Author Topic: Charge 12 volt system from 48 volt source  (Read 8204 times)

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(unknown)

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Charge 12 volt system from 48 volt source
« on: May 15, 2007, 02:53:11 AM »
Hello all,


I recently removed the ICE from my sailboat and installed an DC motor running against 48 volts.  The 8 6 volt batteries are charged by a regulated wind generator...  www.atlantecgroup.com/oldblog (please excuse the unfinished nature of my "old" blog).


I'm looking to tap into the 48 volt system to charge my 2 12 volt house batteries.  I purchased a 48 to 12 volt dc to dc converter, but dont like the fact that this powersource is unregulated.


Does anyone have any suggestions as to how i might best charge these batteries?


Christian

« Last Edit: May 15, 2007, 02:53:11 AM by (unknown) »

Nando

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Re: Charge 12 volt system from 48 volt source
« Reply #1 on: May 14, 2007, 09:17:48 PM »
CRSTIAN:


The best you could do is supply the specifications of the 48/ 12 converter to see what can be done or to have an alternative solution.


There are available cross battery chargers which means if you have a system where the power is taken from a lower battery in the bank, the cross charger takes power from the other batteries and recharge the one being discharged, also if the lower battery is being charged ( like 12 volts ), it takes energy form the battery being charged and charge the upper one.


I do not know if there are for 48/ 12 cross operation, may be somebody in the group can give some indication and possibly the web sites.


In a more exotic way, you could get an MX60 set for 12 volts and that will solve the problem.


why did you remove the ICE ?.


Nando

« Last Edit: May 14, 2007, 09:17:48 PM by (unknown) »

willib

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Re: Charge 12 volt system from 48 volt source
« Reply #2 on: May 14, 2007, 11:07:58 PM »
that is the easiest thing to do , i do it all the time .

but i go from 12 to 14 volts down to 3Volts or so ,but the idea is the same .

you need a varible pwm , and a FET .

the fet pulses current into the lower voltage battery , till its charged.

its not automatic or anything like that but it could be.

as it is now i just monitor the voltage of the battery to be charged
« Last Edit: May 14, 2007, 11:07:58 PM by (unknown) »
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jimovonz

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Re: Charge 12 volt system from 48 volt source
« Reply #3 on: May 15, 2007, 02:20:03 AM »
willib, I don't think its quite as easy as you are making out.... It sounds like you are talking about a low side switch directly between the 48 and 12V batteries. Do you realize how much current would flow if these two batteries are directly connected??? I don't think your FET (or even a great bank of FETs) would do that job for any length of time (maybe a few cycles...) A bit different to switching a 12V battery across a bit of nichrome... This really needs a proper buck converter where the current flow is kept in check by an inductor. This requires switching from the high side which at 48V is not so easy. At ~12V ground is well within the typical Vgs max of +/-20V so switching a pFET on the high side is not too hard. At 48V comming up with a low impedance ~+33V to drive a p or ~+63V to drive an nFET is not that straight forward. Switching that voltage from a logic level output at more than a few kHz is not that straight forward either... Don't forget - ANY inductance in the system at the current levels expected will require a hefty freewheel diode to stop nasty voltage spikes taking out all your components too....


If the 48/12 converter puts out a voltage suitable for charging a 12V battery (~14V) then it may be possible to do as you suggest and use a low side switch to taper off the charge as the battery nears full. Depending on the specs of the converter, it may also need some form of current regulation to avoid drawing too much current at the start of the charging cycle (when the battery is relatively flat).

« Last Edit: May 15, 2007, 02:20:03 AM by (unknown) »

Flux

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Re: Charge 12 volt system from 48 volt source
« Reply #4 on: May 15, 2007, 02:38:19 AM »
I totally agree. It does need a true buck converter, it's not like a limited current source of a solar panel or small alternator.


What you end up with for decent charge control will be effectively the basics of the MX60 that Nando suggested but you may get away with a lower cost for this specific application.


There are probably commercial converters that do 40 to 12v that can be adapted but it is a project for someone with enough knowledge of electronics. Anyone with this knowledge would not have asked the question.


Flux

« Last Edit: May 15, 2007, 02:38:19 AM by (unknown) »

Nando

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Re: Charge 12 volt system from 48 volt source
« Reply #5 on: May 15, 2007, 06:53:30 AM »
Yes FLUX.


One needs to examine the question before answering, to determine why the question was made, under what conditions that may reflect the technical knowhow of the requester.


It is not an easy job with a simple circuit.


This type of converters are available in the industry like 36 to 12 volts and 48 volts to 12 volts, that most of the time have a fixed conversion ratio and not a variable ratio to be able to charge a battery under charging profiles to give long battery life.


Christian may not need a MX60 ( price too high or current capability)


Cross charging chargers are around 100-150 dollars for a 20 amps units, but most I have seen is for 24 to/from 12 volts ranges, which is easy to make since the inductor is connected to the center of the voltages to send the energy where is needed.


With 48 to 12 the case is a bit different and the circuitry for equal ratio power transfer the circuit needs to have a ratio of 4 to 1 in duty cycle --- the same way that the MX60 functions, which by the way can have up to about 8 to 1.


The converter, he now has, may be adapted more easily to do the job to keep the 12 volts at Float and maybe a minor manual over ride for 14.2 equalization but technical information is needed to see if that is possible - I have converted some to that capability with a simple piggyback board added.


