# Does Auxiliary Battery Work Like Power Supply?



## iruraz (Sep 4, 2012)

Hi All,

My electrical system is in the picture below. I want to use dc-dc converter and auxiliary battery together. But I want use auxiliary battery like power supply. It should work only when dc-dc converter is failed and aux. battery is charged by dc-dc converter when it is low. I need your advice about "?" . What should it be? Thanks in advance.

Regards.

(switch 1 and 2 represent stage1 and stage2, stage1: start, stage2:ignition)


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## Ziggythewiz (May 16, 2010)

Why would you want to use it only when the DC-DC is failed? That means your DC-DC has to put out the peak used by your system, instead of just the average.

Also, if usage is less than the DC-DC's output, the system will effectively take power from the DC-DC and not the battery anyway. When system usage is greater than DC-DC output, the battery supplies the difference, and gets charged when the usage drops below the output.


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## iruraz (Sep 4, 2012)

@Ziggythewiz thanks for your interest. I haven't chosen dc-dc converter yet. But I haven't decided usage of auxiliary battery. I have to use it against dc-dc converter failures and greater power consumption. In this case should I run aux. battery and dc-dc converter parallel? If so, how can I do (I mean wiring or connection)?

Regards.


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## Ziggythewiz (May 16, 2010)

Yes, just hook the output of the DC-DC to the battery terminals with a fuse in between. 

Use an AUX batt that can handle your full usage, and get a DC-DC that provides above your average usage. Make sure the DC-DC is isolated. Depending on the DC-DC and your controller you may need a diode and/or inductor to protect it. Be sure to read up on that, there are several threads here.


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## iruraz (Sep 4, 2012)

Ziggythewiz said:


> Yes, just hook the output of the DC-DC to the battery terminals with a fuse in between.
> 
> Use an AUX batt that can handle your full usage, and get a DC-DC that provides above your average usage. Make sure the DC-DC is isolated. Depending on the DC-DC and your controller you may need a diode and/or inductor to protect it. Be sure to read up on that, there are several threads here.


Could you show it on a basic schematic (like mine)? Because I couldn't understand what you mean.

Regards.


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## Joey (Oct 12, 2007)

This shows the battery and DC-DC in parallel with your 12 volt fuse box. The output of the DC-DC is fused. The Diode and inductor go on the input side of the DC-DC to keep the ripple from the controller from killing the dc-dc.


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## iruraz (Sep 4, 2012)

@Joey thanks for your help. 

I wonder that does how dc-dc converter charge aux. battery until it's full? Because output of the dc-dc 12V and potential of aux. batt. is 12V. In ICE car alternator output about 15V and it's bigger than aux. batt. It is easy to understand. But in my design it is different. Should I use relay/switch or sth like that cut charging aux. batt. when it's full? And in the case of 12V accessories load bigger than dc-dc output, how does aux. batt. work with dc-dc together?

Regards.


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## Ziggythewiz (May 16, 2010)

Neither one is really 12V. Your ACC batt will be around ~12.8V and the DC-DC should be ~13.5. You have to size it according to your usage. An always on DC-DC should be a lower voltage than one that only runs while driving.

If usage is less than the DC-DC's output, the system will take power from the DC-DC and not the battery. When system usage is greater than DC-DC output, the battery supplies the difference, and gets charged when the usage drops below the output.


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## iruraz (Sep 4, 2012)

@Ziggythewiz thanks for your answer.



> An always on DC-DC should be a lower voltage than one that only runs while driving.


I didn't understand what you mean. What does "one" refer?



> If usage is less than the DC-DC's output, the system will take power from the DC-DC and not the battery. When system usage is greater than DC-DC output, the battery supplies the difference, and gets charged when the usage drops below the output.


Is there any circuit, switch, relay or anything to change runing only (dc/dc converter) or (dc/dc + battery together)?

I am sorry for my ignorance.

Regards.


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## Ziggythewiz (May 16, 2010)

An alternator runs a fairly high voltage because it has to replenish the lead's self discharge, as well as a significant startup drain typically within a short time.

A DC-DC does not have to be quite so high because it doesn't not have any starter to compensate for. An always on DC-DC should be lower because it is acting as a float charger when the accessories are off.

There's no switch to select between just DC-DC and DC-DC + battery together. That's just physics, the power comes from where it's available.


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## iruraz (Sep 4, 2012)

> An always on DC-DC should be lower because it is acting as a float charger when the accessories are off.


Does every Dc-Dc converter behave as a float charger?



> There's no switch to select between just DC-DC and DC-DC + battery together. That's just physics, the power comes from where it's available.


Do you mean "potential difference"?


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## Ziggythewiz (May 16, 2010)

If it's on just some of the time, it will be acting like a power supply/charger, but if it's on all the time it's acting like a float charger because there's no real load on it. If it puts out a higher voltage than your battery can float at then it will act as a charger and charge your battery away.

