Manipulating 12 volts DC - 8V to 12V and 12V to 8V


I have two challenges and I am hoping to get some advice from the experts in this forum.

I run a Ford jeep for Anzac Day. It runs off an 8V DC car battery. I have bought a vintage siren for the jeep however it is 12V DC. I am unsure of the Amps. The siren, which sounds like a small air-raid siren, is powered by a positive wire. The metal base of the siren is the ground.

Problem 1: I want to connect a momentary switch to it as most of the time the siren button is depressed for a few seconds and then you stop it. Someone suggested a boost connector with an in-line fuse to boost the voltage.

I am after some advice on how to make this work please.

Problem 2: Because I am running my jeep on 8V, there is no charger to charge the battery. They are usually 6V or 12V. Because the vehicle is only used a few times a year, the battery goes flat so I would like to trickle charge it while it remains in the garage.

I have a CTEC 12V 4A trickle charger. I am wondering if there is a way to use use this charger with some kind of converter that I can connect to it which would reduce the voltage to 8V DC to charge the battery and possibly keep the amps around 4A? I’m told that the chargers are so smart that they would detect anything connected to it between a battery and therefore it would not charge.

Your help would be greatly appreciated.

Thanks Jim

Hi Jim,

Have you measured the amps being drawn by the siren? I doubt it would be more than a few hundred mA, and on that assumption, you could use something like this to create a handy 12V5A power rail.

For the charger, DIY Trickle Charger circuits are fairly easy to find. You should be able to reverse engineer something from one of those, it may end up using a variable linear regulator or the like ($3 part).

Hi Graham

I am yet to measure the Amps so once I do I will come back to you.

In relation to the DIY trickle charge, where do I find these?

Regards Jim

Some light Googling should get you to some simple enough options.

If you had a bench power supply, the battery could be charged by dropping the amps right down to something low such as 100mA, though any fixed voltage output system would mandate periodic temperature testing (by hand or a measured means) to ensure the battery is not going into thermal runaway. And of course, charging batteries create hydrogen, so do it in a ventilated area to avoid blowing something up.

It might be worth getting in touch with the manufacturer of the battery, often they are well connected with the industry for the products they sell and can help connect you with people that can help.