Battery Understanding

[diverryan]

When it comes to electronics, I am a bit behind. Can someone please explain to me what wattage is being put out from these HID canister lights with 4.5 amps and 18 amps? I know this is probably a dumb question, but I am a bit confused.

 

[jonnythan]

The wattage is a function of the light head. The most common HID lights have 10 watt, 18 watt, 21 watt, or 24 watt heads.

The battery pack rating is in a unit called "amp-hours." A 1 aH battery pack can deliver a current of 1 amp for 1 hour. A 4.5aH battery pack can deliver a current of 4.5 amps for 1 hour, or 1 amp for 4.5 hours.



Watts = volts * amps

So a 10 watt light head with a 12 volt battery pack means:

10 watts = 12 volts * x amps

So the battery pack must deliver about 0.83 amps to power the head. In reality, it's not 100% efficient, so you can just say 1 amp.

A 4.5 aH battery pack can, ideally, deliver that 1 amp current for 4.5 hours. Typically you will see about 4 hours in reality.


For an 18 watt light head:

18 watts = 12 volts * x amps

x works out to be about 1.5 - 1.7 amps because the power used by the lighthead is actually a little bit more than 18.

So, a 4.5aH battery would last about 2.6 hours ideally.. in reality a bit less, say a bit over 2 hours. A 9aH battery pack would last twice as long - a bit over 4 hours.

 

[cool_hardware52]

When it comes to electronics, I am a bit behind. Can someone please explain to me what wattage is being put out from these HID canister lights with 4.5 amps and 18 amps? I know this is probably a dumb question, but I am a bit confused.

Volts x amps = Watts (A watt is a unit of power.)

Watts x time (hours) = watt-hours (Watt-hour is a unit of energy)

In your example "4.5" refers to the amphour capacity of the battery, likely 4.5 amphours @ 12 vds for a total of 4.5 x 12 = 54 watthours of stored energy

I'm guessing that the "18" in your example is an 18 watt HID lamp. Here things get a little bit confusing. Lamps consume power (watts) A popular HID buld consumes 18 watts, but the "ballast" or voltage converter required to operate the bulb also consumes a bit of power, typically ~15% of the bulb wattage, lets say 3 watts. Total power required to drive the bulb and ballast 18+3 = 21 watts.

This 21 watts is the power required. You have in theory 54 watt-hours of battery

54 watt-hours/ 21 watts = ~2.6 hours or about 150 minutes

Wattage does not equal brightness or lamp output. All thing being equal, i.e. same type of lamp, same efficiency sam reflector etc., more wattage equals more light.

Clearly some bulbs are more efficient, i.e. producing more "Lumens per watt" or more lightoutput per unit power consumed. That's where HID lamps outperform incandescent. HID's are ~2.5-3.0 times as efficient than incandescent lamps.

Hope this helps,


Tobin

 

[Don Burke]

Most small batteries are rated for a twenty hour discharge.

A 4.5 amp-hour will put out .225 amps for twenty hours before the output voltage drops to a specified percentage (which I have forgotten) of the rated voltage of the battery.

Shorter discharge times (higher current) will reduce the useable capacity of the battery. Somewhere around here I have a graph describing how much the reduction is.

 

[cool_hardware52]

Most small batteries are rated for a twenty hour discharge.

A 4.5 amp-hour will put out .225 amps for twenty hours before the output voltage drops to a specified percentage (which I have forgotten) of the rated voltage of the battery.

Shorter discharge times (higher current) will reduce the useable capacity of the battery. Somewhere around here I have a graph describing how much the reduction is.

Not necessarily true. Most SLA batteriers are rated at a 20 hour discharge rate, some at 10. Most cylindrical Ni-Cads and NiMh are rated at a 1 or 5 hour rate.

This rating convention springs from the typical commercial apps. SLA's are often used for UPS units, or alarm backup, where low currents for long periods are necessary.

Ni-Cads, and now NiMh are often used for period high current loads like power tools.

You most certainly need to derate a "20 hour" SLA id you are using it in an application (like a scooter for example) where the batery will be discharged in 1-3 hours.

For most lighting apps using Ni-Cads or NiMh, the "burn times" correspond to the "rating times" well enough that little of no derating is required.

Ni-Cads and Nimh batteries typically have a "minimum rated" capacity, with the better quality cells delivering more than the min. Quite often with a new set of cells you will get slightly more than "rating".


Tobin

 

[diverryan]

Jonnythan and cool_hardware52, thanks so much for your info. It really helped!