Battery mA Query.

Thread Starter

biferi

Joined Apr 14, 2017
203
I am from the USA am I Right a Standard 9. Volt Battery has 500 mA and a Standard D Battery has 1,3000 mA?
 

Yaakov

Joined Jan 27, 2019
3,768
I am from the USA am I Right a Standard 9. Volt Battery has 500 mA and a Standard D Battery has 1,3000 mA?
Don't confuse current—mA—with capacity—mAh. The h is hour, time. Capacity is current over time.

The PP3 (9V battery) has a capacity of 0.8-1.2Ah, and the D cell 12000–18000 mAh.
 

Audioguru again

Joined Oct 21, 2019
3,872
One brand of Ni-MH rechargeable batteries has a C cell inside a D cell. It is shown in the Mah and in the weight.

I bought Energizer alkaline batteries for years because older Duracell batteries leaked.
I heard that Duracell batteries do not leak anymore so I bought some of their Ni-MH cells at Cosco with a charger at a price that is so low that the batteries are free. The fine print says, Made in China and the mAh is half of what Energizer has.

Here are the datasheets of Energizer alkaline batteries:
 

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DickCappels

Joined Aug 21, 2008
8,000
While at it, have you seen those really cheap carbon-zinc AA or AAA batteries where the paste only goes 1/4 to 1/3 the length of the battery? Pretty funny if you don't mistake them for being useful.
 

Audioguru again

Joined Oct 21, 2019
3,872
The really cheap carbon-zinc batteries are called "Super Heavy Duty". Some are dead and leaking in their package since they do not last long even if they are not used.
 

Irving

Joined Jan 30, 2016
2,322
A PP3 9v battery is typically 450 - 600 mAH at the C/20 rate (i.e approx 22 - 30mA) down to a terminal voltage around 6v or so. They don't do well with high discharge rates; 500mA will give typically <40 minutes useful life.
 

MrChips

Joined Oct 2, 2009
24,650
I am from the USA am I Right a Standard 9. Volt Battery has 500 mA and a Standard D Battery has 1,3000 mA?
We keep having to say this. Pay attention to units. Units matter.

mA is current. You cannot establish a current unless you state the load with resistance R.
Ohm's Law:
I = V / R

mAh is charge.
charge C = current I x time t
(sometimes we use Q for charge)
( Q Coulomb = I Amp x t second)

Thus, a 9V 500mAh battery can deliver 50mA @ 9V into a 9V/0.05A = 180Ω load for 10 hours.
This would be called a C/10 discharge rate.

For another example, C/50 discharge rate is 10mA @ 9V into a 900Ω load for 50 hours.

Real life performance may differ.
 

Thread Starter

biferi

Joined Apr 14, 2017
203
I have a Lead Acid Batter that I got a while ago and I use it to Test things.

It it Rated 12 Volts at 18 AH.

I do not think I Understand this?
Does 18 AH Tell me it will Supply 18 Amps Per Hour?
Or does it Tell me it will Supply 1. Amp for 18 Hour?
 

MrChips

Joined Oct 2, 2009
24,650
I have a Lead Acid Batter that I got a while ago and I use it to Test things.

It it Rated 12 Volts at 18 AH.

I do not think I Understand this?
Does 18 AH Tell me it will Supply 18 Amps Per Hour?
Or does it Tell me it will Supply 1. Amp for 18 Hour?
Pay attention to units.
Units are very important.

amps per hour = current / time

amp hour = current x time

18Ah can mean anything from
1mA for 18000 hours
to
180A for 6 minutes
(for an ideal battery only).
 

crutschow

Joined Mar 14, 2008
28,227
Does 18 AH Tell me it will Supply 18 Amps Per Hour?
Or does it Tell me it will Supply 1. Amp for 18 Hour?
An ideal battery would provide 18A or 1 hour or 1A for 18 hours, but a real battery has internal resistance which causes more loss at 18A than 1A (since power dissipated is proportional to the square of the current), so an actual battery will not provide as many Ah (at its rated voltage) at high current as at a lower current.

The battery manufacturer typically specifies the battery Ah rating for some long time-frame, typically 20 hours, to get a high value.
 

dcbingaman

Joined Jun 30, 2021
498
It's just the product of curr
I have a Lead Acid Batter that I got a while ago and I use it to Test things.

It it Rated 12 Volts at 18 AH.

