battery calculator

[Kross]

i don't want to be a battery expert, I just want to know how to calculate how many hours run time I will get, and which battery i should buy to run my stereo all day.

two 600-watt amps, running 12 speakers = 1,200 watts
I have a starting battery already, so I only need deep-cycle battery to run the stereo. So, which battery setup works best? I can get 2 golfcart batteries at 6volts, hook them together for 12-volts. Or, maybe an SRM29 battery?

I've googled this and gone crosseyed at the calculations. Does anyone have a simple explanation of how to calculate how long battery will last, or what i should get? thanks

- Kross

 

[Bruce]

What kind of amps are they?

Wattage = Voltage x Amperage.

At 12 volts 1,200 watts would be 100 amps.

Lead Acid and AGM batteries are damaged with discharged below 50%.

A group 31 battery has around a 100 Ah (amp hour) capacity. You can use half of that before you damage the battery.

If your amps actually drew a constant 1,200 watts they would use all usable power in a group 31 battery in 30 minutes. But music is not a linear load the volume goes up and down frequently and most manufacturers overrate their amplifiers.

If your amplifiers actually consume 600 watts I would guess that you could play for around 2 hours on a group 31 battery near max volume.

 

[Kross]

What kind of amps are they?

JL Audio M600/6

 

[Kross]

A group 31 battery has around a 100 Ah (amp hour) capacity. You can use half of that before you damage the battery.
If your amps actually drew a constant 1,200 watts they would use all usable power in a group 31 battery in 30 minutes. But music is not a linear load the volume goes up and down frequently and most manufacturers overrate their amplifiers.
If your amplifiers actually consume 600 watts I would guess that you could play for around 2 hours on a group 31 battery near max volume.

great info thx. What are battery "groups"? Can I go for a different group that has more power? Or should I just add an additional group31 battery and wire them together? Ideally I'd like 6hour play time.
Also, whats your math to get to 30minutes play time? 100 amps on a 100ah battery = 1 hour. So 30 minutes if always peaked out, and only using half of the battery to avoid damage. Just a guess to go from 30min to 2 hours? thx for ur help here

- Kross

 

[Bruce]

JL Audio M600/6

Those JL amps are the real deal. If they say 600 watts they can consume more. Although that 600 watts is at 3 ohms. If your speakers are 4 ohms they draw less if the your speakers are 2 ohms they will draw more.

What are battery "groups"?

Batteries sizes are expressed as groups. Group 31 is largest common size. Most of our boats come with smaller group 24 or 27 batteries. Group 24 is around 70 Ah.

Also, whats your math to get to 30minutes play time? 100 amps on a 100ah battery = 1 hour

If you use more than 50% of the battery you will soon have to buy a new one.

Just a guess to go from 30min to 2 hours?

It is a guess that the amps are capable of drawing 100 amps but the music will likely consume closer to 25% of that.

Some guys with big systems have 3 or 4 batteries just for the their stereo.

 

[Bruce]

If you want a longer run time go buy two 6 volt golf cart batteries and wire them in series. Same and Costco stock them.

For example two of these will cost less than a group 31 marine deep cycle and run your stereo longer https://www.samsclub.com/sams/durac...rod3590228.ip?xid=plp3990105-auto:product:1:1

They also double as ballast.

 

[Kross]

If you want a longer run time go buy two 6 volt golf cart batteries and wire them in series. Same and Costco stock them.

will do! I'm curious, what would be the downside of running the 2, 6volt golf cart batteries in series? I don't need any starting power, i have another battery for that. Is there any other downside besides no starting power?

 

[Bruce]

The two 6 volt golf cart batteries in series produce 12 volts just like a single 12 volt battery. When charging you would charge from the positive on one battery to the negative on the other.

When batteries are wired in series the voltage is additive and the amperage remains the same when wired in parallel the voltage remains the same but the amperage doubles.

 

[Kross]

The two 6 volt golf cart batteries in series produce 12 volts just like a single 12 volt battery. When charging you would charge from the positive on one battery to the negative on the other.

When batteries are wired in series the voltage is additive and the amperage remains the same when wired in parallel the voltage remains the same but the amperage doubles.

thanks. Any downside to using the golf cart batteries over others you suggested (group 31 etc)?

 

[Bruce]

thanks. Any downside to using the golf cart batteries over others you suggested (group 31 etc)?

Twice the capacity for a lower cost is the upside. You might compare weight as a downside. I use golf cart batteries to power a remote tower for a network connection.

 

[swatski]

thanks. Any downside to using the golf cart batteries over others you suggested (group 31 etc)?

Great info @Bruce !
@Kross the one disadvantage I can think of is maintenance - those 6v batteries are not sealed and need to be watered up which - in a boat - can be a hassle.

