# how to build simple charge controller/limiter for 12V batt?



## Stunt Driver (May 14, 2009)

I would like to continue using my 16v/9A power supplies from IBM laptops - surely they provide enough power, but batteries will boil if left overnigh, wasting electricity and aging batts.

To avoid setting timer every time - need to come up with charge control - I'd like to come up with some simple scheme, possibly based on Zener diode to shut off charge once voltage reaches 15v. Just to make sure I am not inventing a bycicle - decided to ask for help first. 
I can do some soldering, but looking for a good idea to implement.

Here is what came up to my mind - zener diode in parallel with NC relay and limiting resistor. Once voltage reaches 15v - current should run thru ZD and trrigger relay, thou disconnecting the battery charge, and sending voltage on ZD even higher. LImiting resistor should be just big enough to allow small current thru ZD without overheating it, while it will keep relay open. Light bulb will be a good one to use - it will also indicate Done for particular battery charging

One of converns - ZD are some what flexible on breakthru voltage - for 15v part it can be from 14.3 to 14.8v, while i would like to get better accuracy 

ps. this may be a temporary solution until I get settled on batteries/charger AND finish the convertion. So whatever I do - has to be cheap, so I don't have regrets scrapping it later, if, for example, i'll win lottery and will go LiFePo.


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## Stunt Driver (May 14, 2009)

Come on, DIYers, haven't anyone done their own charger? 

I am buying set of zener diodes now, so will test it in real life.

Additionally will add light bult on the fith relay leg to be both signal and current limiter for trickle charge. 5W bult should be good for 0.3A or so.


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## SimonRafferty (Apr 13, 2009)

I think if I were doing this - and I will be after my charger is finished, I would probably go for a preset variable resistor acting as a potential divider between +ve & -ve. The wiper on the VR connects to the base of a transistor (BFY51 I use a lot) Emitter to -ve Collector to the coil of a relay and the other coil connection to +ve. A diode across the coil will stop the inductance of the coil nuking the transistor.

The transistor will start conducting when the wiper reaches 0.7v or so and the relay will energise. At least with a VR you can tune each of them to precisely 14.7v.

Si


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## Stunt Driver (May 14, 2009)

Simon, many thanks for advice.

I have played with ZD a bit, and no good result yet. I'd like to try transistor+potentiometer as you suggest.

What value/size of variable resistor should be used with this transistor and in 12-16v appliance?


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## SimonRafferty (Apr 13, 2009)

I would go for a 10k pot with 1k between the wiper and the transistor base - that should work OK

Si


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## Stunt Driver (May 14, 2009)

Thanks! Is there a place to buy a few of BFY51 at reasonalbe prices? Only a few stores offer it at ebay, and at more than 4 bucks a piece


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## SimonRafferty (Apr 13, 2009)

In the UK - RS, Rapid or Maplin.
In the US - No idea I'm afraid! It doesn't need to be a BFY51 - just something that will switch enough current for your relay. A BC108 may be enough. Try searching eBay for "1A NPN" - in the UK it turns up several for as little as $0.15 each.

Si


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## Stunt Driver (May 14, 2009)

Wanted to double check - would something like this work for my purpose, to drive 50mA relay?
http://www.datasheetcatalog.org/datasheet/kec/BC546_7_8.pdf


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## Roy Von Rogers (Mar 21, 2009)

Instead of cutting the charger off, put two diodes in series, this way when it gets to 15 volts, it will still keep the batteries at 13.8 float.

Roy


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## dataman19 (Oct 7, 2009)

Stuntdriver,
Roy Von Rogers had the best suggestion....
..
This is why...
Your batteries are boiling because you are charging them from a source that is above the 14.6V Boiling Point of the battery.
.
12V Batteries charge at a voltage above 13v (they like 13.8). Any voltage above the 14.6V level will cause the electolyte to boil (it also drastically heats up the battery cells and shortens useful life if left un checked).
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Putting a couple of rectifiers in series with the power supply will drop the voltage to a use able voltage.
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Explanation: All Rectifier Diodes have a forward voltage drov (vfwd). This is the voltage loss of the device. Since rectifiers have voltage losses of 2.2V to 2.8V putting a 2.2V loss diode will drop the 16V to 13.8V. If you have shottky diodes you will only have about 1.2V to 1.8V voltage drop and will need two in series.
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If you don't know the foreward voltage drop of your diodes - take it to an electronics parts supply store and ask them.
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Oh yea - make sure the PIV (peak Inverse Voltage - the operating voltage) of the diode is at least 10-25 Amps. Or at least 2 times the charging current you are looking to supply. I would recommend it be 2.4 times the maximum that would be needed (at least this is my power supply rule of thumb - but then I strive for zero circuit failure designs).
...
Dtatman19


