Charging circuit for my camera external battery pack

[Wuey]

Hi. I have succesfully adapted a battery pack from my RC model car to power my digital camera. The battery pack consists of 6 sub-C 1800mAH ni-cad cells connected in series.

I intend to embark on a long trip crossing the Australian continent by car in a few months time. I intend to camp out on unpowered sites. Does someone know if there is a simple charging circuit for me to build a basic charger to charge the battery pack from the 12V source in the car? I intend to use the camera's original battery as a backup.

 

[TillEulenspiegel]

If your cars electrical system varies a large amount you want to put a fixed (7812) voltage regulator before the LM317.

 

[Sebi]

Don't forget the decoupling cap for input....

 

[Oznog]

I don't understand why there's a diode at the input.

Adding a 7812 at the input wouldn't serve any purpose. The 317 can take an input up to 40V. Also, the common 7812 has a 2V dropout, it would only put out 12V if the input was >14V.

Be aware that a good deal of heat can be generated, it's proportional to the output current. Heatsinking would be a good idea. There's also no charge termination, there's no "simple" way to do charge termination.

Actually, best thing might be to bring your battery charger from home and power it with a power inverter.

 

[Wuey]

Thanks for you input fellows! I think the 317 alone should suffice as most cars have a maximum output of about 14-15V.

TillEulenspiegel, can you please let me know the exact values of R1 and R2. I can't read their values even after magnifying your diagram.

Sebi, what sort and value of a decoupling cap would that be.

Oznog, thanks for suggesting the heatsink. I enjoy DIY projects but I might just take up your suggestion by using an inverter instead. I'm also going to carry along an x-drive for mass digital photograph storage. The inverter would come in handy to charge its li-ion battery.

 

[TillEulenspiegel]

Oznog : Prevents backfeed ? Because I'm anal about separating two supply sources with different potentials? Because it costs $.10?

Adding a 7812 at the input wouldn't serve any purpose. The 317 can take an input up to 40V. Also, the common 7812 has a 2V dropout, it would only put out 12V if the input was >14V.


True that but the caveat was a wide swing of power in the car's charging circuit, I have seen swings of 5 volts in car systems when the regulator kicks on . Most 12V car electrical systems are 13.8V in reality (14-15 V charging) and 12V nominal Again an optional feature probably not necessary that costs < $1.00. Actually I should have put a fuse @1.5A in line with the supply side pre diode .

Yes heatsink is advisable and the LM137T to use is the To-3 pkg.

Weuy: Io= 1.25 /(R1+VR1) in Ex.1 R1=6 ohms with VR1 @ 10 ohms that will give you a window of ~75-200 mA
a slow charge on a 1800 mAH is ~ 180mA ( slow charge is usually 1/10 of the capacity). Measure the charge rate by putting your VOM ( mA scale ) between the battery + and VR1 out and adj the current.
Sorry about the drawing I just got an image format converter and it plays hell with text and even occasionally swaps pins and such > sigh. Time to look for a new one.

 

[k7elp60]

charging battery

Wuey,
I would replace the diode D1 with a schottky diode such as the 1N5817,
as the forward voltage drop is a lot less. I would limit the charge current to .095C or 1800 x .095= 171 Ma. This will insure that you do not overcharge the batteries. The value for the resistor between the output and adjust terminals = 1.25/171Ma or 7.3 ohms. I would use the standard
value of 7.5 ohms 1W. I would put a 0.1uf cpacitor between the input pin of the LM317 and ground or - of the circuit. I would use a small heat sink on the LM317, and a LM317T will work fine. By doing this you can leave the battery on charge overnight with out overcharging. Generally a short time after the engine is turned off the car battery goes down to about 12.6 volts or so, so you can leave the charger on and the car battey will not run down. My only other commet is that Nicad batteries are somewhat
sensitive to heat so try not to have them in the automobile with all the windows closed when charging.
Good Luck

 

[Wuey]

Once again thanks for your suggestions everyone. I didn't realize a simple little diode would cause so much voltage drop until I started reading other posts in this forum.