# Battery capacity and duration

I have a light that needs to be powered by battery. It is a 12V light that draws 4.5 A of current. If I purchase a SLA battery that is 12V rated 5.0 Ah would that battery be sufficient? I am confused on the whole Amp/hours rating. Is a 5.0 Ah battery able to deliver 4.5 A of current? And for how long? Will be using deep cycles probably lighting the light for 10 seconds at a time. Any help appreciated.

• It depends what kind of light it is and the V vs I curve. Commented Sep 13, 2017 at 22:03
• Unless you find the spec sheet that tell you the discharge rating, any attempt to infer it from capacity is only as good as a educated guess based on the type of battery. Commented Sep 13, 2017 at 22:22

The total capacity rating ("5 Ah") has little relationship with how much current the battery can deliver. I would guess that your SLA battery will be able to deliver much more than 4.5A.

CORRECTION: Tony is correct, the 5AH is usually based on 20-hours discharge time. This means that to get 5AH capacity, the discharge current shouldn't exceed 250 mA. Here is an example of typical SLA 12V/5AH battery. The datasheet says that the battery will last only 30 minutes under 5.7 A load, so the battery capacity gets down to 2.85 Ah. So at 4.5 A it might last about 40 minutes of continous discharge. Impulse-type discharge, however, is more favorable, so my initial estimate of 360 bursts might be correct.

The "initial current" of 1.5A is a requirement for charging mode. And, BTW, the battery under my reference can deliver 75 A, but for a really short time of 5 seconds.

• It says on the battery "initial current less than 1.5 A". That's where I am confused. Would it deliver the 4.5 A needed when the switch is turned on?
– NJG
Commented Sep 13, 2017 at 22:49
• no @NJG It is under needed capacity for your load. Commented Sep 13, 2017 at 22:57
• @NJG, yes, if this is a regular commercial-grade SLA battery, it will deliver 4.5 A, easily. See above. Commented Sep 13, 2017 at 23:28
• Ali, I see your battery and data sheet. But this makes it confusing. You say yes, others say no. Here is the battery I was going to purchase: homedepot.com/p/…
– NJG
Commented Sep 13, 2017 at 23:46
• But others say I should get a higher Ah battery such as 15-20 Ah range instead of 5. I'll only use it for maybe 15-30 times at 10 seconds of use before putting it on a charger.
– NJG
Commented Sep 13, 2017 at 23:48

AH rating = Amps*20 for 20 hour load the standard test time.

   Capacity = "5Ah"   >>>>>>>>  4Ah  >>> 3Ah

(total hours * Amps = Ah)
20h* =    10h*  =     5h*  =      1h*  =     ?56 minutes
Amps Ah    Amps Ah    Amps Ah     Amps Ah    Amps  Ah
0.25  5.0   0.43 4.3   0.8  4.0    3.0  3.0   4.5   2.5?


So 10 second bursts of 4.5A may yield on a fresh full charged "PowerSonic 5Ah SLA" about 150 amp-seconds or ~ 33 bursts.

Ref data

• Thanks Tony, however it says on this battery "initial current less than 1.5 A". That's where I am confused. Would it deliver the 4.5 A needed when the switch is turned on?
– NJG
Commented Sep 13, 2017 at 22:58
• @NJG, why don't you post a link to actual datasheet for your battery? Commented Sep 13, 2017 at 23:03
• There are reasons why it states initial current must not exceed 0.3*C or 1.5A in your case so you need to do this or get a 15Ah rating besides at this rate you only get <50% capacity from SLA's Commented Sep 13, 2017 at 23:08
• read your datasheet or mine if you don't understand. These are not LiPo's Commented Sep 13, 2017 at 23:11
• Ok, so I would need a battery where on the specs it says it can deliver more than 4.5 A on initial current like this one?
– NJG
Commented Sep 13, 2017 at 23:24