Two leisure batteries or larger alternator?

[albertramsbottom]

Hi

I have recently been planning to fit two 6 volt Trojan Deep cycle batteries for my interoir electrics. However, I have since started to think that the extra weight in the van wouldnt be worth it so I am now thinking of going down the more powerful alternator route.

Can anyone tell me how many amps is the standard alternator on a 2ltr air-cooled van and possibly what size I could upgrade this too.

Also are there any figures or ways to work out how long one would need to run the engine with a given alternator ampage to charge a given battery size. i have been told that with a 65amp alternator 15 mins of engine should be enought to charge a 60amp hour battery from 50% to full.

Any advice on thsi matter would be great

Cheers

[Davy MacLean]

Standard alternator for yours is 45amp I'm fitting a 90amp one to mine and plan to have 2 batteries for power in the van

[Grumpy Midget]

Hi Albert, Standard size alternator on a 2Ltr CU is 45amp. - A 65amp will be ok for powering most electrics and charging your leisure battery an running a fridge (on 12v only when the engine is running) You can fit a 90amp but I would only fit the 90amp if I was running a high powered sterio system that would take alot of output power, then the bigger alternator would put back, without kiling it, a 65amp would be ok.
(2) I would have thought about 15mls of driving to put back into a battery what has been taken out after cranking. I run a Diesel with 2 leisure batterys plus an Engine battery and my 65amp Alternator dont strugle, Hope this info is of help to your questions.----------

[ambivert]

Yes, the more current that flows into a discharged battery, the less time it will need to become fully charged.

BUT you can't just replace your alternator with a higher current one and expect your battery to charge faster.
This is because the typical max. current ouput of an alternator (eg. 50 to 100 amps) is usually MUCH higher that the typical current that a battery takes when it is charging (eg. 1 to 8 amps).

And that is because a battery has a certain resistance to being charged, and that resistance depends on how charged up it already is. Think of Mr Creosote, the more he eats, the fuller he gets, the more pressure builds up in his belly, the harder it becomes to shove any more in, cos you're fighting against the pressure inside.

So whilst a TOTALLY flat battery might in theory accept a huge current for, say, a minute... after that time, it will be slightly charged and have its own voltage which pushes against the charging voltage. A battery will only accept charge if the charging voltage is higher than the battery's own voltage. The bigger the difference, the more current can flow. As the battery charges up, its own voltage increases, so (unless the charging voltage also increases), the current dwindles. The amount a battery's own voltage increases as it charges up is highly specific to each type of battery (but in general, the MORE capacity a battery has, the LESS its voltage will increase as it charges, so the MORE charging current it will take, but the MORE time it will need to reach a useful output voltage, ie. to charge up).

Most alternators have some kind of clever circuitry in them, so that they can adjust the charging voltage a bit, but they don't just keep upping the voltage as high as they can in order to force more current in as quickly as possible, because the battery heats up as it charges, too much and it will explode. Thinking of sick flying across a restaurant now...

Some types of battery are designed with high current input and output in mind, they are fast to charge up and also fast to run out (the typical car starter battery) and once discharged, they need to be charged up again soon or the plates buckle and they become useless. Others have much higher capacity and take longer to run out but they also take longer to charge up, they are typically are actually described as leisure batteries and can often handle being left uncharged for a longer time (perhaps because the plates are thicker?). I too have been thinking about what sort of battery to put in, and would probably go for one of the latter, and carry a mains charger, so I can also charge it from electrical hookup when camping (rather than expect my alternator to charge the whole thing).

[Matt The Hat]

Do most folk happily run a leisure battery from the standard alternator without any issues?

[Titus A Duxass]

Do most folk happily run a leisure battery from the standard alternator without any issues?

I have been doing just that for the last 5 or 6 years without problem.
Standard in my case being 65amp.

[bigherb]

As ambivert says a battery will only take a bulk charge for a short time. Alternator output is measured at it's normal running temperature, a cold alternator can exceed it's rated output by 10% which compensates for the bulk charge time. A larger alternator will not charge a battery any quicker. A larger alternator would only be needed if there are extra electrical consumers fitted which when switched on continuously when the engine is running exceed the alternator output.

[albertramsbottom]

Excellent advice!

So a battery will only charge according to its charge rate, so I would only need a larger alternator if i was running heavy drain electronics whilst the vehicle is running.

Two leisure batteries are in order then

Thanks great advice