Where to find a 7.2V 250mAh NiCd battery

[LunarOne]

TL;DR Where can I find a replacement for a 7.2V 250mAh NiCd battery 40mm long x 25mm diameter car alarm backup battery.

This evening I completed retrofitting cruise control to my 1992 E32 BMW 730i by purchasing a stalk assembly, steering column cowl with hole for stalk, cruise control module, throttle actuator, and various ancillaries such as brackets, a Belden cable, screws and fittings

Anyway after a couple of weeks of trying (including sourcing the wrong cruise control module initially), I've got it all working and I'm very please. This car is my daily driver barge/shed combo and in the six months I've owned it, my only real annoyance was the lack of cruise control, since I live in an area beset by average speed controlled motorway works and all my other cars do have it.

One slight problem I'm yet to solve, is that the BMW dealer-fit alarm/immobiliser system siren was fitted to the bracket where the cruise control actuator is supposed to be attached. So I've moved the siren (which doesn't sound) and fitted the cruise control actuator in its proper location.

So now I have a siren dangling by a cable running through the firewall to the rest of the alarm/immobiliser/central locking system, the rest of which works fine. It's not really dangling - I wedged it into a gap I found where it hopefully won't come to any harm. But some research I found from a forum years ago says that the siren contains a NiCd battery which goes bad, and that when replaced, the siren usually comes back to life. So I opened it up, found the swollen NiCd battery and desoldered it from the board. And also found one loose corroded wire which is supposed to connect to the speaker inside the siren. Which is why it doesn't sound at all.

I thought it would be easy to find a replacement NiCd battery, but it doesn't seem easy at all. I thought I could maybe find a NiMH battery to replace it, but research tells me it's a bad idea. In this application, the battery is presumably constantly charged, something which apparently NiCd batteries take without trouble, and NiMH batteries don't, as they overheat and swell.

So my question is, do you know where I can find a suitable battery? It's cylindrical in form, approximately 40mm in length and 25mm in diameter. The dimensions don't have to be exact as although this is soldered directly to the board, I could use any battery which will fit in that space or smaller. it was secured to the board with a cable tie strap. The battery itself seems to be made of three smaller sections, presumably each made of two 1.2V cells, so a total of six 1.2V cells making 7.2V. The crusty example I've removed is somehow making 0.5V after all these years. I'm amazed it isn't zero volts!

I thought I could try to contact the original manufacturer in Italy if they still exist, but I doubt they do. I tried searching the likes of Farnell and RS Components without success.

Any thoughts? Thanks!

The car

The displaced alarm siren after fitment of the cruise control actuator (just to the right of the siren in the picture)

The battery that came out of the siren unit

[Benni]

They do exist, but the chart shows only bigger amps at 7V :

http://www.accu-italia.com/prodotti/prodotti-custo...

There seems to be a "contact us" button, the number on the accu was not found in their system.

[Murph7355]

Handsome car.

Might it be worth buying the whole siren unit until you find some way of replacing the battery?

~£30:

https://www.ebay.co.uk/itm/304181274409?var=0&...

You might end up needing to make up a battery or using a "best fit"...I doubt the capacity is an issue, just the voltage, size and material.

[RicksAlfas]

Scratch this.

[rxe]

Capacity is not an issue. Doesn’t matter whether you have a 10 Ah battery or a 250 mAh battery, the only thing it is interested in is the voltage.

If you’re so inclined - buy a NiCd PP3 battery and carefully cut the case open. You will find 7 cells in there, giving 8.4 V. Remove 1 and solder some legs to the remaining pack. You’ll then have a 7.2v pack at about 400 mAh and about the right size.

[Tye Green]

if a pp3 fits use a 9v pp3 with series connected diodes - each diode will lose about 0.7v so 2 in series will drop 1.4v. however many you use, you need the same quantity again but series wired in reverse to the other set so that you charge the battery from a pp3 9v charger.

[LunarOne]

Murph7355 said:

Handsome car.

Might it be worth buying the whole siren unit until you find some way of replacing the battery?

~£30:

https://www.ebay.co.uk/itm/304181274409?var=0&...

You might end up needing to make up a battery or using a "best fit"...I doubt the capacity is an issue, just the voltage, size and material.

