Outer Limits 4x4 Board

I just got a Waeco so i'm doing some sums.

My car Battery has a 160minute RC rating. Apparently the RC rating is the time in minutes the battery would last with a 25amp load put on it.

So is it right for me to say that my 160mins RC = 2.66hours X 25amps......= 66A/H

Apparently my waeco will draw on average 1 amp per hour over a 24 hour period, so my single battery after 24 hours would still be around 65% full.

I would like to hear ppls commenst on what i wrote......bored at work today.

I think RC also means to a quite low discharged voltage too, not just the current (IIRC about 10.4 volts, too low for the waeco to operate)

Other issue is the battery you have would be a cranking battery, which by their construction are not really suited to low current drains/long periods.

You will only have about half the total AH capacity as you calculated (33 AH), as cranking batteries and most wet deep cycle batteries do not like being run dead flat (dead in 5-10 cycles). Some of the AGM battries are able to cope with 60-70% discharge for many cycles, as well as act as both cranker and deep cycle ("hybrid").

Safety sake, get a second battery.

Nothing wrong with the maths (apart from being closer to the safe recovery point of the battery than you realised) but the figures/guestimates were out.

Waecos generally use about 2-2.5amps when fully loaded, and this would put you close to stuffing you battery in 24-30 hours (more so if it is cranker.

At the very least get yourself one of those jump packs to start your vehicle.

Worst case 2.5 amps x 24 Hours = 60 AH, = one stuffed battery in you next 5 trips.

I agree with Damkia - on paper, you're OK, but in reality, Murphy's a bastard!

On a technical level, the Amps x Time figure depends on the Amps. At higher currents, more energy is lost to internal resistance, so it's not a linear relationship. Quoted AH ratings are typically based around a 20 hour discharge - so a 20AH battery will (theoretically) deliver 1A for 20 hours, but it WILL NOT deliver 20A for 1 hour - at a (wild) guess, 40 minutes is more likely.

So, your 66AH figure would be below the manufacturer's claimed capacity, which is an error working in your favour. But, your firdge's "average 1A over 24 hours" won't take into account continual opening and closing of the fridge, or replacing cold beers with warm ones.

Also, your charging system won't necessarily fully recharge your battery in a couple of hours driving - so, unless you're an avid tourer who drives long distances every day, you may find your battery is being discharged lower than your "calculated" level - i.e you're operating between 40% and 80%, not 60% and 100%. So you'll kill your battery faster.

For one or two trips a year, you may be OK - batteries don't often last much longer than a few years anyway. But you may get caught out on your third or fourth trip. Travel with friends, and pack jumper cables.

Cheers guys.