Alternative Power sources - Recommended battery capacity?

Author

Recommended battery capacity?

broughcut

2006-01-23, 4:21 pm

Hi, I've checked google and searched this group and rec.boats.cruising
but I'm struggling to answer the following and could do with some advice
choosing a battery system.

Off-grid with a 4.5 kva diesel generator and upgrading an old 24volt set
of 2v traction cells (unknown amp/hr capacity) to a decent 12v system.

First of all, although this will double the amps on the DC side, will
going for 12v as opposed to 24v actually mean the batteries discharge
twice as quick? Reading past responses, the primary consideration seems
to be cost of thicker battery cable rather than drastically diminished
capacity?

Usage: Would be to run one LCD TV (which is drawing 1 amp from the AC
side) and 80 watts of lighting (all AC) for say 6 hours a day during the
evening. Perhaps 3 amps total max draw (to cover the odd laptop etc.).

This will be off a 12v-230v AC inverter (UK 230 volt, so less amps
extracted from the batts than US systems?).

I believe inverters will take about 10 amps from the battery for each 1
amp on the AC side--is this correct?

Would like to go a week (down to 30-50% DOD) before recharging.

All other appliances taken care of by the genset--which will also be
used to charge the cells. 14 hr recharge would be the max, hopefully
this is doable at 20% of the 20hr capacity.

3 stage chargers that can output 80-100 amps seem to be quite expensive,
so I don't want to buy more battery capacity than is required.

What capacity batteries would people recommend? I'm not sure how the
20hr rate translates to usage over a period of a week or more.

Thinking 2 x 6v 500 AH Surrette Solar cells will do the job. Really
would appreciate any advice!

cheers.

Ecnerwal

2006-01-23, 5:21 pm

In article <Xns9754CA76467C9broughcutplusdslnet@84.92.1.10>,
broughcut <broughcut@plusdsl.net> wrote:

> First of all, although this will double the amps on the DC side, will
> going for 12v as opposed to 24v actually mean the batteries discharge
> twice as quick? Reading past responses, the primary consideration seems
> to be cost of thicker battery cable rather than drastically diminished
> capacity?

The big issue is that you often need more parallel strings at 12V .vs.
24V .vs. 48V. Parallel strings are bad, battery-maintenance-wise. A
single 48V string connected to a 48V inverter will have the same energy
storage as the same number of batteries connected as 12V, but will have
fewer maintenance issues, and not need such big wires. If you are buying
new equipment, rather than sticking with some outdated thing you already
own, 48V seems to make the most sense, AFAICT. I infer that you are
chucking whatever 24V thing you had converting before, since you are
changing battery voltage; without some significant outside reason to use
12V, I can't see why you would...

> Usage: Would be to run one LCD TV (which is drawing 1 amp from the AC
> side) and 80 watts of lighting (all AC) for say 6 hours a day during the
> evening. Perhaps 3 amps total max draw (to cover the odd laptop etc.).
...
> I believe inverters will take about 10 amps from the battery for each 1
> amp on the AC side--is this correct?

No, especially for the one you mention - 12V in, 230V out, 20 amps DC
per amp AC at 100% efficiency, there is no such thing as 100%
efficiency, so 22-25 amps DC (12V) per amp AC (230V).

> Would like to go a week (down to 30-50% DOD) before recharging.

Simple Math. 6 hours per day drawing 75 amps from the battery (to make 3
amps AC) is 450 amp-hours per day. You want that for 7 days - 3150
amp-hours. You want that to be 30-50% DOD - 6300-10500 amp-hours. Hope
you need a lot of ballast on that boat...

--
Cats, coffee, chocolate...vices to live by

broughcut

2006-01-23, 5:21 pm

Ecnerwal <LawrenceSMITH@SOuthernVERmont.NyET> wrote in
news:LawrenceSMITH-0A9F07.15272723012006@news.verizon.net:

> The big issue is that you often need more parallel strings at 12V .vs.
> 24V .vs. 48V. Parallel strings are bad, battery-maintenance-wise. A
> single 48V string connected to a 48V inverter will have the same
> energy storage as the same number of batteries connected as 12V, but
> will have fewer maintenance issues, and not need such big wires. If
> you are buying new equipment, rather than sticking with some outdated
> thing you already own, 48V seems to make the most sense, AFAICT. I
> infer that you are chucking whatever 24V thing you had converting
> before, since you are changing battery voltage; without some
> significant outside reason to use 12V, I can't see why you would...
>

We had two 24volt banks in parallel at one stage, which explains why
cells kept dying and acquiring reverse polarity. I didn't realise at the
time this is the same as cycling them 24/7.

They've lasted 5 years, but are now completely shot. They were old milk
float cells, which we got very cheaply. Now lost that contact... So it's
twice as expensive to get a 24v battery based on 2v or 6v cells.

From what you say about the 25 amps on the 12v DC side, perhaps we do
need to discount a 12v system.

When the batteries were in good condition, we were getting at least a
week out of them running lights and large CRT tv a few hours a day.

It is for a house, off-grid in North Yorks. I've just founf the boating
ng's have a lot of info on the subject.

Anyone have experience with Hawker Powersafe VH cells (VH51). They're
around 1300 AH (2v in a 6v battery). There is a guy selling them on ebay
but reading the manual I think they are predominantly (telecom) standby
cells and don't like to be cycled too much or left for days between
recharges.

Ecnerwal

2006-01-23, 6:21 pm

In article <Xns9754D3B22D5ECbroughcutplusdslnet@84.92.1.10>,
broughcut <broughcut@plusdsl.net> wrote:
> They've lasted 5 years, but are now completely shot. They were old milk
> float cells, which we got very cheaply. Now lost that contact... So it's
> twice as expensive to get a 24v battery based on 2v or 6v cells.

Given a fixed load, and a fixed depth of discharge, it costs exactly as
much to get a 24V battery, a 12V battery, or a 48V battery. The fixing
of capacity fixes the number of cells of a given size - after that it's
just how you hook it up.

ie, a 1500 ah 48V bank is exactly equivalent to a 3000 ah 24V bank or a
6000 ah 12V bank, in terms of cost and watt-hours stored. If built from
the same 2V or 6V cells, the 48V has a lot less maintenance issues,
however. If you can find cells twice as large (in terms of AH) to build
the half-voltage battery from, the limiting factor becomes wire and
connections.

In my opinion, it is best (if at all possible) to get battery cells of
the amp-hour size needed for the entire battery, and connect up a single
series string. If this does not make fiscal sense (and sometimes it does
not, as costs for certain smaller batteries are significantly less per
watt-hour stored) then you have to commit to more maintenance.

If you have a 24V inverter that works now, I'd be quite surprised if
replacing your 24V bank cost more than putting in an appropriately sized
12V bank and a 12V inverter. A 12V bank that "costs 1/2 as much as a 24V
bank" also has 1/2 the capacity. Check with battery distributors and
forklift sales/maintenance places. Over here one would also check with
golf-cart places, but golfers in your part of the world appear to
actually walk around the course, as far as I recall (being a non-golfer).

> When the batteries were in good condition, we were getting at least a
> week out of them running lights and large CRT tv a few hours a day.

Which may mean that you are overestimating your needs (I based my rough
calculations on your stated worst-case consumption and hours of use and
my estimated worst-case inverter efficiency). It may also mean that you
were driving your old battery bank to the knackers, rather than limiting
the discharge to 30-50%.

--
Cats, coffee, chocolate...vices to live by

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