The thing about buying another one is that it will be about the same age and there's no way the internal NiCd battery will still be good, so I'd be no better off than I am now, but then need to redo the wiring to wherever it goes - I think the alarm control unit is under the rear seat.

[LunarOne]

Benni said:

They do exist, but the chart shows only bigger amps at 7V :

http://www.accu-italia.com/prodotti/prodotti-custo...

There seems to be a "contact us" button, the number on the accu was not found in their system.

Thanks Benni - I'll fire off an email!

[LunarOne]

Tye Green said:

if a pp3 fits use a 9v pp3 with series connected diodes - each diode will lose about 0.7v so 2 in series will drop 1.4v. however many you use, you need the same quantity again but series wired in reverse to the other set so that you charge the battery from a pp3 9v charger.

Kind of like a bridge rectifier, but without the AC to DC conversion - clever!

[LunarOne]

RicksAlfas said:

Scratch this.

I scratched it. Does it feel better?

[LunarOne]

rxe said:

Capacity is not an issue. Doesn’t matter whether you have a 10 Ah battery or a 250 mAh battery, the only thing it is interested in is the voltage.

If you’re so inclined - buy a NiCd PP3 battery and carefully cut the case open. You will find 7 cells in there, giving 8.4 V. Remove 1 and solder some legs to the remaining pack. You’ll then have a 7.2v pack at about 400 mAh and about the right size.

I'm aware that the capacity is largely irrelevant, apart from it being related to the size of the battery. I really like your idea and it might actually fit. I was thinking about trying to stack a bunch of coin-type cells but your idea seems much easier and better. Are the cells in a rechargeable PP3 rectangular?

Thanks!

[CharlesElliott]

Finding a 7.2V NiCd battery is really easy, there are 100s of them. Now, finding one of the right phyiscal size is a bit harder. How much space is there in the box?

[LunarOne]

CharlesElliott said:

Finding a 7.2V NiCd battery is really easy, there are 100s of them. Now, finding one of the right phyiscal size is a bit harder. How much space is there in the box?

It's difficult to say. The speaker takes up most of the space, then there's the circuit board that fits vertically between some locating grooves in the case. When the battery is fitted, to the board, it only just fits into the slot as the battery practically touches the speaker. Something the same volume but squarer would physically fit in the box more easily, but it wouldn't fit on the board directly as there's only a thin strip corresponding to the edge of the cylinder clear of other components like resistors. Stupidly, I didn't take a photo of the inside of the alarm siren (it was getting dark and I just wanted to complete the cruise control install and pull out the battery as quickly as I could while I had the siren out of its mounting bracket. I'm sure a mini flat pack or rectangular battery that I could hot-glue and connect with wires if I can't find a cylindrical battery would be fine. It's just occurred to me to ask a model shop. The kind where big boys buy R/C model aeroplanes! Maybe they've all moved over to LiPo now though...

[Skyrocket21]

You need to throw out the dimensions, there's similar alarm batteries for houses

GP250BVH6AMX
https://www.wkhuloos.com/index.php?main_page=produ...

Different size
GP320BVH6A6
https://www.buyabattery.co.uk/alarm-bell-box-batte...

https://www.ebay.co.uk/itm/153472705686?hash=item2...

Obviously don't know the size, would need some soldering.

[LunarOne]

Thanks Skyrocket that’s really useful. I wonder why they say it’s acceptable to replace NiCd with another chemistry. Everything I’ve read says it’s a bad idea with problems ranging from shortening of battery life to overheating and swelling potentially causing a fire. I doubt the circuitry in the siren is designed to manage battery charging properly, relying on NiCad battery tolerance for being charged continuously.

[shedweller]

I would give overlander batteries a call for advice on wether NiMH chemistry is suitable - I would say it will be fine in your case as the charge rate will be very low.

Overlander are knowledgeable and can make custom battery packs.

https://www.overlander.co.uk/

[LunarOne]

shedweller said:

I would give overlander batteries a call for advice on wether NiMH chemistry is suitable - I would say it will be fine in your case as the charge rate will be very low.

Overlander are knowledgeable and can make custom battery packs.

https://www.overlander.co.uk/

Thanks for this - I'll give them a go!