Converting mains battery charger for 12v operation

--- Unless your charger has some smarts in it which will keep it apprised of the battery's condition (state of charge, temp, etc...) or your camera does it and talks to the charger, there's probably no reason why you couldn't just connect the output from your 8.45V regulator (the one you're going to build) directly to the camera.

Having said that, though, and as far as connecting 8.45V to the charger goes, I think you're courting disaster (not to mention voiding any warranties which may exist for the camera _and_ the charger, unless you know _exactly_ what the circuitry in there looks like.

For $165, a CB-910 or CB-920 sounds to me like a better way to go.

-- John Fields

Hi John,

Many thanks for your reply. Unfortunately, I can't connect my regulator directly to the camera to charge the battery because the charger connects to the camera using a 'battery-shaped' plug! In other words, the ONLY way to charge the battery is using the battery charger, which only accepts 240v.

Yes, you're probably right! I thought I'd look into it though.

Yeah... but for that price I might as well buy myself a 12v to 240v inverter (not as efficient, I know - but I'd like to be able to plug other mains devices into my car, so a 12-240v inverter would be useful).

Thanks, Jack

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--- An easy way around that problem (if you want to take the risk :-) is to cut the cord and affix mating connectors to the cut ends so that you can plug them in and out. Then, you'll have the end which plugs into your camera, and you could plug the other end into either the mains charger or the output of your regulator, to which you've also connected a connector identical to the one on the charger. Make the connectors on the charger and your regulator female, and the one on the charger cable male. RCA plugs and jacks work fine, or if you wanted to you could use some nice little locking circulars.

-- John Fields

Thanks for your second really helpful reply on this topic!

Yes, you're right - I should do that. However, this solution wont actually charge the battery - which is really what I want to achieve.

Thanks, Jack

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I'm probably missing something here, but why don't you get a power inverter which coverts 12V DC to AC. (And which can also be useful for other things.)

Bill

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I would go with the inverter.

It's not a good idea to connect a voltage regulator (a voltage source) to a battery (another voltage source). What would limit the current when you hook it up with a flat battery?

The charger will be designed to include a current limit if not an actual constant current source.

Why don't you just buy a cheap, low power inverter for $20 to $30 and use this to power your existing charger?? These things are made by all sorts of companies and plug right into a car's cigarete lighter.

The topology of a mains charger is TOTALLY different than the topology needed for a charger operating off 12V. You aren't going to be successful in trying to "convert" a mains charger to 12 V input operation.

Bob.

Dear Collin,

Thanks loads for your reply.

Sure, I wouldn't dream of connecting a battery directly to my voltage regulator. What I'm proposing to do is to connect my 12v battery to the battery charger, just downstream of the 240v to 8.4v transformer and upstream of the charge controll circuitry. In other words, I hope my battery charger will think it's connected to the mains, even though it's connected to my car. So the charger will look after all the current limiting etc.

Thanks, Jack

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Yes, I think you're right - I will go for the inverter.

Thanks, Jack

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Jack, Don't they make a car charger for it? As for modifying the charger, you would have to open the unit up and see what the raw DC is off the rectifier. My guess would be around 12V for an 8V battery.

Tam

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Hi Tam,

Yes, I've taken the lid off the battery charger. It turns out the voltage across the smoothing capacitor (downstream of the step-down transformer and diode rectifier) is 8.45v.

Thanks, Jack

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This is the unregulated voltage, right? I can't come up with a reason why it should not work if you run 8 - 9 V from a 3 terminal regulator to this point. Your rectifier diodes will prevent the transformer from shorting out the DC. I assume the actual battery voltage is 6V nominal. I guess this is what you proposed originally. The thing to watch for is if the thing uses positive ground. Could cause fireworks if any grounded metal on the camcorder touched grounded metal on the car. Of course, the camcorder probably has no exposed metal. At any rate I would be sure to include a fuse. Measure the DC current when running off AC.

I think some of us are confused as to what the topology is. Is the actual charger in the camera, in the brick, or do you remove the battery from the camera and connect it to the charger? Any power jack on the camera should be labeled as to what the voltage range is. Either on the camera, or in the instruction book. My Ricoh Hi8, for instance, uses a 6V battery. The camera has a label that states 6 - 7.5VDC. You remove the battery for charging.

Let us know how things work out

Tam

Jack,

I want to point out that there are basically two ways the charger can work:

A. The AC line is stepped down to a low voltage, rectified, and fed to the charger. The 8.45V is the input to the charger. This is what I am assuming.

B. The AC is rectified to give 160 -340 VDC, which then goes to a switching regulator. The 8.45V is what goes to the battery. If this is the case, forget it.

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Hi Tam,

Thanks loads for your reply.

Yup, exactly! I didn't explain it very well - but you've figured it out.

The Brick has a battery mount on it. To charge the battery you have to take the battery out the camera and plug it into The Brick. The camera has no charge circuitry. All the charge circuitry is in the brick. So I want to pump 9V into The Brick onto the smoothing capacitor (which is downstream of the transformer and rectifier diodes).

Thanks, Jack.

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Hiya,

I'm 99.999% sure my charger works in way "A".

All the control circuitry for the LiIon charging is on a little daughter board, which is definitely downstream of the 8.45v I measured across the smoothing cap.

Thanks, Jack

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Hmmm... having looked again at the PCB, I'm not so sure!

Take a look:

I want to put 8.45v onto C22 (it's marked on the last 2 JPGs). There are a total of 3 transformers. 2 of which have 240v on both sides (i.e. their coils are symetric). And there's definitely circuitry to produce 320v DC (D1 is a high voltage rectifier).

Urg. I dunno anymore. Any thoughts?

Thanks, Jack

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Jack, What is the nominal battery voltage? I will take a look later, but the

320VDC sounds bad. The 8.45V sounds about right for a charging voltage for a nominal 7.5V or so battery. Unregulated 8.45V would be on the low side for a charger input that charges a 6V or higher battery.

The charger for my Motorola GSM phone appears to use a switching regulator type of charger. The blob that plugs into the wall is not big or heavy enough to contain a 50/60 Hertz transformer.

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Just a few thoughts, I have been watching the posts but haven't had time to absorb all of the detail.

The Li-Ion battery will have its own regulating circuitry in the battery, and most batteries take a raw DC feed from whatever is charging them. They monitor overvoltage and undervoltage and charge current. We have done very large series/parallel Li-Ion battery chargers at 110V 50A X 4 outputs for a subsea application and also a more conventional multi-way version for a series/parallelled cells, but generally the battery defines what it needs.

Re the high DC voltage on the charger, it sounds like a conventional switchmode step-down and isolating circuit, probably to keep the volume and weight down. It's cheaper to buy a volume switcher in from thr far east that make a conventional transformer/rectifier capacitor DC supply over here, and it has the advantage of being universal input, 90 - 260V at almost any frequency.

My 2c worth.

Peter

-- Peter & Rita Forbes snipped-for-privacy@easynet.co.uk Engine pages for preservation info:

Whoa!!! 8v45 sounds VERY much like the CV charge potential across two Li-Ion cells in series. If that's the case, it sounds also VERY much like a "type B" system.