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captaincaveman avatar image
captaincaveman asked

Sailboat - charging starter battery from solar

All Day Wiring Diagram.jpgHi, we recently purchased a boat, and i am trying to understand all the wiring currently. I have pulled together part of the puzzle so far, and in the attached i have shown the positive wiring only for now. (But there is a smart shunt going to a BTMiii unit, so i assume all the negatives will be attached to this)

Currently the solar panels and the large Auxiliary alternator go to the house bank (3 x210ah AGm batteries in parallel). The starter battery seems to be wired to the smaller alternator, but I am not sure that is working properly, as an initial test only showed the output as 12v when the engine was running (more investigation needed). The starter battery also connects directly to the house bank via an isolator switch - which would seem to be the equivalent of attaching jumper cables if needed. I assume though this would not be a prefer way to charge the starter battery? Would it not require some form of DC to DC charger in between? (the starter battery drains overnight if i forget to set the isolator switch to off, which is annoying and a risk, as i can't often jump off the house bank until the sun comes up, and starts to add volts to the house bank)

Ultimately i am looking at some way of topping up the starter battery from the solar

orion-tr smartsolardc-dc charger
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2 Answers
christern avatar image
christern answered ·

As nobody have yet answered your question for a week I jump in and have a few suggestions.

'and in the attached i have shown the positive wiring only for now'

I cannot see any attached drawings of your electrical system. This would be needed to give much more precise advice.

'The starter battery seems to be wired to the smaller alternator, but I am not sure that is working properly, as an initial test only showed the output as 12v when the engine was running'

12V flat is definitely not enough to keep the starting battery charged. Look further into this. Please also describe what the "smaller alternator" is and how it is wired.

'The starter battery also connects directly to the house bank via an isolator switch'

The most common way, to my experience, of connecting a house bank to charging via the alternator is to have the alternator connected to the starter battery (directly) and then some kind of isolator switch which connects the house battery to the starter battery when the voltage is above a certain level, mostly around 13,8V. This will enable the starter battery to get what has been used for starting the engine and then let the house battery consume what it could take above that.

Please note that this is a simplified description and may be disputed in details by some professionals. On an overall level though I hope this gives you some input for futher investigations.

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captaincaveman avatar image captaincaveman commented ·

sorry, not sure why the image didn't upload - have attached it now.

will investigate the smaller alternator some more, but at this point, as this one is not working, then the larger alternantor is the one charging the house bank, and there is then an isolation switch which needs to be set to "On to enable charging to the Starter battery. The problem is, if i forget to turn this off when i stop the engine, the Starter battery is also drained along with the House bank. I was hoping there might be a better way to wire this? ie not having reliance on the house bank to charge the starter battery - perhaps by having the solar also come into a dc to dc charger and then to the Starter battery

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pwfarnell avatar image
pwfarnell answered ·

As you normally should have 2 alternators then all you need is a small trickle charge unit to keep the starter battery topped up as longbas you do not have large loads on it. A few options are linked to, but these may not be available in your location, so look for similar.

https://sterling-power.com/products/battery-to-battery-charger-maintainer

https://www.12voltplanet.co.uk/votronic-12v-standby-charger-trickle-charger.html

https://www.roadpro.co.uk/product/02e02-battery-maintainers/ablemail-amt12-2-trickle-charger-c8450/C8450

https://www.sunshinesolar.co.uk/Item/BATTMA01

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captaincaveman avatar image captaincaveman commented ·

thanks, they llok like they are what i need. Was hoping to move to Victron for everything - do you think any o these might work? https://www.victronenergy.com/dc-dc-converters - they seem to suggest that they detect the alternator running to charge, but i am hoping it will trickle charge the starter battery when the house/leissure bank is being charged with either the alternator or solar

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