12v car project. Charger question

Hi all
Got a new project lined up using a 20M2. Basically an alarm system for my truck to protect the rear load bay and also the spare wheel (been pinched twice now).
This is going to involve an SRF005 ultrasonic module watching the rear load area and a closed loop cable system that will go through the spare wheel underneath.
Turning it on and off will either be by keypad or wireless switch.

My question is running the 20M2 off the truck battery.

I think it would be wise to build a charger system to charge a 1.2Ah Ni-cad while the car is being driven.
This way, when the ignition is off, I know the system is not draining the main car battery.

Anyone got a circuit diagram or al least some pointers to a suitable 12v Nicad charger that will run from a car DC 12v? I have Googled with little results and they seem to vary wildly.
I could use a DS1302 to change the power over and I will need to add some decent spike protection.

As I develop the idea, I will post progress.

Thanks, Steve

I would build my own NiCad or NiMH charger. Use the calibadc10 command to read the voltage of the PICAXE's power source (the NiCad battery) to determine if it needs charged or not. Use one pin on the 20M2 with a voltage & current regulator to determine if the truck is started or not (so the PICAXE doesn't try charging the battery pack while the truck is not running). You'll need to connect this to a 12V source that only operates when the key is on. Usually, the cigarette lighter is a keyed 12V source. Then use a relay driven by another pin on the 20M2 to switch on/off the keyed 12V source that is connected to the NiCad battery with voltage & current regulation to get the correct charging rate for your battery. Have a look at this site for battery charging ideas: http://robotroom.com/Solar-Recharging.html

There are many threads concerning the safety requirements of adding electronics to a vehicle. After those are considered and dealt with, I would question the need to be concerned about the picaxe draining the truck battery. With proper design, you can have a near-negligable current draw. Here, for instance, is a project which has been monitoring an input for 5 years using the same three AA batteries: http://www.picaxeforum.co.uk/showthread.php?8353-Low-Power-Battery-Backup-Reference-Design

Your draw with the SRF005 would be a bit higher, but I would breadboard it first to see if you could get the surveillance you want without having to add battery charging.

Well Ummm.

If this is a 10 cell Nicad pack, at 1.2v nominal 10 cells gives 12v, then you will need about 14v maybe 15v to charge the '12v' battery.

Vehicle systems may go up to 13 to 14v with the engine running, but this may not be enough to charge a Nicad Pack properly.

Besides, you are in the UK, so according to the EU you should be using NiMh.

I just did a test of a HC-SR04 utrasonic sensor and it draws about 13mA @ 5v (0.064w), not much but it adds up when you're running off batteries.

You could use a lithium ion solution, with an appropriate IC to safely handle the charging when the car is switched on (pretty cheap) and a DC-DC voltage booster to get 5v for running the Picaxe, ultrasonic sensor & 5v relay to power the alarm from the main battery when triggered.

Mmm... Lithium batteries fill me with fear!
Well the ultrasonics will be a constant draw, but I am also planning to add flashing leds to the rear lid - still not much current draw however.
BUT.. I have a future upgrade in mind. I would like to add a DVR at a later date (hidden behind the rear seats) which will record the area around my truck. I have 4x small pinhole cameras here that will be ideal.
Therefore, I thought a chargeable battery system would be very handy. Plus my siren/sounder is 12v as well.
Otherwise I could actually just build a 6v backup system to run the 5v Picaxe and ultrasonics.
Some good suggestions there - thanks all

If you're planning on adding more things that require power like the DVR, what about a 2nd lead acid 12v battery of a capacity which can handle the charging ampage from the alternator?

Car audio fans do this and there's enough info out there to do it safely, I had a quick look and it can basically be achieved by using a 30A relay which gets switched on when the alternator is running to isolate it from the main battery when the engine is off. With appropriate fuses etc. of course

If your power usage is set to grow how about taking the 2nd battery a little further and using a split charge relay as used in camper vans etc. ? Would that work?
There are loads of kits and designs - and it's a tried-and-tested method if done properly.
Obv you'd need appropriate protection & regulation for all your sub-circuits.

