On Saturday, May 17, 2014 2:10:06 PM UTC-7, wrote:
As Jon says, it is always good to have a look at the spec.

Taking the PowerSonic PS1290 spec as an example

http://powersonic.com/images/powerso...10-PS-1290.pdf



At the 5-hour discharge current of 1.44A, the specified capacity for this PS1290 9Ah SLA battery is 7.2Ah.

Richard measured 180 min = 3 hours @ 1.4 Amps = 4.2Ah.

4.2Ah of remaining capacity out of the specified 7.2 Ah means that this battery has deteriorated to 4.2/7.2 = 58% of the original capacity.

If this is a used battery, with a number of deep-discharge cycles, this deterioration is quite normal.

If this a new SLA Lead Acid battery, this is a very poor specimen.



Any rechargeable battery is a consumable.

The capacity decreases with each and every discharge-charge cycle.

The use case has a big influence on the rate of deterioration, e.g: shallow vs. deep discharge.

Looking further down the spec sheet of the Lead Acid battery, in the "life Characteristics in cyclic use" graph, you can see that this battery has a life expectancy of only about 200 deep-discharge cycles (100% discharge) and about 1100 cycles of shallow (30%) discharge.

This is why I run my batteries in parallel.



Similar calculations can be made for the LiFePO4s. I have no specification for this chemistry.

Assuming that the LiFePO4 9Ah battery derates similarly to 7.2Ah at the 5-hour current draw, Richard's measurement would indicate:

280 minutes = 4.66 hours @ 1.4 Amps = 6.53Ah or 6.53/7.2 = 91% of capacity.

Not too bad.



As said, I have all 3 of my fused SLA batteries switched in parallel. I am very comfortable with that (and no, one battery does not charge the other !)

As far as I know, the LiFePO4 packaged batteries have build-in discharge and charge protection circuits. With those, I would need to understand the spec in much detail before I would be comfortable switching them in parallel.

3U

I think more relevant to gliders is how deep the battery can usefully be discharged. SLA specs usually regard completely discharged as 10.5V. Modern electronics have DC-DC power supplies inside, as the input supply voltage deteriorates the current consumption goes UP, in order to supply the same currents at 5 or 3.3V to the electronics internally. Also when you hit the transmit button to announce your pattern at the end of the day, in my glider at least the current goes from about 0.6A to 1.6A. Add another 20% due to low voltage and I am trying to draw 2 amps from a mostly depleted battery, dropping the voltage further.

I don't care if I only get 200 deep discharge cycles from a $30 SLA - that represents many years of flying, the battery will decay from age before that. But the LiFe will hold its output voltage up higher when it is deeply discharged, according to Starkpower well above 12V even at 90% discharge. Another potential advantage of LiFe is that they need not be recharged fully immediately after use as they do not sulfate. Most Li battery vendors seem to recommend leaving them in a partially charged state (50-80%) for storage. So if you forget to charge it after the weekend's flying, you will do less damage to it.