Charge 12 volt battery from 24 volt battery bank

OffGrid+LovinIt · March 2014

Howdy,

Since I've switched my remote off grid system over to 24 volts, I been looking for a way to maintain my 12 volt emergency backup battery bank (2 T-105's).

The backup bank is only used to power up my alarm system, video cameras, system monitoring and cellphone link, when/if my 24 volt battery bank fails.

My problem is, how can I efficiently maintain the 12 volt battery bank.

I would like to have the capability to Bulk/Absorb/Float when/if needed.

Any suggestions ???

Thanks DennisB

westbranch · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

welcome. What charger etc do you have? May be easiest to have 2 separate charging systems...

BB. · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

You can use a good quality MPPT solar charge controller to go from XX volts to 12 volts. They will work fine when connected directly to a higher voltage battery bank (always use the correct fuse and wiring).

This is one of the less expensive controllers:

Morningstar SunSaver 15 Amp MPPT Solar Charge Controller

You could go with any sort of DC to DC converter and run without the 12 volt battery bank:

http://www.powerstream.com/dc1-isolated.htm

Or, simply get a small AC to 12 VDC battery charger off your main inverter... You have conversion losses anyway. Do you have 10-20% extra power (over your 12 VDC needs) in the 24 volt system?

-Bill

OffGrid+LovinIt · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

The 12 volt battery bank (2 T-105's) is primarily a backup system (except for occasional use on weekends to power some radios and led lighting in the solar building while I'm working) so I really hated the idea of committing a panel and a controller (24x7) for it's maintenance.

I had hoped to use a small PWM controller, powered from the main 24 volt battery bank.... but the Morningstar people say not to....

After looking at the Midnight Solar controllers, I'm convinced I want make use of their 'waste-not' functionality, and I will switch my main controller to a Classic 150, but I'm still faced with finding an easy way to charge my 12 battery bank from the main 24 volt bank.

Thom · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

How would it b a back up if it's charged from the 24v system ?
If the 24v failed the 12 would fail soon .

Thom

OffGrid+LovinIt · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

BB Thanks for the suggestion to use a MPPT controller.
I actually have a SunSaver MPPT 15 controller that I originally planned on using for this purpose.
But on the Morningstar site, they say.....

Can I use a power source other than a PV panel to charge my batteries?
We do not recommend using a DC power supply with any of our controllers. While they appear to be similar to PV modules in function, DC power supplies have considerably more output capacitance. Connecting a DC power supply to our controllers may cause excessive heating and premature failure. AC powered battery chargers are a better alternative for systems requiring AC battery charging.

And... that's what started my current search for a solution.....

I will call them tomorrow morning and see what they say..... Thanks !!

vtmaps · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

OffGrid+LovinIt wrote: »

I had hoped to use a small PWM controller, powered from the main 24 volt battery bank.... but the Morningstar people say not to....

They might say yes to a small MPPT controller.

OffGrid+LovinIt wrote: »

so I really hated the idea of committing a panel and a controller (24x7) for it's maintenance.

considering your options, it might be more cost effective to just buy a cheap PWM controller and a 12 volt panel.

--vtMaps

BB. · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

Should not be a problem when connecting to a 24 VDC battery bank (controller input capacitance is not an issue).

There was a minor software bug (a few years ago) when charging from a 24 VDC battery bank, Morningstar did go through the trouble to fix it.

-Bill

OffGrid+LovinIt · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

Thom,

The 12 volt battery bank only supplies backup power to the security and cellphone link (which ranges from 2 to 4 amps), when the 24 volt system drops below 23 volts.
So the 12 volt bank is in float mode 99.99% of the time.
The idea is to provide me with security camera data and the ability to monitor and control some basic functions remotely, while I make a decision to drive 2.5 hours to resolve an issue.
Thanks

OffGrid+LovinIt · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

BB
Thank you. The SunSaver MPPT was my original plan and it looks like that is a perfect solution !!

I thank you all for the fast responses to help resolve this issue. I'm one happy guy !!

BB. · March 2014

Re: Charge 12 volt battery from 24 volt battery bank

There is another option... You can use a DC to DC converter on a 24 volt battery bank. The converter connects to the high side 12 volts of the 24 volt bank and connect the other side to the low 12 volt side.

The converter will keep the two halves in balance. As the low side is drawn down, it takes power from the high side:

http://www.solar-electric.com/12to24or24to.html

This rugged and versatile unit is used to power a small load at 12 V from a 24 V battery, or to power a small 24 V load from a 12 V battery. It functions both ways, producing 12 V @ 20 Amps from a 24 V input or can be connected backwards to produce 24 V @ 10 Amps from a 12 V input.

When connected as a battery equalizer, it allows large transient 12 V loads to be taken off the centre tap of a 24 V battery (made of 2 X 12 V batteries) without fear of upsetting the voltage balance of the battery and destroying the 24 V battery system that would otherwise occur.

Gives you the advantage of 1 battery bank instead of two.

-Bill

Skyko · April 2014

Re: Charge 12 volt battery from 24 volt battery bank

A bigger version of the equalizer BB mentioned is made by Vanner. I am going to go with the 65-60 or 66-60 60 amp version to get 12V out of my 24 volt bank while keeping the batteries equal. It looks like a well made unit, no fans and only draws 17mA idle, runs at 97% efficiency.

OffGrid+LovinIt · May 2017

Howdy, Just an update, for those looking to maintain a 12 volt backup system from a main 24 volt system.

On my semi-remote system in western Washington, I wanted to maintain a separate 12 volt system to maintain CCTV, cellphone communications and alarm functions, if/when my main 24 volt system failed.

After getting a number of positive suggestions from the forum, I contacted the people at Morningstar to confirm that a SunSaver MPPT 15 would fit the bill. They said that it would work without problem.

I installed the SunSaver MPPT 15 with it's solar input connected to my 24 volt batteries. The SunSaver MPPT 15's output went to my 150 amp 12 volt battery.

I'm happy to state that the setup has worked flawlessly for the past 3 years, as an efficient way to maintain that 12 volt backup battery.

After a year, I got fancy and added a relay between the 24 volt input and the SunSaver MPPT 15. The relay is controlled from the Midnite Classic so we only charge the 12 volt system, when the Classic is in Float.

And for my application, this has worked perfect.

BB. · May 2017

Thank you for the update Dennis.

Always nice to hear how things work out.

-Bill

Charge 12 volt battery from 24 volt battery bank

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