Charge a 6V battery with a 12V charger?

[erick]

I'm going to buy a small (around 4Ahr) 6V sealed gel battery to power my Pentax K100D when I'm away from AC mains. Those 4 AAs just don't hold enough juice to drive many long exposures. I've read in a camera forum that a fully charged battery (7.2V) with series protection diode (-0.7V) will deliver the 6.5V the Pentax loves.

Now, how to charge it? I am planning to get a 6V charger, but before I spend the $s, I wonder if I can use the el-cheapo 12V charger (240V mains input) that I use to charge my 12V batteries? Could I do something as simple as run the output through a voltage regulator - say a +8V, 1amp regulator? Fit a reasonable size heatsink? Reckon it would work? Or will I just make a mess of the charger?

[Starkler]

If you have some sort of voltage regulation at about 6.8V thats fine, but dont connect a 6v battery directly to a 12v charger.

You could however connect two 6V batteries in series directly to the 12v charger.

Why not get one of those multi voltage regulated adapters that you could power off 12v to give you 6v for the camera?

[bojan]

Just an adequate series resistor with 12V power supply will do..
The good charging current for your lead gel batteries is around 10% of capacity, that is 400mA.
So, put 1.5 ohm/2W in series with 12V power supply adapter (for one battery). Charging time is 10 hours or a bit more but not much more than that.

Another way to do it is to add a small charging circuit (actually a series voltage regulator), with output adjusted to 13.6V. I can send you a schematic if you go this way, it is just a transistor (mounted on heat sink) and couple of components (zener, resistors, capacitor or two).

[Barrykgerdes]

A multi voltage battery charger could be built by anyone with some reasonable knowledge of electronics.

Use a current source (high wattage reostat or solid state transistor design) from say 36 volts (charge all up to about 28 volts).
Then a switched arrangement of voltage sensors (Zener diodes) to automatically turn off the charge when the battery terminal voltage reaches the prescribed fully charged state.

Barry

PS I have never thought of this before. It is the solution to my broken 48 volt charger on the golf buggy.
B.G

[tempestwizz]

I believe Bojan has the right idea, but question his math.
A fully charged '6v' battery is actually 6.8 v ( 1/2 of a nominal 12v battery's 13.6v) For a 400mA charge rate you need to drop 6.8v across the series resistance ( leaving 6.8v for the battery) which to me works out at 6.8/.4 = 17 Ohms, with a power rating of 17*.4*.4 = 2.72 Watts. Hence a 20 Ohm 5W resistor in series should do the trick.

One thing we've found with sealed lead-acid batteries over some years is that they do not like to be over-charged. When over-charging, the excess is converted to heat which cruels the battery. They seem to respond better to a little charge, but often. Hence, don't leave charging at 400 mA for too long.

In fact if it were me, I would use a few series resistor options; something like a 100 Ohm 1W for trickle charge rate, a 47 Ohm 2W for medium charge rate, and the 20 Ohm 5W for initial high charge rate. Start off on high for a couple of hours or so then switch to trickle for overnight or extended charging.


HTH,

BC

[erick]

Thanks, guys. You've given me the right ideas. I'll make up something around dropping resistors.

I can walk into Radio Parts and buy a little Arlec 6V/12V charger for $29.95, but I'm a bit stingy! Why spend that when I have a perfectly good 12V charger to which I can add some cheap resistors?

[bojan]

Sorry, I was too quick, and it was too early in the morning (before espresso... ).

[Starkler]

I wouldnt do the series resistor thing to charge the battery. Forget once to turn it off in time and your battery is cooked and its guts possibly ooze out everywhere.

Assuming you will have a 12V battery on hand to power everything else, my vote for best solution is a voltage dropping regulator to power your camera from the 12v.

http://jaycar.com.au/productView.asp?ID=KC5446&keywords=regulator&form=KEYWORD

[erick]

Geoff, I want it for those situations where it is just camera and tripod and my existing 12V batteries would be at home or otherwise engaged with the scope. But, it doesn't take much encouragement for me to build up a kit - I love waving the soldering iron around. I'll think about this option with a small 12V battery. :)

[Glenhuon]

A small 12v gell cell with variable voltage reducer built around an LM317 type variable regulator IC with some current regulation would do the trick. There's probably a circuit someplace on the net for one of those. Google is your friend :)

Bill

[I.C.D]

Erick,
Why don't you go to your local Battery person's (Battery World)and tell them what you want to do , they will sell you the right battery and charger to suit it may be cheaper in the long run
Ian.C

[erick]

Outcome:- Went to Jaycar and decided that the 6V battery was the one I wanted. It was the right physical size and capacity and only $19.95. Any 12V battery which was the right physical size was too low in capacity and at least $10 more. And they had a little 6V charger for $19.95 as well. So I have those. The battery has been on the charger overnight so I can check its fully charged voltage. I want to check that putting a protection diode in place doesn't drop the voltage too far. I read that some people had problems if the input voltage dropped too far below the nominal 6.5V. All being well, I'll add a 500mA fuse as well and be in business. Just need some way of carrying the battery. Usually it will hang off or sit under the tripod. I might want to sling it around my neck, but under such short exposure applications I have a set of rechargeable AAs that took hundreds of shots on several occasions before they started to show signs of draining.

