How significantly is alternator effected when using it to recharge flat batteries?

Question

Relates to: Best way to recharge battery using alternator?

Didn’t drive car for several months and battery was less than 1 volt. As I didn’t have a battery charger rather than go out and buy one or buy a replacement battery I decided every time I make a trip I’ll just use the jump starter and ensure I’ve driven enough to restore some charge.

Took me about 6 jump starts over 3 days of driving with £60 of fuel to get it from 0 to 12 volts over 3 days of driving. I used a jump starter not jump leads so it wasn’t much hassle.

Charging via the alternator saved me from having to go to the shop and buy a new battery and remove the battery and put it on a charger for 48 hours. However I understand that this is not good for the alternator(or battery) as mentioned in the linked answer.

How significantly does it wear the alternator and how does it affect the battery as well? This is not the first time I’ve done this, but the question is how severe damage is occurring, what are the implications(it’s easier to diagnose and replace a battery than an alternator) and instead of doing what I did would it be better to have bought a new battery or just be patient and get a charger?

If what I done reduces alternator life by say 2% that’s not that bad because this doesnt happen often and the battery is usually ok. So is any damage significant enough that next time I should buy a new battery or use a battery charger?

Another thing to note is that people do this all the time due to leaving lights on etc, however in my case I guess the battery is far more depleted and one journey wasn’t enough to recharge.

Thanks

Answer 1

It’s not very abusive of the alternator to charge a dead battery for two main reasons:

1. When a battery has been discharged to a very low voltage, the electrolyte becomes diluted, meaning that it has a low specific gravity and low conductivity. With such a high electrolyte resistance, a normal charging voltage does not initially result in many amps of charging current. I recently helped a neighbor attach their battery charger to their nearly dead battery, and it took only 2 amps when first hooked up. The amps slowly increased as the battery chemistry increased the sulfuric acid concentration of the electrolyte and the battery gained charge.

2. A typical car battery charging at the maximum possible rate achievable at the typical alternator’s maximum generated voltage does not draw anywhere near the maximum capacity in amps of the alternator. If you’re charging a heavily discharged battery and at the same time your headlights and taillights are on, your blower is running at full speed, you have your seat heaters on, and you are opening the electric windows and adjusting your seats, then OK, you may briefly reach the maximum capacity of the alternator. Just charging the battery won’t hurt the alternator.

However, allowing the battery to discharge to a very low voltage is extremely abusive to the battery and will dramatically shorten its life. Federal Battery publishes a graph that illustrates this effect:

You can read the full story here.

If your lifestyle prevents you from running a car for several months at a time, then ideally the car should be parked near an electric outlet where you can plug in a battery maintainer that you install permanently under the hood.

If you can predict in advance that the car will not be driven for months, you can bring the battery indoors and leave it attached to a battery maintainer.

If neither is possible and the car is in a sunny location, you can leave a solar panel on the dashboard connected to a charge controller to maintain your battery state of charge.

At a bare minimum, you can install a heavy duty battery disconnect switch to your battery terminal. This won’t keep the battery charged, but it will prevent it from discharging so quickly. This is not ideal, because when disconnected, the car will forget settings and engine tuning.

In the long term, all of these solutions will be less expensive than shortening the life of your battery by allowing it to run down so low that it won’t start the car.