House battery sizing and discharge rate

[muskyfins]

I know there is a way to calculate how long my house battery will power my electronics while on the hook, but can't quite remember the formulas.

I'm adding small amp to the stereo this year, so with that running half total power output, I'm expecting somewhere around 40-50 amps total draw. We don't use much else. After dark we add the anchor light, but that is LED with minimal draw. The occasional light in the head for the admiral. No fridge. No A/C. Very basic.

So how do I appropriately size my house battery for 4-6 hours at the sand bar tie up or 4 hours waiting for fireworks?

PS-I think I know the answer, but it's better to not say anything and let people think I'm an idiot than open my mouth and remove all doubt.:disillusionment:

Thanks for everyone's input in advance.

 

[ssobol]

40-50 amps seems a lot for a stereo. Most stereos only have a 10 or 15 amp fuse. A good condition type 24 battery has about 65 amp-hours. A 27 battery is about 90 amp-hours. You can deplete a starting battery about half way before you start to hurt it. A deep cycle battery can be depleted more. Therefore a type 24 battery will run your load for about half an hour.

If you don't have a battery monitor that shows the battery current, I would get an amp meter (that works with DC) and actually measure your loads. You can do the math from there.

 

[muskyfins]

Its not the stereo as much as the amp. 240 watts/12.5 V= 19.2 amps. Plus the stereo itself plus a little err on the side of caution=40 amps.

But i get your point.

This will be off the house battery which will be a deep cycle. Ahalf hour or even a full hour seems awful short. On the fishing rig, I can run the trolling motor all day plus some. I understand the trolling motor doesn't run continuous, but over the course of 8-10 hours on the water, I know it runs more than 30 minutes. In stiff winds I bet she runs 15 minutes per hour. Maybe a bit more. Thats 2 hours of run time or more per day. I don't usually see a decline until late in the day and even then it's still not dead. Maybe I better look into the specs on the trolling motor?:confusion::noidea:

 

[Silvertip]

When you shop for a house (deep cycle) battery look at the label. A group size 27 will generally have about 185 minutes reserve capacity with a 23 amp draw. What that means is the battery can provide 23 amps for 185 minutes before it can no longer meet that spec. So to make this simple, consider you have a 46 amp continuous draw. That's double the rated capacity of a group 27 battery so you simply cut the minutes in half. In this example, you would be out of spec at about 90 minutes.

 

[muskyfins]

Silvertip OK thank You.:yo:

Just so I'm clear, where did the 23 amp draw come from? I'm looking at one in particular that is rated 90 AH with 175 reserve minutes. Would this mean 350 minutes at 45 amps?

Also, that's just to meet spec. So in your example in the 186th minute I would have less than 12 volts for some time period. Right??

I should have paid more attention in electrical portion of physics in high school.

 

[muskyfins]

Calrification--Sorry. Yes I know. 12.7 volts. So after the 185th minute then 12.6 volts?

 

[muskyfins]

Silvertip OK thank You.:yo:

Just so I'm clear, where did the 23 amp draw come from? I'm looking at one in particular that is rated 90 AH with 175 reserve minutes. Would this mean 350 minutes at 45 amps?-NO

Also, that's just to meet spec. So in your example in the 186th minute I would have less than 12 volts for some time period. Right??

I should have paid more attention in electrical portion of physics in high school.

OK-I've half answered my own question. It seems like the answer is at the 186th minute the voltage would drop to 10.5 volts. Dead.

And the 23 volts is an industry standard figure. Got it.

But the specs on the battery still don't add up. 90 Ah/25 amp draw = 3.6 hours (216 minutes) but the battery is rated 175 minutes??? Unless the meaning is that at 175 minutes, you still have 12.0 volts???

 

[UncleWillie]

The Ratings are not linear. If you lower the current you will increase the AH rating.
Kind of like if you drive slower, your MPG's will go up.

If the spec states 90AH for 175 minutes ( 2.92 Hours), that means 90AH / 2.92 = ~31 amps for the 175 minutes.
The other battery stated 23 amps for 185 minutes(3.08 Hours) means 3.08 x 23 = ~71AH.
The 185 minute reserve sounds like a better battery compared to the 175 minute reserve until you also compare the Current and AH specs.
The 185 is longer because the current is lower to make up for the smaller AH. The 175 is the better battery. 90AH vs. 71AH.

Drain the 175 at the 23AH rate and you might get something like 100AH for 4.3 hours = 260 minute reserve.
Same battery, 2 totally different specs. 90AH/175Min = 100AH/260min. With the current ratings hidden. 31amps vs. 23 amps.
It is all smoke and mirrors! They just make up specs that make their products sound better.

They like to make it confusing so the average customer will just give up, and buy the battery with the best price or the most impressive name.
DieHard, DuraLast, Mega-Power-Platinum-Premium-Performance-Plus! MP[SUP]5[/SUP]! Wooooo!

 

[Silvertip]

I visited my local farm store (Fleet Farm) recently and a group 27 deep cycle was $80 with the specs I used in my example above. You can certainly pay double that price and not get much, if any, additional capacity. I've used those batteries for decades on my trollers and never had one fail. The battery went into my new boats and were still in there when they were sold -- functioning normally. Unless you need to squeeze every amp out of your batteries you might want to pay more to get that extra run time. But the option would be two cheaper batteries to really extend run time. But then space and weight may be an issue so one needs to do what one needs to do.