Prophex

I have here a 12volt car batterie, which I use as a field batterie with the following specs. (as I read em of of the batterie itself): 44Ah / 360A (ENG)


I know this is offtopic and has little to nothing to do with RC-stuff, but would I be able to power my mini frige with it ? and for how long ? It has the following specs: it uses 12volt and 4A
the problem is that I don't know what they mean with the '4A' I know that discharging a batterie that has for example 3600mah (= 3.6Ah) at 1.8Ah would make it last for 2 hours. But it just seems rediculous that my small frige, which only works at 12volt would use 4Ah ... I'm confused.

What is the difference between Ah and A ?

jooNorway

Your battery could in theory give away 4,4A for 10(h)ours making the amount of 44Ah (Ampere-hours). Or 44A for 1hour, also giving 44Ah.
But...; your battery could deliver 360Amperes for a short while, telling me this is a starter-battery. Because it can supply such extreme current. And this kind of Pb-batteries do not like to be drained too low (like LiPoes) and will soon be destroyed if they don`t get charged quite soon after draining. This is not a suitable battery for a fridge, but of course it can be used. But don`t suppose it will work for a long time. What you need is "the other kind" of Pb-battery. Don`t know what it is called in english, but you need a battery which don`t deliver high current, and withstand being nearly empty.

mrasmm

yeah, that's the right info. Automotive battery vs deep cycle or marine battery. You can use this battery for your fridge, but it will not last as long because the plates are thin to provide the greatest surface area possible for CCA or cold cranking amps.

With that said the 10 hour figure could be close. Basically when they say it uses 4A, they mean 4 amps per hour, but if it's UL listed that's a max rating, not the average rating. So it could draw the full 4 amps under average running conditions, but then again it might only draw 3 amps. Also there is apparent power vs power, and you're gona have to go off the apparent power or VA instead of watts, but that is a whole different discussion.

Also your fridge will only be running 4A when it is running. This is called cycle load. The most efficient fridges run very low cycle loads, like 3 or 4% if the door is kept shut. If it's lower quality and doesn't have expanded foam filled doors and stuff, then you could be more like 50% duty cycle.

That brings another question, are you going to cool the fridge and the contents before you go, or on the road? If they are cooled before, then your battery will last much longer. Also the more you keep the door shut, the longer the battery will last, so with that said. If you cool what you have in there, pack the extra space with some bags of ice if there is any, and keep the door shut most of the time, and the battery will run the full 44Ah (assuming that the plates are clean and not sulphated with the right amount of liquid in the cells), you could be looking at a day or more depending on the temps

boy oh boy if that wasn't complicating things =P

the simple answer would be probably 8 to 36 hours or so depending on conditions.

mrasmm

oh, I forgot one. Ah is a capacity rating, like a tank of water, how many gallons the tank holds. Amps is more like gallons per minute. You rate flows in gallons per minute, and tanks or capacities in gallons. So you rate battery capacity in amp hours, and usage rates in amps.

Really though when you are talking amps, you are talking about amps consumed in one hour unless otherwise specified. When you say amp hours though, nearly always you are talking about something like battery capacity, or some other kind of storage.

Prophex

haha, all very nice and very helpfull posts thx

So in conclusion, as stated by mrasmm, I would be able to run my frige for 8-36 hours with a 12v 44Ah batterie, depending on the conditions (e.g.: cool it at home first, add some frozen cooling-blocks, open it up alot, etc.)



However, I do still have a small remark to jooNorway's post. You're saying that the 360A (ENG) refers to the maximum discharge amount and that it would not be wise to drain the batterie at 2-4A .... However, everyone at my flying club uses old car batteries to power their field charger and if I'm correct most of em charge their batteries at arround 3-6 amps, so are you saying that alle those lead-acid batteries of the people at my club will be ruined if they don't discharge em harder ? [&:]

jooNorway

Have a feeling my poor english may have confused you []

The battery is able to deliver a current at MAXIMUM 360Amperes for a very short time. Telling me this is an automotive battery, because marine batteries can`t deliver at so high "C"-ratings. When you start your car there is a huge load from the battery to the starter. Usually between 60-200 Amperes as you crank. But this last only 2-4 seconds (if your car is serviced ) Then your generator starts to recharge the battery as soon as the red batterylamp goes out in the dashboard. The battery is able to deliver 360A for a short while, but the capacity is 44Ah. And the battery of course could be loaded at 0A or 4A without problems.

The clue is that there are two kinds of Pb (Lead-Acid) batteries. As mrasmm learned me they are called automotive or marine batteries, and he explained the difference in their construction. What I tried to explain is the fact that automotive batteries get harmed when they are discharged and don`t get a charge at once (topped up). And the marine batteries can`t deliver the large amount of amperes, but don`t get harmed if they are left discharged for a while. A marine battery at the same capacity (44Ah) might propably not be able to deliver more than 10-20A as max load... But their capacity are equal. Max amp rating is different though... See the difference?

An other way to explain: You have two bottles of water. Each contain 44 litres of water. They have equal capacity. One of them have a huge valve in the bottom, the other one a small one. If you want a lot of water fast you open the huge valve allowing a lot of water to drain quickly, dercribing the Amperes needed to start a car. This is the automotive battery. If you would like a small amount of water flowing for a long time you open the small valve on the other bottle. This is the marine battery. They both contain the same amount of water (=Ah) or have same capacity. Simplified...

You could use a automotive battery on the airfield for charging, but if you allow the battery to stay discharged for a long while the lifetime is dramatic reduced.
Or in other words, again; automotive batteries get harmed quiet soon if they are discharged and left discharged.

This days most confusing explanaition I think

mrasmm

hehe yeah, what he said =)

one more thing I might add is that marine batteries are made for deep discharges, where automotive are not. The less you can discharge it the better, 20% or less is really the ideal of where you want to be for max cycle life, but 50% would probably be more where would be useful for you. Make sure to always charge it when you get home after you use it so it lasta a while. Automotive batteries, because of the extra surface area suphate more easily at low charge levels, and that is what ruins these lead acid packs.

hopefully that answers your q. If not hit us back and well keep at it =)

Prophex

Nono, I understood everything I just found it odd that those type of batteries are able to supply that much current and wanted to know if it was true that I couldn't discharge that type of batterie at such a low current. (= 4A)

nynway, I do have 2 more questions now, since I have no idea on where to find a marine batterie:
1) So, I'm not able to discharge this batterie completly, what will happen if I do ?
2) You're saying I would have to recharge it asap, when I drain it, now how fast are we talking about exactly ? a few hours or a few days after discharging it ?



But nyway, thx for all of your replies up untill now,
Prophex

mrasmm

1) anyplace that sells lead acid sells marine, or at the very least would know where to get one, heck, even walmart sell's em =P

2) it'll suphate the plates ending up in a premature battery death
I'd probably go the max 1 day or so, but the sooner the better. You could even hook up a rig to charge it with the cig lighter on the way home or something.