Author Topic: Brat to charge 12v battery from 24 volt battery??  (Read 719 times)

bigal

  • Newby
  • *
  • Posts: 31
    • BigAls Repair Shop Weather
Brat to charge 12v battery from 24 volt battery??
« on: April 17, 2018, 11:02:23 PM »
I need to keep a 12v Battery hot. It gets occasional but heavy loads, so needs 20 amps or so to get charged. My solar Battery bank is 24 volts.

Thinking a Brat might do the trick, charging from the 24 volt source.

What ya think??

27-235 watt Canadian panels, 2 CLASSICS,  Magnum Inverter,  Forklift Battery,  powers entire house easily.
Lots of sun here, recharge by 1 pm normally. 2 Fujitsu mini splits for summer AC.  13 solar thermal panels for heat.

boB

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 2817
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #1 on: April 18, 2018, 01:36:06 AM »
No.  You don't want to do that.  Unless you like bangs and pops and smoke  :)

Do you really need to heat the battery up ?  You could heat it up without hurting it by over-charging it.

There are ways.

boB
K7IQ

mike90045

  • Hero Member
  • *****
  • Posts: 647
    • The Ranch (via facebook re-direct)
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #2 on: April 18, 2018, 02:47:06 AM »
PWM chargers don't often have current limiting components, except for a fuse.  Solar panels have limited current output, so they do work well with PWM chargers
http://tinyurl.com/LMR-Solar

Classic 200| 2Kw PV, 160Voc | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph )| Listeroid 6/1, st5 gen head | XW6048 inverter/chgr | midnight ePanel & 4 SPDs | 48V, 800A NiFe battery bank | MS-TS-MPPT60 w/3Kw PV

bigal

  • Newby
  • *
  • Posts: 31
    • BigAls Repair Shop Weather
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #3 on: April 18, 2018, 02:50:03 PM »
Dang, I thot you MidNite guys covered all the bases...just not mis-use!!

Thanks for explanation Mike...did not know panel did the limiting!

Ok if there are ways, I'm listening!

Should I start a new post??

PS Guess I should post some pics and info about using the Classic in RV application, using 24 volts and 1400Watts of panels.  Lots of fun!
« Last Edit: April 18, 2018, 03:07:24 PM by bigal »
27-235 watt Canadian panels, 2 CLASSICS,  Magnum Inverter,  Forklift Battery,  powers entire house easily.
Lots of sun here, recharge by 1 pm normally. 2 Fujitsu mini splits for summer AC.  13 solar thermal panels for heat.

boB

  • Global Moderator
  • Hero Member
  • *****
  • Posts: 2817
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #4 on: April 19, 2018, 12:55:39 AM »

Nahhh....  No need to start a new subject or thread, although there may be another one about charging a 12V battery from a 24V battery.

Solar (PV) is inherently current limited to around 10 amps per panel or single string of panels.  parallel strings or panels and the current goes up of course.

The Brat does actually have internal current limiting.

So, charging one battery from another high voltage battery can be done but what I'm wondering about is the requirement to heat the 12V battery up.  Yes you could overcharge it and get it hot but that kind of reduces its life as a battery.

One way to charge 12V from 24V is by using a DC to DC converter.  Some people use an MPPT charge controller but that by itself can also be bad for the controller because it's not made for it.  They are expecting a photovoltaic solar panel or array that has its maximum power point voltage very close to the open circuit voltage of the source.  A solar panel has its MPPV at around 80% of open circuit voltage.  It's not expecting the maximum current and voltage as high as a battery supplies.  Sometimes they work great though but you can't count on it.  If it fails, it can be expensive.

I am assuming you have an inverter.  If so, you could just go buy a 12V battery charger from a hardware store and charge from 120 VAC.

It's the heating of the 12V battery that has me perplexed.  I suppose you could wrap an electric blanket around it or put some other heater next to the battery ?

boB

K7IQ

ralph day

  • Jr. Member
  • **
  • Posts: 85
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #5 on: April 19, 2018, 07:16:53 AM »
Did "hot" perhaps mean always on charge?

bigal

  • Newby
  • *
  • Posts: 31
    • BigAls Repair Shop Weather
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #6 on: April 19, 2018, 11:19:44 AM »
I have considered a 120v charger.

It just seems there should be an"elegant" solution....simple and efficient....oh well.

Here is a pic of "hybrid" MH power supply.
On left,2 starting bats. Back is single 12 v. to satisfy 12v needs of coach interior.

Remaining space is for 3x 8volt GC8 Batteries to power  the entire AC loads via Magnum 24 volt sine wave inverter.  No genset on board, carry no power cord.

MidNite Classic does the charging from 4x  340watt 72 cell panels!!  You can see why I had to go to 24 volts.

This includes Solar powered Air Conditioning by way of inverter style mini split unit. This is the second MH I've equipped, and the concept is a solid success!


« Last Edit: April 19, 2018, 11:22:00 AM by bigal »
27-235 watt Canadian panels, 2 CLASSICS,  Magnum Inverter,  Forklift Battery,  powers entire house easily.
Lots of sun here, recharge by 1 pm normally. 2 Fujitsu mini splits for summer AC.  13 solar thermal panels for heat.

CDN-VT

  • Hero Member
  • *****
  • Posts: 827
  • Thats a C/V-107 pulling a barge on candled ice. ^
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #7 on: April 19, 2018, 11:06:15 PM »
Adjustable converter with a timer OR photo cell to tun on . You need to explain the systems of what they do & what your trying to do or dream !!

