Simple electrical theory question

[JSYmartini]

Simple for some, not including me, I'm just simple...

Following on from the recent thread re 12v and 24v systems together, if I have a house battery bank made up of 4no 225ah 6v batteries, wired in series to give 24v, do I still end up with 900ah on tap at 24v?

 

[nigelmercier]

... if I have a house battery bank made up of 4no 225ah 6v batteries, wired in series to give 24v, do I still end up with 900ah on tap at 24v?

If the batteries are wired in series, you increase the voltage, not the capacity. The result is 24V 225Ah.

 

[macd]

+1 with nigelmercier. But lest the OP feels in some way robbed, 225Ah at 24V is of course capable of four times as much 'work' as 225Ah at 6V.

 

[David2452]

+1 with nigelmercier. But lest the OP feels in some way robbed, 225Ah at 24V is of course capable of four times as much 'work' as 225Ah at 6V.

Indeed, available power (watts) are what is imprtant in a battery bank. 6v @ 2a draw = 12w 24v @ 2a draw = 48w, as has been rightly said, 4 times the work, and probably better due to cable resistance importance reducing with increase in voltage.

 

[VicS]

+1 with nigelmercier. But lest the OP feels in some way robbed, 225Ah at 24V is of course capable of four times as much 'work' as 225Ah at 6V.

Indeed, available power (watts) are what is imprtant in a battery bank. 6v @ 2a draw = 12w 24v @ 2a draw = 48w, as has been rightly said, 4 times the work, and probably better due to cable resistance importance reducing with increase in voltage.

But since he has 4 x 225 Ah at 6 volts; connected in parallel they would also be capable of four times as much work as a single 225 Ah at 6 volts.

The advantage of the higher voltage is that the current is lower for the same power ( watts) enabling lighter cables to be used.

 

[savageseadog]

Remember that anything above 50V is getting into electric shock/cardiac arrest territory.

 

[JSYmartini]

+1 with nigelmercier. But lest the OP feels in some way robbed, 225Ah at 24V is of course capable of four times as much 'work' as 225Ah at 6V.

Thought it was a bit good to be true!
I currently have 2no 24v battery banks, one for each engine, each consisting of 2no 12v truck batteries (Lucas 627). Some house loads are taken from one bank and some from the other, an arrangement I'm not all that keen on.

I'm looking at putting in a third house bank, Trojan T-105 deep cycle batteries seem to be highly regarded and well priced from a £ per ah perspective.

I wonder if ill have problems with my battery charger, mixing deep cycle and starter batteries, do they have different charging requirements?

Thanks for the replies so far

 

[JSYmartini]

With the stereo on and the VHF listening out plus a few led lights and the AIS, I'm using about 4 amps. The fridge with compressor running burns about 2 amps so call it 1 amp if its cycling at 50%.

So 5amps from a 225ah bank would last 45 hrs in theory right? Obviously a lot less in practice.
And if I leave the boat with everything off except the fridge using 1A then I get 225 hours of use?

 

[macd]

So 5amps from a 225ah bank would last 45 hrs in theory right? Obviously a lot less in practice.
And if I leave the boat with everything off except the fridge using 1A then I get 225 hours of use?

Your 'Obviously a lot less in practice' suggests you may already know this, but that's a very big NO with lead-acid batteries. Regularly draining them below 50% substantially shortens their life (although true traction batteries like the Trojans you mention are more tolerant of deep discharges). So you should really regard your 225Ah set-up as offering only around a usable 110Ah (and some would suggest even less).

 

[JSYmartini]

This is why I want an extra bank just for house loads using deep cycle bats. I can run it down without worrying about damaging them or not having power to start the engines.

 

[Keith-i]

If you are going to put all your domestics onto a dedicated house bank, then would it be worth using one of your existing start banks for the job and buy a new lower capacity set for the engine start. The logic behind this is that Ah's cost money and your start batts don't need 225 capacity.

 

[JSYmartini]

If you are going to put all your domestics onto a dedicated house bank, then would it be worth using one of your existing start banks for the job and buy a new lower capacity set for the engine start. The logic behind this is that Ah's cost money and your start batts don't need 225 capacity.

