Rechargeable 12v battery advice please.

[JennyMorgan]

Right, must be honest, yours truly is as thick as two planks when it comes to iron donkeys and electricity. I have a Mercury outboard that charges batteries at 2.98 kilowatts. I really don't want to lash out on a leisure battery just to power a navigation light and a fish-finder. Would one of these do the job and could it be recharged underway by my engine?
http://www.screwfix.com/p/sealed-lea...3-x-97mm/63554
All advice & alternative suggestions will be appreciated!

Edited August 10, 2015 by JennyMorgan

[Baitrunner]

What about getting a leisure battery from someone who is changing their batteries? I changed mine last year as they were not 100% but would have powered your fish finder and nav lights all day and night. 

If I had known you could have had all 5! I  sure they charged me for recycling them. 

In answer to your question I don't see why your idea wouldnt work. The charging value seems odd though in KW?

[JawsOrca]

HI Peter, I'm no expert in this.. but I guess your outboard would overcharge that and blow it great yarmouth.  Although that should power your nav lights (if they are LED) I don't quite think it would keep your fishfinder going for too long but you could take it out and charge at home with a suitable charger.

You should be able to get a small car battery from Euro car parts (theres one in lowestoft or there's a motor factors behind halfords at oulton broad) for a about the same price (but it wont last for ever), You certainly don't need a leisure battery. http://www.eurocarparts.com/063-car-batteries

[Hockham Admiral]

Right, must be honest, yours truly is as thick as two planks when it comes to iron donkeys and electricity. I have a Mercury outboard that charges batteries at 2.98 kilowatts. I really don't want to lash out on a leisure battery just to power a navigation light and a fish-finder. Would one of these do the job and could it be recharged underway by my engine?
http://www.screwfix.com/p/sealed-lea...3-x-97mm/63554
All advice & alternative suggestions will be appreciated!

Let's call it 3 kW, Peter.. for simplicity.... at 12V that's 3000/12 = 250 AMPS..... I can't help you if that figure is anywhere near correct, sorry.  

[kfurbank]

Peter,

Alternator's are normally quoted in amps and not kilowatts, however as John has worked out, that figure cannot be correct because most boats with full inboard engine would normally have somewhere around a 60amp alternator.

However to answer your question about the battery from Screwfix, it will not be suitable unless you can recharge it from a different current limited suitable source. It's a 1.2Ah battery which if 50% discharged would need 600mA (milliamps) to recharge it. If completed over say three to four hours you would need a charging source limited to around 150-200mA. An alternator is designed to give as near as possible its full output when it senses discharged batteries and then gradually turns off as the batteries become fully charged. Even if your outboard had a 10A alternator it would be way to powerful for that battery and attempt to recharge it way too fast.

My boat has 3 x 100Ah batteries on the 12V side. With a 60A alternator this means that if the batteries are quite discharged each battery could only receive 20A, a figure somewhat smaller than their overall capacity of 110Ah. With the Screwfix battery your alternator output would be many times bigger than the battery capacity.

Due to the way alternators work, I would assume that if the battery was always full, or very nearly full it would be ok., but the first time you discharged the battery heavily and then started the engine would probably result in the death of the battery. Overcharging or recharging batteries too fast can lead to them exploding.

As Alan has said, go for a cheap car battery, or if you can find some other figures for the alternator post them back here. 

The other alternative would be a small solar panel charger and the Screwfix battery and never recharge it from the alternator, but by the time you add in the cost of the solar panel you might as well buy a small car battery.

 

 

Edited August 11, 2015 by kfurbank

[Jonathan]

A couple of thoughts to add to kfurbank's reply:  If the navigation light is 12W then it will use about 1A, so that screwfix battery would last about 1.2 hours.  If the fishfinder uses another amp (probably less) then the battery will be drained after 0.6 hours.   The 7Ah version of the battery would last six times longer. These sealed lead-acid batteries require careful charging and there are a lot of 240V chargers available, or solar chargers, because these batteries are often used in burglar alarms or UPS supplies for computers.  However, you will need something to "regulate" the output of the engine alternator to stop it from cooking the battery.   I agree that a cheap car or motorcycle battery having a much greater capacity than 1.2 Ah is probably the way to go.  It will be much more tolerant of erratic charging too.  The 2.98 kW mentioned is 4 hp, so perhaps that is the engine output?

Finally:  Although it would be more "hassle"; you could have one or two 7Ah batteries and take them home to charge them from 240V. .That would give you about 6 hours of use.

[JennyMorgan]

From the sparse Mercury website:

Alternator amp / Watt	4 lighting / 2 charging amp / 50 lighting / 25 charging watt By the way, thanks for the info todate.

Would this do?

http://www.eurocarparts.com/ecp/p/accessories-car-batteries/car-batteries/other-batteries/motorcycle-batteries/?445770171&0&cc5_989

Edited August 11, 2015 by JennyMorgan

[Jonathan]

That says to me that your alternator can put out 25 Watts which is almost 2A at the 13+V needed to charge a typical lead-acid battery.

[Hockham Admiral]

From the sparse Mercury website:

Alternator amp / Watt	4 lighting / 2 charging amp / 50 lighting / 25 charging watt By the way, thanks for the info todate.

Would this do?

http://www.eurocarparts.com/ecp/p/accessories-car-batteries/car-batteries/other-batteries/motorcycle-batteries/?445770171&0&cc5_989

The problem with Lithium Ion batteries is that they can overheat dramatically if overcharged... see below:

 

http://www.scientificamerican.com/article/how-lithium-ion-batteries-grounded-the-dreamliner/

[kfurbank]

Lithium ion batteries are not generally found on motorbikes, certainly not at that price. Lion, in this case,  is a brand of battery, as opposed to the method of construction.

The website is rather poor as it doesn't give the capacity of the battery, but a look at the picture shows it as a 4Ah battery. Assuming, (assumptions can be dangerous) that Jonathon's suggestion that the alternator is a 2A one is correct, then that battery would be fine.

 

[JennyMorgan]

Thanks folks, really appreciated. As space is an issue, as is portability, the motor bike battery option seems to be the one for me. If it all goes pear shaped then I know who to blame! 

[ChrisB]

It must be secured so as not to move more than 10mm in any direction, I failed my last BSS on that last time, mine could move vertically!  My old Seafarer echo sounder would take a 9v lantern battery, Navisafe make battery nav lights that last up to 50 hours. I would like to see more equipment for small boats that does not require hard wiring.