Nando

« Last Edit: May 15, 2007, 06:53:30 AM by (unknown) »

willib

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Re: Charge 12 volt system from 48 volt source
« Reply #6 on: May 15, 2007, 08:53:54 AM »
Jim , this is what i had in mind .





sometimes i tend to oversimplify things, but yall tend to overcomplicate stuff too

the origional poster wanted to charge a 12V battery with 48 volts , this circuit will do that , he did not mention just how much current he wanted to use , but fuse in the circuit will prevent a fet mishap?

if yall think this will not work , i'll take yer word for it , i have other fish to fry at the moment
« Last Edit: May 15, 2007, 08:53:54 AM by (unknown) »
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jimovonz

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Re: Charge 12 volt system from 48 volt source
« Reply #7 on: May 15, 2007, 12:18:25 PM »
willib, yes if your sole objective is to charge the 12V battery then what you propose will work. However I think you can safely presume that the energy stored in the 48V bank has been hard won and that Christian is not inclined to waste it. The specific characteristics of the batteries involved play a part, but however you look at it you are wanting to drop some where in the region of 48-12=36V across R1 to limit the charging current and keep the voltage seen by your 12V battery down to something thats not going to cause it to gas violently. What ever charging current you design for, R1 is going to be loosing roughly 3 times as much energy as the battery recieves. Its bad enough that charging a lead acid battery is only ~80% efficient at the best of times without using a charging system that brings the overall efficiency down to around 20%...
« Last Edit: May 15, 2007, 12:18:25 PM by (unknown) »

kell

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Re: Charge 12 volt system from 48 volt source
« Reply #8 on: May 15, 2007, 08:57:44 PM »
You could make a simple buck circuit by replacing the resistor with an inductor and put a freewheel diode with its anode at the fet drain and its cathode connected to the +48.  The ground of the 12 volt battery is flying up and down, though.  So if you want to close the loop for regulation you will have to resort to an opto or something.
« Last Edit: May 15, 2007, 08:57:44 PM by (unknown) »

willib

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Re: Charge 12 volt system from 48 volt source
« Reply #9 on: May 15, 2007, 10:45:52 PM »
not sure why i drew that resistor in there , it isnt needed

because the varible pwm can control the current fine without it


it can pulse a few milliAmps or tens of Amps depending on the Fet used ,of course

and if one uses a fet with a low Rds(ON) , the power loss in the system is minimal

« Last Edit: May 15, 2007, 10:45:52 PM by (unknown) »
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wooferhound

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Re: Charge 12 volt system from 48 volt source
« Reply #10 on: May 15, 2007, 10:53:42 PM »
Seems to me that you could use a pulse charger setup where you would charge a large capacitor up from the 48 volt batteries, then discharge the capacitor into the 12 volt battery. Repeat as necessary.
« Last Edit: May 15, 2007, 10:53:42 PM by (unknown) »

jimovonz

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Re: Charge 12 volt system from 48 volt source
« Reply #11 on: May 15, 2007, 11:37:43 PM »
How much current do you think will flow when the 48V battery is swithched to the 12V (even for a few micro seconds)? Batteries are a very low impedance source. Although internal battery resistance will be a much larger factor here than normal, lets ignore it for now and consider how we calculate power into a battery from our alternators. We take the unloaded output voltage, subtract the battery voltage and divide by the line resistance. Say the open voltage of the alt is 30V and the line resistance is 2 Ohm into a 12V battery. 30-12=18V 18/2=9Amps into our battery. In this case the 48V battery takes on the role of the alternator and the winding resistance is the Rdson of the FET + some cable resistance - say 0.01Ohms. Doing the same calculation to find current: 48-12=36V 36/0.01=3600A. P=I^2R=3600^2*0.01=129kW loss through your FET and cable whenever the FET is on. Which FET are you proposing to use to meet this spec?

 Charging at more than ~2.4V/cell will only serve to liberate copious quantities of hydrogen from the electrolyte which is even more inefficient than normal (unless perhaps you collect and use the H...) and leads to more maintenence and less battery life than is necessary.
« Last Edit: May 15, 2007, 11:37:43 PM by (unknown) »

(unknown)

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Re: Charge 12 volt system from 48 volt source
« Reply #12 on: May 17, 2007, 08:15:47 AM »
Sorry for the delay in response... I thought i was going to get a notification upon posting of comments... preferences since edited.


I removed the ICE as it was over 30 years old, had swallowed a bit too much salt water, spent much time alone, and was about to cost me between 3-4k in rebuild...


Instead, I spent a bit less than 3k and have myself a SUPER quiet electric solution that is ready on demand.  No one part of the entire system weighs more than 75 lbs (the batteries are the heaviest) and no one piece costs more than 600 bucks (the wind generator was the pricey piece).  Regarding weight.. you should have seen the event that ensued humping that atomic 4 engine out of the cabin.


The loss is that I currently don't have the run time I had with gasoline... but that can be cured running a generator hooked into the system.  I've yet to run out of juice, though, as i really only use the motor when I'm single handing the boat and trying to land it on my mooring.


The DC to DC converter I purchased is this one: SM2412-10 Converter


"Heavy Duty DC to DC converter. Less than 40 mv p-p ripple. Output voltage: 13.7/13.9 Output current (full load) 10A, Input Current (no load) 100ma/200ma Load regulation .015/.250 VDC"


The intent was to initially wire it into the 48 volt system like this:





Note, the 12 volt system is already connected to the 48 as it is needed to power the contactors.


There is no way to just connect the output of this converter to something that will regulate current flow for healthy charging?  


Christian

« Last Edit: May 17, 2007, 08:15:47 AM by (unknown) »

jimovonz

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Re: Charge 12 volt system from 48 volt source
« Reply #13 on: May 17, 2007, 01:32:15 PM »
A solar regulator (pwm not shunt) might do the trick. It would have to limit max current to 10A as any reasonable size (flat) 12V battery will try to draw more than this when charging at 13.9V
« Last Edit: May 17, 2007, 01:32:15 PM by (unknown) »