Battery for WS2812 Strip? (Something's not right)

Question

I have posted this in a few places, so sorry if you have seen it somewhere else

Im working on a battery powered led strip project,
iv put the project together and have been testing with a mains power supply
Iv run some tests and found at max I'd be pulling 4.5 amps
Averaging around 2 ish

I rounded out to about 3.5 amps, and said for 90 mins run time id be looking at the 7ah battery
7/2 = 3.5 so 2 hours runtime
I got a 7ah SLA 12v battery, running though a 12v to 5v converter to power the led strips
(Arduino is running off a usb battery pack)

However after about 45 mins I tested the battery and it said 10.9 v
Which I understand to be too low for SLA batteries (11 should be the minimum?)
so I shut it off

My question is did I mess up somewhere and get my math wrong?
Many thanks in advance

Answer 1

There are a few things that may contribute to your lower than expected battery lifetime.

Effective capacity: you should aim to never use more than 50% of the capacity of a lead acid battery or you will shorten its lifespan considerably.

Also, note that lead acid battery capacity is usually given for a discharge time of 20 hours (C20 rating). The “C” rating could be different but its value should be on the label of the battery.

In the case of a stated capacity of 7Ah at C20, this means the capacity has been calculated at a discharge rate of 7Ah/20h = 350mA. For higher discharge rates, such as in your application, the effective capacity will be much lower.

Battery voltage: measuring battery voltage under load does not say much. Measure the battery voltage “at rest”, when completely disconnected from the load for about 1 hour.

Charging: Are you charging the battery with a proper charger?