Battery Discharge Prevention

[RSole]

battery

The link above is a page on this site about preventing battery discharge. I must say that discharging a car battery to 50% of it's capacity it not recommended. Unless it's a deep-cycle battery designed to be discharged safely down to a max of 50%, I would never go below 80-90% charge level on a regular basis. Going down to 50% (or even 75%) several times will hasten the death of a car battery.

"When the automotive type battery is optimized for high cranking current, it has some limitations in other functions. You should keep the battery near full charge (between 90% and 100%) at all times. Allowing the charge level to drop below 90% of full capacity can reduce the ability to produce full voltage during high current cranking. It also promotes sulfation of the plates, which can shorten the life of the battery. Fully discharging can shorten the battery life severely, and leaving it fully discharged for even a short period of time may kill it completely. If you allow your car to sit for long periods of time unused, you might consider hooking up a regulated trickle charger to maintain full charge level in the battery. This device may be called a "battery maintainer". " -http://mgaguru.com/mgtech/universal/ba103.htm

 

[DashCamMan]

Hi Rsole,

Thanks for the info! Let me investigate this further and I will update the page as necessary.

 

[oddpoo]

RSole is completely right. Lead-acid batteries should always be kept as fully charged as possible. As soon as your lead-acid battery is not fully charged the sulfation process begins. The problem is the combination of charged levels and time. As soon as your battery is not fully charged the sulfation timer is ticking. And the deeper the discharge lever the faster the timer is ticking.

Lead-acid starter batteries are designed to be storage fully charged. When you want to start your car the battery is designed to give a huge amount of current (without too much voltage drop) for a short period of time and then the battery want to be fully charged again as sons as possible.

 

[DashCamMan]

Very interesting. I did some quick reading and you guys are right. Lead Acid start batteries should not be routinely deep cycled or that will reduce the life of the battery.

Hence, if you never want to deep cycle your battery the cutoff needs to be 80% or 90%. This equates to a voltage cut-off of around 12.4V. No battery protection device that I have seen has this setting.

Now the real question is, is it a big deal to allow the battery to deep cycle occasionally to 50% (approx 12V, a common voltage cut-off)?

I will update the page with this info as well but perhaps we can gather some more info on the subject.

 

[RSole]

"Now the real question is, is it a big deal to allow the battery to deep cycle occasionally to 50% (approx 12V, a common voltage cut-off)?"

No, if you don't mind buying a new battery often and risking no-starts in cold weather.

Yes, if you want to keep the same battery for a long time. I've had some cars with the same battery for 8 yrs, because they were always kept topped up (driven daily)

I've read that every time you deep-discharge a car battery, you lose at least 10% of it's previous capacity:

Deep-discharge #1: 90% remaining capacity
#2: 81%
#3: 73%
#4: 66%
#5: 59%
#6: 53%
#7: 48%
#8: 43%
#9: 38%
#10: 34%

So even 5 discharges will result in almost half the capacity being gone.

 

[oddpoo]

It is really not a big problem to discharge a lead-acid battery to 50%. The problem is for how long time the battery is exposed to that low level of charge.

Let’s say that your car has problems to start and that you have to run the starter motor multiple times before the engine finally starts. This might lead to a 50% charge. But as long as you let the motor run for a long time so the battery get a chance to recover to a full charge level then it is not a big deal. This is basically what a starter battery is design for.

The problem occurs if you leave the battery at the 50% level. Then the sulfation process starts and you will lose capacity. You will not lose voltage, the battery might actually look fine, but you will gradually lose capacity (Ah). Always running the dash camera from the battery when the engine is of is a typical case when the battery will be exposed to low levels of charge for a long time. This might not be a problem if you live in a fairly warm climate and are willing to change the battery every second year.

 

[RSole]

Very well said, strangefeces. Of course loss of Amp-hours capacity also means loss of short duration cranking Amps. Cell plates coated with sulfur will have a reduced surface area to take part in the chemical reaction that produces power.
I built a 120V desulfator that I crammed into a computer power supply case that reversed the sulfation a bit and will let me use my battery until winter comes again. That was caused by an alternator drain (bad diode), not by accessories left on.

 

[tofu]

let's not forget that many modern vehicles no longer use lead-acid batteries

i know for certain my e92 doesn't

 

[RSole]

tofu: "Let's not forget that many modern vehicles no longer use lead-acid batteries

i know for certain my e92 doesn't"

Actually, I would say almost all cars use lead-acid batteries. Your e92 may use an AGM (Absorbed Glass Mat) battery, which still uses lead and acid. The acid is held in a sponge-like mat instead of sloshing around like in a old style flooded cell battery. AGM have about 20% less capacity than flooded cell types, so the risk of deep-discharging is even higher. They are also difficult and take longer to bring back to full charge.
But, if you are certain, then free feel to correct me.

 

[tofu]

earlier models came with AGM batteries, but they have been superseded by a gel battery. i was under the impression that these are of a different chemical composition. is this incorrect?

 

[jokiin]

earlier models came with AGM batteries, but they have been superseded by a gel battery. i was under the impression that these are of a different chemical composition. is this incorrect?

probably something similar to an Oddessy or Optima gel cell, some of these are deep discharge versions and some aren't though so I don't know that just being gel means you won't have problems

 

[RSole]

Tofu: No, you are correct, the chemistry is different. But they still use lead and an acidic electrolyte (lead-acid). It's mainly how the electrolyte is kept in suspension. A gel cell has the electrolyte mixed with silica dust to form an immobilized gel.

Absorbed Glass Mat batteries are ruined by heat, so in some BMWs they are in the trunk, away from the engine. High heat kills all types of batteries, but the old flooded cell style ones can tolerate heat the most. They are in the engine compartment to lessen the distance between them and the starter and alternator. Those thick copper cables are expensive, so the shorter the better, also to minimize Voltage drop.

Here are some good sites for info about batteries:

Battery University

Wikipedia - Gel batteries

 

[tofu]

That's unfortunate news. And here I thought I was out of the woods!

Well, either way, thank you very much for correcting me

 

[tofu]

let's explore this topic some more

does anyone know how much power per hour the top popular cams draw in parking mode (blackvue, finevu, itronics, etc)? i supposed it would vary depending on how much motion is going on in front of the car, but we should have some kind of average at least.

then we can tell how many hours it would actually take to "deep discharge" a battery.

it seems people have been using parking mode for years now without any complaints that I see, or this issue would have been brought to light much sooner.

I'd assume the average person leaves their car parked about 22 hours of the day (2 hours total commuting time).