how do I calculate 12v battery life?

[ScottyKM]

Bit random, but I am trying to sort out a 'bollerwagon' (its a kind of pull along trolley for days out etc)

the idea at the moment is to fit a small car battery under the seat to power a 12v coolbox.

the blurb on the coolbox says it draws 4.5ampsx 12vdc= 54w.

thats fine but how big a battery would I need to power it for say, 10 hrs without completely killing the battery. It will be charged at home from a normal 12v battery charger.

Evenually I may fit a stereo & speakers, but lets not get ahead of ourselves!

 

[(RIP) Kaister]

you need to know tha amp hour rating of the battery you want to use

eg 10AH will give 10 amps for an hour or 1 amp for 10 hours

for 4.5amps you need to juggle you battery ah figure to give you time

10ah / 4.5A = 2 hrs

etc etc

 

[Deleted account 220427002]

well get a deep discharge / leisure battery to start with

 

[ScottyKM]

cheers guys, tuned- the bollerwagon is quite small- 1mx.60m so no big battery

So a 45AH batery will run 4.5amps for 10hrs.

ages since I did this stuff

 

[Deleted account 220427002]

you need to know tha amp hour rating of the battery you want to use

eg 10AH will give 10 amps for an hour or 1 amp for 10 hours

for 4.5amps you need to juggle you battery ah figure to give you time

10ah / 4.5A = 2 hrs

etc etc

Sort of... a battery will last longer than the amp hour rating if a low current draw item is attached a 50 a/h battery should keep an 600 watt item going for an hour and it likely will do that
you could also work out that a 60 watt item would last for ten hours... that also will be pretty close but a 6 watt draw will likely last longer than the calculated 100 hours it should last.

Batteries have an Amp hour rating but they also have a time period for that too... usually about 20 hours is quoted... but that 50 a/h could be more like 70 or 80 a/h with low current draw over a longer period of time

 

[ScottyKM]

Thank you muchly,
Always rely on the guys on here for good technical answers.

 

[Engineer]

cheers guys, tuned- the bollerwagon is quite small- 1mx.60m so no big battery

So a 45AH batery will run 4.5amps for 10hrs.

ages since I did this stuff

Well sort off - most batteries specify a maximum discharge rate - if you exceed this rate the figures don't add up and you may damage the battery - also depends on the battery technology, NiMH, Li-ion, NiCd, lead acid etc - exceeding the discharge rate will reduce the life of the battery - oh and ambient temperature has a big effect. Lead acid probably most tolerant of high discharge rates, hence their use for starter motors.

Spent ages designing battery chargers for Tetra Radios - got to learn quite a bit about different types batteries!

But as a first rough calculation the above is correct.

 

[Marjorie Proops]

For the definitive answer go here Scotty

(I've used it for years and it has never let me down - I can vouch for the kit, the info & the people)


http://www.victronenergy.com/upload/documents/Book-EN-EnergyUnlimited.pdf

 

[(RIP) Kaister]

its rough enough tho

but the thread will run and run

 

[Myke Rocks]

Battery life

If you check far enough, you will find that most batteries are also rated for the number of charge/discharge cycles they will take before performace falls off.
E.G. Golf trolley batteries & computer batteries.
Thus, for example, as I recall, a Yuasa Lead Acid golf trolley battery is rated at around 500 cycles.
If you only 1/4 discharge the battery on each cycle, it will do 2000 cycles.
Same applies to computer batteries, so it pays to keep it plugged in when using it, because the battery life is extended.
(This does not apply to Nickel Cadmium batteries, whch develop a "memory" & stop discharging properly unless regularly deep discharged.)
Myke

 

[Engineer]

If you check far enough, you will find that most batteries are also rated for the number of charge/discharge cycles they will take before performace falls off.
E.G. Golf trolley batteries & computer batteries.
Thus, for example, as I recall, a Yuasa Lead Acid golf trolley battery is rated at around 500 cycles.
If you only 1/4 discharge the battery on each cycle, it will do 2000 cycles.
Same applies to computer batteries, so it pays to keep it plugged in when using it, because the battery life is extended.
(This does not apply to Nickel Cadmium batteries, whch develop a "memory" & stop discharging properly unless regularly deep discharged.)
Myke

Yes - battery life is specified in terms of the number of charge discharge cycles as well as charge rate, discharge rate etc. The manufacturers often specify that charge-discharge cycles must be complete discharge (but not over discharged) and complete charge at the 1C rate - it goes on and on, but unless you want the ultimate in performance just stick with the basic spec BUT DO NOT exceed the charge and discharge rates otherwise you will significantly reduce the life of your battery.

 

[Fanum]

On a less geeky note, put a couple of two litre water bottles in the freezer a day or two before than stick them in the cool box on this trolley thing of yours-The less hard the cooler has to work, the less current it'll draw.

By the same logic, decent insulation will buy you a lot more time that the battery will last for as well

 

[Bem]

And if you have any sort of thermoswitch in the circuit then it will not be running all the time either.
Plus, if you have any idea of the temperature gradient of the "goodies" going in the box, and how often you open the lid........

 

[WindyChuffer]

45Ah in theory, but that will be flattened at 4.5A current draw after 10 hours. Best double it, so go for minimum 90Ah.

And use a leisure battery too, not a car battery. Leisure batteries are designed for low current draw over extended periods, and car batteries for v high current draw for v short periods. Both will suffer is used incorrectly.

 

[Bear]

45Ah in theory, but that will be flattened at 4.5A current draw after 10 hours. Best double it, so go for minimum 90Ah.

And use a leisure battery too, not a car battery. Leisure batteries are designed for low current draw over extended periods, and car batteries for v high current draw for v short periods. Both will suffer is used incorrectly.

Correct but:- the amount of cold beer you will need to help you drag a 90AH battery about will not fit in the coolbox the battery powers. Check the weight of these things before you buy one. They are heavy.

 

[DollyRocket]

4.5 A for 10Hrs is 45Ah . Battery capacity is normally calculated with a low current draw (i.e. 2A, although they are normally stated in the specifications).

Your voltage will also drop a little during this current draw which means a constant power device will increase the current draw to compensate (accelerating the rate of tail off.) - I normally factor in a 25% overhead so a 60Ah battery would suit.

This is a large battery which means charging time to maintain the lifetime of the battery within acceptable limits mean the main charge rate would be ( 60 / 4) i.e. 15A which is higher than most home battery chargers. You will also need to trickle (or float charge) for approximately the same time you bulk charge (normally dealt with in automatic chargers) so a very rough approximation a 5A charger could take 60hrs to charge, i.e 5A for 60/4) = 30hrs + 30 hrs @ C/20 (or 3A or lower).

The quickest you could charge it would be 8 hrs (60Ah/15A)x2 (for a wet cell)

As someone also mentioned on here, you don't really want a car battery, they are wet cells and are designed for very high currents (400A) for short bursts, 5 secs. You probably want a deep discharge battery(leisure) which are based on AGM technology (gel cells) which will give you a better lifetime and have more charge / discharge cycles. Probably weigh around 20Kg though.