--

 

[David Analog]

Forget the calculations. A DC equation doesn't translate to an AC (music) application. There are dozens of other variables too. System design. System tuning. EQ. Type of music. Amplifier efficiency at a) different volume levels, b) different impedances, c) different supply voltage levels, etc. etc. etc.
6 volt batteries in series sum voltage but do not sum amp/hours. The advantage of 6 volt golf cart batteries are that they generally have more tolerance for deep cycles versus hybrid marine 12 volt deep cycle batteries, will perform better a season from now, and will last longer, if they are properly shore charged and maintained with proper fluid levels.
With each Group 29 battery you should get about 95 A/Hs, 105 A/Hs with a Group 31, and about 175 to 225 A/Hs with a pair of good 6V golf cart batteries. So the A/Hs of two 12V Group 29s should be roughly similar to two 6V golf cart batteries.
Either option would be a ton of reserve capacity for two M600/6s.

 

[Kross]

Great info @Bruce !
@Kross the one disadvantage I can think of is maintenance - those 6v batteries are not sealed and need to be watered up which - in a boat - can be a hassle.

Interesting. So i just take the caps off every few months and put water in them? thanks for the info

 

[Kross]

Forget the calculations. A DC equation doesn't translate to an AC (music) application. There are dozens of other variables too. System design. System tuning. EQ. Type of music. Amplifier efficiency at a) different volume levels, b) different impedances, c) different supply voltage levels, etc. etc. etc.
6 volt batteries in series sum voltage but do not sum amp/hours. The advantage of 6 volt golf cart batteries are that they generally have more tolerance for deep cycles versus hybrid marine 12 volt deep cycle batteries, will perform better a season from now, and will last longer, if they are properly shore charged and maintained with proper fluid levels.
With each Group 29 battery you should get about 95 A/Hs, 105 A/Hs with a Group 31, and about 175 to 225 A/Hs with a pair of good 6V golf cart batteries. So the A/Hs of two 12V Group 29s should be roughly similar to two 6V golf cart batteries.
Either option would be a ton of reserve capacity for two M600/6s.

excellent info thank you. Which option would you go with personally?

 

[tdonoughue]

You need to fill with distilled water to the right level (not over filled). That is simple until you try to hide the batteries way in the back or something. Invest $10 in a battery filler jug. Got mine in Amazon. Automatically fills to the right level.

Personally I would not let that deter me from going with the cart batteries.

 

[Bill D]

I run two group 31 AGMs for my 1,000 watt stereo. I have not had a need to, but if I wanted more Ah's I would add a 3rd AGM. The combination of maintenance, weight, and inconvenience made the golf cart batteries a less attractive option. To make the boat balanced, I would have had to have a pair on each side of the boat with extra cables running between. Four 6V golf cart batteries would also take up a lot of storage space.

Although some could claim that they withstand harsher treatment and are slightly cheaper, for me the best solution was 12V group AGMs. I've had no issue in the last season. We usually are on the water for more than 12 hours at a time and at least 6 of those are at anchor or beached listening to tunes.

 

[David Analog]

excellent info thank you. Which option would you go with personally?

An unavoidable truth in deep cycle batteries is that weight is the best indicator of capacity. And guess what. 6 volt batteries are heavy!
I do believe that 6V golf cart batteries have performance and longevity on their side. The flipside is the ease of two AGM 31s.
Either will start your boat with ease in an emergency. Either will play your system for several hours without falling below the 50% charge (12.0 volt) threshold. Both options will require a quality & high amperage AC shore Smart charger (like 3-bank, 30- amp, 3-stage).
Personally, either option would be great in my book.

 

[Bruce]

Two 6 volt golf cart batteries will cost ~$130 and provide 225 Ah of capacity with 112.5 of that useable.

In comparison two group 31 AGM batteries will cost more than $300 and provide around 230 Ah of capacity with 115 of that useable.

 

[MattFX4]

I went the two 31 AGM route. I thought about golf cart batteries but really liked the fact the AGM's are sealed. No maintenance, no leaking, no acid boiling out while charging if overfilled, etc. I also wondered how the golf cart batteries would hold to the vibrations and movements of a boat. Seems like the AGM's would be better in the boating environment

 

[Jgorm]

I've done 6v batteries and will not do again. It's not double capacity. You can't add the ah like with 12v because they are in series. If one dies, your done. When my boat batteries die, they're getting agm for sure. The last thing I want to do I'd have to top off the water 4x a year at minimum. Spilling battery acid on carpet is less than ideal, even with the mandatory battery filler (well worth $12) you'll spill sometime.

There is no way to know how much your stereo pulls unless you play some typical music, at typical volume, and measure the amps using a current shunt. Adding peak watts for all channels will represent driving the speakers at max volume continuously during the largest bass hit. You would need a frequency generator to play a ~40hz tone at full volume to even come close to using peek watts.