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How much will it affect battery life when you drained the battery?

I borrow my roommate’s every now and then since this May. During the time I use the car I left the lights on and drained the battery for 4 times (evenly spread). Since winter is coming we think it’s better to change the battery. I googled it and most people say the average battery will last about 5-6 years. His battery is about 3-4 years old when I first start using the car. But since I’ve jumped it for so many times I’m wondering should I pay half for the battery? I want this to be fair for both of us but I almost know nothing about cars so could someone with more experience help me here? Or maybe what do you think is the fair way to do this.


If you discharged the battery that many times replace it.

Starting battery’s aren’t designed to be deeply discharged. This reduces the cold cranking amps and the reserve capacity of the battery.

Deep cycle battery’s on the otherhand, are designed to be deeply discharged and recharged.


Thanks Tester! We are definitely gonna replace the battery. I’m just wondering if I should pay for half of the price. What do you think?

Well? If you’re the one that caused the battery to drain down, you own it.


Batteries unlike wine, are one of those items that does not improve with age. Replace it and assume responsibility. I wonder why your roommate keeps lending you the car ???

I think you should pay half. And let your thnner wallet be a reminder to you to turn the lights off, when you borrow the car after the new battery is installed.

One thing you could do if you feel lucky is have the existing battery load tested. There’s a lot of retail auto parts stores that will do this job for free. Knock on wood, it might test ok and it will last another year.

If you do decide to buy a new battery, be sure to look up what Consumer Reports says about who makes the best value battery that fits this car. I’ve had very good luck w/Kirkland batteries purchased at Costco, but according to CR, how good the Costco batteries are varies between the different groups (types).

Usually if you drain a car battery more than 3 times to nothing ( totally dead ) it will be junk in the long run… Car batteries are only meant to be drained no more that 85 % of capacity. The battery will sulphate during these dead times. Car batteries are designed for a short term discharge unlike "deep cycle batteries " I f were far you I would be the kind gentleman you are and buy a new battery or at least 1/2 of the cost. The strain on the alternator could be another problem as alternators are not designed to re-charge a depleted battery…Count your blessings so far.

My vote is that half the value is fair, as he got 3-4 years service out of that battery.

However, since he most likely would not be out of pocket ANY money this year if the battery had not been abused, and now he has to cough up $50 this year, if you ever want him to loan you the car (or anything else) again, you would be well-advised to pay for the battery.

BTW, if the battery is working well now, and there are jumper cables in the trunk, I would roll the dice and try to get another year out of it unless your friend really needs it to be reliable.

The question is how long did the battery sit in a fully discharged state? A few hours isn’t a big deal a few weeks is a problem. When fully discharged the material inside the battery can flake off and eventually kill the battery. Usually one cell goes first and battery gets “weak” but sometimes it just shorts out and dies a sudden death.

This battery might be OK, but due to age and the repeated deep discharge cycles replacing it makes sense. The battery can be load tested. Since a battery will run about $100 the OP might just give the car’s owner $50 and let the owner decide whether or not to replace the battery now or wait until it dies someday.

I would get the battery replaced. If it does go dead then the cost of the service call will probably double the cost of a new battery. If the battery was only one year old then I might chance it. Since it’s 3-4 years old…no way. You do owe your roommate half the cost of the battery.

Out of curiosity, how are you managing to leave the lights on so many times? Isn’t there a warning chime when you’re leaving the car? Perhaps that chime isn’t working correctly.

@Meghan If I were in your shoes I would feel obliged to pay for the whole amount to replace the battery. I once lent my car in college to another student, who never shut off the windshield wipers, letting them rub over a dry windshield! It burned out the wiper motor, and he never offered to pay for any of it! That was in 1963! I still remember and I only lend out my cars to relatives.

Just take the amount you would have paid a car rental company those 4 times (instead of borrowing your friend’s) and it is small potatoes compared to the cost of a new battery!


I would pay the full cost for the new battery for these reasons

You admittedly killed the battery several times

You want to remain on good terms with the roommatet

I will politely disagree. I think you should have the battery tested. To have it tested, it should be fully charged. Take it to a parts store, they will happily test it for you. If they are like ours, they will tell you the truth.

If you don’t have a Consumer Reports access, let me say that on average the Everstart Max batteries sold by Wal-Mart are the best of the "not-super-expensive’ ones, according to them.

Half sounds right on a battery that old.

About charging a fully discharged battery … this can be tricky b/c a fully discharged battery develops high internal resistance. It then acts like it won’t take a charge. A lot of times these fully dishcharged batteries are tossed, when they may in fact remain serviceable if properly charged. The trick is to charge it, but slowly. It might take several days, maybe an entire week, as at first it will accept only a small amount of current, but as it charges, the internal resistance decreases, and then at some point it will charge the remaining amount in a normal amount of time.

Half would be a good since you have used it also.