normal 2 diode splitter would need regulator sensing on one battery +ve terminal to get the alternator to provide correct charging voltage after the losses of the diode.
As said a VSR is the best bet or just manual paralleling with a switch.
If it has a 15amp alternator then I imagine the system is far more like a diesel alternator system than the smaller old outboard lighting coils.
Just as a wild idea the alternator will have 6 diodes. 3 effectively going to -ve and 3 going to the +ve output terminal. If you could find these diodes and take 3 wires out from the 3 phase stator you could add another 3 diodes to go to the second battery +ve. The engine battery system would remain as original and the aux battery would be charged some what the same as the engine battery almost in parallel in terms of regulation and same voltage. (don't tell em I suggested it)
good luck olewill
 
William_H said:
Yes definitely unless you can find the battery sense wire to the regulator for the alternator you won't get much charge into either battery if you use a normal diode splitter. (some electronic type splitters have 0 volt drop which would be oK but a normal 2 diode splitter would need regulator sensing on one battery +ve terminal to get the alternator to provide correct charging voltage after the losses of the diode.
As said a VSR is the best bet or just manual paralleling with a switch.
If it has a 15amp alternator then I imagine the system is far more like a diesel alternator system than the smaller old outboard lighting coils.
Just as a wild idea the alternator will have 6 diodes. 3 effectively going to -ve and 3 going to the +ve output terminal. If you could find these diodes and take 3 wires out from the 3 phase stator you could add another 3 diodes to go to the second battery +ve. The engine battery system would remain as original and the aux battery would be charged some what the same as the engine battery almost in parallel in terms of regulation and same voltage. (don't tell em I suggested it)
good luck olewill
Click to expand...
Sorry.
I think you are wrong about the "alternator" I think you will find it is the usual outboard style stator coil under the flywheel feeding out to a rectifier/regulator. So no sensing wire involved. No problems using a diode splitter although I personally would also have preferred to use a VSR.

See http://www.boats.net/parts/search/Tohatsu/Outboard/2011/MFS15C/MAGNETO/parts.html
and
http://www.boats.net/parts/search/Tohatsu/Outboard/2011/MFS15C/ELECTRIC PARTS/parts.html
 
Had I found the cable I thought I had in my garage, I'd be able to tell you all how things went. As it goes, I've got to go and buy the cable :o.

On http://www.boats.net/parts/search/Tohatsu/Outboard/2011/MFS15C/ELECTRIC PARTS/parts.html, there's a red wire that goes from the rectifier to the right hand terminal/post on the starter solenoid. I've identified this wire as the charge wire. I'll be disconnecting this wire at the starter solenoid, extending it and taking it to the split diode input. Split diode in question has a 0.2V drop. Not sure until it's all wired up if the OB will be excited on start up but will let you know.

I'll also be using two separate battery isolator switches in accordance with the BSS. If, for whatever reason, the starter battery goes flat and can't start the OB, I'll just have to unbolt the domestic battery and, if it's got enough umph, reconnect it to the starter circuit. Yes, I could have put another OFF-1-2-BOTH switch in the circuit but it's another component and more £'s :o.
 
Deefor said:
If, for whatever reason, the starter battery goes flat and can't start the OB, I'll just have to unbolt the domestic battery and, if it's got enough umph, reconnect it to the starter circuit. Yes, I could have put another OFF-1-2-BOTH switch in the circuit but it's another component and more £'s :o.
Click to expand...

Stash a single car jump-lead away somewhere. Much easier than unbolting things and moving them around.

Pete
 
Take care to disperse any hydrogen gas that may have been produced before connecting a jump lead.

Engine not have rope starting as an option ?
 
prv said:
Stash a single car jump-lead away somewhere. Much easier than unbolting things and moving them around.

Pete
Click to expand...

Good idea, I like the simple things in life :).

Had better remember, should the need arise, to close the vent on the OB tank :eek::eek:.
 
OB Alternator

Thanks Vic I did assume that if it is rated at 15 amps it must have some sort of regulator. Perhaps not. I wonder what sort of charging current you can get in real life.
If it were me I would be thinking about additional rectifiers from the 3 phase leads out of the alternator. But for the OP probably far to difficult. good luck olewill
 
Eveything's now up and running but I've just one concern :confused:.

Both the starter and domestic batteries were showing 12.52v when they went back on the boat, measured across the terminals with a multimeter. They'd been sitting uncharged in my garage for around a week.

Started the engine up and on tickover, both battries were reading 14.14 volts. At WOT, (5k rpm) under no load for 20 seconds, the volt readings on both batteries shot up to 15.15 volts but no higher.

Is this 15.15 volts something to be concerned about as I thought 14.7ish would be the most I should be getting?

As an aside, what's the lowest volts I should let my 85ah domestic battery drop to before I have to start unplugging things, such as the wife's 40w coolbox (aka wine cooler lol)?
 
Last edited:
Charge volts

Someone will be along will more accurate detail. However 14.2 volts at 5k RPM I would think is oK. The 15.15 volts is perhaps OK also. If the batteries are wet cells with filler caps keep an eye on fluid level. If they are using too much water then they are being overcharged. You could try a cheap solar regulator to reduce the charge voltage a little or even fit a silicon diode which will drop about .7 volt.
I would think if the battery drops below 11.4 volts it is time to turn off the cooler. A huge power waster. Get something like a WAECO CF25 compressor type fridge. Actually uses more power but really cools so will be off much of the time. These have an internal turn off circuit at about 11.4 volts or so. good luck olewill
 
Thanks for the moral support William ;).

As for the coolbox issue, we're boating on a budget and are generally only away for one or two nights at most without shorepower. I did find last year that after unplugging the coolbox overnight, the temperature inside was still around 5 degC with an ambient air temp. of 15 degC, some 12 hours later.

What I have noticed, which is good, is that with the engine running under load (in forward gear) @ 2krpm for 5 minutes, both batteries are reading 13.9 volts. Under no load conditions, the charge is much higher.

Perhaps VicS can explain the load/no load anomally, but as I said, it looks like the charging is working as it should :).
 
You must log in or register to reply here.

Share this page

Top