ALRM BATTERY BACKUP

[fiddle]

If I fitted a 12v domestic alarm battery to provide a memory supply to my car stereo which is fitted to my boat, and the battery was connected to the ground (of course) and my 1 2 both switch, so that when I left the boat the alarm battery could do its bit, yet when I am on the boat the boats batteries could recharge the small alarm battery, would I have any problems?

 

[Joe_Cole]

I think that the alarm batteries have a different charging "profile" and will be damaged, or a least short lived, if you connect up to an alternator.

In any event are you sure you need to do this? The power consumption for the memory of most car radios is very low indeed. (You can go on holiday for 2 or 3 weeks and the battery will still start the car on your return)

 

[VicMallows]

I cannot see any problem, except that perhaps you should add a diode into the circuit so that you do not discharge the small battery when using the other electrics on the boat. I too prefer to totaly isolate the boat batteries when I leave. (I even have a seperate battery just for the automatic bilge pump). But I find it no effort to reprogram the car radio each visit.

Vic

 

[LadyInBed]

Yes, a diode in the circuit will stop the small battery discharging to the main circuit.
On my last boat I used s similar arrangement with a small 12v gel battery and a rectifier diode to connect to the autohelm and gps as they reset when I started the engine.

 

[bonny]

Firstly, The alarm battery would not be damaged by the alternator. The alternator voltage regulator will limit the voltage to 13.8 - 14.2V and most of the charge current would be going into your normal batteries.

Secondly, using a diode to prevent the small alarm battery discharging into other boat circuitry would be a must. Unfortunately this creates another problem in that your alarm battery would not fully re-charge due to the forward volt drop of the diode. For a typical silicon diode this will be around 0.7V.

Use a "Schottky rectifier diode" since these have a lower Vf (forward voltage drop). You can get a 1N5821 from Maplin (pt no JA49D) for 24p which should do the trick. It has a max continuous forward current of 3A which should be sufficient for a small alarm battery (or use 2 of them in parallel for 6A) and has a Vf of 0.5V @ 3A which reduces to 0.38V @ 1A and typically reduces to 0.25V at 150mA.

 

[William_H]

It is not clear from yo9ur post if the alarm battery back up battery is a Lithium dry cell or if it is a sealed lead acid which you would wish to recharge. Most car radios have a separate wire for memory supply which goes to battery on all the ti9me and another positive supply which goes via the key switch to actually run the radio. So you could simply connect a dry battery to the memory wire and see how long it lasts. My experience on an old car radio was that the memory current was appreciable and actually soaked up all my 2watt solar panel could provide. Modern radios may be better Now if you use a lead acid rechargeable then connect a diode as described from the supply from your battery switch to the memory power wire (anode to battery switch cathode (end with the band) to the memory power wire which is also connected to the lead acid memory battery. This will charge the memory battery when the ships power is turned on. However I would suggest you connect a 47 ohm 3 watt resistor on the battery positive terminal between it and the diode and radio. This will act as fuse (make sure it can get hot without igniting anything) and will limit the discharge current in the case of a fault to 1/4 amp and will limit the charging current to a lot less than that so the sealed lead acid battery is never hit with excess charging current. A lower value around 20 ohms will permit more charging current while highe value will give more protectionj but less charging current. (when engine is running) regards will

 

[Benbow]

This is good advice - a diode would be essential and a current limiting resistor highly desirable.

The alternator certainly can damage these batteries. Details of their charging limits are on the rs website and they are very different from your main batteries both in terms of max voltage and max current. If ever it became significantly discharged it could be thoroughly zapped by the charging current.

However, these batteries do have amazingly low self-discharge rates. I kept one on the boat as an emergency source of 12V totally independent of the main system. I just charged it at home at the start of the season and again at the end. I would bet that one would easily maintain your radio memory with hardly any re-charging.

On the other hand, why not take power direct from the 'battery' side of your main battery switch? As long as it is protected by a correctly sized fuse well secured and right next to the switch it is probably safer than the other systems discussed here.

On the other-other hand, as was said above, many radios will retain their memory with no external power for a long time.

 

[boatmike]

Just a thought, but on my own boat I have a seperate very small 12v motorcycle battery that maintains power to auto bilge pumps, radio, intruder alarm and a few other things that I like to leave on when away. It's not a complicated set up. I simply change over to this battery on a rotary switch before switching off main battery banks before leaving boat. In my case I also switch solar panels on to this battery if leaving the boat for any length of time but this is optional.

 

[Ruffles]

fiddle,

Might be easier to get a newer radio. New ones don't seem to use a PIN number - rely on a removable panel instead.

You can get a CD/radio from Argos from £50.

Rob.

 

[pvb]

Check the \"permanent live\" first...

There seems to an assumption that the "permanent live" connection on a typical car stereo only powers the memory circuit. This usually is not the case - the permanent live usually also supplies the main power that the unit needs to operate. The "switched live" connection simply activates the unit. Because of this, adding a little battery to give the "permanent live" power wouldn't work.

In fact, there's no need for a separate battery anyway. Just connect the permanent live via a suitable fuse (with the fuse as close to the battery as possible) and there's no real problem unless the boat is left unattended for a very long time.

 

[bruce]

there is a product on market over here that will alarm when battery gets into area where starting may be a problem, automatic cut off of all gagets running off battery leaving enough energy to start motor.