No room in the engine bay for another car battery. Only room really is behind the rear seats and that isn't big enough for a car battery.
Plus... a massive car battery is overkill. I am not going to be drawing that kind of amperage.
Perhaps an 'off the shelf' charger for a caravan could be used however.... Google time

A small Sealed Lead Acid 6V battery (like used in older burglar alarm panels) would work fine and are less than £10

Typical car battery is 55A/Hr, so with a load from this unit of say 10mA, that's enough capacity to power it for at least 220 days. Add to that it is likely to be switched off when not required, then it's not worth the trouble of installing a Nicad IMO, just run it straight from the vehicle battery. The probability of the vehicle not being started in 6-months is very low.

Fair point.

Do conventional car alarms have their own batteries ?

The one I have fitted did not.

No - my car for example has a car alarm, with an interior ultra-sonic detector (as do most today), a GPS that is ready for use the moment the car is started (and so must be on) and the radio records traffic annoucments, so adding this topic load would be trivial in that context.

But... the load of a future 12v DVR running 24hrs @ 10-15w, + 4x IR camera modules @ 160mA each + this alarm unit is more than I want constantly drawing off the main car battery.
I think a 1.2Ah or larger Nicad/Nimah hidden behind the rear seat - charging as I drive, and used as power when the ignition is off will be the best option.

Look for radio control (RC) battery systems. You will find plenty of capacity in those battery systems.

Steve2381 said:

I think a 1.2Ah or larger Nicad/Nimah hidden behind the rear seat - charging as I drive, and used as power when the ignition is off will be the best option.

Click to expand...

Hi Steve,

IMHO that seems completely illogical. You seem satisfied that the "alarm" system will continue to work from a 1.2 Ahr battery (what's the point of an alarm that only works some of the time?), but that it might completely drain the main 40+ Ahr car battery. And the "shelf life" (self-discharge) of a NiMH battery is probably shorter than that of a lead-acid car battery.

As others have implied above, KISS (Keep It SimpleS).

Cheers, Alan.

I just don't want to get in the car and find it flat. If it draws off a different battery while the ignition is off, then I do not have that worry.
Also, modern car alarms sometimes trigger on current draw (e.g. interior light). I was trying to keep my system isolated.
I would work out the required battery for a good 14 days of constant use, once I have established the full current draw.
It could always switch across to the car battery if the Nicad/Nimah has drained. I would rather it was that way around than the car battery first.

I will investigate the full load and make a decision. I see both sides of the argument here.

Am I reading this wrong?
"the load of a future 12v DVR running 24hrs @ 10-15w, + 4x IR camera modules @ 160mA each ..."

Let's say 12W at 12Volts for easy calcs. How much current is that? Plus your other bits.
How long would your 1.2Ah last? (Per charge and overall life).
Is it the "best option"?

steve2381, I think you should work out what Dippy has summarised, you wil be surprised at the result and battery size required to keep your equipment list operational...

Yep.. I know, already done the calcs.
1.2Ah is not going to be anywhere near large enough. I didn't realise that the DVR would pull 1.25 amps per hour on its own. Certainly don't want that drawing off my main battery.
Perhaps I should install a diesel generator instead

http://www.maplin.co.uk/outbackcam-589600

Just fit one of those and get stills and video.

Actually, looking back to your orginal post, that should be all you need, but will need to be weather proofed though. Camera above already is.

Its Maplins. It will have terrible unreadable instructions from some far flung country, and it will pack up after a week!
The cameras where going to be mounted on each wing mirror looking along the sides of the truck. Sick of people door dinging my car and also watches people trying to get in the back lift lid (which has happened already).
I will re-think the plan then

Maplin stuff - yes have to agree, so I don't know why I suggested that

Maybe instead of dinging your car they'll just pinch the cameras