Do you discharge a battery a little before you charge it?

[Sawyer Rob]

When I was a kid helping out is a shop, the meticulous owner told me to ALWAYS turn the lights on for two minutes before a battery charge to drain off surface resistance to a charge.

Is this a wive's tale?

YES

SR

 

[oosik]

I've sure never heard of this idea. Besides - when I hook up a trickle charger to any of my batteries - they are already in a shallow state of discharge.

 

[WranglerX]

IF battery needs a charge it sure is missing any surface resistance..... Never heard of such nonsense.....

 

[tjandrews]

And the LiPo batteries don't need to be discharged. They keep going until dead anyway. I've been converting all my battery hungry items to rechargeable LiPo. Their output is a straight line on a graph then drops off instantly. I especially like the rechargeable AA and AAA batteries now. They output 1.5-1.6v. Not like the old NiCad of 1.2v fully charged.

Li-ion batteries aren't bad, either. A few years ago I bought an old Dewalt cordless drill at a church sale for a dollar. The original Nicad battery in it barely made it grunt. I found a replacement with nimh cells in it, and that has worked well, but is beginning to show its age. Recently, I had a chance to get a discounted Chinese-made Li-ion replacement battery, and there's a world of difference. The drill turns faster, and lasts longer on a charge. The down side, if you can call it that, is that unlike the nimh battery the li-ion battery won't charge with the drill's original charger. I have to use the charger that was bundled with the battery.
No idea how long it will last, but so far, I'm happy.

 

[John_Mc]

Draining the surface charge is necessary prior to testing a freshly charged battery, but I have never heard anyone say to do that prior to charging a battery.
For batteries that I want an accurate test on, I let them sit for 24 hours before testing.

This is exactly what I've heard & read. A weak battery may hold a surface charge for a time, making the voltage look as though it's up to snuff. Letting the battery rest for a bit gives a more accurate reading.

I've never heard of a surface charge interfering with how a charger works, but I suppose the temporarily higher voltage could fool a smart charger into not starting the charge cycle? I've not seen it happen on my multiple BatteryMINDer chargers. However, they do run through a testing cycle when first connected. I wonder if that part of their cycle drains the battery a bit during the test? I also have two NOCO chargers. I don't pay as close attention to those, because they are either hidden on-board on my antique truck, or on my wife's motorcycle that rarely gets used. I can say that the batteries on that equipment is always ready to go, so they seem to be functioning as they should.

 

[aircommuter]

Definitely not necessary. If it has a surface charge why would one be charging it?
For testing these days a conductance type tester like a Midtronics it superior. It will give you the battery’s actual capacity compared to its rating. It also can decide if the percentage of its actual capacity is sufficient for service or not.

 

[MarblemountLane]

The tale I’ve heard was turn on lights for a moment before cranking, to “wake up” the battery. I know that in tests of a really big battery (powerhouse station battery) I’ve seen the quick initial drop when chargers stop, then slow drop and stabilize then gain slightly under load, before voltage goes back to slowly dropping. That voltage drop under load is not exactly linear.

 

[aircommuter]

The tale I’ve heard was turn on lights for a moment before cranking, to “wake up” the battery. I know that in tests of a really big battery (powerhouse station battery) I’ve seen the quick initial drop when chargers stop, then slow drop and stabilize then gain slightly under load, before voltage goes back to slowly dropping. That voltage drop under load is not exactly linear.

You see that initial drop because the charger voltage is higher than the battery voltage or it wouldn’t charge it. Then you have absorption time which is usually about 2 hours for a lead acid battery. Batteries don’t sleep. They basically have 3 stages, charged, discharged and dead. The dead stage is when can’t be charged. Often misspoken.

 

[AnotherBueller]

I'm calling wives tale on that. Another tale I used to hear was RJ Corazacompletely drain the battery before charging. Science has proven that batteries have a cycle life. If a battery has a cycle like of say 1500 full recharges, then it has a life of 3k 50% recharges. And everything runs better, draws less amperage, on a full charge vs low voltage. Less amperage draw reduces motor and component heat increasing efficiency.

Like RJCorazza said, you need to take the surface charge off a newly charged lead acid battery to get an accurate state of reading, or let it sit for 24 Hrs.
For NiCad batteries, they can take a certain numbers of discharge / charge cycles with out getting a memory before they must be serviced to start the series over. That process can be hazards.

 

[OldPaint]

Did he invite you to go snipe hunting with burlap sacks too? I think he may have been yanking your chain just a little bit

Lead acid cells do indeed build up sulfate residue on the lead plates, but as others have already pointed out, charging the battery will make the residue come off. Trying to discharge a dead battery is much akin pulling the plug on an empty sink. Trying to charge a fully charged battery is sorta like cramming 10 pounds of sh** into a 5 pound bag. The worst case is overcharging which will start discharging hydrogen into the air around the battery. If you’ve never seen a battery explode, you don’t want to. Been there, done that, and had to call the ambulance.