How long to charge battery

[super99]

Maybe a dumb question but here goes. How long do you charge a dead battery versus one that is low? My charger has 10,40 and 200 boost rates. If the battery is down enough it won’t crank I usually give it 40 minutes on 40 amps.if it has set for quite a while I give it 15 minutes on 40 amps. I’m afraid of overcharging the batteries, how long should I be charging?

 

[showcrop]

I don't claim to know much about battery charging, but my understanding is that 40A is only for starting and that it is not good for a battery. 20A is for day to day recharge of a battery that has lost most of its charge. 20A should be followed up by much longer 2A charge.

 

[Russ from MN]

I agree with showcrop, fast charging is not good for a battery, it's only to get an engine started. So, charge it on the lower setting if you have the time. Does that charger taper off automaticly as the battery reaches full charge? I think once charging voltage is up into the mid 13 volt range the battery is charged, but I don't know for sure, but someone else will. Most of my chargers are automatic and taper off and stop charging.

 

[Red Dave]

Maybe a dumb question but here goes. How long do you charge a dead battery versus one that is low? My charger has 10,40 and 200 boost rates. If the battery is down enough it won’t crank I usually give it 40 minutes on 40 amps.if it has set for quite a while I give it 15 minutes on 40 amps. I’m afraid of overcharging the batteries, how long should I be charging?View attachment 79424

40 amps for 40 minutes (if that's not a typo) is a lot. It could heat up the battery and possibly damage it.

A safer way would be a lower amperage for a longer time, like 2 amps for 8 - 12 hours.

 

[BarnyardEngineering]

Charging a battery is a matter of knowing the battery's capacity in Amp-hours (Ah) then dividing by Amps to get hours. Google can give you the general idea of the capacity of a particular battery type (e.g. "group 24 battery amp hour capacity"). For the example group 24 battery google says "70-85 Ah." So charging on the 40A setting it would take roughly 2 hours to bring a battery from "flat dead" to full, not that you'd do that. Charging on the 10A setting would take roughly 8 hours to bring the battery from "flat dead" to full. Charging on the 2A setting would take 35-40 hours. There's no need to get more exact than "roughly" here.

Of course you'd never charge a "flat dead" battery. Flat dead batteries are scrap lead. There's no bringing them back. Chargeable batteries are always at some percentage of charge, so it won't take the full XXX hours to charge at YYY Amps.

What you'll notice by watching the meter, as a battery gets near "full" the charge rate drops as the supply voltage and battery voltage equalize. On the lower settings, 10A and especially 2A, you really can't "overcharge" the battery. The 40A setting you just use to give the battery a boost for a few minutes to get a machine started. You wouldn't use 40A or 200A to charge the battery.

 

[Russ from MN]

I have one old 6 amp charger that the rectifier failed in, so I replaced it with a diode. Now it doesn't taper off, so I put a timer on it. I mainly only use it if a battery is so low that a smart charger won't turn on. I think if the voltage is below about 8 my smart charger won't turn on. I did this many years ago so I don't remember all the details, but I still use it occasionally.

 

[MarkB_MI]

I like to monitor the charging voltage. If you keep it between 13 and 14 volts you're unlikely to damage the battery.

 

[ASE guy]

Maybe a dumb question but here goes. How long do you charge a dead battery versus one that is low? My charger has 10,40 and 200 boost rates. If the battery is down enough it won’t crank I usually give it 40 minutes on 40 amps.if it has set for quite a while I give it 15 minutes on 40 amps. I’m afraid of overcharging the batteries, how long should I be charging?View attachment 79424

Flooded lead acid battery's are best done on a trickle charger in my opinion. 1 amp for 24 hours works well on most of them. AGM's are different. For those, I would use a smart charger.

 

[buickanddeere]

Depends on the Ahr , temperature , age , composition and design of the battery .
At first glance it would appear that a 100amp hour battery could be charged in one hour at a 100 amp rate .
Closer to 25amp for three hours . Another 2-3 hours on a 10 amp max trickle .
Deep cycling and running to dead will rapidly ruin a lead acid battery .

 

[buickanddeere]

Flooded lead acid battery's are best done on a trickle charger in my opinion. 1 amp for 24 hours works well on most of them. AGM's are different. For those, I would use a smart charger.

Yes , no and maybe .
Sometimes some vigour is required if the plates are sulphated.
I agree that charging too fast and too long has ruined about as many batteries . As batteries that have been deep cycled or ran to complete discharge .

 

[JD Dave]

A SLOW, - LONG charge is best for any battery and may even recover an old battery. Fast charging may ruin a good battery and even cause an explosion. Think of a person sitting in an electric chair. They shoot the victim with high voltage and amperage because they want to get the job done in 60 seconds. Life ceases.

