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#4695487 - 03/15/18 10:03 AM 15 amp Chgr - Hrs to charge
CarlGR Offline


Registered: 06/06/16
Posts: 18
Loc: Massachusetts
how long to charge 12 volt battery on 15 amp charger with 100 amp boost. unit is from the 70's and meter only has low/good/boost on screen. Currently screen is on line of low and good.
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#4695506 - 03/15/18 10:26 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
Trav Offline


Registered: 11/20/06
Posts: 19319
Loc: MA, Mittelfranken.de
How many amps is the battery? If its a "normal" size car battery charge for an hour then test the battery repeat on the hour.
Without knowing what the charger is capable of, the size of the battery and its current state of charge its impossible to give an answer that's even close.
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#4695516 - 03/15/18 10:32 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
CarlGR Offline


Registered: 06/06/16
Posts: 18
Loc: Massachusetts
Battery is Voltage: 12
Format: BCI Group 24F
Lead Acid Type: Starting
Cold Cranking Amps: 725
charge state is unable to start vehicle without boost
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2013 Rav4 XLE
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#4695523 - 03/15/18 10:40 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
JHZR2 Offline



Registered: 12/14/02
Posts: 41896
Loc: New Jersey
Assuming say, 80-90% charge is the same as "full" to the layperson, it's a matter of calculating the following:

(0.85 * Nameplate Ah)-(initial SOC * Nameplate Ah)/ 15 = how many hours you need to charge.

It's far more complex in reality. And, usually there is a current decay that makes it much longer to get to a full SOC.

But for "good enough" ballpark, that's sort of your start.

Issue is that most batteries don't provide nameplate Ah values.

For an old school charger like yours, Is not want to be topping the battery.

Id target running two-ish hours for a known low battery (unloaded voltage around 12-12.4v.

Reduce time before checking from there.

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#4695526 - 03/15/18 10:44 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
Wolf359 Offline


Registered: 04/27/12
Posts: 4891
Loc: MA
Yeah, it depends on the battery. You can convert reserve power to amp hours. Typical batteries are anywhere from 40-80 amp hours. So at 15 amps, anywhere from about 2.5 hours to 6 hours at 15 amps if the battery is fully discharged, less if it's not.

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#4695527 - 03/15/18 10:44 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
Quattro Pete Offline


Registered: 10/30/02
Posts: 35968
Loc: Great Lakes
Originally Posted By: CarlGR
Battery is Voltage: 12
Format: BCI Group 24F
Lead Acid Type: Starting
Cold Cranking Amps: 725
charge state is unable to start vehicle without boost

Need to know Ah value.
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#4695530 - 03/15/18 10:46 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
CarlGR Offline


Registered: 06/06/16
Posts: 18
Loc: Massachusetts
Thanks all. Appreciate help.
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#4695533 - 03/15/18 10:49 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
Snagglefoot Offline


Registered: 12/31/17
Posts: 1909
Loc: SE British Columbia, Canada
Chargers are a bit better now than 40 years ago. Maybe ask for one for your birthday!. smile


Edited by Snagglefoot (03/15/18 10:50 AM)

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#4695640 - 03/15/18 12:42 PM Re: 15 amp Chgr - Hrs to charge [Re: JHZR2]
440Magnum Offline


Registered: 02/01/09
Posts: 8756
Loc: Texas
Originally Posted By: JHZR2
Assuming say, 80-90% charge is the same as "full" to the layperson, it's a matter of calculating the following:

(0.85 * Nameplate Ah)-(initial SOC * Nameplate Ah)/ 15 = how many hours you need to charge.



FWIW, if the battery label doesn't list its amp-hour rating but only gives "reserve capacity" in minutes, the amp-hour rating is approximately 0.416 times the reserve capacity. IOW, AH=RC/2.4

For example, if the battery lists a RC of 100 minutes, then its roughly a 42 amp-hour battery.
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#4695696 - 03/15/18 01:35 PM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
wrcsixeight Offline


Registered: 10/15/10
Posts: 1747
Loc: california
A group 24 battery is generaly 75 to 85 amp hours.

I'd not necessarily believe a 15 amp charger is capable of 15 amps.

No voltmeter and no ammeter is like no speedometer and no mile markers or clock.

but in this case it is like how long does it take to go from a to b, not knowing where a or b or located, or the speed one will travel.

So, my answer is yes
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#4695713 - 03/15/18 01:48 PM Re: 15 amp Chgr - Hrs to charge [Re: Snagglefoot]
tmorris1 Online   content


Registered: 07/15/03
Posts: 2371
Loc: MN
Originally Posted By: Snagglefoot
Chargers are a bit better now than 40 years ago. Maybe ask for one for your birthday!. smile

Not really for lead acid. Supplying current to a battery is pretty much the same now as 40 years ago.

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#4695781 - 03/15/18 03:13 PM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
CharlieBauer Offline


Registered: 03/29/16
Posts: 1181
Loc: CA
A 15 amp smart charger would take about 8 hours to charge a dead 85Ah battery. 5 hours for bulk and the rest for absorption.
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#4696020 - 03/15/18 07:18 PM Re: 15 amp Chgr - Hrs to charge [Re: tmorris1]
Snagglefoot Offline


Registered: 12/31/17
Posts: 1909
Loc: SE British Columbia, Canada
Originally Posted By: tmorris1
Originally Posted By: Snagglefoot
Chargers are a bit better now than 40 years ago. Maybe ask for one for your birthday!. smile

Not really for lead acid. Supplying current to a battery is pretty much the same now as 40 years ago.


Smart chargers are worth the money. That 70ís charger belongs with the platform shoes. Not to be unkind. smile


Edited by Snagglefoot (03/15/18 07:21 PM)

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#4696504 - 03/16/18 09:34 AM Re: 15 amp Chgr - Hrs to charge [Re: Snagglefoot]
tmorris1 Online   content


Registered: 07/15/03
Posts: 2371
Loc: MN
Originally Posted By: Snagglefoot
Originally Posted By: tmorris1
Originally Posted By: Snagglefoot
Chargers are a bit better now than 40 years ago. Maybe ask for one for your birthday!. smile

Not really for lead acid. Supplying current to a battery is pretty much the same now as 40 years ago.


Smart chargers are worth the money. That 70ís charger belongs with the platform shoes. Not to be unkind. smile

I agree they are worth it, but they are not anything magical. They do the same thing as the old ones except they are smarter on when to shut off. They don't really charge any better. My point was an old charger will probably just charge just as well as a new one, but you will have to babysit it more.

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#4696605 - 03/16/18 11:18 AM Re: 15 amp Chgr - Hrs to charge [Re: CarlGR]
CarlGR Offline


Registered: 06/06/16
Posts: 18
Loc: Massachusetts
If I was to replace the circa 70's charger, what 15 amp auto charger (medium grade)with approx 50A starting ability would you look to buy. The schumacher sc1359A for $51.52 look good but I'm reading quite a bit bad info on that maker.
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2013 Rav4 XLE
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