Yes, the DC-DC should be a little higher than the battery's normal full voltage so the loads will draw from the DC-DC instead of the battery. As you add additional loads it causes voltage sag which increases the potential difference, and the output of the DC-DC, until you reach it's limit where you start drawing from the battery.


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## iruraz (Sep 4, 2012)

> but if it's on all the time it's acting like a float charger because there's no real load on it.


But DC-DC is not on all the time because in my design it opens and it begins to charge aux. batt. in stage1. Does the above things valid in my design?

And I think there is load out of dc-dc everytime, sometimes it is aux. batt. sometimes it is 12V accessories. What did you mean with "real load" ?

Thanks again.


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## Ziggythewiz (May 16, 2010)

When the DC-DC is switched on, it will top off the battery (from self discharge while off) plus provide power to your accessories. Once the battery is fully charged if there are no accessories on there is no real (significant) load on the DC-DC. It will just run it's own phantom (wasteful internal power) load.

Depending on how you switch it, you may or may not always have a load on it (such as a contactor) when it is on.


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## iruraz (Sep 4, 2012)

> Depending on how you switch it, you may or may not always have a load on it (such as a contactor) when it is on.


I switch it in stage1. Stage0 only vehicle management system works. Stage1 opens 12V accessories. Stage2 starts motor. Do you have any advace for my design?


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## Ziggythewiz (May 16, 2010)

Just figure out what your typical load will be and get a DC-DC that's bigger than that. Hook it up like Joey showed plus your switch. It's not too complicated.


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## Joey (Oct 12, 2007)

If you charge a battery and then turn the charger off, the battery can take a certain voltage. If you leave the charger on all the time (float charge), it is common to use a lower voltage because holding the battery at the higher charge voltage for extended periods will damage the battery over time. 

So you need to decide if you will leave the DC-DC converter on all the time, or turn it on and off. Then you will need to know the charge and float voltage of your 12 V battery chemistry, and set your DC-DC converter voltage based on what is best for your battery and use case.

Current will flow from the DC-DC converter to the battery whenever the voltage at the battery is less than the voltage on the DC-DC converter and the battery will charge. Once the voltages are equal, the battery will automatically stop charging. If there is no voltage difference, there will be no current flow. You need to make sure the battery is happy being held at the voltage setting of the DC-DC convertor. I've seen DC-DC converter voltages anywhere between 12.5 and 14.4 volts.

When there is a load on your 12 volt system, the DC-DC converter and the battery will both work together to supply the current. Once the load reduces, and the DC-DC has some spare capacity, the DC-DC converter will be able to recharge the battery. This will all happen automatically, as long as you pick a DC-DC converter than can supply a little more than your average current load, and you battery has enough capacity to handle the duration of your peak loads. If you have no 12 volt battery, then the DC-DC convertor would have to be big enough to handle your peak loads.


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## iruraz (Sep 4, 2012)

@Ziggythewiz @Joey thanks for your help. 



> So you need to decide if you will leave the DC-DC converter on all the time, or turn it on and off. Then you will need to know the charge and float voltage of your 12 V battery chemistry, and set your DC-DC converter voltage based on what is best for your battery and use case.


I turn DC-DC converter on and off. And I start searching for 12V battery. Maybe little motorcycle battery provides my needs (if i'm not mistaken there is motorcycle battery in tesla roadster)



> You need to make sure the battery is happy being held at the voltage setting of the DC-DC convertor. I've seen DC-DC converter voltages anywhere between 12.5 and 14.4 volts.



Should I prefer 14.4V output converter to satisfy potential difference between converter and battery? Are there any important differences between 12.5V and 14.4V outputs about effective working with 12V battery (auxiliary)?

Regards.


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## dtbaker (Jan 5, 2008)

if you get a dc-dc that outputs 13.8 volts, that is a comfortable float voltage for the little sealed ATV batteries. get a GOOD dc-dc.... like a curtis, belktronix, or ACME that can put out at least 400 watts. put them in parallel, and wire the dc-dc on ALL the time powered from pack voltage. The aux battery will cover sudden voltage sag when things like headlight or vacuum pump flick on and then 'recharge' as dc-dc picks up the load.

a little sealed ATV 12v batterie, like a 3ah or 4ah is all you need.... They are about $30-40, and the size of a box of index cards. don't go smaller than that though because the tiny ones for RC toys can't really handle the higher amps loads.


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## Ziggythewiz (May 16, 2010)

iruraz said:


> Are there any important differences between 12.5V and 14.4V outputs about effective working with 12V battery (auxiliary)?


12.5 and 14.4 were just examples of the ranges possible. Really 12.5 is too low to charge most batteries, and 14.4 is too high for most. Somewhere in the middle should be good.

You should start by figuring how much power you need. I'm looking for one myself, but I only need 20-30W, which I don't think exist in EV voltages.