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## Stunt Driver (May 14, 2009)

thanks for input. Using diode for enabling float charge is definilyte a good idea, but lowering cut-off voltage may be harmful because full charge can't be obtained at 13.8v from what I know. At least once in 10-15 times charge up to 16v is needed for desulfation reasons.
Also, batteries under target are used starting LA obtained at $15 each, so i'd rather get full charge out of them every time at expense of shorter life.
Also, I believe VFWD can be as low as 0.7v on Germanium based diodes. But i'd rather go with 4A power supplies (from smaller laptops) to provide less current and so deeper charge. Still, 4A in overnight 10h charge will give full capacity to any starting battery (with 50% dod)


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## dataman19 (Oct 7, 2009)

Stuntdriver- 16V is not for desulfating your batteries... It is so you buy new batteries..
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13.8VDC is the float voltage to charge a 12VDC Battery (even an deep cycle battery).
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You can use 14VDC if you absolutely have to have than 16ozs of water in your glass to justify taking a sip....
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In the solar power world you only use 14.6VDC to gass off/desulfate your batteries. This voltage causes the electrolyte to boil - which breaks up the hydrogen sulfate crystals caked onto the plates. Gassing off your batteries shorten their life span - but allowing them to cake with hydrogen sulfate crystals will also shorten the battery life span. So what do ya do...
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If you routinely discharge your batteries in excess of 40% Depth of Discharge (DOD) you should not have to desulfate your batteries. BUT - If you leave your EV in storage for two to three years and don't use a desulfating battery tender - YOU WILL GET SULFATED BATTERY PLATE.
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Thinking that over heating your battery by applyiong 16VDC for prolonged periods is desulfating is like thinking you are extending the life of your tires by deflating them (letting all the air out) every night, then reinflating them in the morning before you drive the car.
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The diode as a voltage reduction device is a necessity if you are using a 16VDC power supply to charge your batteries. If you have 14.2V or less (preferably less than 14V in my worrld) you don't need to be concerned, and therefore don't need to use the diode.
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I have personally seen hundreds of batteries destroyed by people who think charging voltages above 14V helps to charge their batteries more quickly. Same is true for people who think high charging voltages keep sulfate deposits from forming. All these people are doing is killing their batteries before they have a chance to sulfate.
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Case in point:: I have a solar Power Installation in Turky (the country) that has been operational since 1976. The batteries have only been replaced once (in 1989). They were replaced because a caretaker had adjusted the battery charging voltage on the Battery Maintenace Computer to 14.8VDC. He told me that he was told by a mechanic that this high voltage would desulfate the batteries and make them perform better. He even added that he thought the system was actually working better. What does he know? The well pump worked (just like it always has), the night lights came on (Just like they always did), the tracking arrays south the eastern sun every morning (just like they always have) and they tracked the sun accross the sky (you guessed it). So what exactly did the system do better - never found out.
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We put this system in this small village because they were 80-miles away from the nearest power grid (not that Power grids in Turky are that reliable). It is taken for granted, and has performed flawlessly for 99% of the time. Just like 3-Mile Island - had not a human being interfered - it would have never failed.
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Stick to 14V less on your charger for 12VDC Battery sets and you will be happy. Mess with them by upping the voltage and the battery salesman who told you to do it will make another comission check when you buy the replacem,ent set.
..
Dataman19


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## Roy Von Rogers (Mar 21, 2009)

Do yourself a favor if you going to want to charge those type of batteries and buy on of these, and you will know that its done right, and you wont kill your batteries.

http://www.northerntool.com/webapp/...6706?cm_mmc=Housefile-_-SHIPPED-_-1206-_-CONF


Roy


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## Stunt Driver (May 14, 2009)

*Roy Von Rogers* - would you think that charger is much better from one I see at the same store (except desulfation mentioned)?

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200331939_200331939

*dataman19* - thanks, interesting. But what are typical discharge rates on those solar battery banks? I fear that having up to 250A peak, and 150A constant discharge for EV use on 125AH batteries shortens battery life alot, with any charge policy.
And to be more precise - Schumacher charger by link above does not "charge at 16V". It keeps constant current, and voltage starts from about 12.8v, rising slowly. At about 14v - current is lovered to near 2A. In next hours voltage reaches 16v, charging is nearly complete, current is dropped to very low so 15v voltage maintained for absorbtion stage.
I Also have VRLA profile on charger - it will only go up to 14.4v and then drop for absorbtion. like here. but I see much less range out of my batteries after such charge. How would you comment this? 


