Battery re-charge time

Submitted: Monday, Apr 05, 2004 at 08:59
ThreadID: 11828 Views:2702 Replies:4 FollowUps:1
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How long does it typically take to recharge a "normal" cranking battery. I am thnking of borrowing the "wrinkly one's" Auto Fridge for a few day trip up the beach. But since I do not have a DB setup, I will be relying on my main battery (Overlander 700). How long will I have to drive the car for to recharge the batteries? These fridges can be run twice a day for 2-3h, each drawing ~10Ah. How quickly can an alternator replace this? 10min, 1h, 10h? I will be travelling with others, so a jump start it always possible, but would obviously prefer not to. Will also be running a couple of flouro's at night.

Thanks in advance

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Reply By: Wayne (NSW) - Monday, Apr 05, 2004 at 11:14

Monday, Apr 05, 2004 at 11:14
If you are running a Auto fridge, then it is designed to be turned off over night. The wall cavity has a fluid in it which freeze while the fridge is running. The drive to the camp site will be enough to freeze the fluid and the fridge can be turned off overnight. The next day when you are driving, turn the fridge back on and the fluid will freeze again. The Auto Fridge is made this way so that a second battery is not required.
PS Don't forget to thank the "wrinkly one's" for there choise of fridge.

AnswerID: 53227

Reply By: Roachie - Monday, Apr 05, 2004 at 12:06

Monday, Apr 05, 2004 at 12:06
Sorry, I can't help you with an answer, just some advice. To me, the idea of running a fridge or anything else off the main battery when the truck isn't running is like fingernails being dragged down a blackboard....... LOLOLOL. I know that I'm just being stupid, but that's why I've never had any embarrassing problems with a flat battery in the bush.
If I were you, the least I'd be doing would be to invest in one of those battery pack type things that you can get. They are versatile enough to run fridge and lights and you can take from car to car. They are pretty EXY, so if you're planning to keep your existing truck for a fair while yet, it may be worthwhile installing a DB system; even a very basic one like mine with just a solenoid and a couple of switches. You can always remove them later and take to next truck too.
AnswerID: 53230

Reply By: Crackles - Monday, Apr 05, 2004 at 19:27

Monday, Apr 05, 2004 at 19:27
Tim. It really depends on what size altenator is charging the battery and what else is turned on at the time. As a rough guide I would suggest it would take at least one hour to recharge the battery to 95%. But if you are often opening the fridge adding more warm cans of beer or if the temp gets above 35 degrees it may need as much as two hours. The best option is to run the fridge flat out on high when you go for your day drives leaving it off overnight. A cheap insurance would be to fit a Low Voltage Disconnect Switch for the lights & fridge set around 11.8 volts which ensures just enough power to start the car.
The Autofridge is one of the most efficient brands on the market so you shouldn't have too many problems with just the standard battery. Craig...............
AnswerID: 53289

Reply By: Eric Experience. - Monday, Apr 05, 2004 at 23:16

Monday, Apr 05, 2004 at 23:16
the battery is labeled with its amp hour capacity, IE if it is a xx70 it has a capacity of 70 amp hours [ah] . If you discharge at 10 amps you will be completely flat in 7 hours. This is not good for the battery, When you recharge the same 70 ah applies but the normal charge rate for a battery is 1/10 of its capacity, that is 7 amps in the case of a 70 ah battery, it makes no diference what size the alternator is it will only charge at 7 amps, so you will have to run the motor for 10 hours every day, that is why others are saying in effect don't do it. Eric.
AnswerID: 53323

Follow Up By: Rod - Tuesday, Apr 13, 2004 at 10:38

Tuesday, Apr 13, 2004 at 10:38
Done some research here....

Tried recharging a Marine Battery from the vehicle's alternator. Voltage before charging was 11.91 V

In the first minute of charging, the battery was drawing over 20A of current. As the voltage of the battery rose, the current tapers off so after 10 minutes, the current was 16.5 A and after 15 min, 14.5A.

I believe a flooded cell 12V battery is only 75% efficient at charging. ie. 25% of the current going in is wasted in heat etc.

So, after 30 minutes of charging, I estimate only 5.56 A had actually been stored in the battery. After 60 minutes, this had tapered to 8.89A.

Within reason, I think the size of the alternator is irrellevant (unless you are running big spotties at the same time). A 12V battery will only accept a high current for a short period of time before this tapers off to around 10A. Aparently deep cycle batteries will accept less current and AGM batteries more. No experience here.

Therefore, I doubt you will come close to recharging a battery significantly in an hour.

From memory, the doco with my Rotronics Isolating DB setup recommends 4-5 hours to charge a battery. To complicate matters even more, I believe a standard alternator will not charge a battery beyond 70% anyway.

Interested to hear others findings ...
FollowupID: 315646

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