Someone check my math on my amp draw?

Here's what we've got... I've decided to put a dedicated battery bank for my stereo totally seperate from my charging/electrical system and I've trying to figure out how many batteries and size, etc. Ok my amps are putting out close to 2400W at 14.4V. Even though I know the amps aren't putting out that the majority of the time, we'll just use those numbers to be safe and we'll use 12.6V to be safe as well. So 2400W at 12.6V is 190amps. Is this correct?

The sales rep from Optima was trying to tell me I'm only using 20amps and only need 3 of their D31A batteries if I want to run my stereo for 8 hours. That'd be nice if that was true, but something tells me that's not right. So am I right on my amperage draw?

Power (W) = (I^2)(R)

or W = IV

2400W / 12.6V = 190.5A (I)

Your math is right. I don't know about the batteries, and their Amp Hours.

Thanks Rob, he had me thinking I was crazy and had somehow got my math all screwed up. I mean it's been a while since I took physics but I Didn't think I was that screwed up in my math.

Now it's just a matter of finding some batteries that will put out 200 amps/hour for 8 hours without having to spend over $1300.

Anyone know much about the Trojan DC-500ML's? In particular retail price?

I use the trojan t-105 golf carts batts. is the 2400 watta RMS or peak? your amp draw is not linear, the formula above assumes all variables are constant, which is not true on your boat.

That's RMS .... it's actually like 2340 BUT I was playing safe. And yes I realize that I'm not really pulling close to that very often, if at all even though it is RMS. How many are you running adam and what's your set-up?

I should add that what I'm trying to do here is keep from adding a larger alternator. Although that seems like it'd be the cheaper thing to do, there are days on holiday weekends when the lakes are busy and choppy, where we sit for 6-7 hours at a time and just listen to music and party back in the coves. I hate having to turn my boat on and run it for a while when doing this. I just get scared that I'm gonna run down my deep cycles to low for my alternator and fry it. That's why I'm wanting a dedicated battery bank that I can just charge at the end of the night when the boat is out of the water.

Tanner we haev similar sized systems. I have two 6 v t-105's. I can run 8 hours ( fairl loud) sitting w/ the motor off and my batteries will drop to about 50%, on the drive back to the dock and try and keep the volumes lower than normal on the stereo so I dont; run in to any potential overheating of the alt. or worse burn it out, I do have an upgraded 105 amp alt.

adam, i like your setup - do you have a schematic drawn out? How are you charging the cart batteries?

no schematic, I charge the GC battereis with a 7 amp trickle charger/battery tender and the startign battery with a 1.5 amp trickle charger/battery tender. I have them attached with qucik disconnects, but they usually just stay in the boat.

So you charge them with a battery tender overnight?
Do you run them in series then to get 12V?

I charge witht he battery tenders overnite, the 7 amp will not recharge the GC batts 100% if they have been run down to 50%. I have a bigger charger that will charge them if I need them back up to 100% in a short time.

they are run in series

Tanner:

You would need about 24 golf cart batteries, or about 36 Opitma batteries to provide 200 amps for 8 hours. It would certainly solve any need for ballast, but I doubt that you want to do that.


The circuit breaker/fuse and wiring between the battery and amps need to be sized for the 195 amp peak. The draw on the batteries will be dramatically less.

Unless you are simply being a jerk and trying to outblast everyone else within a mile of your boat I don't see why you would need to run 2400 watts of power when the engine isn't running.

Your average power draw out of the batteries, at full volume, is probably like 1/3 of peak, or 65 amps. Turn the volume down to 1/2 and your power drops another 1/3, or about 20 amps. An Optima battery should be able to handle that for one to two hours.

Why not just use the switch+isolator system? That way if you are wakeboarding you can be charging your stereo battery at the same time. The isolator will take the excess strain off the alternator and the switch will allow you to make sure you can startup.

I would seriously doubt you push anywhere near 2400 watts at any given time. You shouldn't be or need to be maxing out any of the components you're using. I would bet you're closer to 20 amps than 95.

Rod,

Actually in the cove that we go to usually everyone that we're tied up to asks me to turn on my stereo and "blast it"... of course I take requests and give people access to my ipod. But in a cove w/ 500 boats you kind of have to turn your music up just so you can hear it inside your own boat over the other stereo's.

I think I've narrowed down to either three of the Trojan DC-500ML's, 3 Crown CR-400, or 6 of the Trojan L16H

Brad,

I already have a switch isolator system, the problem w/ that is worried about killing my alternator b/c of running down my deep cycles to far. I don't like having to turn my boat on and let it run for 20-30 mins and gassing out all my friends swimming back behind my boat.

Watts divided by volts equals amps. 2400 watts divided by 12.6 equals 190.4 amps.