 

[Watthour]

If you want reliable information about charging and maintaining flooded wet cell batteries, locate and download the guide published by the navy. They have a LOT of experience in battery service. I can't find the link right now, but one thing I do remember is that a low rate charge for 21 hours was a recommendation for peak battery life.

 

[Watthour]

Some overview: https://archive.org/details/25634usnavytheoryoftheleadacidstoragebattery

 

[John M]

I have that same charger. Ive put it on 2 amp and left it oh 16 - 18 hours and not thought twice about it. Heck Ive had it on 10 amp and forgot about it overnight. Oh it even charged my 6 year old AGM battery when I left the switch on on my tractor once.

 

[bc]

Your charger is not a smart charger.
Some smart chargers won't work on dead battery.
I would only use your charger long enough to put enough charge for a smart charger to work.
Set the timer to 15 minutes and watch the amps.
My favorite Smart charger is a DeWalt charger.
I have two, one at each property.
The DeWalt charger is also a maintainer after it reaches full charge.
It has a desulfating mode as well.
You can put it on 100 amp engine boost too.
Slow charge is better than a fast charge.
The Dewalt charger is the best smart charger I've used and controls the charge, fast in the beginning and slows down as needed.
I think your charger is almost like a jumper battery designed to give a battery a fast charge to start the vehicle or tractor.
If you were to use your charger you may what to charge it a few minutes at a time.
My brother and I many years ago used a charger like yours. It was very cold winter day and he couldn't get his car to start.
He had just put in a new battery and he ran the battery down trying to start the car.
We decided to get inside car while charger put a fast charge in the battery. A good thing we both got inside the car.
As soon as I turned the key there was Big bang, a ball of fire and the battery exploded throwing acid all over.
If were outside close to the battery we would have taken an acid bath and got pieces of battery case in our face.

Dewalt Battery Charger/Maintainer, DXAEC100, 100Amp Engine Start, 12V is by far the best charger I've owned in my lifetime.​

https://www.ebay.com/str/friendly200?_trksid=p4429486.m3561.l161210

Wow George, you were lucky. Now your brother and the rest of us know not to let you in our cars and turn the key on.

 

[504]

I have a Harbor Freight Viking smart charger that’s been plugged in for four years now. It will not charge a dead battery so you need to boost with another for a few minutes before you use the smart charger. Camper batteries,mower batteries and even the tractors that set too much get the Viking. I have used it on the camper as the only charger after the inverter quit, for those who don’t have experience with campers nothing works without batteries on the camper. The power goes from the battery to the inverter to the appliance. As soon as this charger quits I will buy another.

 

[MeanGene1]

As I have a 48V Genie lift with 8 6V golf cart style batteries, I tried a NOCO Genius 3500 smart charger, and it has been great for special purposes, settings for AGM, lithium, 6V, 12V, and the "repair" setting that actually works pretty well. It has brought back more than half of the weak ones I've used it on, and when on repair, you can watch it spike the battery, back down, repeat if you have your voltmeter hooked up. Let it run overnight, it'll gain some, run it again, keeps getting better. I will gently tap the battery case with a rubber hammer and gently shake it a bit, then run the repair again, brings 'em back unless they are very bad. You do have to run a "normal" charger first if the battery is very low, it won't bring them back from very low, but it works better than I ever expected

 

[buickanddeere]

A dead or low battery will freeze in cold weather . Explosions are more common on a boosted or high charge rate frozen battery . Or a battery that has been frozen in the past .

 

[BarnyardEngineering]

A dead or low battery will freeze in cold weather . Explosions are more common on a boosted or high charge rate frozen battery . Or a battery that has been frozen in the past .

Explosions are precisely why these modern "smart" chargers will not charge a truly flat dead battery. I would not be surprised that if you could navigate your way around the NDAs, you'd find a litany of lawsuits against battery charger companies stemming from explosions.

So now you have to do something dumb to explode a battery in your face and end up looking like Deadpool, and it's your fault and your problem.

 

[Phil T]

I have a Harbor Freight Viking smart charger that’s been plugged in for four years now. It will not charge a dead battery so you need to boost with another for a few minutes before you use the smart charger. Camper batteries,mower batteries and even the tractors that set too much get the Viking. I have used it on the camper as the only charger after the inverter quit, for those who don’t have experience with campers nothing works without batteries on the camper. The power goes from the battery to the inverter to the appliance. As soon as this charger quits I will buy another.

Is yours the $39.99 one? I have probably 8 of those and put most of my tractor batteries on them down cellar all winter. Next spring the battery functions like a new battery. I have got over 5 years out of most of my batteries.