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## iruraz (Sep 4, 2012)

@Ziggythewiz, @dtbaker thanks. I make a list for my power neeeds (for 12V system). All values are approximate below:

1- Computer for VMS ~2A
2- Headlights(x2) and Stop Tail Light ~20A
3- Wipers ~20A
4- Horn ~7A
5- Blower ~20A
6- Power Windows ~30A
7- Dook Locks ~5A
8- ABS ~20A
9- Coolant Pumps ~8A

And my instant maximum current need is ~132A (I assume that all system works together for a while). 
And basic calculation: 132A x 12V = 1584W . Maximum power need is 1584W. What do you think about this calculation? When I calculate "avarage power need" which values do I take into account? Because 132A seems big.

Regards.


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## dtbaker (Jan 5, 2008)

iruraz said:


> And my instant maximum current need is ~132A (I assume that all system works together for a while).


you are way too high.... you'll never have 'everything on' all at the same time. you'll find that the max draw is not what the fuses show, it is significantly lower. 

with 'normal' equipment most people do fine with 300-400 watt dc-dc. If you have power windows, extra 12v cooling fans, or other stuff like that you might be safer with 500-600 watt units like Belktronix(sealed) or ACME(open w fan).


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## Ziggythewiz (May 16, 2010)

Unless your car is starring in Zombie Apocalypse, I doubt you would use the wipers, windows, door locks, and horn at the same time...and peak usage only matters if you go no AUX.

Only 1,2,3 and 9 look like possible continuous loads, so the most you could need is 600W. 

If you let the AUX carry the wipers and some of the headlights (both of which you don't need all the time) you should be fine with a 300-400W unit.

Also, are those real numbers or sticker ratings? Actual usage for most electrical stuff is 1/2-1/4 of the max sticker.


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## Joey (Oct 12, 2007)

The number you calculated is peak load, and you would need to size the dc-dc converter this large if there was no 12 v battery.
Take each item and multiply by the percentage of on time to get average load. The numbers you listed seem an the high side. They may be only that high for a second when they first turn on. If possible, try to measure the load with a current clamp meter.


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## dtbaker (Jan 5, 2008)

if the dc-dc is either 13.5-13.8v thats not a bad 'float' voltage for most little aux batteries. in the moments the input pack voltage sags or demand causes dc-dc to sag, then aux battery is 'fresh' and picks up the peak..... when load lowers, dc-dc recovers and recharges battery until its back to 'float' voltage....

dc-dc always on, little 3ah or 4 ah ATV battery is all you need for aux if they are in parallel.


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## iruraz (Sep 4, 2012)

@dtbaker , @Ziggythewiz , @Joey thanks for your comments.



Ziggythewiz said:


> Unless your car is starring in Zombie Apocalypse, I doubt you would use the wipers, windows, door locks, and horn at the same time...and peak usage only matters if you go no AUX.


@Ziggythewiz maybe you see my car in "Zombie Apocalypse Reloaded" 



Ziggythewiz said:


> Also, are those real numbers or sticker ratings? Actual usage for most electrical stuff is 1/2-1/4 of the max sticker.


They are not real. Because I haven't chosen stuff yet. They are approximate value for now. But I considered maximum value of them. 



Joey said:


> Take each item and multiply by the percentage of on time to get average load. The numbers you listed seem an the high side. They may be only that high for a second when they first turn on. If possible, try to measure the load with a current clamp meter.


As I stated below. It is not possible for now. I try to choose proper dc-dc converter and auxiliary battery.



dtbaker said:


> dc-dc always on, little 3ah or 4 ah ATV battery is all you need for aux if they are in parallel.


Do you mean motorcycle battery is enough beside 400-500 watt dc-dc converter?

Regards.


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## dtbaker (Jan 5, 2008)

iruraz said:


> Do you mean motorcycle battery is enough beside 400-500 watt dc-dc converter?


yes, plenty.... if wired always on, in parallel w dc-dc.

a little 12v 3ah AGM battery (for ATVs, scooters, small motorcycles) is plenty in parallel with a decent 400+ watt dc-dc. with power windows, you might want to step up to a 500+ watt dc-dc like a belktronix (closed/finned design), or an ACME (open fan-cooled design)... they are both solid units as far as I can tell, under $300, and output 600 watts.

I plan to avoid Chennic in the future.... they are cheaper at $99-ish, but I had two fail after less than a year of service. Searching around threads here there are reports of problems with Iotas as well. Curtis seems very solid, but more expensive.


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## iruraz (Sep 4, 2012)

dtbaker said:


> yes, plenty.... if wired always on, in parallel w dc-dc.


Yes, auxiliary batt. and DC-DC will work parallel. 



dtbaker said:


> I plan to avoid Chennic in the future.... they are cheaper at $99-ish, but I had two fail after less than a year of service. Searching around threads here there are reports of problems with Iotas as well. Curtis seems very solid, but more expensive.


I will consider your warnings. Thanks.

Regards.


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