# Re: Formula for charging time of 12v battery

Discussion in 'Electronic Basics' started by Grant, Feb 14, 2011.

1. ### GrantGuest

On Sun, 13 Feb 2011 18:37:49 -0800 (PST), Bill Bowden <> wrote:

>On Feb 12, 4:45Â am, John Fields <> wrote:
>> On Fri, 11 Feb 2011 19:49:07 -0800 (PST), Bill Bowden
>>
>>
>>
>> <> wrote:
>> >On Feb 10, 8:25Â am, John Fields <> wrote:
>> >> On Thu, 10 Feb 2011 07:23:37 -0800 (PST), George Herold

>>
>> >> <> wrote:
>> >> >On Feb 10, 7:33 am, John Fields <> wrote:
>> >> >> On Wed, 9 Feb 2011 14:15:42 -0800 (PST), George Herold

>>
>> >> >> <> wrote:
>> >> >> >On Feb 9, 4:57 pm, Northern Night Sky <>
>> >> >> >wrote:
>> >> >> >> On Feb 9, 5:37 pm, "Phil Allison" <> wrote:

>>
>> >> >> >> > "Northern Night Sky"

>>
>> >> >> >> > >I have a Powerpack 300, which is a 12v battery at 12Ah.

>>
>> >> >> >> > > Using the AC adapter with the house outlet (20amp at 120v) it takes 17
>> >> >> >> > > hours to fully charge the battery. How would I calculate the
>> >> >> >> > > (approximate) time it would take to charge the battery, using a solar
>> >> >> >> > > panel that draws 1amp at 15v?

>>
>> >> >> >> > ** Don't solar panels only work when the sun shineth brightly ??

>>
>> >> >> >> > Any such predictions would have to involve detailed knowledge of the local
>> >> >> >> > weather, cloud cover and solar illumination levels.

>>
>> >> >> >> > Wouldn't it ?

>>
>> >> >> >> > .... Phil

>>
>> >> >> >> You still can get an approximate time of how long it would take to
>> >> >> >> charge the battery, giving the variables I've provided...Duh! If I
>> >> >> >> get 5 hours of sunshine/day, then I'll do the math in consequence.
>> >> >> >> I'm still interested on getting the FORMULA, as per my original
>> >> >> >> question.- Hide quoted text -

>>
>> >> >> >> - Show quoted text -

>>
>> >> >> >The 12Ah is the capacity of the battery. 12 amp x hours. So if your
>> >> >> >solar charger provides 1 amp, then it will take about 12 hours.

>>
>> >> >> ---
>> >> >> Sorry, but since there's no free lunch, that's just not true.

>>
>> >> >> Rule of thumb, as I recall, is that the charging efficiency of a
>> >> >> lead-acid cell/battery is about 60%, so to get 12Ah into the battery
>> >> >> using a 1A constant-current source which will take it to its terminal
>> >> >> voltage will take about 20 hours.

>>
>> >> >As bad as 60%? Â I didn't know that. Â The only time I've used/measured
>> >> >battery capacity was in rapid charging of NiCad's. Â And there the
>> >> >efficiency was pretty good.

>>
>> >> >George H.

>>
>> >> ---http://photovoltaics.sandia.gov/docs/PDF/batpapsteve.pdf

>>
>> >> ---
>> >> JF

>>
>> >I don't get it. The article suggests battery efficiency drops off near
>> >full charge, however figure 1 shows about 68 AH input for about the
>> >same out, while an input of 116 yields only about 97 out. That would
>> >indicate less efficiency at deeper discharges, and higher efficiency
>> >near full charge.

>>
>> >What did I miss?

>>
>> >-Bill

>>
>> ---
>> Let's say we have a 12V lead-acid battery which has been discharged to
>> the point where its terminal voltage is 10.5V
>>
>> Then let's say the battery has a capacity of 97Ah, but to get that out
>> of it we have to put in 116.
>>
>> Since efficiency is equal to output divided by input, that gives us an
>> efficiency of about 0.84, or 84%.
>>
>> Now, with the battery discharged to 10.5V, (since we took 97Ah out of
>> it) let's pump 68Ah back into it.
>>
>> We find that in this case we can get, say, 65Ah out of it until it
>> gets discharged to 10.5V, so thats's an efficiency of 65Ah/68Ah, or
>> about 96%, so efficiency decreases as the battery approaches full
>> charge.
>>
>> ---
>> JF

>
>Yes, I was looking at it from the top down rather than the bottom up.
>So, it appears efficiency is better when the state of charge (SOC) is
>less than maximum. But that brings up another issue of battery life
>operating at lower SOCs. Maybe it's incorrect, but I thought lead acid
>battery life is extended by keeping the battery fully charged. So, the
>trade-offs might be better efficiency with reduced life, or visa
>versa?

Yes, like that. It's very hard to push the top 20% of capacity back into
the battery, that's why car batteries quickly fail when the car is used for
mostly short trips, they're never fully charged.

If you look at voltage (or energy) while charging the battery, you need to
push 30 to 50% more into the thing than what you got on discharge. The
industry talks in AH and expect about 1:1 AH charge/discharge for a good
battery, the failure modes for LA batteries is a race between corrosion and
sulphation, difficult to get right at times.

Other day I noticed a 12V 5AH SLA had split it's case. They do that sometimes,
all by themselves, just crap out -- that battery was on standby float charge
for several years, had no useful capacity, output voltage dropped to about 8V
on load, which is 2 of the 6 cells shorted? I floated them at the low end of
the range too (13.5V).

Grant.
>
>-Bill

Grant, Feb 14, 2011