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Old 04-02-2015, 00:57   #1
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Charging 12v start battery from 24V system

we have, as all Amel boats do, a 24VDC house bank, and a 12V start battery for the main engine and the genset.
The 12V battery is only charged by the alternators on the main engine and genset.
The 24V house bank is charged by wind generator, solar panels and shore power when plugged in.
Does anyone know of a device that could be placed between the 24V house bank and the 12V start battery to keep the start battery charged ?
I've Googled it and can't find anything...
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Old 04-02-2015, 01:07   #2
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Re: Charging 12v start battery from 24V system

google 24v to 12v charger. lots come up.


here is a new looking promariner one. (never seen this product, but I use their ac chargers)


Marine DC to DC Charging Systems | ProMariner




though personally I'd be looking the other way. I would be trying to charge the house from the engine alt. so you'd want a 12-24v charger. but one that only shares charge when the engine battery is actually charging (13v+) some do this


the engine / gen battery should lasts months with no charge, especially with agm. just run the engine and gen every month. or add a small ac 12v charger aswell.
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Old 04-02-2015, 01:40   #3
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Re: Charging 12v start battery from 24V system

Not that many come up...there's a lot of stuff that comes up but its not relevant.

We have a 180 A 24VDC alternator on the main engine that charges the house bank when the engine is running.
We have a 60A 12VDC alternator on the main engine that charges the start battery when the engine is running.
We have a 7.5kVA 220VAC generator which can run both the 100A and 30A 24VDC chargers to charge the house bank.
We have a 50 A 12VDC alternator on the generator that charges the start battery when the generator is running.
We have 600W solar panel array with MPPT controller that charges the 24VDC house bank when the sun shines.
We have a D400 wind generator that charges the 24VDC house bank when the wind is blowing.

BUT

we have nothing charging the start battery when neither the main engine nor the genset are running.

But thanks for the comments,

I thought just now while I'm writing this that I could get a small 220VAC powered 12v battery charger and plug it into one of the small Mastervolt inverters which are powered from the 24VDC house bank and charge the start battery via the 24VDC house bank that way !

Cheap solution !!

Cheers
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Old 04-02-2015, 01:48   #4
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Re: Charging 12v start battery from 24V system

cheap but not efficient.


a dc to dc charger would be a better solution.


if you are at dock often just a 12v charger on shore would also be good.


an engine battery charger is really only for long term boat storage. nothing should be draining your start battery.
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Old 04-02-2015, 02:01   #5
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Thumbs up Re: Charging 12v start battery from 24V system

Quote:
Originally Posted by smac999 View Post
cheap but not efficient.


a dc to dc charger would be a better solution.


if you are at dock often just a 12v charger on shore would also be good.


an engine battery charger is really only for long term boat storage. nothing should be draining your start battery.
My start battery is a wet flooded battery, so it will self discharge....

I don't like being on shore power for long periods of time...galvanic corrosion issues from improperly grounded neighbours etc...
I have plenty of charging power to the 24VDC bank so the efficiency of the inverter / charger is probably not much of an issue

BUT

I think I'll replace the flooded start battery with an AGM...they don't self discharge anything like flooded batteries...and that's probably the best solution...

thanks
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Old 04-02-2015, 05:10   #6
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Re: Charging 12v start battery from 24V system

Quote:
Originally Posted by Albro359 View Post
My start battery is a wet flooded battery, so it will self discharge....

I don't like being on shore power for long periods of time...galvanic corrosion issues from improperly grounded neighbours etc...
I have plenty of charging power to the 24VDC bank so the efficiency of the inverter / charger is probably not much of an issue

BUT

I think I'll replace the flooded start battery with an AGM...they don't self discharge anything like flooded batteries...and that's probably the best solution...

thanks
AGMs don't like overcharging, and start batteries , paralleled into charging systems, get regularly overcharged, I wouldn't have AGMs on a boat , waste of money.
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Old 04-02-2015, 05:43   #7
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Re: Charging 12v start battery from 24V system

Albro,

Why do you think you need this? Is the engine start battery getting discharged for some reason? Millions of cars and trucks do not require separate charging of their engine start battery.
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Old 04-02-2015, 08:31   #8
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Re: Charging 12v start battery from 24V system

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Albro,

Why do you think you need this? Is the engine start battery getting discharged for some reason? Millions of cars and trucks do not require separate charging of their engine start battery.
I agree-you should not need to charge a good start battery.
But,if you want the comfort of knowing you can boost/jumper the start battery,in the unlikely event it does go dead,I suggest a H/Duty Bat.on/off switch,that ties your start bat to one of the 12V bats. of your 24V bank.
ONLY used for emerg. boost start,& turned off after eng. is running.
Make sure your 24V bank is common neg. with start bat. neg.
Tap the 12V from the bat. of 24V bank,that has it's neg. common to start bat. neg.
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Old 04-02-2015, 08:55   #9
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Re: Charging 12v start battery from 24V system

i trivial solution would be to add a small solar panel (with a cheapest PWM controller maybe) dedicated to just keeping the start battery topped off? a 30W flexible one could fit on top of a hatch, be out of the way, and not interfere with the looks of the boat?

alternatively, maybe smth like this:
Step Down DC-DC Battery Chargers
(no first hand experience)
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Old 04-02-2015, 11:54   #10
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Re: Charging 12v start battery from 24V system

Quote:
Originally Posted by Albro359 View Post
Not that many come up...there's a lot of stuff that comes up but its not relevant.

We have a 180 A 24VDC alternator on the main engine that charges the house bank when the engine is running.
We have a 60A 12VDC alternator on the main engine that charges the start battery when the engine is running.
We have a 7.5kVA 220VAC generator which can run both the 100A and 30A 24VDC chargers to charge the house bank.
We have a 50 A 12VDC alternator on the generator that charges the start battery when the generator is running.
We have 600W solar panel array with MPPT controller that charges the 24VDC house bank when the sun shines.
We have a D400 wind generator that charges the 24VDC house bank when the wind is blowing.

BUT

we have nothing charging the start battery when neither the main engine nor the genset are running.

But thanks for the comments,

I thought just now while I'm writing this that I could get a small 220VAC powered 12v battery charger and plug it into one of the small Mastervolt inverters which are powered from the 24VDC house bank and charge the start battery via the 24VDC house bank that way !

Cheap solution !!

Cheers
I believe you have answered your own question. Someone said it is inefficient, how often will you need to do it and it sounds as thought you believe you have sufficient 24V capability. So stick with it, sometimes all the experts want to make the proverbial mountain out of a mole hill.
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Old 04-02-2015, 11:57   #11
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Re: Charging 12v start battery from 24V system

Thanks all for your replies.....
The concern is that I am often not on the boat for quite long periods of time and there is no facility to keep the start battery charged.

Thanks for the link to the DC-DC chargers, I'll look into that.

I have had great experience wit AGM batteries over the years, no battery likes being overcharged, or undercharged for that matter.
With good multistage battery chargers to look after AGM batteries, they work very well IMHO.
And they have a very low self discharge rate
And they're sealed, no corrosive fluids.
I like them !
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Old 04-02-2015, 12:00   #12
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Re: Charging 12v start battery from 24V system

Charge your house bank from the engine alternator, wind and solar and use somthing like the Balmar duo charge connect the input voltage from the house bank to the duo charge. This device takes upto 30 volts in and 12v at 30 amps out, you can set the type of battery it is charging and when full will shut down. power on from your alternator regulator power switch as you would when running the engine, if you have a motor that starts quickly the re-charge time will be short .

John
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