VT
Canadian Solar 350W 37.6 VOC  30.6 VMP 8.22 ISC 7.87 IMP ,-15 c +30c max  4 strings in 2 in Series for 24v Classic 150 -1020 Ah  Freezers & fridges ~~~ Second Array same panels of 3sx3 parallel for 24 V Classic 150 -440 Ah Outback Barns & out blds.
48Vdc almost done,11Strings up of 3s11P same panels

bigal

  • Newby
  • *
  • Posts: 31
    • BigAls Repair Shop Weather
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #8 on: April 20, 2018, 09:13:22 AM »
It's really pretty simple.  I need two dc voltages. Solar charges 24 volt batteries. A Single deep cycle 12v battery,
has some use and needs to be kept hot from the 24 volt system.  That part is not changeable.
The method of charging the 12volt is the item for discussion.
An  mppt charger is perfect in concept if it has sufficient circuitry  not to burn up from the high current availability of the Battery.

I've gotten some input from a guy on  the NAWS forum , that has successfully done this with a Morningstar mppt. 

A Kid likely would work, but is overkill for the job at hand. I'm a dyed in wool MidNite guy, but I may need to turn to another source, since the Brat won't do it.
27-235 watt Canadian panels, 2 CLASSICS,  Magnum Inverter,  Forklift Battery,  powers entire house easily.
Lots of sun here, recharge by 1 pm normally. 2 Fujitsu mini splits for summer AC.  13 solar thermal panels for heat.

bigal

  • Newby
  • *
  • Posts: 31
    • BigAls Repair Shop Weather
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #9 on: April 22, 2018, 09:24:02 AM »
Just ordered victron Mppt 75/15   $88
Can't get a definitive answer from them. 
" Should work"
Guess I will try it.  Really good solution if it works.

Will try small fuse, maybe 5-10 amps on 24 volt side
And 15 On output to start with.
It is rated 15 amps output. Plenty since it will be active 24/7
27-235 watt Canadian panels, 2 CLASSICS,  Magnum Inverter,  Forklift Battery,  powers entire house easily.
Lots of sun here, recharge by 1 pm normally. 2 Fujitsu mini splits for summer AC.  13 solar thermal panels for heat.

bigal

  • Newby
  • *
  • Posts: 31
    • BigAls Repair Shop Weather
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #10 on: May 08, 2018, 05:01:59 PM »
Well I tried the little Victron 75/15, which would make a neat little 3 stage charger.

However, it is NOT current limited on input. I started with small fuses and worked up.  Long story short, it won't work without modifications.

 I played around with various bulb filaments to limit input current. ...see picture.  It worked,  but not well, I never hit a sweet spot.

Some kind of mosfet current limiter circuit would probably work, but hardly worth the effort.

I am reporting this for the sake of information , as I have another solution which will work.
27-235 watt Canadian panels, 2 CLASSICS,  Magnum Inverter,  Forklift Battery,  powers entire house easily.
Lots of sun here, recharge by 1 pm normally. 2 Fujitsu mini splits for summer AC.  13 solar thermal panels for heat.

ZoNiE

  • Jr. Member
  • **
  • Posts: 55
  • Controls Engineer
    • FourPAC
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #11 on: June 17, 2018, 08:00:18 PM »

Here is a pic of "hybrid" MH power supply.
On left,2 starting bats. Back is single 12 v. to satisfy 12v needs of coach interior.

Remaining space is for 3x 8volt GC8 Batteries to power  the entire AC loads via Magnum 24 volt sine wave inverter.  No genset on board, carry no power cord.


That looks like a Safari Battery Tray. I have a 96 Sahara. My slides are shot. Need to build a new one. Where did you mount the condensing unit for the Mini Split? I have a center door, so I am thinking of using the space in front of the right front wheel well where the stairs would be on a front entry coach. Some louvers in the side panel and it should work.

What size Mini split did you use, and does it cool the entire coach or just the living area?
Update:
1996 Safari Sahara Class A Motorhome with:
4X Hanhwa 280W panels
1X Midnight Classic-150
4X  Dying WallyWorld Energizer GC2's bought by the PO
1X 2,000W Heart Interface Freedom 20 Inverter/Charger (OEM)
1X Onan Emerald 6300LP RV Genset

ZoNiE

  • Jr. Member
  • **
  • Posts: 55
  • Controls Engineer
    • FourPAC
Re: Brat to charge 12v battery from 24 volt battery??
« Reply #12 on: June 17, 2018, 08:13:51 PM »
Just found your other post about the Condensing unit being where the Onan was, although the photo links are down. Do you have a new link? We plan to keep the onan. I, too, want to bust the norm with regards to the way these things are wired and cooled, but we still want the genny. I have a Sensata 24V pure Sine that I really want to use for the coach, but the 12V loads pose the problem, plus the battery boost circuit also needs to be thought of or eliminated. DC-DC converters for some of the 12V loads is what I was thinking of doing, since the 24V charging a 12V battery idea using a charge controller idea won't work without some sort of current limiting circuitry... Hmmm. I wonder if there is a market for such a setup. I am an electronics and controls engineer, so maybe I can figure something out...
Update:
1996 Safari Sahara Class A Motorhome with:
4X Hanhwa 280W panels
1X Midnight Classic-150
4X  Dying WallyWorld Energizer GC2's bought by the PO
1X 2,000W Heart Interface Freedom 20 Inverter/Charger (OEM)
1X Onan Emerald 6300LP RV Genset