Alternators big enough for dual battery?

[Old.Tony]

Wow. Let's clear something up here.

Any size alternator is good enough, as long as it provides enough power to:

* recharge the starter battery
* operate all of the electrics while at full load (high beam, night, wipers on)
* some spare capacity for the auxiliary battery.

The thing with the aux battery isn't whether or not you have a 6 gazillion amp alternator. It only needs SPARE capacity.

The D22 alternator is typically about 90A (there are variations, some made by Mishitibus, some Hitachi). If everything running uses about 40-50A, there's plenty of capacity to charge an auxiliary battery.

That's the important aspect - spare capacity. If you've got a lot of driving lights or other loads, that capacity won't be there.

I like the cabling you're using, that's going to nearly eliminate voltage drop. If you're just using an isolator, that's important. If you're planning on inverting the power and using a charger, then maybe you could go lighter - but if you already have it, may as well use it!

 

[Old.Tony]

The rumour is alternators wont charge batteries properly but then you admit that in under an hour both batteries could be fully charged?

So the rumour is true but the batteries charge anyway, seems contradiction is king on the internet.

Not quite. The alternator will charge the SLI battery up to its normal state of charge (around 75%) within that time, which is what I think they were saying.

For an auxiliary battery, this is not good. Since you should never discharge a battery below a certain level (flooded = 50%, gel/AGM = 20%) you're reducing your available capacity. For a flooded battery of (say) 105Ah of reserve capacity (RC = 105) you only have 0.75*105-0.5*105= 26.5Ah of capacity instead of about 52Ah.

That's why I use an inverter and a good charger - I get 100% charge in my auxiliary battery. At the moment it's a dodgy gel and I couldn't give a flying firetruck about it, since although it's rated at 100Ah I'm only getting about 15Ah out of it. I'm about to throw a 50Ah flooded back in (that's what was in there before) but next time Rays have a 25% off I'm grabbing another 160Ah AGM spiral wound. I can use 80% of that capacity = 128Ah. Just powering my Engel 1.5A * 12hrs per day = 18A/day or 7 days before I need to worry about it.

As for charging times ... my 7A C-Tek will take 128/7 = 18 hours to charge the battery fully. Since it will never get discharged that far anyway (at most I'm idle for 3 days - 18*3 = 54A = 8 hours to charge) by the time I next stop the battery's full again.

 

[Old.Tony]

I had a spare wet cell of 550cca I think. That now resides in the tub & runs my fridge. When it is shagged I will buy another. Maybe a deep cycle although most probably not.

No intention of running the fridge for days without using the car so it always has enough charge for my needs.

Have I done something wrong?

Joe, don't count the CCA. Look at the RC value - that's "Reserve Capacity" and is roughly equal to the Ah capacity that you can draw from it before the battery is considered terminally dead (I think it's 10.75v). Discharging to this level will kill the battery really fast.

Starter batteries don't like being discharged, period. Using them as a deep cycle is generally terminal for them - they're not built for it. They are designed for a short high-current discharge followed by a recharge to its nominal level (75% of capacity). Deep discharges will damage the plates.

However, you can minimise the damage. Charge the battery to 100% capacity, keeping it topped up with fluid, and don't discharge below 50% and it shouldn't suffer too badly. Once every couple of months hook it up to a C-Tek charger and press the "Recycle" button.

 

[Krankin]

Wow.


a 6 gazillion amp alternator.

I want one of those.....who sells em...gimmie gimmie gimmie

 

[joe2006]

Joe, don't count the CCA. Look at the RC value - that's "Reserve Capacity" and is roughly equal to the Ah capacity that you can draw from it before the battery is considered terminally dead (I think it's 10.75v). Discharging to this level will kill the battery really fast.

Starter batteries don't like being discharged, period. Using them as a deep cycle is generally terminal for them - they're not built for it. They are designed for a short high-current discharge followed by a recharge to its nominal level (75% of capacity). Deep discharges will damage the plates.

However, you can minimise the damage. Charge the battery to 100% capacity, keeping it topped up with fluid, and don't discharge below 50% and it shouldn't suffer too badly. Once every couple of months hook it up to a C-Tek charger and press the "Recycle" button.

Yeah, nah, yeah, but...

I was just demonstrating it was a starter battery and because it was a spare I had nothing to loose putting it in the tub. To date it has done a sterling job & fills my requirements.

No point spending $$ if the battery I already have is just sitting there deteriorating. Besides if I buy yet another car & might end up with another spare battery.

 

[Old.Tony]

No argument there Joe, if you can save a few quid then go for it!

 

[KraftyPg]

Geez I am glad we cleared all that up.