My start batts are actually only 120Ah but 750 CCA. I totally agree, I'd like to use something smaller for the start banks as all they'd have to do is start an engine each. Unfortunately, enormous as they are, they are only just above the min requirements specced in the engine manual.

 

[Keith-i]

How about an Optima red top or two for each engine then put your existing start batteries into one bank as house. Should only set you back about a grand

 

[JSYmartini]

Is that all?! Still, current batts are best off where they are as they're no good for running down.
The Trojan 6v 225Ah batts can be had for just over £100 so 4 of those wouldn't be the end of the world

 

[Mistroma]

With the stereo on and the VHF listening out plus a few led lights and the AIS, I'm using about 4 amps. The fridge with compressor running burns about 2 amps so call it 1 amp if its cycling at 50%.

So 5amps from a 225ah bank would last 45 hrs in theory right? Obviously a lot less in practice.
And if I leave the boat with everything off except the fridge using 1A then I get 225 hours of use?

Almost everything works against getting stated Ah out of a flooded battery, like the T105, on a regular basis.

Won't reach 225Ah when new (Builds up from new over 1st 50 cycles)
After that, capacity drops off with age & how it is treated
Going below 50% charge regularly shortens life
Not easy to get 100% charge level quickly

Many people cycle batteries between 85% & 50% so only using ~80Ah per T105

Temperature also affects capacity with higher temp. increasing it (at expense of battery life) & vice versa with lower temp.

OP mentioned an example with very low current draw and this is one case where capacity would actually be more than 225Ah. Trojan quote a capacity of 250Ah if discharged over 100 hours at around 2.5A. They don't quote for lower currents but it would be reasonable to expect ~300Ah at 1.5A

So you will be able to get more Ah out if drawing a small current. However, I'd be surprised if wasn't more cost effective to buy a very low capacity battery in that case (or use different battery technology). It was only an example but it is still valid to note that 225Ah quoted is for a T105 in perfect condition at 25C with constant load over 20 hours.

Any change will result in higher or lower capacity (Guess which way it usually swings? ) T105s are certainly good value for money if looked after reasonably.

 

[angelsson]

Suggest reading this: http://www.sterling-power.com/support-faq-2.htm
Makes a lot of sense, has been discussed here before, also this is very helpful making connections to get the best from your batteries: http://www.smartgauge.co.uk/batt_con.html

 

[angelsson]

Be aware what the article says, leisure/deepcycle batteries do not exist, except in the minds of those wishing to separate you from your beer tokens.

 

[danielefua]

Theoretical but quite practical addendum

As the title of the thread is about theory I like to pinpoint the fact that what is called ampere-hours (Ah) is NOT a good acceptable physical unit (i.e. a non ambiguous unit). No wonder it is subject to misunderstanding and indeed it will never be defined in a serious textbook of physics.
In particular in order to be used as a measure of energy (joule or whatever unit) it implies a fixed conventional voltage, usually 12V; therefore it cannot be used spanning over different voltages.

...please pardon my professional distortion...
Daniel

 

[nigelmercier]

As the title of the thread is about theory I like to pinpoint the fact that what is called ampere-hours (Ah) is NOT a good acceptable physical unit (i.e. a non ambiguous unit)...

It isn't a unit. It is a combination of units*, just like Nm (Newton metre) which is far more ambiguous.

(* OK, technically it should be As, but that is just silly)

 

[macd]

As the title of the thread is about theory I like to pinpoint the fact that what is called ampere-hours (Ah) is NOT a good acceptable physical unit (i.e. a non ambiguous unit). No wonder it is subject to misunderstanding and indeed it will never be defined in a serious textbook of physics.
In particular in order to be used as a measure of energy (joule or whatever unit) it implies a fixed conventional voltage, usually 12V; therefore it cannot be used spanning over different voltages.

...please pardon my professional distortion...
Daniel

Fair enough, but if that's advocating anything, surely it's Watt-hours instead, which allows for the issue of differing voltages? Yet it's abundantly clear that plenty of people on here struggle enough with Ah without introducing another variable.