Also, how would you comment on 15.5 equalization recommendation by world leading Trojan batteries - http://www.trojanbattery.com/BatteryMaintenance/Charging.aspx





ps, and back to my idea once again - I am not planning for constant "voltage charge". 
Power supply is capable of 16v max, BUT only 8A max (also have 4A). This means when 4A psu connected to 50% DoD battery - I will measure ~13v across battery, slowly rising with battery getting charged. The question to me is - at what voltage do I shut off the charge, and maybe switch to float charge of ~13.8v or so. I sure do not think that charging at constant 16v is the right thing. Would appreciate good & argumented discussion here.


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## Roy Von Rogers (Mar 21, 2009)

Stunt Driver said:


> *Roy Von Rogers* - would you think that charger is much better from one I see at the same store (except desulfation mentioned)?
> 
> http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200331939_200331939


 
Yes, I just recently bought one, it cant be beat in its amperage range. Considering some of the junk I see out there, its refreshing to come across something so well build.

I can assure anyone who purchases one, you will love it.

Roy


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## Stunt Driver (May 14, 2009)

I don't understand - you say 2/4/8 range can't be beat by 2/6/10 ??? To me those are effectively the same. Both are 3 stage, microprocessor controlled. 
Is there anything you actually find better about $100 charger?

Also, do they have different cutoff voltages? Because I use 12A schumacher, and don't see any issues with them. I see major cost advantage thou.


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## Roy Von Rogers (Mar 21, 2009)

Stunt Driver said:


> I don't understand - you say 2/4/8 range can't be beat by 2/6/10 ??? To me those are effectively the same. Both are 3 stage, microprocessor controlled.
> Is there anything you actually find better about $100 charger?
> 
> Also, do they have different cutoff voltages? Because I use 12A schumacher, and don't see any issues with them. I see major cost advantage thou.


 
Look at the specs and compare, I'm giving you my professional opinon. Its a well build and thought out unit. I'm not afilliated or a seller of the product, if it wasnt worth the money I wouldnt recommend it.

Roy


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## Stunt Driver (May 14, 2009)

that is exactly why i'm asking - specs are IDENTICAL, except desulfation mode.


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## alvin (Jul 26, 2008)

http://www.evdl.org/pages/hartcharge.html

Try this.


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## Stunt Driver (May 14, 2009)

i don't know details on BatteryMinder charge profile, but chumachers do equalization after each charge when in LA profile. I do EXPECT that batteriminder does the SAME.
Schumachers can also do GEL and VRLA profile, where they don't go above 14.6v. I think i'll switch to that profile for daily charge. However, batts are covered by warranty, so it's a good ground for experiments 

thanks for link, very crisp and short but nothing new.


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## alvin (Jul 26, 2008)

Are you trying to limit volts on a power supply? Or did you decide to
buy a smart 12 volt charger. Your'e right nothing new. I think that
is all there is. 
Alvin


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## Stunt Driver (May 14, 2009)

Buying is too simple, and drains funds, not for my case. Just have other priorities in life now. Besides, saving up to buy Aptera. My reservation # is 2224.


So, back to simple (~$1) limiter, that can turn 16V DC 9A power supply into a charger: I have put together transistor, pot and relay - it works, but will oscillate for a while once trigger point reached (set to about 15v).
Schematics below.

Also, it is very difficult to adjust cut-off voltage. Very slight tough to the pot - and cutoff jumps from 14.2 to 15.8, so i'm spending ages to set unit to 15.2 (my desired cutoff voltage)


If someone can guess why this may happen and how to fix this - please give me a hint!


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## dataman19 (Oct 7, 2009)

In response to your decision to not use the "blocking diodes"....
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Blocking diodes are not a "convenience" they are a "necessity".
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The blocking diodes protect your system from reverse bleed. darkness will cause the solar panels to "suck" power from your batteries. The blocking diodes "block" the power reversal on a dark winter night. Hence the term "blocking Diodes".
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Most commercial panels also have three diodes a + Blocking Diode, and a - blocking diode. A third diode provides bypass current as a safety in the event the panel is blacked out (covered- as in shade). This third diode will keep the panlels from over heating due to uneven power generation. This is also a strong argument to include blocking diodes in any and all panels.
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If you look at the specifications for panels you will find that most are rated at about 19V - which is why a 185W panel still only produces 145Watts of actual used power. The 19V rating is a safety to allow for inefficient wiring (a problem that plagues most DIY Solar Installs), and device voltage drops in the Power Buss Combiners, charge controller, etc.
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19Volt Panels are used in a 12V (13.8-14.6V) system, 32-40V panels are used in a 24V (27-28V) system, and 60V Panels are used in 48V (52V-56V) systems.
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Incidentally, 13.8VDC is the charge voltage for a 12V battery, Not 16 Volts. ...
...
Dataman19


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