[Stunt Driver]

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.

[Stunt Driver]

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.

[SimonRafferty]

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

[Stunt Driver]

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?

[SimonRafferty]

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

Si

[Stunt Driver]

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

[SimonRafferty]

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

[Stunt Driver]

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

[Roy Von Rogers]

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

[dataman19]

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).
..
Putting a couple of rectifiers in series with the power supply will drop the voltage to a use able voltage.
..
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.
..
If you don't know the foreward voltage drop of your diodes - take it to an electronics parts supply store and ask them.
..
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