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arduino ups

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Dr_Doggy

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1)
I have arduino that says input 7-12v

I have several 12v lead battery 7 amp hours, sticker says charging max voltage is 15v

I have power adapter, one 12v 2500ma(prolly too low volts) and several rated @ 15v 3amp

is it ok to just hook it all up, or maybe i need some resistors and diodes in between? usually i would try to push the limit and let it all slide, but i worry that the 15v charger will overvolt the arduino input..... should i not worry so much?


2)
the other part of circuit requires 24 volts, is it ok if I put both batteries in series, power arduino from 12v battery, and use 24 to power device.......actually the part i wonder about is charging, can i use a 15v charger in series for each battery(so total of 2 chargers in series)? while 2 batteries are in series too>?
(I do know to watch out to ensure grounds are isolated)
 

ronsimpson

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If you series two batteries to make 24V, this is good but if you load the bottom battery more by adding the Arduino, the batteries will not keep the same charge. There may be a imbalance. If the Arduino load is small you are OK.

If the max is 12V to the Arduino then don't apply 15 to it.
Here is an example of a DC to DC supply that will buck down a voltage as high as 35V and make a voltage like 8V for yor Arduino.

http://www.ebay.com/itm/LM2596-DC-D...694546?hash=item3d15ad7612:g:o70AAOSwv0tVFO8c
 

Dr_Doggy

Well-Known Member
but wont the battery pull the volts from charger down? maybe just not enough?
also do i need to think about disconnecting the chargers when the batteries are fully charged and there is little draw on the 24v load?
arduino load will be constantly draining ... but not very much

here is my idea.....
Image2.jpg
 

ronsimpson

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Look at a automobile voltage meter on the dash of a car. (assuming you have a meter not a light) The "12 volt" battery of a car runs 14.5 volts +/-.
For you to charge a 12V battery the voltage charger must be able to lift the battery above 12V. You charger might have 1 amp current limit and 15V voltage limit. Which is to say the charger will output 1A up to 15 volts then it goes into voltage limit mode.

If the charger is removed the battery voltage will drop to about 12V quickly and then stay about there for a long time before it starts to drop at the end.
 

Dr_Doggy

Well-Known Member
ya, was hoping the "12v, but actually 15v" methodology applied to arduino internal buck too.....

what about a diode on either side of the battery, like from the charger to battery, then battery to circuit, that would drop 1.4v from 15 = 13.6v going in to arduino, would this work?
 

Dr_Doggy

Well-Known Member
... my mistake, it seems the arduino is 20v max, so that solves that problem,

but please confirm #1)
....that it is safe to stack my 2 chargers in series just like i am going to stack my series batteries, also is it good idea to put that center node in there between the 2 batteries &chargers , i figure this will make for more balanced charging, rite??

also is it essential to disconnect charger if batteries are fully charged? or can i leave it all hooked up and continuously feed..like my alarm system?
 

dr pepper

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Why not leave the batteries in series, and use a regulator like the one ron pictured to regulate the 24v down to a voltage suitable for the 'duino.
The you can leave the charger connected to the batteries all the time, when mains is on the charger will keep up with extra power for the 'duino the regulator circuit taking care of the extra voltage, then when the mains goes the regulator circuit will run off the batteries, some of the better ones also have an undervoltage shut off you can use to protect the batteries.
 

MikeMl

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If you plan to leave any constant-voltage charger connected to a 12V SLA indefinitely, the "charger" must be voltage regulated to 13.75V at 21degC, reduced by 100mV for each 6degC temp rise above 21degC. Connecting a 12V SLA to an unregulated charger that can raise the voltage of the SLA above that cited above will ruin the battery in a couple of weeks...
 

dr pepper

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Yes just reading other posts you mention power adaptor not chargers, if you leave a charger permanently connected to a battery it as mentioned needs to be a maintenance charger.
You could add such a circuit to your power adaptors if the voltage and current are suitable, there are chips made to do this linear make them for one.
 

Dr_Doggy

Well-Known Member
well, arduino has internal buck to 5v, but not enough current for all the 12v fets, so i have additional buck(like rons) for [email protected], plus i found some of the load devices(water pumps) work better at the 15v, was hoping to keep it there, plus another load device at the 24v end.... aiming for less circuitry.... I think existing plan will work fine,... now that i realize that arduino can go to 15v

.... i think i am just hooked on the idea that 2 parallel chargers is better since they would individually charge their own batteries, (and not need to send current through battery 1, just to charge battery 2 ... since there will be high(er) power draws but for smaller periods of time in the 24v circuit, at the 12v side there will be constant low power draws with additional low period(1minute every 4)-high power (12v) draws plus the 24v draws. ... so the 12v side battery will be used more, there is no reason to pass additional current through first battery... ..... this maybe a concept i should drop...

but NOW an adequate charger may be problem instead, as im not using proper chargers, actually just regular 15v power brick cos i got no 24v + ones laying around...

dont really want to spend on maintenance charger, got some 2n3055's and some adc s on arduino left , or definitely some power mosfets , or maybe some parallel lm317's, but i can just imagine what kind of heat sink required.... or what is the name of linear chips?
 

spec

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Most Helpful Member
Hy Dr_Doggy,

Your serial connected lead acid batteries with separate isolated chargers for each battery would be ideal as each battery is charged according to its needs. But, as in automobiles, the chargers must have a current capability at least equal to the maximum current drain from each battery

You have 15V supplies to charge each battery and, as Mike says, you need 13.75V, with a negative temperature coefficient, for constant charging. Well, you can make a pretty good attempt at this by simply putting two 20 amps or so silicon rectifier diodes (not schottky) in series with each 15V supply. The temperature coefficient will be the wrong way (2mV per degree Centigrade) but, while not ideal, it should be OK- lead acid batteries are pretty tolerant. Just look at how they are treated in automobiles.:)

It would be best if the diodes were mounted on a decent heatsink. TO220 case diodes would be ideal.

spec
 
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dr pepper

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Yes I think I got that, and I agree if the load on the 2 12v batts isnt equal then separate chargers might be better.
One thing you must make sure though is that both the supplies are isolated from each other, server and pc supplies often have the - output grounded, so if you connected 2 of them to 2 batts then you'd short one of them out, the o/p's must be floating with respect to mains.

Yes why not use the 'duino to control the state of charge, so long as the ground of the 'duino is one end of the batteries (ie not the tap between them) you can measure the voltage of both, you would need some way to control the charge current transistors though, maybe an opto isolator, or I might be inclined to use a fet and drive its gate from the arduino via a gate drive transformer and maybe a gate driver ic.
Power for this can come from your supply bricks, if the 'duino is powered from one you only need to isolate the other.
From memory lead acids seem to like being charged at a constant current, then when a certain overvoltage is reached the voltage held untill the current drops to about 1/20th, then the voltage dropped and maintained to about 2.3v per cell.
Dont forget fuses!
 

spec

Well-Known Member
Most Helpful Member
Hy dr pepper,

It is always better to monitor and charge battery cells separately as they all have different requirements. It is unfortunate that an automobile battery is six cells connected in series with only access to one terminal of the outermost cells.

If you just stick 12.75V across a lead acid battery and limit the charge current to the max for the battery all will be well, but the battery will not charge in minimum time. For that you need to jack the voltage up to around 15V.

Yes, you could make a Rolls Royce system with current controllers and voltage monitors interfaced to an MCU but probably overly complex this application.:wideyed:

spec
 
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