Houseboat on Batteries

[joshua]

I want to achieve 8 hours of use, at 40 amps, to run lights, tv, fridge, etc. on a houseboat.

Its a 24 volt system, so how do I figure out how many batteries are needed?

For example, if I wanted to use Trojan T605's which are 105 minutes at 75 amps, how can I convert it to 40 amps to figure out the run time?

Any help would be greatly appreciated!

[henjulfox]

Joshua,

The usual unuit of measure is amp-hour, 1 amp for 1 hour. Convert both your need and capacity to that unit and you get your answer.

You want 40 amps for 8 hours. 40 X 8 - 320 amphours

Each battery is good for 75 amps for 105 miniutes. 105 miniutes / 60 = 1.75 hours. Therefore 75 X 1.75 = 131.25 amphours. 320 need / 131.25 per battery = 2.4 batteries.

But, this will leave you with a battery that is absolutly dead. No battery will live long under this abuse. The less you remove from a battery the longer it will last. The rule of thumb is to only take around 20% of capacity from a battery before recharging. Therefore the usefull capacity of your batteries is 131.25 amphours X 20% (.2) = 26.25 amphours Now we get 320 / 26.25 = 12.2 batteries.

Then you price out 12 batteries and get sticker shock.

Do you really need the full 40 amps for the full 8 hours? Can you get by without some of the load? Can you charge the batteries at some point during the 8 hours?

Hope I helped.

-Henry

[wdyasq]

Henry,

You failed to catch has a 24V system. I calculate 320 AH at 24V as ~1.7 "T605" ~ 20 hour 190 AH rating. Four for a 24v bank and 8 would give him a 70% drain in a day.  This woudl quickly kill the batteries.  "T105" batteries are about 220AH and might take the load for a day's use.  EITHER of these choices would quickly destroy a battery bank.  

A proper battery bank will seldom draw to 50% capacity and preferrably draw down only 20% or the 80% charge state.  This would mean - where I am at least - a bank of 1600 AH capacity at 24V or 32 "Golf cart batteries". These are $65 each here.  32 X $65 is $2080 and about a ton of batteries.

Enjoy,

Ron

[BT Humble]

I want to achieve 8 hours of use, at 40 amps, to run lights, tv, fridge, etc. on a houseboat.

Its a 24 volt system, so how do I figure out how many batteries are needed?

8 hours * 40 amps = 320Ah (amp hours).

If you want the batteries to last any reasonable length of time, you'll only use the top 20% of capacity:

320Ah / 0.2 = 1600Ah

You might be able to stretch it a bit to (say) 30%:

320Ah / 0.3 = 1060Ah

That's still pretty big.  For example, here's a picture of my 24V/1000Ah battery bank:

To give you a sense of scale, the frame for the rack is 2"x4" timber.  The whole set weighs about 1500kg.

How are you planning to charge these?  

Assuming that you're recharging these with solar, and you have 5 "peak sun" hours per day, you'll need 320Ah / 5 hours = 64 Amps of charging.

Under real-world conditions, each of my 140W solar panels puts out about 4.5A into the battery bank while in full sun.  You'd need 14 of them.  They're each roughly the size of a standard house door:

If you're planning to charge your batteries with a windmill I'll let someone more knowledgable do the calcs, but I expect you'll need something like a 12-foot windmill.

If you're planning to charge via a generator/battery charger it'll have to be pretty substantial too.

Also note that this doesn't allow for cloudy days (reduces your energy harvest by ~80%), batery inefficiency (you'll get out about 90% of what you put in) or inverter inefficiency (another 15% loss of power.

320Ah at 24V is 7.7kWh.  If you're serious about using renewables, you'll need to get it down to a more manageable 1kWh - 2kWh first.

BTH

[zap]

WOW... I hope he has plenty of freeboard!

[BigBreaker]

Buy a generator and use fewer batteries.  You probably only use the houseboat once in a while.  Those batteries will usually just sit there self discharging.  A fuel tank is light and nearly free.  You'll need a generator or some other power source to charge them anyhow unless you charge off the grid anyhow.  The only reason to run off batteries is silence and pristine air.  Perhaps your lake doesn't allow generators?

[WXYZCIENCE]

In the book, "Death of a battery", it was proved later that they were actually murdered.

[joshua]

Hey guys,

Thanks for all the great input!  This is helping me out a lot.

I have one other question.  I am having trouble with the solutions I am getting with the equations I am using.

The T605 is rated at 190AH, and is also rated at 105 minutes at 75 Amps.

If I do:

105 minutes / 60 = 1.75 hours * 75 Amps = 131.25 AH

Shouldn't I get 190 AH?  Or is it because the 190AH rating is based on lower amperage, and the battery has lower AH rating the higher the amperage being used?

Josh

[alcul8r]

That is correct.  

A 200AH battery might supply 10A for 20 hr if that was how it was rated, but no way would it supply 200 amps for 1 hr.

Remember they said don't take off more than the top 20-25%.  This affects battery life adversely.

Can you cover your roof with solar panels?

If  the boat will be at anchor, would a small wind generator help out?

It is much cheaper to conserve than to generate electricity.  Look at your loads and see what you can do without before you sink that boat.

Rex

[joshua]

Thanks Rex,

Its actually not my boat, its a customer of mine.  Usually when someone contacts me, they already know how many batteries they need.

He did not know, and only estimated the amps for the 8 hours.  I lurk on this forum quite a bit so I knew if I asked here you guys would be able to help me

Im getting ready to call the customer now and give him his different battery options.

Thanks again guys!  And if you need any batteries and your in the North Florida area contact me, Ill give you a great deal.  Just mention your from this forum.