[Paul Hatchman]

Hi Eric,
I'm glad you found another solution, but for anyone else reading this, please DO NOT USE RESITORS to drop the charging voltage. Modern battery chargers have electronics to do either a 3 or 4 stage charge, using different currents and voltages at each stage. Because the resistors do not act like a battery they will totally confuse the electronics and you may well damage the battery.

A 6 volt charger or a 12 volt charger with 2 x 6 volt batteries connected in series is the only safe solution.

Cheers,

Paul.

[Starkler]

Eric you shouldnt need any diode and in fact adding one will drop you below your desired 6.5v. Whats the tolerance of the camera of voltages up to say 7v ?

[Barrykgerdes]

Hi
What Paul says is very important. Those people wanting a cheap way to charge some of those expensive special equipment batteries. If you don't understand the electronics of the system you may end up ruining a very expensive storage cell.

Most of the equipment storage batteries these days require a current source to charge them safely. ie a maximum current of a few amps (reducing as the terminal voltage increases) depending on the size till they reach a specified terminal voltage that indicates full charge where at the charge rate drops to zero.

The car batteries that the cheap chargers are designed for are much more forgiving and will take much more kindly to rough use. (I used to jump start my car with a 24 volt 70 amp supply through a coil of earth wire, no problem).

So if the price of the factory supplied charger for your special battery looks prohibitive it still may be cheaper than a new battery.

Barry

[erick]

Don't know, but I am suspecting the 6.5V input spec is because the camera has its own protection diode (which I would expect!).

Charged the 6V battery it it seemed to settle to about 6.9V after a day. I'll plug it directly into the camera! I'll let you know how it goes.

[Starkler]

This is another reason why I was in favour of a 12v battery and a regulator as it would always provide a constant 6.5v as set.

If worried you could try discharging the battery a little with a torch bulb or something first to bring the voltage down. Becomes a bit inconvenient then though to mess around like that.

[ausastronomer]

Eric,

Why not just buy the right tool for the job instead of trying to cobble up a half pie solution to save a few bucks when you are risking damage to some expensive equipment?

There is the right tool for every job and trying to use a tool for something it wasn't designed to do usually results in something getting damaged. Like using an axe as a marriage counselling device :)

Cheers,
John B

[GTB_an_Owl]

GEEZ i don't know John !

my mum used to keep an axe beside the old fuel stove in the kitchen - never heard my dad complain about her cooking once :D

geoff

[ausastronomer]

Here's what you want Eric

http://cgi.ebay.com.au/6-AMP-6-12-VOLT-BATTERY-CHARGER-CAR-TRUCK-MOTORBIKE_W0QQitemZ180352020098QQcm dZViewItemQQptZAU_Power_Tools?hash= item180352020098&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A1%7C39%3A1% 7C240%3A1318%7C301%3A1%7C293%3A2%7C 294%3A50

Cheers,
John B

[erick]

Yes, John. I also saw a combined 6V/12V charger and 4A 12V power supply in the Autobarn catalogue last night - $89. If I was starting with nothing, I'd have one of these - it would do a number of tasks for which I have separate units.

But I do now have a 6V battery and 6V charger - I'll connect up this weekend (when I can get a power plug to fit the camera. Looks like I'll be carefully drilling out the one I bought :rolleyes: ).

This has been an interesting exercise together with my other thread on AA batteries and better chargers. I've learnt a lot. I had heard the term "trickle charge" but had thought that charging was not much more than stuffing current into a battery until it was "full"! There is clearly much more to charging batteries than that!

I've realised my 12V charger is a "stuff it in @ 3A" type charger and I should be careful not to overcharge my gel batteries. No more leaving them sit on the charger overnight, but rather remove when that green LED comes on.

[Paul Hatchman]

Hi Erick,
Is it a standard car-battery charger? If so, go and grab yourself an intelligent charger with a gel cell setting if you can afford it.

Gel batteries need to be charged at a lower voltage than wet cells to prevent them from outgassing. Since there is no way to replace the outgassed hydrogen in a gel cell, you're going to shorten the life of the battery.

[erick]

"an intelligent charger with a gel cell setting if you can afford it."

Thanks - any recommendations?

[Starkler]

Indeed gel cells require different handling to wet cell lead acid batteries.

Nominal trickle voltage is 13.8V for wets and 13.65V for gels. You can boost charge wets at 14.4V I think. Doing the same to a gel is very inadvisable.

[Paul Hatchman]

Hi Erick,
As always, it depends. But for charging a range of smaller lead acid batteries, the name brand chargers are Xantrex and CTEK. Both provide high quality multi-stage chargers and I believe both brands are available from Battery World. The CTEK XS800 is about $90 or the next model up, the XS3600 is $140.

Jaycar have knock-offs of the CTEK range e.g. This one would be equivalent to the XS3600 http://www.jaycar.com.au/productView.asp?ID=MB3604&CATID=18&form=CAT&SUBCATID=295. I'm not sure how well the Jaycar ones actually work, and unfortunately they don't have a seperate "gel" charging mode, but claim to be able to charge gel batteries correctly. I have both a Xantrex for charging my batteries at home and a Jaycar CTEK knockoff which I take with me to Wiruna.

Cheers.

[erick]

Thanks Paul. :)