Automotive

dead battery charging question - 2a or? it's been charging 17 hrs....how long can i leave it charging

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  • Jun 2nd, 2021 5:59 pm
[OP]
Deal Addict
Aug 18, 2003
1464 posts
535 upvotes
YVR

dead battery charging question - 2a or? it's been charging 17 hrs....how long can i leave it charging

hi. so battery was dead. it was new about 15 months ago. long story but car wasn't driven for 7 mos. life happens.

anyways i boosted the battery and ran the car for 30 mins before charging. Then i removed the battery and plugged it into the charger (said 60% charged) and it's now been running 16 hrs at 2amp (it's on auto) - and now it's only 70%. Should I up the amperage from 2a to 6 or 8? Also should I leave it on for more then 24 hrs? thx

ps..im using a motomaster smart charger
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12 replies
Sr. Member
May 2, 2017
863 posts
1144 upvotes
Charles wrote: hi. so battery was dead. it was new about 15 months ago. long story but car wasn't driven for 7 mos. life happens.

anyways i boosted the battery and ran the car for 30 mins before charging. Then i removed the battery and plugged it into the charger (said 60% charged) and it's now been running 16 hrs at 2amp (it's on auto) - and now it's only 70%. Should I up the amperage from 2a to 6 or 8? Also should I leave it on for more then 24 hrs? thx

ps..im using a motomaster smart charger
How long it will take to charge depends on the capacity which should be indicated as Ah (amp hours) on the label (not CCA). E.g. if you have a 70Ah battery, that would mean it could supply 70 Amps of power for 1 hour. So if you are charging it at 2 amps, divide the 70 by 2 to figure out approximately how many hours you'd need to feed it 2A for it to fully charge. In this case, 35 hours. Or if you were to charge it at 10A, it would take 7 hours.

Of course, if the battery is old and at half it's original capacity, it will take half as long.
Sr. Member
Sep 4, 2006
973 posts
605 upvotes
Ottawa
It also depends on the actual charger. A good "smart" charger? Put it on 10a, let it charge then it will switch to maintain.
[OP]
Deal Addict
Aug 18, 2003
1464 posts
535 upvotes
YVR
thanks!
To learn who rules over you, simply find out who you are not allowed to criticize.
Sr. Member
Aug 8, 2005
554 posts
177 upvotes
Toronto
Set it to the highest (8a) then when it hits 100% you can change it to 2a or just disconnect and start driving the car regularly. Smart charger won’t overcharge it. Especially if you’re disconnecting it within a day or two of it hitting 100%
[OP]
Deal Addict
Aug 18, 2003
1464 posts
535 upvotes
YVR
it's weird = i have a "smart" charger and i switched it to 10a and it refused to charge above 2.1a. It won't let me charge at the higher rate...still only 80% full

edit...voltage shows 14.4v (that's max i think) and now charging at 1.5a and still just 80% full. I think my battery is whacko.
To learn who rules over you, simply find out who you are not allowed to criticize.
Deal Expert
Jan 27, 2006
17163 posts
9905 upvotes
Vancouver, BC
Charles wrote: it's weird = i have a "smart" charger and i switched it to 10a and it refused to charge above 2.1a. It won't let me charge at the higher rate...still only 80% full

edit...voltage shows 14.4v (that's max i think) and now charging at 1.5a and still just 80% full. I think my battery is whacko.
You ran into a common misconception when it comes to chargers and the current output. The battery will ONLY accept a charge at a certain current regardless of what you set the charger for. The setting on the charger only controls the MAX current to be used for charging and nothing more. As the battery state of charge gets closer to 100%, the current the battery requires drops like a rock. You might only be able to charge a battery at 10A when the state of charge is close to zero and once you get to 30%, that might drop to 5A and at 80^ drop to 1.5A. This is NORMAL and why I tell people that paying lots of money on a higher current charger doesn't make sense for most people as the charger will typically not run at that level for very long.

You could have left the charge current at 2A and got to the same spot you are now as the battery is getting full.
[OP]
Deal Addict
Aug 18, 2003
1464 posts
535 upvotes
YVR
craftsman wrote: You ran into a common misconception when it comes to chargers and the current output. The battery will ONLY accept a charge at a certain current regardless of what you set the charger for. The setting on the charger only controls the MAX current to be used for charging and nothing more. As the battery state of charge gets closer to 100%, the current the battery requires drops like a rock. You might only be able to charge a battery at 10A when the state of charge is close to zero and once you get to 30%, that might drop to 5A and at 80^ drop to 1.5A. This is NORMAL and why I tell people that paying lots of money on a higher current charger doesn't make sense for most people as the charger will typically not run at that level for very long.

You could have left the charge current at 2A and got to the same spot you are now as the battery is getting full.
thank you. so it may not ever show 100% since the voltage is 14.4 and the charging has dropped to 1.3?
To learn who rules over you, simply find out who you are not allowed to criticize.
Sr. Member
May 2, 2017
863 posts
1144 upvotes
Charles wrote: thank you. so it may not ever show 100% since the voltage is 14.4 and the charging has dropped to 1.3?
It should eventually show full. Let it run, at 1.3A it's still taking a charge. That will drop even further as it gets up to 100%, but eventually it'll stop accepting any more current and you'll be fully charged.
Deal Expert
Jan 27, 2006
17163 posts
9905 upvotes
Vancouver, BC
Charles wrote: thank you. so it may not ever show 100% since the voltage is 14.4 and the charging has dropped to 1.3?
No. You are making the wrong conclusions about what is happening. As the battery gets closer to FULL, the charging current will drop which is NORMAL. Once the battery charger detects that the charging current basically hits zero or close enough to it, the charger will declare the battery to be FULL which NORMAL. Everything you are seeing is NORMAL.

In other words, you are worrying too much for a day like today. Go outside, get some sun, grab and drink while chilling out.
Deal Expert
Mar 23, 2004
29127 posts
9657 upvotes
craftsman wrote: No. You are making the wrong conclusions about what is happening. As the battery gets closer to FULL, the charging current will drop which is NORMAL. Once the battery charger detects that the charging current basically hits zero or close enough to it, the charger will declare the battery to be FULL which NORMAL. Everything you are seeing is NORMAL.
Everything this guy^ says.

However, OP, if you had a battery sitting connected for 7 months, unused, you may well have sulfated the crap out of it so yeah...even if the battery is still okay now you've probably shortened it's useful life. Next time don't do stuff like this. Because when you do, death happens--to your battery.
Deal Expert
Jan 27, 2006
17163 posts
9905 upvotes
Vancouver, BC
And if you are concerned about the battery is whacko, then get the battery tested after the battery has been fully charged. Without testing the battery, you really can't tell the health of the battery.

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