Charging a 12v bank from a 48v bank

rolandr

Hi,

Full disclosure I live on a sailboat with an electric inboard propulsion system using a 400Ah 48v bank of AGMs at 6v each. I've also got a single 12v 105Ah AGM which is used for house loads, lights, etc. The 48v bank is charged with a 120v --> 48v shore power charger and soon solar and wind as well. I'd like all the charging sources to be 48v and feed into the 48v bank. Is it possible to connect the 48v bank into a solar controller as the input load to smart charge my single 12v AGM to keep it topped up since it gets used more than the propulsion bank on a daily basis? If so, would a simple PWM controller suffice? I could use a SevCon 48v to 12v converter (which I already own) to input into the solar controller, or is there a simple (read: cheap) controller which would take a 48v input and smart charge a 12v bank? Is this feasible and does anyone have any recommendations on models or configurations?

Regards,

Roland
s/v Miss Teak

Cariboocoot

Re: Charging a 12v bank from a 48v bank

Welcome to the forum.

Yes, it is possible to charge your 12 Volt battery from the 48 Volt bank. It's not easy, though. You can't connect them with just a PWM controller as it will put the full 48+ Volts to the 12 Volt battery; too much current potential there. (This is not the same as using a "24 Volt" panel through a PWM to a 12 Volt battery, where you just lose power because panels are current sources and don't really care what Voltage they are limited to.)

Some ways you can do it:
DC to DC converter. Basically stepping down the 48 Volts to about 18 and regulating it for charge. You have to find (or build) the right one for your particular needs. This would be the most efficient way. The output of the SevCon unit you have would determine if that is feasible. If the actual output Voltage is regulated to 13.8 it won't be high enough to charge the battery should it be seriously depleted, but it would "maintain" it. You might consider this in combination with a separate AC charger used when shore power is available.
48 Volt inverter power 12 Volt charger. This way tosses efficiency off the stern and watches it sink, but it sure is easy.
MPPT type charge controller. This method was actual researched on the forum and applied to a boat whose owner was seeking a solution to the same problem. In this case it was a 24 Volt starting/charging system to supply 12 Volt for emergency communication equipment. Shortfall: not cheap. $300 range for a Morningstar 15 Amp MPPT controller.

vtmaps

Re: Charging a 12v bank from a 48v bank

You didn't mention whether you have an inverter (48 DC to 120 AC) on the boat. If so, the cheapest option would be to use an inexpensive 120 volt automotive 'smart' charger. --vtMaps

rolandr

Re: Charging a 12v bank from a 48v bank

Thanks guys for the suggestions. It appears the MPPT controller is the way I'll need to go (nothing on a boat is cheap and the 12v bank powers safety equipment ).

I did read that emergency communication equipment thread and I've looked at the Morningstar product website. A couple of things I couldn't find in the literature or thread: Can I simply leave the 48v bank connected to the controller and will it shut off or float the AGMs when they're fully charged or do I have to micro-manage it by turning it off once the 12 bank is charged? Also, does it down-convert to 12v on it's own or do I still need the SevCon to do this?

If I can eliminate using the SevCon, I'll keep it aboard as a back up in case I need to directly power our emergency VHF radio, instruments, radar or some other 12v equipment should a worst case scenario occur with the 12v bank while we're away from shore. We also will have a propane powered generator which can plug into our shore power inlet to provide 120v charging for both banks while underway as a backup if solar and wind aren't available. We've got kids on board and will be doing long ocean passages and I'm trying to engineer as many layers of redundancy as possible.

Thanks again for any info you can provide to the questions above!

Regards,

Roland

Cariboocoot

Re: Charging a 12v bank from a 48v bank

The Morningstar 15 Amp MPPT will be set-and-forget. When it sees the 12 Volt battery is fully charged it will cut back to Float Voltage and stay there. It will also handle the down converting of 48 Volts to 12 Volts fine. Its maximum input Voltage is 75, so not even an equalization charge on the 48 Volt bank will be a problem.

Brock

Re: Charging a 12v bank from a 48v bank

I would second the morning star. I have a large 48v bank and a bunch of 12v load. I just leave the morning star running all the time and it keeps the 12v topped off, no messing with anything.