I do not think I Understand this?
Does 18 AH Tell me it will Supply 18 Amps Per Hour?
Or does it Tell me it will Supply 1. Amp for 18 Hour?
It's just the product of current over time. Another way of stating how much charge the battery has.
Thus 18 amps for one hour, 9 amps for 2 hours, 1 amp for 18 hours.

An 18AH battery is like a super capacitor providing 1 Amp for 18 hours or 1 C/s * 18 * 3600 s / h = 64,800 C of charge.

A 12V battery with an 18AH rating has an energy storage capacity of 12 J/C * 64,800C = 777,600 Joules of Energy Storage, assuming it stays at 12V the entire time which it will not.
 

Alec_t

Joined Sep 17, 2013
12,321
It's not a good idea to rely on the full Ah rating for driving a load, because completely flattening a battery will damage it.
If the voltage of a 12V Lead Acid battery voltage gets below ~10-11V when discharging it may well suffer damage.
 

Thread Starter

biferi

Joined Apr 14, 2017
203
Well I know my Battery is Rated 12 Volts at 18 AH.

I know if I Connect it to a Load that will need 12 Volts at 5. Amps this will Run Down the Battery in a different Time Frame then if I Connect it to a Load that needs 12 Volts at 12 Amps.

I always thought if somethings Says 18 AH it means it could Supply 18 Amps for 1. Hour?

Just to let you know what it can do not that you use it like that?
 

Irving

Joined Jan 30, 2016
2,322
I always thought if somethings Says 18 AH it means it could Supply 18 Amps for 1. Hour?
Lead acid batteries are quoted at the 20H rate, so when brand new they will supply 1/20 of their capacity for 20H, or in this case 900mA for 20H, but that is to 100% discharge. Here is MK's definition (one of the few SLA battery manufacturers I trust)


20-hour rate
The number of amps that can be withdrawn at a constant rate for 20 hours @ 80 F before voltage drops to 1.75 volts per cell (discharged).

1.75V per cell = 10.5v

If you discharge it that far regularly you'll get maybe a couple of 100 charge cycles.... consider80%, 11.5v the maximum you should go.
 

dcbingaman

Joined Jun 30, 2021
498
It's not a good idea to rely on the full Ah rating for driving a load, because completely flattening a battery will damage it.
If the voltage of a 12V Lead Acid battery voltage gets below ~10-11V when discharging it may well suffer damage.
Astutely true! The discharge rate and how long must be considered. I was not clear about that on the theory. There are limits in the real world.
 

Irving

Joined Jan 30, 2016
2,322
There are many other factors you need to consider: One, as already discussed, is that the discharge time reduces as discharge rate increases. Here are the curves for an MK 18Ah SLA . If you limit Depth of Discharge (DoD) to 80% or about 11v you can see that at 18A you can expect 25min tops and thats for an MK. If your supplier can't provide the curves or a data sheet for the battery (that means most cheap far-eastern batteries) you won't get better... and these are for a brand new battery, after a 100 or so charge cycles, depending on how much you discharge them, you'll be lucky to see 90-95% of this.

1626553492735.png

A second issue is how well you charge them. The absolutely worst thing you can do is put them on some cheap no-name charger and take them off when the ready light comes on... thats the fastest way to kill them - a generic charger rarely charges effectively, and they almost certainly won't be fully charged when it says they are. They need to be charged at the manufacturers specified voltage/current until they reach a specific voltage, then float at a specified lower voltage for as long as possible to absorb as much charge as they can. There are good configurable chargers out there, more expensive but depends on how important your battery is to you, and how long you want them to last.
 

dcbingaman

Joined Jun 30, 2021
498
Nice information. Thanks. I am currently working on a wireless temperature sensor that checks the temperature every minute. Stores it in memory and downloads it to a computer upon request via a wireless connection. I am trying to find you how long some D cells will last (Alkaline) when running at around 1mA average current. The processor pulls 20mA but I put it to sleep for 59 seconds and only wake it up for one second out of 60. Same for the wireless UART. It consumes another 20mA, but I keep it turned off except for 1 second out of every 60. When the computer wants to download the information it queries the device until it responds (1 second window in 60 seconds) then stays on until all data has been transmitted. Then goes back to sleep. I thought about using a battery smaller than a D cell but even at just 1mA max average current the D cell seems to work best at 6.5 Amp-Hours lasting 6.5/0.001 according to my calculations or about 270 days. Am I making any mistakes in my calculations?
 
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