Battery Capacity Calculation

[RichardAllen]

I am trying to work out what capacity battery I need for the leisure battery in a dual setup.

To get a handle on it I have worked out the following schoolboy physics problem:

Question:

If I have a 12V systems with a 65Amp-hour battery and it powers a single fridge which takes 45 Watts, for how long will a fully charged battery power the fridge ?

Answer:

The battery stated capacity is 65 Amp Hours. Reduce by a nominal 20% to allow for battery not in reality being either fully charged or fully discharging, gives an effective capacity of 65 x 80% = 52 Amp-hours

Fridge consumptions is 45Watts, for a 12V fridge this means it takes 45/12 = 3.75 Amps

Therefore the battery will power the fridge for 52/3.75=13.87 hours say 13.5 hours to be on the safe side.

Is this a correct calculation, or have I fundamentally failed to understand something about vehicle electrics ?

[SteveRK]

I agree with your calculation but battery terminal voltage may drop before getting to fully discharged.

A normal lead acid battery is typically rated for 300 cycles at 70-80% depth of discharge. This mean that batteries don't like being discharged too much. 300 discharges of 75% capacity should keep your beer cool for a few years if you only use the fridge occasionally though.

[beekay]

Hi Richard,

Basic calculations for I=P/V are good but remember as the battery terminal voltage decreases with use, so the available power to the cooler decreases and so it becomes less efficient (sees the cooler as a resistive load). For a standard lead acid battery (not deep cycle) regular deep discharging below about 10.5 volts can seriously affect the long term recharging characteristics and general condition. 65 Ah is quite a small capacity battery, if you plan on using the cooler often it would be better to get a larger capacity battery and recharge it before the terminal voltage drops too much.

Bryan.

[FridgeFreezer]

There's a few extra factors:

- The fridge probably won't be drawing current all the time. My LR (Wabco) one only runs at ~5% duty cycle keeping stuff cool in normal British ambient temperatures. If you want to make ice lollies in the middle of summer then it runs more often The peltier effect "coolers" (many also will warm stuff up) do draw more current more often than proper fridges.

- Many fridges have a cutoff at a certain battery voltage so they will not run below that voltage to avoid flattening your battery.

- Normal lead-acid batteries don't like being properly discharged very often, as has been said. Leisure batteries should be more resilient to that though.

[RichardAllen]

I have just spotted the Waeco 35l fridge spec which runs on average 15% at 20deg C. By the same calculation, assuming a deep cycle battery and ignoring the voltage drop-off that gives 13.5/15% = 90 hours. This may well be optimistic and assumes no use of lighting, camera battery charger, water system etc etc, but gives me feel of being on the right lines

I am currently trying to design the detail of a dual battery system and spotted in some issue of LROi what I think is the setup on Vince Cobley's white 110 truck cab. He had two Odyssey PC1500's. I think Odyssey's being dual purpose is a useful backup for engine starting and this size allows space in the battery box for fuse/distribution boxes to keep the wiring neat, out of the way but accessible, and therefore fixable. The PC1500 is 65AH and the largest long term drain is see is the fridge hence the calculation.

[RichardAllen]

Other electrical drains such as two striplights, 8W each mean that one hour's lights trades off against almost 2.4 hour's fridge, so no great deal.

However, showering is a new question. I have no figures for a Brownchurch shower kit. Does anyone know what current the pump draws ? This becomes an issue with a wife and 2 daughters !

Not to mention the hair dryer which I guess may pull 150A out of an inverter. So which is it to be dry hair or cold beer. I am designing this system, so do I get the casting vote ?

[disco_al]

this may be useful

http://www.alternate-energy.net/batteryamphours03.html

[Mark]

Not to mention the hair dryer which I guess may pull 150A out of an inverter. So which is it to be dry hair or cold beer. I am designing this system, so do I get the casting vote ?

do you mean 150A or 150W? either way you are well out for a hair dryer...

You will be looking at 1500 to 2000W for a hairdryer, and if you want to plug that into an inverter, you will be looking at quite a large (read expensive) inverter. Also given that inverters are not even close to 100% efficient, you will be loading your battery even more... I would either forget about hairdryers, or look at specific travel ones....

Mark

[dollythelw]

adivse wife and daughters that the Sigourny Weaver/Sinaed O'Conner convict look is about to make a big comeback therefore removing the hair dryer issue and saving valuable beer cooling time

[beekay]

Sorry, think I may have missunderstood.

Most thermo-electric coolers (Peltier as previously mentioned) are quite ineficient, typically 40 - 60% and tend to run constantly. They can generally only cause a fixed reduction in temperature between ambient and product (typ 15-20 deg) so would put constant drain on the battery. Traditional compressor based refrigerators are a lot more efficient (1 watt of electrical power = 2-3 watts cooling capacity) and as previously stated have a low duty cycle at normal ambient temp so would not be a constant drain. The down side is the pukker ones are a lot more expensive .

[FridgeFreezer]

Hairdryer use a towel, or if you really want to produce volumes of hot air in-car, install a Webasto/Eberspacher heater and get the missus to stand in front of that. It'll probably be cheaper (and use far less power) than buying an inverter setup that will happily run 2000W of hairdryer.

[headhunter]

I use an Odyssey 2250 for main and a 2150 for "hotel" services, giving 126 and 100 amp hours respectively.

They will take a discharge to 80% without any harm and will recharge quickly. They also will hold a fuller charge than conventional batteries.

The 2250 will fit in a Defender battery box. I mount the 2150 under the driver's seat and use a VSR for split charging and an isolator switch to prevent main battery drain if required.

The Engel 45 fridge draws 2.4amps. I don't know if this is an average per hour or on a duty cycle, I suspect the former given the marketing literature. 2.4amps is pretty good when you look at the coolers sold in accessory shops usually draw 5+amps and thats only keeping them at 20 degrees below ambient.

Have you thought of carrying a remote area charger as sold by BOAB - http://www.boab.biz/Pages/Remote%20Area%20...y%20Charger.htm. When the ladies drain your batteries, just top them up at up to 55 amps using the generator for 20 minutes or so.

John

[Turbocharger]

There's already a source of large volumes of hot gas on the car, but their hair will smell a bit afterwards...

Actually, this started as a flippant answer but you could try sleeving the exhaust downpipe and blowing air up through the sleeve? You'd certainly need less 'pumping' watts than heating watts.

The problem with extracting (or sinking) any large quantity of heat is that you usually want your shower in the morning, when the car's cold and you're reliant on stored (eg battery) energy. If you can alter your lifestyle slightly you can use less energy in many many ways.

[RichardAllen]

As always, loads of good stuff there guys. I already have a 2000W inverter because I got a new one (relatively) cheap and I want to run a drill / angle grinder in the field separastely from going camping. The hair dryer stuff was a bit flippant, but gave rise to really innovative ideas. Best of all it is good to know on my own part that the setup I have in mind is not only similar to that chosen by people with lots more knowledge and experience than me, but is also borne out by re4asonable quality calculations on the load I am expecting

Thanks again

Richard

[RichardAllen]

PS Had ywo beers and decided to try the Sigourney Weaver suggestion. Received a more than usually withering glare.

Oops.

Richard

[RichardAllen]

this may be useful

http://www.alternate-energy.net/batteryamphours03.html

In my calc I guessed a 20% reduction on the quoted amp-hours figure, ie 100 amp hour battery can effectively deliver only 80 amp-hours in practice. The calculation on this link uses a reduction of 16.6666%, ie a 100 amp hour battery can effectively deliver only 83.333 amp-hours in practice. So the claculations pretty much agree.